nanobubbles and the nanobubble bridging capillary force

Advances in Colloid and Interface Science 154 (2010) 30–55

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Advances in Colloid and Interface Science

j ourna l homepage: www.e lsev ie r.com/ locate /c is

Nanobubbles and the nanobubble bridging capillary force

M.A. Hampton ⁎, A.V. NguyenSchool of Chemical Engineering, The University of Queensland, Brisbane, 4072, Australia

⁎ Corresponding author.E-mail addresses: [email protected] (M.A. Ham

0001-8686/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.cis.2010.01.006

a b s t r a c t
a r t i c l e i n f o
Available online 22 January 2010

Keywords:NanobubblesNanobubble bridging capillary forceDissolved gasWaterAtomic force microscopy

Interactions between hydrophobic surfaces at nanometer separation distances in aqueous solutions areimportant in a number of biological and industrial processes. Force spectroscopy studies, most notably withthe atomic force microscope and surface-force apparatus, have found the existence of a long rangehydrophobic attractive force between hydrophobic surfaces in aqueous conditions that cannot be explainedby classical colloidal science theories. Numerous mechanisms have been proposed for the hydrophobic force,but in many cases the force is an artifact due to the accumulation of submicroscopic bubbles at the liquid–hydrophobic solid interface, the so called nanobubbles. The coalescence of nanobubbles as hydrophobicsurfaces approach forms a gaseous capillary bridge, and thus a capillary force. The existence of nanobubbleshas been highly debated over the last 15 years. To date, experimental evidence is sound but a theoreticalunderstanding is still lacking. It is the purpose of this review to bring together the many experimental resultson nanobubbles and the resulting capillary force in order to clarify these phenomena. A review of pertinentnanobubble stability and formation theories is also presented.

pton), [email protected] (A.V. Nguyen).

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© 2010 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312. Evidence of nanobubbles revealed by AFM imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.1. The effect of dissolved gas and surface pre-treatment on nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.2. Coalescence and Ostwald ripening of nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.3. Effect of temperature and pressure on nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.4. Influence of surface roughness on nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.5. The effect of electrolyte concentration on nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.6. Influence of surface tension and contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.7. Electrochemical formation of nanobubbles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3. Evidence revealed by atomic force spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.1. Variability of the force measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2. Steps in the force curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.3. NBCF model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.4. The effect of dissolved gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.5. Increased NBCF range with surface scanning due to nanobubble coalescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.6. The effect of electrolyte concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.7. Influence of surface tension and contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4. Other experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.1. Rapid cryofixation/freeze fraction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454.2. Infrared spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5. Nanobubble theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.1. Nanobubble stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.1.1. Why nanobubbles should not be stable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475.1.2. Line tension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.1.3. Surface forces between the vapor–liquid and vapor–solid interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485.1.4. Dynamic equilibrium and the density depletion layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

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31M.A. Hampton, A.V. Nguyen / Advances in Colloid and Interface Science 154 (2010) 30–55

5.1.5. Reduction of surface tension. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.1.6. Summary of nanobubble stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.2. Nanobubble formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.2.1. Spontaneous formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505.2.2. Exposure to gas super-saturated liquid environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.2.3. Surface perturbation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515.2.4. Gas-trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525.2.5. Summary of nanobubble formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6. Applications of nanobubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.1. Hydrophobic coagulation and hetero-coagulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.2. Electrochemical antifouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.3. Friction and adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526.4. Dissolved air flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536.5. Boundary slip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

7. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1. Introduction

The discovery of nanobubbles and the nanobubble bridgingcapillary force (NBCF) resulted from research into the long rangehydrophobic attractive force (LRHAF) between hydrophobic bodies inaqueous solution. Since the first experimental evidence by Blake andKitchener [1], the debate over the mechanism of the LRHAF has beenactive. It is now clear that in many cases the “hydrophobic” forcemeasured was an artifact due to the presence of gaseous domainscalled nanobubbles at the liquid–hydrophobic solid interface. Thus,what was thought to be a hydrophobic force was actually a capillaryforce resulting from the gaseous bridge formed from the coalescenceof nanobubbles, that is, the NBCF.

The concept of the NBCF was introduced in 1994 by Parker et al.[2], investigated further [3–5], then atomic force microscopy (AFM)tapping mode images confirmed the existence of nanobubbles [6,7].Many groups have investigated nanobubbles, with some refuting [8–14], but many more supporting the phenomena. Research onnanobubbles and the resulting capillary force has focused on forcespectroscopy and direct imaging of nanobubbles utilizing the AFM.There are also a number of other experimental and theoretical studieswhich suggest nanobubble existence. An in-depth discussion andcritical analysis of previous research on nanobubbles is presented, andis separated into five sections: AFM imaging; AFM force spectroscopy;other experimental methods; nanobubble theory; and future researchand applications of nanobubbles.

Before proceeding with the review it is important to point outthree issues. Firstly, the meaning of the word “hydrophobic” must beconfirmed. In this article a hydrophobic surface refers to a surface thatis not completely wetted by water (contact angle larger than 0°). It iscommon place for a hydrophobic surface to be described as a surfacethat results in a water contact angle above 90°. This terminology is notused within this article as many of the experimental results presentedare for surfaces less than 90°, such as Highly Oriented PyrolyticGraphite (HOPG) which has a contact angle of 80°, but is still con-sidered as hydrophobic in many contexts. Secondly, throughout thisreview the contact angle is taken within the denser phase. Finally,much of the review is focused on results measured with an AFM. It isnot the purpose of this article to investigate the function of the AFM,thus for further information the reader is suggested to consult theAFM references here in.

2. Evidence of nanobubbles revealed by AFM imaging

The most direct evidence of nanobubble accumulation at thewater–hydrophobic solid interface comes from tapping mode AFM

images on a variety of hydrophobic surfaces [6,7,15–55]. Opticalmethods for observing nanobubbles are unsuitable as the bubbles aresmaller than the wavelength of light. Also, contact mode AFM is noteffective as the probing force tends to be too high for the softnanobubbles [36]. Some works do use the contact mode methodutilizing a low contact force [24,27], but the quality of the imagessuffers and doubts are cast on whether the domains imaged arenanobubbles or contamination (see Section 2.2). Therefore, tappingmode AFM is the ideal technique as minimum force is placed on thesurface from the cantilever tip.

The first images by Ishida et al. [6] on octadecyltrichlorosilane(OTS) silanated silica in pure water showed that the nanobubbleswere randomly distributed on the surface and of a flat hemisphericalshape with heights and diameters less and 40 nm and 650 nm,respectively. Similar results have been found by a number of othergroups, with excellent examples shown by Zhang et al.[46,50,51,53,54], Yang et al. [43,44] and the present authors [20–23,25]. An example of a nanobubble tapping mode height image and acorresponding cross-section taken by the author for 1-octanolesterified silica (advancing and receding water contact angles of 80°and 65° respectively) in water after solvent-exchange (see Section 2.1for more information on the solvent-exchange process) is shown inFig. 1. The nanobubbles in Fig. 1A are randomly oriented and sized,and as indicated by the cross-section (Fig. 1B), very flat with a watercontact angle of more than 150°.

There was initial skepticism of the nanobubble images, with anumber of suggestions that the domains imaged were in fact con-tamination. The contamination issue raised by Evans et al. [9] appearsto be correct for nanobubble images such as those produced by Tyrelland Attard [36,37], whose “nanobubbles” are irregular and close-packed. Evans argued that the domains imaged were due to poly-merization of OTS on the surface. It is the authors view that theirregular shape of the domains is actually contamination and that thecorrect morphology of nanobubbles is an array of flat hemisphericaldomains, as shown by a multitude of groups. Proving that the hemi-spherical domains imaged are not contamination has been a chal-lenge, but evidence in the remaining sections indicates that thedomains are indeed gaseous nanobubbles.

Another gaseous formation, the so called “nanopancakes” [22,54],have been found on HOPG, as shown in Fig. 2. As the name suggests,the nanopancakes appear as flat gaseous layers that either form aloneor in conjunction with nanobubbles. The shapes of the pancakes areinfluenced by the cleavage steps of the HOPG surface. Phase images ofthe structures, as shown in Fig. 3, indicate that they are softer than theunderlying hydrophobic surface, suggesting that nanobubbles andnanopancake structures are of the gas phase [6,15,16,22,30,34,42].

Fig. 1. A) A 5×5 µm AFM tapping mode height image of nanobubbles on 1-octanolesterified silica (advancing and recedingwater contact angles of 80° and 65° respectively)inwater after 1-propanol–water exchange (see Section 2.1), and B) a corresponding cross-section. Imaging was performed with a Veeco Nanoscope IV AFM using HANC/15 Etalon(NT-MDT) cantilevers (120 kHz, 3.4 N/m) at a scan rate of 1 Hz [20].

Fig. 2. A) A 3×3 µm tappingmode AFM image of nanobubbles and pancakes onHOPG in1 mMNaCl with b) corresponding cross-section. The nanobubbles were produced usingethanol–water exchange (see Section 2.1) and then 1 mMofNaClwas slowly introducedinto the fluid cell. Imaging was performedwith a Veeco Nanoscope IV AFM using HANC/15 Etalon (NT-MDT) cantilevers (120 kHz, 3.4 N/m) at a scan rate of 1 Hz [22].

32 M.A. Hampton, A.V. Nguyen / Advances in Colloid and Interface Science 154 (2010) 30–55

An understanding of the accumulation of gas at the liquid-hydrophobic surface has become even more complex with theobservation by Zhang et al. of bi- and tri-layers of gas, as shown inFig. 4 [46]. Zhang observed that tri-layers were preferentially formedwhen the degree of gas super-saturation was increased, and that thethickness of each layer increases from the bottom to the top layer.Another interesting observation was that the layered gas domainschanged into nanobubbles due to cantilever tip perturbation. Does thissuggest that nanopancakes are the pristine gaseous formation andnanobubbles are just an artifact caused by tip perturbation? Thisquestion will be analyzed throughout this review.

Are the domains imaged in the above figures gas or contamina-tion? This is a controversial aspect of nanobubbles and has beenhighly debated over the past 15 years. The focus of the review willnow explore many of the experimental observations that indicate thatthe domains imaged are actually gas.

2.1. The effect of dissolved gas and surface pre-treatmenton nanobubbles

Anumberof groupshave suggested thatnanobubbles arenotnative tothe hydrophobic solid–water interface and are formed due to contactbetween the imaging tip and the hydrophobic surface [12,34,35] (seeSection 5.2.3). Also, it has been found that one type LRHAF is due to cavityformation between hydrophobic surfaces [56–58] (see Section 3.1). Uponretraction the cavity leaves a nanobubble on each surface. Nanobubbleformation due to cavity formation between the imaging tip andhydrophobic surface is one possible explanation for large hydrophobictips (suchas colloidal tips) andsurfaces (contact angle>90°), butdoesnot

explain the formation of nanobubbles on surfaces and nano-dimensionaltips with contact angles below 90° (see Section 5.2.3).

To date, themost important discoveries in nanobubble understandinginvolve investigations on surface pre-treatment. Previous studies showedthat nanobubbles could only form on HOPG and other low hydrophobicsurfaces (<90°) after solvent-exchange [7,33,41,43,49–54,59]. It isinteresting tonote thatnanobubbles couldalsobe formedonahydrophilicmica or silica after treatment; however the population and volume wasdramatically smaller than on hydrophobic surfaces [15,33,40,48,55]. Thesolvent-exchange process involves flushing an alcohol, typically ethanol,and then water through the AFM fluid cell and over the hydrophobicsurface. The solvent-exchange process appears to be a requirement ifsurfaces are smooth or have a contact angle below90°,whereas for highercontact angle surfaces the pre-treatment is not a requirement [34,52] (seeSection 5.2).

The results from our studies agree with the solvent-exchangephenomena, as shown in Fig. 5.When the hydrophobic esterified silicasurface is not treated with solvent-exchange no nanobubbles areobserved (Fig. 5B). After solvent-exchange the population and volumeof nanobubbles increases, with the higher molecular weight alcoholresulting in the largest gas accumulation. The increased amount of gasat the surface agrees with AFM force spectroscopy studies of the NBCFafter different solvent-exchanges, as discussed in Section 3.4.

The mechanism behind the formation of nanobubbles withsolvent-exchange is believed to be linked to the super-saturation ofgas in the liquid phase [50]. As alcohol (higher gas solubility) isreplaced with water (lower gas solubility) the resulting alcohol–water mixture becomes super-saturated due to the non-linearity ofthe solubility–concentration curve. Because of the higher gas

Fig. 3. A 5×5 µm tapping mode AFM A) height and B) phase image of nanobubbles andnanopancakes on HOPG in 1 mMNaCl. The nanobubbles were produced using ethanol–water exchange (see Section 2.1) and then 1 mM of NaCl was slowly introduced into thefluid cell. Imaging was performed with a Veeco Nanoscope IV AFM using HANC/15Etalon (NT-MDT) cantilevers (120 kHz, 3.4 N/m) at a scan rate of 1 Hz [22].

Fig. 4. A) A 3.1×3.1 µm tapping mode height image of a tri-layer gaseous formation onHOPG and A') a corresponding cross-section. The tri-layer was created by submerging aHOPG surface at 40 °C to 4 °C water. Imaging was performed with a Veeco NanoscopeIIIa AFM using NP-S (Veeco) cantilevers (0.58 N/m) [46].


solubility for longer chain alcohols more gas in the form ofnanobubbles is produced for 1-propanol compared to ethanol, asindicated by the results in Fig. 5. For more information on theformation mechanisms behind solvent-exchange see Section 5.2.2.

The importance of gas super-saturation is confirmed by resultsshowing that significantly less nanobubbles are formed if the ethanoland water used in the exchange are degassed [19,54,55]. Thetemperature of the fluids in the artificial creation of nanobubblesusing solvent-exchange also influences the formation of nanobubbles.As the temperature of the alcohol and water are increased the densityof nanobubbles (after reaching ambient temperature) formed onexchange increases slowly from a temperature of 9 to 30 °C thendramatically increases after 30 °C [55]. An increase in the differencebetween the gas solubility of the alcohol and water at highertemperatures is believed to be the explanation for the temperaturephenomena and further indicates the importance of gas super-saturation in the solvent-exchange formation of nanobubbles.

Further proof that dissolved gas super-saturation is importantcomes from a complementary study by Zhang and Ducker [49], whichinvestigated the creation of nanodroplets of decane using the solvent-exchange method. To create nanodroplets, the surface and the AFM

liquid cell were first flushed with a 25–60% ethanol/water solutionsaturated with decane. Flushing with water saturated with decanefollowed (lower solubility of decane compared to ethanol solution).This process created decane nanodrops of approximately 1–10 µmacross and 10–500 nm thick. Zhang and Ducker also concluded thatmore droplets formed if the difference in decane solubility betweenthe two solvents increased, and also that the first solvent needs to beclose to saturation whereas the second solvent does not. These resultsconfirm that the super-saturation of gas/oil in the liquid mixture isimportant for the formation of nanobubbles/drops.

The importance of other surface pre-treatments on nanobubbleformation has been clarified by results demonstrating that thepopulation and size of nanobubbles on the hydrophobic surfacecould be manipulated in a number of ways including: increasing thetemperature of the solution in which the surface is immersed [43];displacing cold water with warmwater over the surface [54]; and pre-heating the surface before immersion into water [54]. In each of theseformation methods, gas super-saturation is important. Anothermethod of creating nanobubbles effectively is by simply cleaningthe hydrophobic surface with alcohol followed by drying [43]. Thereason for the influence of alcohol cleaning is puzzling but Yang et al.[43] believe it to be the result of residual water removal from thesurface. How this results in nanobubbles in unclear.

The solvent-exchange process created a big leap towards a greaterunderstanding of nanobubble formation and helped solve some of theinconsistencies in nanobubble measurement. A number of groups haveargued against the existence of nanobubbles as they could not be foundusing their experimental system. For example, the X-ray reflectivity studyof Mezger et al. [12] indicates no nanobubbles at the water/OTS–silicainterface, but a thin density depletion layer (1–6 Å thick). Measurements

Fig. 5. 5×5 µm AFM tapping mode height images of 1-octanol esterified silica(advancing and receding water contact angles of 80° and 65° respectively) in waterA) without exchange, B) after ethanol–water exchange and C) after 1-propanol–waterexchange. Imaging was performed with a Veeco Nanoscope IV AFM using HANC/15Etalon (NT-MDT) cantilevers (120 kHz, 3.4 N/m) at a scan rate of 1 Hz [20].

Fig. 6. A 5×5 µm AFM tapping mode height image of nanobubble coalescence. A2×2 µm area of the surface shown in Fig. 1A is scanned in light contact mode. Theshaded area indicates the area scanned by light contact mode. It is obvious that thenanobubbles in this area have coalesced to form larger nanobubbles [20].


of the water/OTS–silica interface using ellipsometry were also unable tofind nanobubbles [10,13]. Neutron reflectivity [8], electrochemical quartzcrystal microbalance [14] and evanescent wave AFM [11] have also

concluded that no nanobubbles are present. The author's agree with thefindings of theseworks, but believes that nanobubbleswould be observedusing these experimental techniques if the surface pre-treatmentssuggested above had been implemented. Further experimentation withthe above techniques will be useful to further validate the solvent-exchange process and give greater insight into the characteristics ofnanobubbles.

The influence of dissolved gas concentration on nanobubble mor-phology and population is the most obvious reason why the domainsfound on hydrophobic surfaces in aqueous solutions are believed tobe gaseous. An example of this is the solvent-exchange process, butother examples are also found in the literature. In the case of highlyhydrophobic surfaces (contact angle more than 90°) no observablenanobubbleswere formedwhen the surfacewas immersed in degassedwater, but were present in gassed water [26]. Additionally, noobservable nanobubbles are formed when the surfaces are renderedhydrophobic in-situ without any exposure to the atmosphere [6].

The authors' believe that the above effects of solvent-exchangeand dissolved gas concentration are solid proof that the domainsimaged at the liquid–hydrophobic solid interface by AFM are indeedgaseous nanobubbles. Additional evidence for the gas effect isdiscussed in Section 3.4, which describes the influence of dissolvedgas on the NBCF. Rejections of the existence of nanobubbles by otherresearchers are based on experimental systems in which nanobubbleformation are not favorable. Also, these results rule out thatnanobubbles are domains of contamination as the influence ofdissolved gas on contaminants is negligible.

2.2. Coalescence and Ostwald ripening of nanobubbles

Another indication that the dome structures on the hydrophobicsurface are gas is the ability tomake the structures coalesce into largerdomes. The phenomenon of nanobubble coalescence has beenobserved by a number of studies using tapping mode AFM imagingbefore and after increased tapping force [15,16,18,19,34,46,47]. Forexample, Simonsen et al.'s tapping mode images of a polystyrenesurface in water showed that nanobubbles covered 61% of the surface[34]. After increasing the tapping amplitude on a small area it wasfound that the nanobubbles in the region fused into one large bubble.A similar procedure was undertaken by our group to coalescenanobubbles on hydrophobic silica, but instead of using higher

Fig. 7. AFM tapping mode height images of a 1H,1H,2H,2H-perfluorodecyldimethyl-sholorsilane silanated silicon wafer (advancing contact angle 105°) in water at asubstrate temperature of A) 25 °C and B) 30 °C. The increase in temperature resulted innanobubble formation at the scratch. The substrate was heated by a heating stagebelow the sample. AFM tapping mode imaging was performed with a PicoSPM(Molecular Imaging) AFM using Si3N4 tipped cantilevers (NSC18/AlBS, MikroMasch,20 kHz, 0.9 N/m) [43].


tapping force to coalesce the nanobubbles, which was found to beineffective, light contact mode was used [20]. In Fig. 6, a 2×2 µm areaof the surface shown in Fig. 1a is scanned in light contact mode. It isobvious that the nanobubbles in this area have coalesced to formlarger nanobubbles.

It must be mentioned that other groups have used light contactmode to image nanobubbles directly, but did not observe coalescence[24,27]. For example, Holmberg et al. [24] imaged nanobubbles onunmodified gold surfaces in pure water (with no solvent-exchange).In light contact mode, domains with an apparent height around 1–1.5 nm and diameters around 100 nmwere imaged.When the contactforce was increased these domains disappeared or were flattened, butre-emerged to the original state when contact force was reduced.Holmberg et al.'s work cannot fully explain why the nanobubbles donot move or coalesce, but suggest that adsorbents from theenvironment or characteristics of the surface not observable areresponsible. It is the authors' view that either contamination orpinning at the three-phase contact line is responsible for the behavior.

In Holmberg et al.'s work it is interesting that the equilibriumwater contact angle of the unmodified gold surface used in theseexperiments is 100°. Other authors have shown that thoroughlycleaned gold exhibited a zero contact angle (due to gold oxide), whichwhen immersed in ethanol had an equilibrium contact angle of 65°(due reduction of gold oxide), and when modified with a C16SHsolution had an angle of 106° (due to hydrophobic layer) [60]. Why isthe unmodified gold surface in Holmberg's research so hydrophobic?The possibility of contamination on the unmodified gold surface ishigh as the surfaces were stored in the ambient atmosphere and thereis no mention of a cleaning procedure. Thus, the domains imaged onthe unmodified gold surface could be contamination, explaining whythe domains could be imaged in contact mode and could not bemoved by a higher contact force. Similar reasons for the domainsimaged by Jeon et al. [27] on silicon are also justified.

Contact line pinning is another alternative for the inability to moveHolmberg's nanobubbles. A good example of contact line pinning is ofnanobubbles on HOPG, as shown by Zhang et al. [53]. As discussed inSection 2.4, nanobubbles preferentially accumulate at the HOPGcleavage steps. These steps pin the nanobubbles in placewhen imagedunder contact mode, and increasing force tends to flatten thenanobubbles which return to the original conformation uponlowering the imaging force. In a recent study, Wang et al. [38]found that the nanobubble coalescence due to tip perturbation wasreduced by nanoindents on polystyrene films and nanoindents andisland structures on partial polystyrene films.

The unmodified gold surfaces used by Holmberg et al.'s do notappear to contain obvious roughness or heterogeneity at which thenanobubbles can pin, further indicating that the domains imaged arecontamination. It is believed that the ability to move and coalesce theimaged domains is an excellent test to prove that the domains onultra-smooth surfaces (i.e., no pinning) are actually gaseous nano-bubbles, otherwise contamination maybe responsible.

Another indication that the domains imaged are gaseous is thenatural coalescence (i.e. without tip interaction) and/or ripening. AFMimaging found that neighboring nanobubbles naturally coalesced[28,42], and smaller nanobubbles decayed and dissolved completelywhile the neighboring nanobubbles became larger [42,52]. Thisexample of Ostwald ripening is characteristic formacroscopic bubbles,and further indicates that the dome structures are gaseous. It must bestated that the above phenomena are also possible for liquids, but dueto the mounting evidence for the gas influence the author remainscertain that the domains imaged are gaseous nanobubbles.

2.3. Effect of temperature and pressure on nanobubbles

If nanobubbles are indeed gas, changes to the temperature andpressure of the surrounding environment should change the size and

shape of the nanobubbles. Studies by Yang et al. [43] have shown thatan increase in the temperature of the hydrophobic surface (25–30 °C)leads to the preferential formation of nanobubbles in the vicinity ofsurface asperities, as shown in Fig. 7. An increase in volume,coalescence and disappearance of those nanobubbles results withfurther increases in temperature (up to 40 °C). The reason for theseresults is not explored by Yang et al., but it is obvious that surfaceasperities and temperature are important for nanobubble formation.Possible explanations for this phenomenon include changes to thedynamic stability of nanobubbles (see Section 5.1.4) and local gassuper-saturation (see Section 5.2.2). Another possible explanation isthat small pockets of gas are trapped within these rough areas onimmersion (see Section 5.2.4), which are not visible, and increasingthe temperature results in expansion to sizes that are measurablewith the AFM.

Yang et al. also investigated the effect of submerging a hydropho-bic surface into waters of different temperatures [43]. It was foundthat an increase in the temperature of the water increased thenanobubble density and from the AFM images it appears that thenanobubbles formed randomly on the surface, that is, they did not


preferentially form at surface asperities. Yang et al. suggests that therapid heating of the liquid results in gas super-saturation, whichwhenexposed to the hydrophobic surface forms interfacial nanobubbles.The formation process is analogous to the solvent-exchange process.

For pre-existing nanobubbles on a hydrophilic mica surface, Zhanget al. [48] showed that an increase in water temperature from 28 to42 °C resulted in no significant change of height, however the contactradius increased up to 37 °C then decreased. The dependence of gassolubility with temperature was used to explain the results. Zhanget al. states that as the temperature is increased the solubility of theair in the water decreases to a minimum around 37–77 °C. Thus, thenanobubbles grow due to the excess gas in solution and the sizereaches a maximum where the gas solubility is at a minimum. Theauthors are not certain with this reasoning. It is not clear why the gassolubility decreases, reaches a minimum then increases. From Perry'sHandbook of Chemical Engineering, gas solubility decreases over thetemperature range of 0–100 °C [61]. There is no doubt that gas super-saturation is implicated in the behavior, but the reasoning is morecomplicated.

The influence of pressure changes on nanobubbles has not beenstudied significantly. Preliminary work by Borkent et al. [19] using ashock wave generator showed that nanobubbles are stable underextreme increases and reductions (±6 MPa over 6 µs) in liquidpressure, as indicated by AFM images before and after the shock wave.The nanobubble super-stability to changes in liquid pressure is unlikethat experienced by microbubbles [62–64]. Borkent et al. suggest thatsurface forces between the liquid–vapor and solid–vapor interfacesmay contribute to this stability (see Section 5.1.3).

Under reduced pressure (i.e. the solid/liquid set up was removedfrom the AFM and exposed to a vacuum) Zhang et al. [59] found thatsome nanobubbles at the water/HOPG interface grew in size, possiblycoalesced with other nanobubbles and detached from the surface dueto buoyancy. Nanobubbles which do not detach reduced in size afterthe pressurewas increased to atmospheric [59]. As shown in Fig. 8, themajority of nanobubbles were unaffected by the pressure changes.However, some grew with the pressure reduction, coalesced,decreased in size on the return to ambient and left a nanobubble

Fig. 8. Large nanobubbles and nanobubble free rims formed from a reduction in theambient pressure at a HOPG surface in water. Nanobubbles were produced usingethanol–water exchange. The nanobubble covered HOPG and the droplet of water wasthen put into a desiccator connected to a vacuum pump (0.1 atm for 4 min). As shown,the majority of nanobubbles were unaffected by the pressure changes. However, somegrewwith the pressure reduction, coalesced, decreased in size on the return to ambientand left a nanobubble free rim surrounding the larger nanobubble. AFM tapping modeimaging was undertaken with a Veeco Nanoscope IIIa and Veeco NP cantilevers (0.32 or0.58 N/m) [59].

free rim surrounding the larger nanobubble. Zhang et al. explained thegrowth in terms of Henry's law, that is, a decrease in the externalpressure reduced the gas solubility, causing gas super-saturation andnanobubble growth. This behavior, along with the fact that a criticalsize of nanobubble was required for growth, further indicates that thedomains imaged are indeed gaseous. In all pressure experiments theliquid–hydrophobic solid sample was taken away from the AFM,exposed to different pressure conditions and then returned to theAFM for further analysis. Currently, there is no in-situ method forimaging nanobubbles under different pressure conditions using AFM.

2.4. Influence of surface roughness on nanobubbles

Typically, gas super-saturation (using solvent-exchange for exam-ple) is a requirement for nanobubble formation on smooth surfaces,but this is not the case for rough hydrophobic surfaces whichspontaneously form nanobubbles on immersion into aqueous envir-onments [19,42,52]. Roughness can cause nanobubble formation dueto concave areas which are unfavorable for water to penetrate,resulting in a gas cavity and nanobubbles with lower curvature andthus greater stability [52] (see Section 5.2.4 for more information).Greater attraction potential of gas at these surface asperities will alsocause formation (see Section 5.1.4 on dynamic equilibrium theory).

A pertinent paper by Yang et al. [42] showed that the nanobubblesformed on rough, silanated surfaces were larger and less denselydistributed than those on a smooth surface with similar hydropho-bicity. Yang et al. believed that the influence of roughness on the sizeand population of bubbles was linked to the larger number of bubbleformation sites on the rough surface. Further growth at these morenumerous formation sites lead to bubble coalescence, resulting in themore sparse population on the rougher surface. Areas of roughnesshave also been found to be preferential sites for nanobubble formationas the temperature of the sample is increased [43], as discussed inSection 2.3. Nanodroplet formation using the solvent-exchangeprocess is also preferential at areas of surface roughness [49].

With HOPG, it has been shown that nanobubbles preferentiallyform at the top of the cleavage steps due to increased hydrophobicityat that location, as shown in Fig. 9 [44]. The nanobubbles also form, toa lesser degree, on the flat terraces between the steps. At the bottomof the cleavage steps, the surface is hydrophilic in character resultingin a 20 nm nanobubble free zone from the bottom of the step. The

Fig. 9. A 3D AFM tapping mode height image of nanobubbles on HOPG indicating thepreferential accumulation at the top of the cleavage steps and the nanobubble free zonenear the bottom of the steps. Nanobubbles were produced using ethanol–waterexchange. AFM tapping mode imaging was performed with a PicoSPM (MolecularImaging) AFM using Si3N4 tipped cantilevers (NSC18/AlBS, MikroMasch, 20 kHz) [44].


cleavage steps also restrict nanopancakes [22,54] (see Fig. 3). Thereason for the restriction of the gaseous domains is probably due tothe bottom of the cleavage steps having a hydrophilic character whichpins the domains in place [44,65].

A recent study by Kameda and Nakabayashi found that HOPG witha high cleavage step density resulted in nanobubbles confined be-tween the steps [28]. The confined nanobubbles coalesced to form anelliptic nanobubble pinned by the cleavage steps. In the case of HOPGwith low cleavage step density, a lower number of solitary hemi-spherical nanobubbles were formed. It therefore appears that thedegree of HOPG roughness influences both the formation and stabilityof nanobubbles.

2.5. The effect of electrolyte concentration on nanobubbles

The studies by Zhang et al. [51] were the first to show that themorphology of pre-existing nanobubbles do not change significantlywith electrolyte concentration and type. The distinct lack of change upto a concentration of 1 MNaCl was not expected as it was proposed byAttard et al. [17] that the strong electrical double layer force betweenthe nanobubbles in pure water was a significant factor in the stabilityof the gaseous formations. Attard et al. probably cameupwith this ideaas the nanobubbles imaged by them were closed packed, therefore itseemed plausible that an electrical double layer force was stabilizing

Fig. 10. AFM tappingmode images of nanobubbles and nanopancakes on HOPG inA) 1 mMandusingethanol–water exchange, and then the salt concentrationwas slowly increased. Imagingw(120 kHz, 3.4 N/m) at a scan rate of 1 Hz [22].

the nanobubbles (N.B. as discussed in Section 2, the authors are notconvinced that these are nanobubbles). Zhang et al.'s results prove thatan electrical double layer force is not required to stabilize sparselypopulated nanobubbles. But what would happen if the nanobubbleswere in a closer configuration? It is expected that an electrical doublelayer force is present between nanobubbles in close proximity, andbecause of this, electrolyte addition may result in coalesce of close-packed nanobubbles. Unfortunately, the image of nanobubbles onHOPG in 0.5 M NaCl by Zhang et al. does not provide a scale bar, thusthe nanobubble proximity is unknown.

Results by the present authors conclude that electrolytes do notchange the morphology of nanobubbles/nanopancakes and does notreduce coalescence between nanobubbles/nanopancakes in closeproximity [22]. As shown in Fig. 10, the two nanobubbles on HOPGin 1 mM and 1 M NaCl aqueous solutions do not change significantly,the only obvious difference between the two images is the merging ofthe pancakes. It is conceivable that the depressed electrical doublelayer force between the nanopancakes results in coalescence, butgaseous domains in closer proximity did not coalesce. Additionally, themerging of nanopancakes has been observedelsewhere as a time effect[54], and in the authors opinion could be due to tip perturbation.

It was suggested that nanobubbles/nanopancakes in close proxim-ity will coalesce as the electrolyte concentration is increased due to areduced repulsive electrical double layer force. This hypothesis seems

B) 1 MNaCl with C) corresponding cross-sections. Nanobubbles/pancakeswere producedasperformedwith aVeecoNanoscope IVAFMusingHANC/15Etalon (NT-MDT) cantilevers


plausible as Jin et al. [66] demonstrate that increased electrolyteconcentration corresponds to coalescence of free nanobubbles insolution.Why is this behavior not found for surface nanobubbles? et al.suggested that the attractive force (i.e. vdW or true hydrophobic)between the gaseous domains is not strong enough to overcomepinning of the contact line. Would coalescence occur for nanobubblesin a close-packed configuration on an asperity free surface? It ispredicted that if the contact lines of neighboring nanobubbles/nanopancakes are within range of attractive forces, coalescence mayoccur if the contact line is free to move. Further studies are required toprove this phenomenon.

Currently, there is no systematic investigation on the influence ofelectrolyte type and concentration on nanobubble formation. It isexpected that electrolyte concentration will influence the amount ofnanobubbles/nanopancakes formed utilizing the solvent-exchangemethod. As an increase in the NaCl concentration reduces gassolubility of water, the difference in solubility between the electrolytefree initial solvent and the electrolyte solution increases. It is thereforeexpected that higher electrolyte concentration will increase nano-bubble formation by the solvent-exchange process.

The solvent-exchange method discussed in Section 2.1 has convinc-ingly demonstrated that replacing a solvent by another with a lowerdissolved gas concentration results in the formation of nanobubbles.Thus, the present author expected that the gas super-saturation formedduring the replacement of pure water with NaCl solutions would createmore nanobubbles or increase the size of pre-existing nanobubbles. It isbelieved that the gradual increase in salt solution and the smalldifference in the gas solubility between the solutions (in comparison toethanol–water mixtures) results in negligible gas super-saturation. It isconceived that if pure water is replaced by a saturated salt solution, thegas super-saturation will be sufficient, and nanobubbles will form.Further studies are required on this topic.

2.6. Influence of surface tension and contact angle

A number of works have investigated the effects of surface activeagents on the morphology of nanobubbles. For example, Simonsen et al.[34] investigated the influence of different alcohols on the formation ofnanobubbles at a polystyrene surface (equilibriumwater contact angle of90°). In pure water, the spontaneous formation of nanobubbles occurredall over the surface, resulting in a closed packed configuration of sphericalcap shaped nanobubbles. In the case of alcohols (ethanol and 1-pentanol)the formationwas restricted, resulting in a decrease in the population andsize of the nanobubbles, with the high polarity 1-pentanol showing thegreatest effect. Simonsen et al. explains the results using a connectionbetween thenanobubbles and thedepletion layer. Asdiscussed indetail inSection5.2.3 thedepletion layer is possibly aprecursor layer.Disturbancesof this layer (possibly from the AFM tip) initiate nanobubble formation.It is believed that the ethanol and 1-pentanol reduce this depletion layerdue to fewer hydrogen bonds being broken.

The images of nanobubbles in pure alcohol presented by Simonsenet al. are very unique. To the best of the authors' knowledge this isthe only example of nanobubbles in pure alcohol. In the case of pre-existing nanobubbles formedusing solvent-exchange, Zhang et al. [53]showed that an increase in the ethanol concentration decreased thepopulation of nanobubbles and completely removed them beyond aconcentration of 20%. These results are in agreement with forcespectroscopy results, as discussed in Section 3.7. Borkent et al. [19]also state that nanobubbles were not present on a smooth polyamidesurface (contact angle 80°). A work by Yang et al. [43] showed that theaddition of 2-butanol (1:5 ratio of 2-butanol to water) to pre-existingnanobubbles on silanated silica slightly decreased the volume and thecontact angle of the nanobubbles. Further additions of 2-butanol werenot undertaken by Yang et al., thus it is unknown if higher concentrationswould have completely removed the nanobubbles for this system. Zhanget al. suggests that the high wettability of ethanol desorbs nanobubbles

and additionally proposes that the non-uniformity of the ethanol–watersolution at the molecular level may influence nanobubble stability. Thisbehavior is also explained in terms of dynamic stability theory inSection 5.1.4.

It is interesting that nanobubbles formed in pure alcohol forSimonsen et al.'s system, but a similar behavior was not found in theremaining literature. Also, Thormann et al. [56] do not measure aNBCF in pure pentanol, thus it appears that tip perturbation is notresponsible for the formation of nanobubbles in Simonsen et al.'ssystem. The reason for this discrepancy is unknown, but it highlightsthat subtle variations in experimental systems can have a dramaticoutcome on nanobubble formation.

A significant paper by Zhang et al. [51] looks at the influence ofcationic, anionic and non-ionic surfactants on pre-existing nanobub-bles formed from solvent-exchange on OTS silanated silica and HOPG.In the case of OTS silanated silica, the addition of surfactants to thepre-existing nanobubbles changed the irregular shape of the contactline to circular, an effect also observed for microbubbles. As suggestedby Zhang et al., the reduction in surface tension due to the addition ofsurfactant relaxes the pinning at the contact line. The contact linebehavior was not present for the nanobubbles on HOPG, because thecontact line was already circular in pure water. Despite the reductionin surface tension with surfactant addition, the nanobubbles did notchange morphology as dramatically as with the addition of alcohols,as discussed above. This result indicates that the surface tension of theliquid has negligible consequence on the stability of nanobubbles.Another interesting finding from the surfactant study was that thecontact angle of the nanobubbles did not change as significantly as themacroscopic angle when surfactant is added. This further indicatesthat the calculation of contact angle based on the Young equation (seeSection 5.1.1) is not comprehensive at the nanoscopic scale. Duckerprovides an alternate response to the results of Zhang et al., asdiscussed in Section 5.1.5 [67].

The hydrophobicity of a surface (i.e. water contact angle) and itsinfluence on nanobubbles is a topic that has been investigated by anumber of groups. From the previous discussion it appears thatsurfaces of low hydrophobicity (<90°) require surface pre-treatmentto form nanobubbles (in the majority of cases). Whereas, surfaces ofhigher contact angle tend to spontaneously form nanobubbles onimmersion, but there are exceptions. It is the authors' opinion thatlinking the macroscopic contact angle of a surface to the formationand stability of nanobubbles is fraught with danger as the classicalcontact angle theory (see Section 5.1.1) does not take into con-sideration other phenomena important at the nanoscopic scale. Anyattempt at linking the macroscopic contact angle to nanobubblesrequires a more significant understanding of the meaning of contactangle at the nanoscale. Factors such as line tension, roughness,interactions between the vapor–liquid and vapor–solid interfaces andother phenomena at the three-phase contact line need to be takeninto account.

2.7. Electrochemical formation of nanobubbles

A recent development involves the electrochemical formation ofnanobubbles at a conductive surface. The first study of this kind wasperformedbyZhanget al. [47] usingAFM tappingmode imaging to viewthe formation, growth, coalescence and release of hydrogen nanobub-bles on a HOPG surface in sulfuric acid solution with controlled voltageand reaction time. It was also found that nanobubbles preferentiallyformednear the cleavage steps ofHOPGand that thegrowthwas limitedby the steps, the applied voltage and the reaction time. The domainsimaged were confirmed to be gaseous using techniques such asdegassing, phase imaging and coalescence using an AFM tip.

A more recent study by Yang et al. [45] showed the electrochem-ical formation of both hydrogen (HOPG as a cathode) and oxygen(HOPG as an anode) nanobubbles at a HOPG surface in pure water and


0.01 M NaCl. It was found that the yield of hydrogen nanobubbles wasmuch higher than that of oxygen nanobubbles due to the highersolubility and lower production rate of oxygen. Further, thenanobubbles formed as thin films and grew in the vertical directionuntil a dynamic equilibrium was reached (unless the nanobubblesbecame too large and were removed from the surface due tobuoyancy), as shown in Fig. 11. The results of this study agree withthe dynamic stability model, as discussed in Section 5.1.4.

The topic of electrochemical formation of nanobubbles waspurposely left last in this section as the authors believe this to be themost decisive proof that the domains imaged by tappingmode AFM areactually gaseousnanobubbles. Arguments against nanobubble existencedue to fundamental problems are now completely unwarranted.

3. Evidence revealed by atomic force spectroscopy

As already briefly discussed, nanobubbles are responsible for theNBCF, a force one mistaken for a LRHAF. A simple pictorialrepresentation of this force is shown in Fig. 12. As two hydrophobicsurfaces approach, the gas filled nanobubbles on the adjoiningsurfaces are forced to coalesce, forming a gas bridge and an attractivecapillary force. Additionally, the gas bridge could also form due to theinteraction of a nanobubble and a hydrophobic solid.

The force due to nanobubble interactions is the combination of twomain mechanisms, the force resulting in the rupture of the filmbetween two nanobubbles (or a nanobubble and a hydrophobic solid)and the capillary force of the resulting gas bridge. As two nanobubbles(or a nanobubble and a hydrophobic solid) approach each other, DLVOforces (possibly repulsive in the case of a nanobubble and a solid) inaddition to other non-DLVO (hydrophobic) forces result in therupture of the film. The rupture of the film is a hydrophobic effect,whereas the force resulting from the capillary bridge is dependent onthe hydrophobicity of the surfaces (the bridge shape is dependent onthe contact angle) but is a capillary mechanism, not a hydrophobiceffect. The focus of this review will now shift to force spectroscopyresults that indicate the presence of nanobubbles, giving further proofto nanobubble existence.

3.1. Variability of the force measurement

Nanobubble bridging has been supported by a number ofbehaviors in AFM force spectroscopy results. One experimentalfinding which is characteristic of the attraction is the variable rangeof the force for the same experimental conditions. The variabilityindicates that the force is not due to some molecular property of thesurfaces and fluid medium like classical surface forces. The variability

Fig. 11. Time evolution of a nanobubble profile on HOPG (as cathode) when 1 V isapplied. The applied potential forms a hydrogen nanobubble on the HOPG surfacewhich grows up to 70 s, at which point a dynamic equilibrium is reached (seeSection 5.1.4). Tapping mode AFM profiles were performed with a PicoSPM (MolecularImaging) with Si3N4 tipped cantilevers (NSC18/AlBS, MikroMasch, 20 kHz) [45].

in the range is consistent with nanobubble bridging as the forcedepends on the interaction of randomly sized and oriented nano-bubbles on the interacting surfaces, as shown in the AFM tappingmode images in Fig. 1. The randomness of the force resulted in muchspeculation on the mechanism of the LRHAF, but as stated, thisrandomness was a nanobubble artifact and not a hydrophobic force atall. The results from our studies indicate this variability, as shown inFig. 13. It can be observed that the range and magnitude of the forcedramatically changes as the probe is moved to different locations,which is unlike a classical surface force.

The variability between groups using similar experimental condi-tions can also be explained by the effect of different surface pre-treatments, originally thought to have no influence on force measure-ments, suchas cleaning the surfacewith ethanol beforemeasurementorthe solvent-exchange process. As discussed in Section 2.1, simplycleaning the surface with ethanol or flushing the AFM cell with ethanolthen water before measurement dramatically increases the amount ofnanobubbles accumulated at the hydrophobic surface. The inconsis-tencies in previous work between authors using the same surfaces andconditions are the result of slightly different pre-treatments, whichwere believed to be trivial, and the inherent randomnature of theNBCF.

Variability can also occur between measurements taken at the sameinteraction area. In most AFM force spectroscopy measurements the twosurfaces come into intimate contact before any data is gathered. The socalled “engagement” process is used to bring the two surfaces into therange of the AFM scanners z-axis movement. Thus, the first interactionbetween the surfaces is usually not measured in an AFM forcemeasurement. By deliberately changing the force measurement proce-dure, other groups have measured the first and consecutive interactionsbetween hydrophobic surfaces in water and concluded that the firstinteraction is always significantly shorter ranged than the later interac-tions, which reach a constant value after 1–5 interactions [56–58]. Thisbehavior is indicatedbyYakubov et al. [58] results, as shown in Fig. 14. It isbelieved that the first interaction is due to the spontaneous formation of avapor capillary between the surfaces at a separation distance ofapproximately 10 nm. Upon separation, this gas capillary ruptures,resulting in the formation of stable nanobubble/s on each surface, whichcause a much longer ranged force in latter interactions. It is important tonote that in theseworks the advancing contact angles of the surfaceswereclose toor above90°, thus the cavitationmechanismof thefirst interactionappears plausible. Additionally, nanobubbles were not deliberatelyproduced in these works.

Yakubov et al.'s results in Fig. 14 show that the range of attraction forthe first interaction is below 10 nm for themajority of cases, but sphere2 is an exception. Yakubov et al.'s states that nanobubbles attached tothe surface before the force measurements should be ruled out. But it isthe authors' view that the first interaction of sphere 2 is due to a pre-existing nanobubble, as a similar phenomena was observed by thepresent authors [20]. Force curves between 1-octanol esterified silicasurfaces (without solvent-exchange or any other nanobubble formationmechanism) occasionally showNBCF forces, but in themajority of casesshort ranged “true” hydrophobic forces occur, as shown in Fig. 15.Despite the fact that the nanobubbles were not deliberately produced,nanobubbles still exist; possibly due to formation mechanisms otherthan gas super-saturation (e.g. gas-trapping as discussed in Section 5.2.4).Thus, Yakubov et al.'s statement that pre-existing nanobubbles are notpresent cannot be substantiated as nanobubble formation is notcompletely controlled in their system.

3.2. Steps in the force curves

The LRHAF curves contain steps which are suggested to result fromthe bridging of nanobubbles [2,4]. Experimental results show examplesof a single step [4,20,21,23,26,60,68–70], due to single nanobubblebridging, or multiple steps, due to multiple nanobubbles bridging[2,56,69,71,72]. These steps are also observed in the withdrawal curve,

Fig. 12. Simplified mechanism of the NBCF between two hydrophobic surfaces, each with one nanobubble (in actual experiments there are likely to be many nanobubbles in theinteraction area), in a liquid. Initially the two surfaces are far apart and there is no interaction between the nanobubbles. As the two surface approach, the nanobubble on each surfaceinteract and coalesce, which results in a vapor capillary bridging the two surfaces. The resulting concave capillary bridge produces an attractive force which forces the two surfacesinto contact.


which are suggested to result from the rupture of the gas capillary[20,21,23,26,56,68–70]. Additionally, the multiple steps found by someauthorsmaynot bedue tomultiple nanobubble interactionsbut the stepwise movement of the bridge contact line [2]. A pertinent study bySakamoto et al. [69] observed that the first interaction of twohydrophobic surfaces contained multiple steps, whereas later interac-tions contained one step. The change frommultiple to single steps is dueto the merging of multiple bridges on the first interaction, resulting in asingle nanobubble on each surface, and thus a single step for theremaining interactions, as shown in Fig. 16a and b. This study indicatesthat force curves which show multiple steps after the first interactionare either between very rough surfaces, so that the bridges do notoverlap (Fig. 16c), or because the contact line of the bridge moves in astepwise fashion due to contact line pinning (Fig. 16d).

3.3. NBCF model

As the NBCF is a result of nanobubble bridging, the attractive forcecan be explained by a capillary based model. The first to analyze theforce due to nanobubbles in terms of a capillary bridge was Attard [5],who utilized minimization of the constrained Gibbs free energy.Minimization of the free energy has been used by other authors[37,73], and a simple capillary force model has also been used[37,70,74,75]. Yaminsky [75] used a constant volume capillary modelto indicate that the “hydrophobic force” is due to lipid like materialsegregated between the interacting surfaces. This maybe the case forunstable surfaces, but it does not explain the force data betweenstable surfaces. Thus, Yaminsky's conclusion does not cast doubt onthe NBCF. Ishida et al. [70] also used a simple capillary model toexplain the NBCF, but unsuccessfully. The authors believe that theconstraints imposed in the model (Laplace pressure between 0 and1 atm) by Ishida are too restrictive.

Fig. 13. Approach force vs. separation distance between a 1-octanol esterified silica sphere anexchange and no surface scanning. The force ismeasured in three consecutive locations. The rlocations, which is unlike a classical surface force. Measurements were performed with a Vee

To date, the most significant model presented is that of Attard [5].In this model the bridge shape is parameterized by an appropriateexpansion and the constrained Gibbs free energy minimized withrespect to the variations in the coefficients of the expansion. Themodel assumes constant contact angle and a fixed number of gasmolecules. The volume is not fixed and the system is able to exchangevolume with an external reservoir. Attard calculates the approachforce of the capillary as a function of separation distance of two equalspheres with contact angle above 90°. Without taking into consider-ation the formation of the bridge the model predicts a submicroscopicsized bridge at large separation distances and a microscopic sizedbridge at smaller separations. The jump-in on the approach forcecurve is due to the transition from a submicroscopic to microscopicbridge. When the bridge formation is taken into consideration, thesubmicroscopic branch of the approach interaction is unnecessary asthe bridge jumps straight to the microscopic regime. This jump-inagrees with the steps found in experimental force curves, as discussedin Section 3.2. An unusual behavior of this model is that the retractioninteraction closely follows the approach interaction up to the jump-inseparation distance, a behavior the author has not experienced orfound in the literature.

Attard's model also predicts a repulsive force for the capillaryinteraction between two equal spheres with contact angles below 90°.In this case, there is no jump to a microscopic branch on approach,resulting in an increased convex shape at small separation distancesand thus a repulsive force. Similar behavior was also found if thecontact radius of the bridge was pinned.

The repulsive component of the NBCF at small separation distancesfound by a number of authors is predicted by an extension of Attard'smodel to include dynamic drag that opposes the horizontal expansionof the bridge in proportion to the velocity of the contact area. Themodel including the drag component strongly resembles experimental

d plate (macroscopic receding contact angle 65°) in pure water after 1-propanol–waterange andmagnitude of the force dramatically changes as the probe is moved to differentco Nanoscope IV AFM and MikroMasch NSC12 (7.5 N/m) cantilevers at a rate of 2 µm/s.

Fig. 14. Jump-in distance vs. interaction number for different experiments using the same experimental parameters. The first interaction is due to the spontaneous formation of avapor capillary between the surfaces at a separation distance of approximately 10 nm. Upon separation, this gas capillary ruptures, resulting in the formation of stable nanobubble/son each surface, which cause a much longer ranged force in latter interactions. Experiments were performed in water between a silica sphere and an oxidized silicon wafer silanatedby exposing to the vapor of 1,1,1,3,3,3-hexamethyldisilazane [58].


force data qualitatively, but with suitable choices of variables (bubblesize, drag coefficient) Attard suggests the model can quantitativelyaccount for the experimental data.

The thermodynamic analysis of the NBCF presented by Attardprovides further evidence that the long range force measured bymultiple authors is due to the bridging of nanobubbles. Despite itsability to predict experimental data, the authors believe that themodel, or more so the variables chosen, could be improved. Forexample, Attard uses the macroscopic contact angle, but it is knownthat the contact angle at the nanoscale (see Section 5.1.2) is sig-nificantly different. Another problem is that the model does not takeinto consideration hysteresis of the contact angle, resulting in unusualbehavior of the retract force, as discussed above. Additionally, the

Fig. 15. Two separate approach force vs. separation distance measurements between a 1-octwater without solvent-exchange exchange and no surface scanning. One measurement resuvdW force, F=−AR/(6 h2), where A, is 1×10−20 J. Measurements were performed with a Ve[20].

method is computationally demanding compared to simpler capillaryforce models used by other authors. Because of these reasons, theauthors developed a different model based on simplified capillarybridge equations, without the constraints imposed by Ishida[20,21,23]. It is not the purpose of this article to review capillaryforces, therefore only a brief overview of capillary forces is givenbelow. For a greater understanding of capillary forces the reader issuggested to view the recent review paper of Butt and Kappl [76] andthe works of Pepin et al. [77,78], Lian et al. [79], Hotta et al. [80] andOrr et al. [81].

The force (attractive or repulsive) due to a capillary bridge arisesfrom two components, the surface tension force acting at the three-phase contact line and the force due to the pressure difference

anol esterified silica sphere and plate (macroscopic receding contact angle 65°) in purelted in a short range (“true”) hydrophobic force and another in a NBCF. Included is theeco Nanoscope IV AFM andMikroMasch NSC12 (7.5 N/m) cantilevers at a rate of 2 µm/s

Fig. 16. Mechanisms of multiple and single steps in the NBCF. A) The first interactionbetween nanobubble covered surfaces is due to multiple nanobubbles which mergeinto a single nanobubble. B) The second interaction between two nanobubbles resultsin a single jump-in step. Multiple steps also occur due to C) high surface roughness andD) pinning of the contact line.

Fig. 17. A) Concave and (B) convex capillary bridge dimensions between a sphere and aflat substrate [23].


(Laplace pressure ΔP) across the curved interface described by theYoung–Laplace equation.

ΔPγLV

=1

y xð Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 + y′ xð Þ2

q − y″ xð Þ1 + y′ xð Þ2� �1:5 ð1Þ

where γLV is the surface tension of the vapor–liquid interface and xand y are the axial and radial coordinates respectively. The Young–Laplace equation relates the mean curvature of the capillary bridge tothe pressure difference across the interface and states that theinterface must have a constant curvature at all points. The Young–Laplace equation cannot be solved analytically (in most cases),therefore numerical procedures must be used. The numericalprocedures required are computationally complex, thus approxima-tions of the interface are usually used. A toroid is a commonapproximation, in which the meridional profile of the bridge isdescribed by a circular arc. In the toroidal approximation the surfacecurvature is not constant along themeridional profile, thus the force isa function of the position of the bridge, unlike that described by theYoung–Laplace (Eq. (1)). The toroidal approximation will not result ina perfect prediction of the NBCF, but it is much simpler than thenumerical solution of the Laplace–Young equation. As the informationgained from this model is used for comparative purposes, the ease ofits use outweighs the concerns for an exact solution.

Following on from the work of Tselishchev and Val'tsifer [82], theNBCF model uses a toroidal bridge geometry to calculate the capillaryforce between a sphere and a flat plate, as shown in Fig. 17. Thisgeometry is the same as that used in the colloidal force measurementsof the NBCF. The y-axis is the axis of symmetry and the origin is takenas the point where the bridge is at its narrowest or thickest point forconcave and convex bridges, respectively.

The capillary force model takes into account the force due to thepressure drop across the interface at the neck and the interfacialtension force, giving

F = −πRγLV sinα 2 sin θ−αð Þ + R sinα1r−1

l

� �� ð2Þ

where F is the capillary force, R is the radius of the colloidal probe, α isthe fill angle, θ is the liquid contact angle, and r and l are the principleradii of the capillary bridge, as shown in Fig. 17, and given by

r = − R 1− cosαð Þ + hcos θ−αð Þ + cosθ

ð3Þ

l = R sinα−r 1− sin θ−αð Þ½ � ð4Þ

where h is the separation distance between the particle and flat surface.The fill angle, α, in the model Eqs. (2)–(4) is unknown, but by

considering the condition of constant interface curvature or constantbridge volume, the fill angle can be calculated as a function of theseparation distance, h. The condition of constant bridge volume wasfound to compare better with the experimental data, given the shorttime scale of the experiments [83].

The volume of gas in the capillary is calculated by [82],

V = π ∫−rcos θ−αð Þ

r cosθ

F yð Þ½ �2y−43πR3 sin4 α

23−2 sin2 α

2

ð5Þ

where F(y), the equation of the interface, is

F yð Þ = r + l−ffiffiffiffiffiffiffiffiffiffiffiffiffiffir2−y2

qð6Þ

for a concave bridge, as shown in Fig. 17A, and

F yð Þ = −r + l−ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 + y2

qð7Þ

for a convex bridge, as shown in Fig. 17B.The NBCF can now be predicted using Eqs. (2)–(7) with the

condition of constant bridge volume and contact angle (measured inthe liquid phase). The contact angle and the force at the jump-in (forapproach data) or snap-off (for retract data) are the only fitting


parameters. In the matching, fill angle, α, was changed and thevolume of the bridge was determined at the jump-in/snap-off point.During the sub-sequential calculations at different separation dis-tances, the calculated volume was kept constant by changing α andthe attractive force was calculated using Eq. (2).

The model presented does not take into consideration intermolec-ular forces between the two hydrophobic surfaces such as vdW in thegaseous environment inside the nanobubble, and vdW and electro-static forces in the liquid environment. These intermolecular forceswill only be of significance at short separation distances or when thecapillary force is comparatively small, as explained by Attard [5].

An example fit of a NBCF for the interaction between 1-octanolesterified surfaces in water (approach interaction) is shown in Fig. 18.The fit results in a receding contact angle of 99.9 degrees and acapillary volume of 7.3×106 nm3. If the capillary force model isimplemented using the measured macroscopic water contact angle(receding 65°), a constant capillary volume of 7.3×106 nm3, and ajump in distance of 230 nm, the capillary force is repulsive. But, therepulsive force is not measured because the water receding contactangle of the bridge is significantly higher (99.9°) than themacroscopiccontact angle. This highlights, that the macroscopic contact angle ofsurfaces must not be used when modeling the NBCF.

Despite the excellent fit and contact angle reproducibility betweenexperiments, the model produces Laplace pressure anomalies. Thepressure difference ΔP across the vapor–liquid interface (i.e. LaplacePressure) is calculated by the Young–Laplace equation (negligibleeffect of gravitation) as

ΔP = PV−PL = σ1l−1

r

� �ð8Þ

where PV and PL are the pressures on the vapor and liquid side,respectively. Using Eq. (8), the Laplace pressure was calculated for theforce curve in Fig. 18 as a function of separation distance, as shown inFig. 19.

At first glance the results in Fig. 19 are a little concerning. Forexample, if PL is assumed atmospheric (1 atm), which is usually the case[70], the pressure inside the capillary at jump-in is approximately 7 atm.This very high pressure should dissolve the gaseous capillary bridge inmicroseconds. Also, the negative Laplace pressures beyond −1 atm

Fig. 18. Fitting the NBCF model (Eqs. (2)–(7)) to the approach interaction between a 1-octawater after ethanol–water exchange and surface scanning (10×10 µm). Measurements wcantilevers at a rate of 2 µm/s [23].

suggests that the pressure inside the bridge is negative. A negative gaspressurewithin thebridgedoes notmake sense. It is thesediscrepanciesthat caused the failure of the capillary force model implemented byIshida et al. [70]. Ishida et al. assumed that PL was atmospheric and−1<ΔP(atm)<0, that is no negative internal pressure and no positiveLaplace pressure. But is it correct to assume these limits?

As discussed in Section 5.1.1, nanobubbles have a high internal gaspressure, but are remarkably stable. A number of theories proposereasons for this deviation from classical thermodynamics, as discussedin Section 5.1, but in the case of capillary bridges two theories seemappropriate, the surface-force theory and the dynamic stabilitytheory. In the surface-force theory, an electrostatic repulsive forcebetween the vapor–liquid and vapor–solid is proposed to account fornanobubble stability. The repulsive force results in aMaxwell pressurethat supplements the internal pressure, lowering ΔP for a givencurvature. In the case of a capillary bridge at large separation distancesthe neck of the capillary (l) is very small. The value of l is 97 nm for theapproach curve shown in Fig. 18. Thus, it is conceivable that theMaxwell pressure from the interacting vapor–liquid interfaces at theneck exists, analogous to the stabilization of bulk nanobubblesproposed by Jin et al. [84]. Additionally, the dynamic stability theory(see Section 5.1.4) may also be valid for the NBCF.

The negative pressures are not correct and are due to assumptionsof the model. The problem could be caused by the toroidal bridgeapproximation, which may break down at small separation distancesand small r radii. Additionally, the NBCF predicted by Eqs. (2)–(7)deviates from the experimental data at a separation distance belowapproximately 20 nm, thus the Laplace pressure calculations in thisarea can be disregarded. The reason for the negative irregularity of theLaplace pressure at short separation distances is not completelyunderstood, but it appears that the model is sufficient for predictingthe NBCF at long separation distances.

3.4. The effect of dissolved gas

Another important experimental finding indicating the role ofnanobubbles on the attractive force is the importance of dissolved gas.Many researchers have found that the range of the attractionsignificantly reduces (but still larger than any conceivable van derWaals (vdW) attraction) in degassed water or when the surfaces are

nol esterified silica sphere and plate (macroscopic receding contact angle 65°) in pureere performed with a Veeco Nanoscope IV AFM and MikroMasch NSC12 (7.5 N/m)

Fig. 19. Laplace pressure (ΔP) as a function of separation distance for the approach interaction between a 1-octanol esterified silica sphere and plate (macroscopic receding contactangle 60°) in pure water after ethanol–water exchange and surface scanning (10×10 µm) [23]. The Laplace pressure was calculated using the results of the fit shown in Fig. 18 andEq. (8).


rendered hydrophobic in-situ without any exposure to the atmo-sphere [69,85–87]. The reduced range of attraction indicates that theamount of gas present on the surface as nanobubbles is reduced orcompletely removed. The gas influence clarifies that nanobubbles areindeed responsible for the attractive force.

Stevens et al. [87] showed that the variability of the force (asdiscussed in Section 3.1) between fluoropolymer surfaces significant-ly reduced in the degassed system. The consistency of the force dataafter degassing indicates that the nanobubbles were completelyremoved, leaving a pristine hydrophobic surface and measurementsof a “true” hydrophobic force, which the authors suggest is due to acavitation mechanism. This result can be compared to the work ofThormann et al. [56], Yakubov et al. [58] and Vinogradova et al. [57]who suggested that nanobubbles form on the first interaction due tocavitation, as discussed in Section 3.1. It appears that the in the case ofSteven et al.'s work no nanobubbles are formed on the first interactiondue to the lack of dissolved gas in the system.

Results from the present authors show the importance of solvent-exchange on the NBCF [20]. The typical dependence of the approachforce on separation distance between esterified silica surfaces at asingle point after alcohol–water exchange is shown in Fig. 20. Afterthe AFM liquid cell is flushed with alcohol and then exchanged withwater the range of the NBCF dramatically increases. The type ofalcohol used in the alcohol–water exchange also impacts the range of

Fig. 20. Typical approach force vs. separation distance measurements between a 1-octanol eafter solvent-exchange with methanol, ethanol and 1-propanol. Measurements were perforrate of 2 µm/s [20].

the force, with 1-propanol exchange showing the longest range forcefollowed by ethanol and methanol.

It has been shown that alcohol–water exchange increases theoccurrence and/or size of nanobubbles on the interacting hydrophobicsurfaces, as shown in Fig. 5. As discussed in Section 3,when two surfacescovered with randomly oriented nanobubbles come into contact, thenanobubbles coalesce to form a gas bridge between the two surfaces, asshown in Fig. 16. In the case of methanol and 1-propanol alcohol–waterexchange, the nanobubbles produced by 1-propanol exchange after thefirst interaction will be larger than that of methanol due to the largeramount of gas initially present on the surface. An increase in the heightof nanobubbles results in a nanobubble interaction at larger separationdistances. Thus, theNBCFbetween surfaces treatedwith 1-propanolwillbe larger thanwithmethanol, as supported by the force curves in Fig. 20.

3.5. Increased NBCF range with surface scanning due to nanobubblecoalescence

Results from our studies showed that the range of the NBCFincreased after the spherical hydrophobic surface is scanned over theflat hydrophobic surface [20]. The typical dependence of the approachforce on separation distance after surface scanning is shown in Fig. 21.It is hypothesized that the interaction of the colloid probe over the flatsurface causes the nanobubbles on each interacting surface to collide

sterified silica sphere and plate (macroscopic receding contact angle 65°) in pure watermed with Veeco Nanoscope IV AFM and MikroMasch NSC12 (7.5 N/m) cantilevers at a

Fig. 21. Typical approach force vs. separation distance measurements between a 1-octanol esterified silica sphere and plate (macroscopic receding contact angle 65°) in pure waterafter ethanol–water exchange, with point force and 1×1 µm, 5×5 µm, 10×10 µm, and 20×20 µm surface scans. Measurements were performed with Veeco Nanoscope IV AFM andMikroMasch NSC12 (7.5 N/m) cantilevers at a rate of 2 µm/s [20].


and coalesce, resulting in a larger gas bridge between the sphere andthe flat surface. Upon retraction, a large nanobubble is formed on thecolloidal probe and flat surface, which increases the range of attractionin following interactions. Using the capillary force model discussedabove (Eqs. (2)–(7)) the capillary volume increased as the scan sizeincreased. The results agree with the ability to make nanobubblesmerge using an AFM cantilever in contact mode imaging, as shown inFig. 6.

3.6. The effect of electrolyte concentration

The experimental results on the influence of electrolyte on theNBCF indicate that the force does not change significantly at lowelectrolyte concentration (<1 M) [2,4,71,85,88], but increases inhighly concentrated electrolyte solutions [2]. The adhesion was alsofound to significantly increase in concentrated electrolyte [2]. Theresults agree with a gaseous capillary force model as an increase inelectrolyte increases the surface tension but does not influence thecontact angle significantly [89], thus the capillary force becomesstronger. Parker et al. [2] found that the attractive force increased 10%in 5 M NaCl compared to pure water, which is about the same as theincrease in the surface tension. This result agrees with the capillaryforcemodels, which are a function of the vapor–liquid surface tension,and further proves to the gaseous nature of nanobubbles. The resultsalso agree with the lack of change of nanobubble morphology withincreasing electrolyte concentration, as discussed in Section 2.5. Theidentical increase in the NBCF magnitude and the surface tensionindicate that the volume of gas in the capillary bridge does not change.

A number of groups demonstrate that an increase in electrolyteconcentration reduces the repulsive force prior to the attractive jump[4,58,85,86,88,90]. Repulsion prior to the attractive jump is believedto be the result of the electrical double layer force between interactingnanobubbles and/or solid surface. The reduced repulsion withincreasing electrolyte concentration makes sense as the double layeris compressed between the interacting nanobubbles.

3.7. Influence of surface tension and contact angle

The previous section conclusively demonstrated that a change inthe vapor–liquid surface tension resulted in an equivalent change inthe magnitude of the NBCF. A similar behavior is predicted for theaddition of surface active substances, that is, the reduction in surfacetensionwill result in a similar reduction in themagnitude of the NBCF.However, unlike salts the addition of alcohols in known to reduce thevolume of nanobubbles (see Section 2.6).

Experimental results from different authors agree with theory thatthe magnitude of the NBCF reduces as ethanol concentration isincreased (decreasing surface tension) [2,72,91]. But it must bequestioned; is the force reduction in higher ethanol concentrationssolely due to a surface tension effect, or is the size reduction of thebridge more significant? The size reduction of nanobubbles and theresulting bridge agrees with force data that shows that the NBCFdisappears in pure alcohol [2,33,56,72].

Questions also need to be asked why the jump-in force range(related to the height of the nanobubbles) reduces [2,91], or isvariable [72] as the ethanol concentration is increased. It is theauthors' opinion that the randomness of the NBCF is contributing tothis lack of consensus. Using a systematic experimental technique thepresent authors were able to accurately measure the influence ofethanol on the NBCF [23]. As shown in Fig. 22A, as the ethanolconcentration is increased the magnitude of the NBCF decreases. Toreiterate, the forces in Fig. 22, are between two hydrophobic esterifiedsilica surfaces, each covered with a single nanobubble produced bythe solvent-exchange method and collection by surface scanning.

Fitting the capillary force model (Section 3.3) to the approach andretract force curves generates information on the geometry, contact angleand volume of the capillary. As the surface tension is reduced the contactangle decreases, changing the capillary from a concave to convexgeometry. At a concentration of 40% ethanol, the receding contact angledrops below 90° (convex geometry) resulting in a repulsive approachforce. The reduced contact angle also lowers the adhesive force.Additionally, the attractive force and the resulting adhesion decreasewith an increase in ethanol concentration as the volume of the capillarybridge dramatically reduces. Above an ethanolmass concentration of 40%,the capillary force cannotbemeasuredas thenanobubbleshavedissolved.

4. Other experimental methods

Much of the success in understanding nanobubbles and the NBCFis due to AFM tapping mode imaging and force measurements.Despite the reputation of these methods, a number of otherexperimental measurements have contributed substantially.

4.1. Rapid cryofixation/freeze fraction method

A novel method of visualizing nanobubbles was developed bySwitkes and Ruberti, which uses rapid cryofixation/freeze fraction andScanning Electron Microscopy (SEM) [92], as shown in Fig. 23. In thismethod a hexamethyldisilanzane (HMDS) silanated silica surface(static contact angle of 92°) immersed in water was quickly frozen by

Fig. 22. A) Approaching and B) retracting force curves between silica sphere and plate (macroscopic receding contact angle 65°) in water/ethanol solutions. Nanobubbles wereproduced using ethanol–water exchange and surface scanning of a 10×10 µm area. Ethanol concentration was then slowly increased by increments of 5%(v/v). As the ethanolconcentration is increased the magnitude of the NBCF decreases but the range does not change considerably. Measurements were performed with Veeco Nanoscope IV AFM andMikroMasch NSC12 (7.5 N/m) cantilevers at a rate of 2 µm/s [23].


contact with a polished copper block immersed in liquid nitrogen.Under high vacuum, the hydrophobic surface was removed from thefrozen water and thin layers of platinum and carbon were evaporatedon the ice surface to form a replica of the surface. The ice was thenremoved by heating, and the replica viewed using SEM.

Switkes and Ruberti observed “nanobubbles” on a hydrophobic sur-face immersed in gassed water, but found no sign of nanobubbles whenthe water used was degassed or the surface was hydrophilic, as shown inFig. 23. The results of Switkes further confirm the influence of dissolvedgas and surface hydrophobicity on nanobubble formation/stability.

4.2. Infrared spectroscopy

Infrared spectroscopy is useful in determining the presence ofnanobubbles at the liquid–hydrophobic solid interface. Attenuated

Total Internal Reflection Infrared Spectroscopy (ATR-IR) has beenused by Zhang et al. to detect the presence of a carbon dioxide layer ata hydrophobic surface (advancing contact angle 112°) in water afterethanol–water exchange (both saturated with carbon dioxide)[50,52]. ATR-IR indicated that the pressure inside the nanobubbleswas between 1 and 2 atm, and that water vapor was present insidethe bubbles. A lack of ethanol adsorption and little change in the C–Hstretch after ethanol exchange indicated that the ethanol or otherorganic contamination from the solvent-exchange process was notpresent. However, Ducker recently suggested that monolayer cover-age of organic contamination may exist at the vapor–liquid interfacedue to instrument sensitivity issues [67].

The IR studies of Zhang et al. also indicated that the stability of existingnanobubbles is dependent on the gas concentration in the surround-ing liquid [52]. It was found that nanobubbles created with air

Fig. 23. SEM images of the platinum replicas of HMDS silanated silica immersed inA) gassed water indicating the presence of nanobubbles and B) degassed water with nosign of nanobubbles. The replicas were produced by quickly freezing the HMCSsilanated silica surface immersed in water with a liquid nitrogen cooled copper block.Under high vacuum, the hydrophobic surface was removed from the frozen water andthin layers of platinum and carbon were evaporated on the ice surface to form a replicaof the surface. The ice was then removed by heating [92].

Fig. 24. Three-phase contact of a bubble to a hydrophobic surface in liquid. The contactangle (θ) is taken within the liquid.


saturated ethanol–water exchange are stable in an Infrared Spec-troscopy (IR) cell for over 4 days, whereas nanobubbles createdunder carbon dioxide saturation are stable for up to 2h. The instabilityof the carbon dioxide nanobubbles is due to the fact that the IR cell wasnot gas tight, thus the liquid was exposed to the ambient atmosphere.Because of the exposure to atmospheric conditions, the concentration ofcarbon dioxide in thewater decreased rapidly, creating a chemical poten-tial, which resulted in the diffusion of carbon dioxide from the nano-bubbles [52]. This behavior agrees with the dynamic stability modeldiscussed in Section 5.1.4 and further indicates that gas does accumulateat the liquid–solid interface in the form of nanobubbles.

Other researchers have utilized infrared spectroscopy to detectbutane accommodation at hydrophobic surfaces [30,93]. Miller et al.[93] were the first to detect butane accumulation in both butanesaturatedwater and in purewater after the butane saturatedwater wasflushedaway. Additionally, an increase in the contact angle (0° to84°) ofthe substrate increased the amount of butane present. A later study byKameda et al. [30] confirmed the presence of butane at the hydrophobicsurface and further discovered that the butane was in the liquid state.The chemical analysis of nanobubbles using IR gives further proof thatthe domains imaged by tapping mode AFM are nanobubbles/dropletsand not organic contamination.

5. Nanobubble theory

The vast about of experimental data presented above clearlydemonstrates that nanobubbles do exist, but questions still remain ontheir formation and stability. This section of the review delves intothese questions with a discussion into nanobubble stability andformation. Throughout the review these concepts have been brieflyreferred to in relation to explaining the experimental results.

Therefore, this section serves as a more in depth analysis of thesemechanisms together with further comments by the authors.

5.1. Nanobubble stability

Once formed, nanobubbles have been shown to be stable fromhours [42,51] to days [52]. Why is the lifetime significantly longerthan that predicted by classical thermodynamics? This section of thereview will focus on the contentious issue of nanobubble stability. Itwill begin with a brief overview of why nanobubbles should not bestable according to classical theories and then expand into a numberof possible mechanisms which stabilize nanobubbles.

5.1.1. Why nanobubbles should not be stableAn understanding of nanobubble stability/instability starts with

the thermodynamics of wetting. Take for example a small bubble(negligible buoyancy effects) on a hydrophobic surface in liquid, asshown pictorially in Fig. 24.

The equilibrium contact angle (θ, the angle of the vapor–liquidinterface to the solid, measured within the liquid) can be calculatedwhen the total surface free energy of the system is minimized.Consider the movement of the gas over the solid by an infinitesimalamount, so as to cover an extra area dA (where A is the interfacialarea), resulting in an increase of the vapor–liquid interfacial area ofdAcos(π–θ). Thus the change in the total free energy of the system is[94]:

G = γSVA−γSLA + γLVA cos π−θð Þ ð9Þ

where G is the total free energy, γSL, γLV and γSV is surface free energyof the solid–liquid, liquid–vapor and solid–vapor respectively. Atequilibrium dG=0, thus

cos θY =γSV−γSL

γLV: ð10Þ

Also known as Young's equation, where θY is Young's contactangle. The measurement of the contact angle is much morecomplicated than this simple theory due to chemical and geometricalheterogeneity of the real solid surface. In reality, the contact angle fallsbetween two values, an upper limit called the advancing contact angle(liquid is advanced over the dry solid surface) and a lower limit calledthe receding contact angle (liquid is receded from awet solid surface).The difference between the advancing and receding contact angles isreferred to as contact angle hysteresis and will be higher for morechemically and geometrically heterogeneous surfaces.

The stability of a bubble at a surface immersed in water isdependent upon the pressure inside the bubble, which is calculated bythe Laplace pressure

ΔP =2γLV

Rcð11Þ

where ΔP is the Laplace pressure (pressure drop across the vapor–liquid interface) and Rc is the radius of curvature of the vapor–liquidinterface. For example, if a nanobubble of contact radius 100 nmwere


to follow the Young equation (Eq. (9)) and have a macroscopiccontact angle (80°), the radius of curvature and internal pressure(Eq. (11)) would be approximately 100 nm and 15 atm respectively(assuming spherical cap and surface tension of 72.8 mN/m). Thus, ananobubble would dissolve in microseconds if it had a macroscopicYoung's contact angle [95]. As discussed in Section 2, the contact angleof nanobubbles is significantly higher than the macroscopic Youngcontact angle. The deviation of the contact angle from classicalthermodynamic theories gives an insight into a possible stabilitymechanism for nanobubbles, that is, line tension.

5.1.2. Line tensionAn added complication to the contact angle theory given above

(Eqs. (9) and (10)), is the influence of drop/bubble size on the contactangle. A number of groups have found that the contact angle of a drop/bubble on a solid surface changes with volume/contact radius[30,42,96–106] a result which is believed to be due to two effects,line tension and pseudo line tension. The concept of line tension wasinitially developed by Gibbs as a one dimensional analogue of surfacetension, that is, the excess free energy of a solid–liquid–vapor systemper unit length of the contact line. Inclusion of line tension in thethermodynamics of wetting results in the so called modified Young'sequation [96,101,107]:

cos θ = cos θY−σSVκγLV

ð12Þ

where κ is the line tension, σSV is the geodesic curvature (whichequals the reciprocal of the base radius, Rbase, for an ideal system of ahomogeneous flat solid surface), and θ is the actual contact angle ofnanobubbles with the solid surface. As the modified Young equationsuggests, the effect of line tension is negligible for larger drops/bubbles.

Theoretically, the line tension is calculated to be in the range of 10−12

to 10−9 N [108–112], but the first measurements of line tension usingdrops/bubbles with a contact radius greater than 10 µm were substan-tially larger (10−8 to 10−4 N) [96,98–100,103,104]. The development ofnanoscopic imaging techniques lead to the measurement of drops at amuch smaller scale (<10 µm), resulting in line tension values moreconsistent with theory (10−12 to 10−10 N) [29,30,42,100,102,106,111].The inconsistenciesbetween theoryand themeasurements at largerdrop/bubbles sizes, aswell as the large deviation between results fromdifferentauthors, lead to the development of an extended line tension theoryincluding the effects of pseudo line tension. The modified Young'sEquation given above (Eq. (12)) is derived for an ideal system (a sphericaldrop/bubble on a smooth, homogeneous, and isotropic solid), butmeasurements on a real surface are not ideal. Heterogeneity androughness of the real surface result in corrugations of the three-phasecontact line, thus the assumption that the geodesic curvature equals thereciprocal of the base radius is incorrect [96,97,100]. Themeasurement ofline tension at smaller drop/bubble sizes is not influenced asmuch by theroughness and heterogeneity (σSV≈1/R) so the measured values arecloser to those predicted by theory. This leads to the extension of Eq. (12)to include pseudo line tension, that is,

cos θ = cos θY−κ* Rbase; xið ÞγLVRbase

ð13Þ

where, κ*(Rbase, xi) is the pseudo line tension, which is a function ofbase radius (Rbase) and other variable such as roughness andheterogeneity. At this point, pseudo line tension is an unknownfunction of contact radius, heterogeneity and roughness. Drelich et al.[97] observed a two regime trend, with pseudo line tensions in theorder of 10−6 N and 10−8 N for larger (Rbase>400 µm) and smallerdrops, respectively. The pseudo line tension value at small drop sizesis close to that expected theoretically, and is more than likely the true

line tension value (i.e. κ constant), or at least very similar. At a largerdrop size the pseudo line tension value deviates from the true linetension value due to the effect of roughness and heterogeneity on thegeodesic curvature. A similar two regime pseudo line tension alsoappears for nanobubbles on HOPG measured by Zhang et al. [51], butdue to error in the contact angles this is not certain.

The deviation of nanobubble contact angle from Young's equationdue to line tension and pseudo line tension effects, and the resultinglowering of the Laplace pressure across the liquid–vapor interfacegives an indication of why nanobubbles are stable. As the contactangle of a nanobubble is significantly higher (>150°) than themacroscopic contact angle due to line tension and pseudo line tension,the radius of curvature increases. Thus, the pressure inside of thenanobubble reduces, increasing lifetime (i.e. metastability). Butdespite this, the pressure difference is still too high for a nanobubblelifetime of days. Thus, other factors must be at play and a number ofdifferent theories have been suggested, as discussed below.

5.1.3. Surface forces between the vapor–liquid and vapor–solidinterfaces

It must be pointed out that the nanobubble lifetimes computedabove using the calculation of Ljunggren and Eriksson [95] are validfor bubbles in bulk solution. Contact with the surface may increasestability due to surface forces between the vapor–liquid interface ofthe bubble and the underlying hydrophobic surface [50,52]. Arepulsive force between vapor–liquid and vapor–solid interfaceswould supplement the pressure, lowering ΔP for a given curvature. Assuggested by Jin et al. [66], the ΔP of the nanobubble can be estimatedby a modified Laplace–Young equation

ΔP + PMaxwell =2γLV

Rcð14Þ

where, PMaxwell is the additional Maxwell pressure gradient generatedfrom a repulsive force between the vapor–liquid and vapor–solidinterface. The air–water interface is believed to be negatively charged,thus a repulsive electrostatic stabilizing force will occur if the vapor–solid interface is negatively charged. Insight into this effect wasdeveloped from the combined IR and AFM results of Zhang et al. [50].In this work it was found that the CO2 pressure within thenanobubbles measured by IR was lower than that calculated fromthe Laplace equation (Eq. (11)). Thus, it appears that an additionalpressure due to surface forces exists.

Objections to this stability mechanism have been proposed byDucker [67]. He suggests surface forces are not present as nanobub-bles have a constant curvature, as measured by AFM tapping modeimaging. The authors agree with this statement, as it is expected thatdistortion will result close to the three-phase contact line, that is,where the surface forces are stronger due to the small separationdistance. Ducker also argues that surface forces will result in differentcontact angles for different sized nanobubbles (a similar effect to linetension discussed in Section 5.1.2). All of these objections to thesurface-force stability theory are valid, but it must be questioned, isthe resolution of the AFM enough to detect such phenomena? To theauthors, surface forces between the vapor–solid and vapor–liquidinterfaces are obvious due to the close proximity (<20 nm) and theincreased Hamaker constants in gas.

5.1.4. Dynamic equilibrium and the density depletion layerAs discussed above, themain theoretical problemwith the stability of

nanobubbles is that they should quickly dissolve because of the largeLaplace pressure and the resulting outflux of gas. But what if there is anequal influxof gas into thebubble fromthe three-phase contact line? Sucha dynamic equilibrium has been proposed by Brenner and Lohse [113].Traditionally, the kinetics of bubble dissolution and drop evaporationhas been found to be proportional to the radius of curvature and the


concentration gradient [114–117]. For nanobubbles however, an influx ofgas at the contact line, due to a reservoir of gas in the water densitydepleted layer at the hydrophobic–water interface, is believed to beimportant.

Apart from nanobubbles, it is believed that gas can also accumulateat the water–hydrophobic solid interface due to the reduced waterdensity layer; also known as the density depletion layer [118]. Anumber of theoretical studies [119–124] and MD simulations[12,125,126] have indicated that the density of water is reducednear a hydrophobic surface, up to a range of 2.5 Å, and that dissolvedgases accumulate in this depleted layer at a concentration that is anorder of magnitude higher than in the bulk [127]. Current under-standing of dissolved gas in the depletion layer is limited, and thereare conflicting reports about the gas concentration in the depletionlayer and the influence of dissolved gas on its thickness [8,128].Neutron reflectivity measurements have indicated that the densitydepletion layer is influenced by gas type, with argon resulting in thesmallest depletion followed by carbon dioxide and air [8]. Addition-ally, degassed water was also found to reduce the depletion thicknessand increase as air was reintroduced [8,35]. However, X-rayreflectivity measurements showed that the type and concentrationof dissolved gas was found not to influence the layer thickness[128,129]. A comprehensive understanding of the density depletionlayer and the gas concentration within the layer is at this pointlacking, but its influence on the stability of nanobubbles is important.

Brenner and Lohse calculated the volume flux rate of gas out of thenanobubbles using a steady state diffusion equation to give

jout Rbaseð Þ = πRbaseD 1− c∞c Rð Þ

� �ð15Þ

where D is the diffusion constant, c∞ is the concentration of gas in theliquid far from the nanobubble and c(R)=c0Pgas/P0, where c0 is thesaturated concentration of gas at atmospheric pressure P0, and Pgas isthe Laplace pressure of the nanobubble. Eq. (15) assumes that thebubble is nearly flat, so the diffusive gradient is perpendicular to thesolid surface.

The volume flux rate into the bubble at the contact line iscalculated from the equilibrium of the diffusion and the potentialattracting gas molecules to the hydrophobic solid, giving

jin Rbaseð Þ = 2πsRbaseD− tan θ

ð16Þ

where s is the attraction potential and θ is the contact angle in theliquid phase. In the model, contact angle is a function of Young'scontact angle and line tension, as discussed in Section 5.1.2. Onceagain, Eq. (16) assumes that the bubble is nearly flat.

The reservoir of gas in the density depletion layer results in an idealnanobubble base radius. If the nanobubble is larger than the equilibriumradius, theoutfluxof gas outweighs the influx, resulting in a reduction ofthe nanobubble radius. On the other hand, if the nanobubble is smallerthan the equilibrium radius the influx of gas outweighs the outflux,resulting in an increase of the nanobubble radius.

The idea of dynamic equilibrium is exciting as it sheds light on anumber of experimental results, which to this point are not explainedsufficiently. For example:

1) Alcohol addition removes nanobubbles: Brenner and Lohse's modelsuggests that as the Young contact angle is decreased, the idealnanobubble radiusdecreases. Theadditionof alcohol towaterwets thesurface, thus the reduction involumeofnanobubbles (Section2.6) andthe lack of NBCF in ethanol/water solutions (Section 3.7) agrees withthe dynamic stability theory. The attraction potential will also reducein ethanol solutions, further validating the behavior.

2) Degassed water removes nanobubbles: In the dynamic stabilitymodel, if c∞ decreases, the outflux increases and the ideal radius

reduces. Some results also suggest the depletion layer is reduced indegassed water, thus the influx could be reduced. This agrees withthe finding that nanobubbles reduce in volume or disappear andthat the NBCF reduces in degassed solutions.

3) Electrolyte addition leaves nanobubbles unaffected: The addition ofelectrolyte is not expected to significantly change any of thevariables in Brenner and Lohse's model. Thus, Brenner and Lohse'smodel validates the lack of change in nanobubble morphology(Section 2.5) and the NBCF (Section 3.6) with electrolyte addition.

4) Electrochemical formation of nanobubbles: As briefly discussed inSection 2.7, Yang et al. [45] produced nanobubbles at a HOPG surfaceby electrolysis. At a constant voltage the nanobubble dimensions didnot change, despite the non-zero current indicating the formation ofgas at the surface. When the voltage was increased, signifyingincreased gas production, the nanobubbles increased in size untilsaturation at non-zero current. This interplay between the gasproduction at thewater–HOPG interface, and the size and saturationof the nanobubbles dimensions further validates Brenner'smodel. Inthis case the dynamic stability is influence by the gas inflow at thethree-phase contact line from the electrolytic gas production.However, if electrolytic gas flow is responsible for the stability,why do the nanobubbles not shrink when the voltage is removed(see Figure 2e ref. [45])?

5) Increase in nanobubble volume with increase in surface temperature:Yang et al. revealed that an increase in the surface temperatureincreased the volume of nanobubbles [43]. The gas super-saturationdue to increased surface temperature will increase c(R), decreasingthe outflux. Results indicate that the depletion layer thicknessincreases with higher temperatures, thus the influx into thenanobubble may also increase [130]. Unfortunately, Yang et al. didnot image the nanobubbles at a set temperature overtime, as it isexpected that as the gas super-saturation subsides the c(R) willdecrease, resulting in smaller nanobubbles.

6) Limited lifetime of CO2 nanobubbles: Zhang et al. IR resultsdemonstrate that CO2 nanobubbles have a much shorter lifetimecompared to air nanobubbles in their experimental system [50,52].As the water in the IR cell is open to the ambient atmosphere theCO2 concentration slowly decreases and as predicted by thedynamic stability theory the nanobubbles become smaller anddisappear.

Brenner and Lohse's model demonstrates that the density depletionlayer is important for the stability of nanobubbles. If this is the case, whydo Zhang et al. [55] observe nanobubbles on mica? A mica surface isperfectly hydrophilic (zero contact angle), therefore there is no reason fora density depletion layer and from Brenner and Lohse's model thereshould be no influx of gas at the contact line. Does this indicate thatnanobubbles are not stabilized by the density depletion layer, or thatZhang et al.'s nanobubbles are contamination, or that surface forcebetween the vapor–liquid and vapor–mica interfaces aremore importantfor nanobubble stability (as discussed in Section 5.1.3)?

Despite the fact that Brenner and Lohse's model explains many ofthe nanobubble and NBCF measurements, it has one major flaw:where does the energy for the cycling gas flow originate? Brennerand Lohse suggest that the dynamic equilibrium is only transient,thus nanobubbles will have a limited lifetime (possibly hours, days,weeks?), but considerably longer than that predicted by classicalthermodynamics. Ducker [67] questions this, as nanobubbles can bestable for over 4 days with no input of energy, except for occasionallaser energy for operating the AFM. Is this intermittent energy fromthe laser enough to facilitate the dynamic equilibrium?

Another problem with Brenner and Lohse's model is that allnanobubbles on a surface are predicted to have the same base radius,behavior not observed in AFM tapping mode imaging (see Section 2).The authors suggest that this discrepancy is due to roughness andheterogeneity of the surface which will result in a distribution of


line tension, contact angle and attraction potential over the surface.Apart from this minor flaw and the energy problem discussed above,the authors belief that this is the most comprehensive model ofnanobubble stability.

5.1.5. Reduction of surface tensionA reduction in surface tension of the vapor–liquid interface

reduces the Laplace pressure and increases nanobubble stability. Butwhat are possible reasons for a reduction of the surface tension?Tolman and others predict a decrease of the surface tension for largecurvature on small scales (several molecular diameters) [131–134],but in the case of nanobubbles this effect is negligible.

Contamination from improper surface preparation, sloppy experi-mentalprocedures or organic contaminants in solventsused for solvent-exchange, will reduce surface tension of the nanobubble vapor–liquidinterface. A pertinent article by Ducker suggests that a film of waterinsoluble contaminant at the vapor–liquid interface decreases thesurface tension and increase contact angle of nanobubbles [67]. Asdiscussed in Section 5.1.2, line tension is often given as an explanationfor the very high contact angles of nanobubbles, but the contaminationinfluence suggested by Ducker is also plausible. Additionally, the layerof water insoluble contaminates acts as a barrier to diffusion of gasesfrom the bubble, further increasing nanobubble lifetime.

A layer of contamination at the nanobubble vapor–liquid interfacesheds light on the surfactant study of Zhang et al. [51], as discussed inSection 3.7. Ducker suggests that the lack of change in the nanobubbledimensions with surfactant concentrations below to the critical micelleconcentration (cmc) is due to a pre-existing layer of an unknownsurfactant on the nanobubble. The contamination effect is further clarifiedby the removal of nanobubbles using surfactant concentrations five timesthe cmc, which cleans the contamination from the vapor–liquid interface.

Despite the sufficient theoretical and experimental validation ofthe contamination mechanism, a number of areas need to beinvestigated before this stabilization approach is taken as standard:

1) Origin of the contamination: Ducker states that “contamination isnot surprising due to the method of generation”, that is, solvent-exchange. He suggests that minute quantities of organic contam-ination in solvents such as ethanol are more than enough tostabilize nanobubbles. However, there are a number of works inwhich nanobubbles and the NBCF exist spontaneously, without theuse of the solvent-exchange or other techniques which couldintroduce contamination. For example, the spontaneous formationof nanobubbles by Simonsen et al. at polystyrene when immersedin pure water [34], and the temperature studies of Yang et al. [43].What is the source of contamination in experiments such as these?As suggested by Borkent et al. [175], the contaminant could beorganic substances arising from the cantilever packaging material.

2) Ostwald ripening: It has been shown in some instances that smallernanobubbles decayed and dissolved completely while the neigh-boring nanobubbles became larger (see Section 2.2). This exampleof Ostwald ripening shows that gas flux is possible across theinterface, which suggests that there is no impermeable barrier dueto contamination [52].

3) Influence of gas type: Why do the CO2 nanobubbles of Zhang et al. [52](as discussed in Section 4.2) only last a few hours, whereas airnanobubbles last for days? The author's expect that the reducedLaplace pressure and the diffusion barrier due to the contaminationwill significantly increase the CO2 lifetime to more than a few hours.

4) Electrolytic formation: Zhang et al. electrochemically formednanobubbles at a HOPG surface, which grew, coalesced anddetached from the surface, resulting in a nanobubble free area[47]. It is the authors' expectation that the nanobubble free areawould be cleaned of organic contamination by the expandingthree-phase contact line and increasing vapor–liquid surface areaof the micro-bubble. Unfortunately, Zhang et al. do not continue

with additional applications of voltage, thus it is unknown iffurther nanobubbles can be grown in the nanobubble free zone.Experiments which use electrochemical formation of nanobubblesto clean the surface of contamination (much like the de-foulingexperiments of Wu et al. [39–41,45], as discussed in Section 6.2)would be integral to an understanding of contamination onnanobubble stability.

5.1.6. Summary of nanobubble stabilityThe exact reason for nanobubble stability is still in question.

Nanobubble stability is concisely stated in a quote by Eriksson andLjunggren [135]:

“…NO stable bubbles can form and adhere to hydrophobicsurfaces insofar as (macroscopic) equilibrium surface thermo-dynamics is applicable. When surface imperfections are presentcausing hysteresis and other kinds of “misbehavior”, we cannot,however, exclude a bridging bubble mechanism of the hydro-phobic force on these theoretical grounds.”

From the discussions above, nanobubble stability is beyondclassical surface thermodynamics. An understanding of nanobubblestability requires further analysis of the surface “misbehavior”mentioned by Eriksson and Ljunggren. The above theories on pseudoline tension, surface forces between the vapor–solid and vapor–liquidinterfaces, and the importance of the depletion layer provideadditional information on surface “misbehavior”.

No single theory can explain nanobubble stability, and is morethan likely due to a number of mechanisms, depending on theconditions of the system. It is obvious to the author than an increase inthe contact angle (due to line tension or some other phenomena) ofthe nanobubbles above Young's contact angle is responsible for thepartial stability in all experimental conditions. The increased stabilityof nanobubbles to timescales of days results from additionalstabilizationmechanisms. The dynamic stabilitymechanism proposedby Brenner and Lohse appears to hold the most merit, due to thesufficient theoretical background, and the agreement with experi-mental results. However, if contamination as proposed by Ducker isthe reason for nanobubble stability, then the surface “misbehavior” ismuch more easily understood.

5.2. Nanobubble formation

The formation of nanobubbles is very much dependent on thesurface characteristics, the liquid properties and the conditions underwhich these surfaces are exposed. From the experimental evidenceprovided is this review it appears that nanobubbles are formed in fourpossible manners; spontaneous formation; exposure to gas super-saturated liquid environments; surface perturbation; and gas-trap-ping. These are discussed in more detail in the following sections.

5.2.1. Spontaneous formationThe review of the experimental results provides many examples in

which nanobubbles are formed on smooth hydrophobic surfaceswithout the use of solvent-exchange. In such cases, spontaneousformation of nanobubbles at the surface has been suggested. But whydoes this spontaneous formation occur? A number of possibilitieshave been suggested.

Brenner and Lohse's dynamic stability model discussed inSection 5.1.4 specifies a criterion for the spontaneous formation ofnanobubbles. Spontaneous formation occurs at small bubble sizes(Rbase➔0) when the influx is greater than the outflux. This occurswhen the surface is sufficiently hydrophobic and when the gas phaseis attracted to the hydrophobic surface (i.e. large s). This theory agreeswith the observation that nanobubbles only appear to formspontaneously on high contact angle surfaces such as Teflon and


OTS silanated silica. Brenner and Lohse's theory predicts thespontaneous formation of nanobubbles at surface asperities withincreases in surface temperature, as observed by Yang et al. [43]. Asthe surface temperature increases the gas super-saturation decreasesthe outflux, while surface asperities increase the attraction potentialand the influx, resulting in the spontaneous formation ofnanobubbles.

Li et al. [136] analyzed nucleation and the accumulation of gas atthe liquid–solid interface using continuum theory where the long-ranged retarded vdW force describes the interaction between thevapor–liquid and vapor–solid interfaces of the nanobubble/nanopan-cake. It was found that the type of formation (nanobubble, nanopan-cake, and nanobubble/pancake composite) was dependent on the freeenergy of the system and the number of gas molecules accumulated atthe interface. A similar study by Wang et al. [137] using moleculardynamics simulations found that gas molecules (N2 and H2) couldseparate from the bulk water and accumulate at the hydrophobicsolid–water interface as either nanopancakes or nanobubbles due tothe competition between water–water interaction and gas–waterinteractions.

5.2.2. Exposure to gas super-saturated liquid environmentsAs discussed briefly in Section 2.1, the formation of nanobubbles due

to the solvent-exchange process is believed to be due to the gas super-saturation of liquid in the vicinity of the hydrophobic surface. Forexample, if 1 mol of ethanol (containing3.66×10−4 mol ofN2) ismixedwith1 mol ofwater (containing1.34×10−5 mol ofN2), 2 mol of 0.5 molfraction ethanol is created which has a solubility of 1.06×10−4 mol ofN2. That is, the mixture will contain 2.16×10−4 mol of N2, but theoriginal water and ethanol had 5×10−4 mol of N2 combined, leaving2.84×10−4 mol of N2 to precipitate [138], as indicated in Fig. 25. Zhanget al. suggest residual gas is expected to accumulate at the hydrophobicsurface as nanobubbles due to an excess of air and ethanol at thehydrophobic solid–liquid interface and a reduction in the free energy offormation at the surface compared to the bulk [49].

Studies by Jin et al. [66,84] have shown the existence of freenanobubbles (in the bulk liquid, not at a surface) in mixtures of waterand organic molecules. For this reason it seems plausible thatnanobubbles could also form in the aqueous bulk solution theninteract with the hydrophobic surface in order to form surfacenanobubbles. The results by Zhang et al. indicate that nanodroplets ina bulk solution can form interfacial nanodroplets when a hydrophobicsurface is immersed in the mixture [106]. Another indication thatnanobubbles are formed in the bulk is that nanobubbles cannot beeffectively formed in AFM liquid cells of large volume. For example,Zhang et al. observed that nanobubbles using the solvent-exchangeprocess do not form aswell in an Asylum fluid cell (5 mL) compared to

Fig. 25.N2 solubility–concentration curve in ethanol–water mixtures (data from [138]). The n

a Veeco cell (0.5 mL) at the same injection rate [52]. It is envisionedthat the fast flowing liquid in the Veeco cell imparts enough kineticenergy to the bulk nanobubbles to overcome surface forces andbuoyancy to strike and attach to the surface.

Another possible idea for the mechanism of the solvent-exchangemethod involves the influence of solvent/water exchange on thedensity depletion layer at the hydrophobic surface. Is it possible thatnanobubbles are likely to form if the density depletion layer isdisturbed by solvent-exchange? As of yet, there is no experimental ortheoretical studies on this phenomena, but as suggested from thedynamic stability theory (Section 5.1.4), the depletion layer appearsto be of integral importance. The formation due to gas super-saturation can also be explained by Brenner and Lohse's theory, thatis, the influx is greater than the outflux, as discussed in Section 5.2.1.The super-saturation from the solvent-exchange will definitelydecrease the outflux. It is unknown, however if the solvent-exchangewill increase or decrease the influx.

5.2.3. Surface perturbationThe evidence that the NBCF appears after the first interaction of

the colloidal probe with the underlying substrate (both withequilibrium contact angles >90°) indicates that separation inducedcavitation is a mechanism bywhich nanobubbles are formed [56–58](Section 3.1). The mechanism is analogous to the formation of watercapillary bridges in air at hydrophilic contacts [139–142]. Manytheoretical and experimental studies predict a drying transitionwhen hydrophobic surfaces (in this case with contact angles >90°)contact in water [122,143–146]. Upon separation the cavity formedcan fill with gas dissolved in the liquid phase and form a nanobubble[56].

A number of groups have suggested that nanobubbles form due tosurface perturbation from the AFM tip [12,34,35]. In terms of largehydrophobic tips (contact angle >90°), such as those used to studythe NBCF (Section 3.1), this mechanism appears valid. However, AFMcantilever tips used for imaging are hydrophilic (e.g. silicon nitride)and have small radius of contact (<20 nm), thus how do suchinteractions produce nanobubbles hundreds of nanometers wide?Also, if tip perturbation forms nanobubbles, why do nanobubbles notreform on the surface after being “cleaned” by increased tapping force,as shown by Simonsen et al. [34]? It is the author's opinion that thedensity depletion layermaybe responsible for the discrepancies. Steitzet al. [35] have suggested that the density depletion layer is a pre-cursor to nanobubbles, and that disruption of this layer formsnanobubbles by an as yet unknown mechanism. Is it possible thatadditional nanobubbles are not formed in Simonsen et al.'s experi-ments because the other nanobubbles restrict the formation of furthernanobubbles, or that the density depletion layer has been removed?

on-linearity of the curve results in super-saturation whenwater and ethanol aremixed.


Further studies are definitely required on this possible formationmechanism.

5.2.4. Gas-trappingOne of the most fundamentally sound formation mechanisms is

based on cavities in the hydrophobic surface that do not allow waterto penetrate, the so called Harvey nuclei [147]. Harvey nuclei can onlyform at surface cracks or inverted cone like holes if the recedingcontact angle is equal to or greater than (90°+ψ)/2, where ψ is theangle of the cone shaped cavity. This indicates that Harvey nuclei willnot form on surfaces with a macroscopic receding contact angle lessthan 90°. However, as discussed in Section 5.1.2 the nanoscopiccontact angle is greater than themacroscopic contact angle due to linetension, pseudo line tension or other possible effects. Thus, surfaceswith a macroscopic receding contact angle under 90° may formHarvey nuclei if the surface is sufficiently rough.

In the vast majority of cases, nanobubbles are imaged on ultra-smooth surfaces such as silica plates or HOPG. Therefore, Harveynuclei are more than likely not responsible for the majority ofnanobubbles imaged throughout the literature. However, Yang et al.[43] did observe that nanobubbles formed at surface scratchesapproximately 2 nm deep as the temperature of the surface wasincreased. Yang did not provide a macroscopic receding contact angle(advancing of 105 °), but the nanobubbles had a contact angle of up to164°, suggesting that the nanoscopic receding contact angle is morethan 90°.

5.2.5. Summary of nanobubble formationNanobubble formation is still a matter of debate, but from the

evidence provided above the matter can be summed up by a fewsimple points:

• In the case of surfaces with low hydrophobicity and roughness, gassuper-saturation is a must for nanobubble formation.

• Increased roughness provides opportunities for nanobubble forma-tion without very high macroscopic contact angles and gas super-saturation.

• Spontaneous nanobubble formation only occurs for surfaces of veryhigh hydrophobicity.

• Tip perturbation appears to be a valid nanobubble formationmechanism for large, hydrophobic (>90°) colloidal probes, butnot for small, hydrophilic imaging tips. That is, unless disruption ofthe depletion layer is important.

6. Applications of nanobubbles

A great deal of research has been performed to prove the existenceand understand the properties of nanobubbles. Now that theexistence of nanobubbles has been confirmed, work should be appliedto understand the use of nanobubbles in industrial systems, that is, toengineer nanobubbles. The following sections give some examples ofthese applications.

6.1. Hydrophobic coagulation and hetero-coagulation

The accumulation of gas at the liquid–hydrophobic solid interfacehas been shown to influence hydrophobic coagulation [148–153] andhetero-coagulation [150,154–156]. After degassing it was observed thatcoal formed smaller flocs in water [148], while the capture efficiencybetween a nitrogen bubble and a silanated quartz particle in waterdecreased substantially after degassing [154]. Coagulation of chargedsolid paraffin particles was also reducedwhen the systemwas degassed[149]. In all cases it is believed that the decrease in coagulation is due toreduction of nanobubble size and/or coverage on the hydrophobicsurfaces. However, the influence of dissolved gas on the depletion layerand the “true” hydrophobic force could also be important.

The influence of nanobubbles on coagulation and hetero-coagula-tion, specifically the influence on DLVO forces between particle–particle and particle–bubble, has been extensively studied in a numberof works by Mishchuk et al. [150,155,156]. The theoretical studies byMishchuk et al. indicate that the size and distribution of nanobubbleson the particle surface can not only change the magnitude of the vdWforce but also its sign [150]. The vdW interaction becomes stronger asthe population and height of the nanobubbles increases and the overallDLVO force becomes less attractive as the concentration of electrolyteincreases [150]. A study by Stockelhuber et al. [157] demonstrated thatnanobubbles cause rupture of the wetting film between a bubble andhydrophobic surface without the introduction of a “true” hydrophobicforce mechanism. They found that the repulsive DLVO forces withinthe wetting film are overcome by capillary wave forces which deformthe film surface.

One of the most interesting applications of nanobubbles is in theflotation of mineral particles, specifically the increased flotation insaline waters. In the early 1930s it was found that coal recoveryusing froth flotation increased with the concentration of salt insolution [89,158,159]. High concentrations of salt (e.g. above 0.1 MNaCl where the electrical double layer is sufficiently compressed)could increase the kinetics and yield of flotation without the addi-tion of frother or collector. According to Mishchuk et al. [150],nanobubbles will facilitate bubble–particle attachment, whileincreased electrolyte concentration will not, which does not agreewith single bubble flotation results that show increased attachmentwith electrolyte concentration [160]. An increase in NaCl concen-tration does not change the morphology of the pre-existing sparsenanobubbles, as discussed in Section 6.1. Thus, an increase in thebubble–particle attachment due to larger, more numerous nano-bubbles, as suggested by Stockelhuber et al. [157] is not expected.The salt effect on mineral flotation and the importance of nano-bubbles, the NBCF and other types of hydrophobic forces is aperplexing problem.

6.2. Electrochemical antifouling

The electrochemical formation of nanobubbles at conductivesurfaces has recently been applied to the prevention of surface foulingand de-fouling surfaces. The studies by Wu et al. [39–41] show thatproteins can be removed from a surface by electrochemically formingnanobubbles at the surface. Additionally, pre-existing nanobubblesreduce the absorption of proteins at a surface, as shown in Fig. 26. It isobvious that the existence of pre-existing nanobubbles on the HOPGsurface reduces bovine serum albumin (BSA) absorption, and leavescircular domains of the pristine HOPG surface.

6.3. Friction and adhesion

The understanding of the influence of nanobubbles on thefrictional force between hydrophobic surfaces in aqueous environ-ments is important in a number of applications including micro- andnanoelectromechanical machines (MEMS and NEMS), microfluidics,froth flotation and shear in particle suspensions. Despite the im-portance of friction in a multitude of engineering and scientificapplications, the significance of nanobubbles on friction has not beenstudied extensively. The present authors studied the influence ofsolvent-exchange on the friction force between 1-octanol esterifiedsilica surfaces in water to understand the effect of the capillarybridge size on friction [21]. The larger capillary bridge size (formedby the use of higher changed alcohols in solvent-exchange) in-creased adhesion, which correspondingly increased the friction forcefor an applied load. It was discovered that the friction due to themovement of the three-phase contact line is negligible as the size ofthe capillary bridge had no effect on the friction coefficient or thevelocity dependence.

Fig. 26. The influence of nanobubbles on the absorption of BSA to HOPG (5×5 µm AFMtapping mode images in 10–20 µg/mL BSA solution). A) shows the adsorption withoutpre-existing nanobubbles, while B) shows the adsorption on HOPG with pre-existingnanobubbles. It is obvious that the existence of pre-existing nanobubbles on the HOPGsurface reduces BSA absorption, and leaves circular domains of the pristine HOPGsurface. The nanobubbles were produced using electrochemical methods. AFM imageswere produced using a Veeco Nanoscope III with Veeco NPS cantilevers (7–9 Hz, 0.32 N/m) [39].


6.4. Dissolved air flotation

An understanding of nanobubble formation due to pressurereduction, a topic which has received little attention, is veryapplicable to industrial processes such as dissolved air flotation,commonly used in the water treatment sector. A dramatic reductionin pressure will super-saturate the liquid with gas, thus it is feasiblethat nanobubbles will be formed in a way similar to the solvent-exchange process. The dynamics of nanobubble growth to micro-scopic sizes and the possibility of remaining nanobubbles on thesurface is an interesting area. Recent flotation studies by Tao et al.[161,162] and Zhou et al. [163] indicate that the formation of gaseousdomains at the coal surface using hydrodynamic cavitation signifi-cantly enhanced flotation recovery.

6.5. Boundary slip

Classically, it is assumed that the relative velocity between a liquidand a solid wall is zero. However, this no-slip boundary condition

does not appear to apply for hydrophobic surfaces [164,165].Nanobubbles are believed to be one reason for fluid slip athydrophobic surfaces, as their presence decreases the solid–liquidinteraction [166]. Theoretical [167–172] and MD simulations[173,174] indicate that nanobubbles will increase boundary slip.Recently, the phenomenon was experimentally proven byWang et al.using AFM squeezing experiments [166]. An understanding on theinfluence of nanobubbles on boundary slip is important for reducingdrag in fluid flow, especially in micro- and nano-fluidic systems.

7. Concluding remarks

From the vast amount of experimental and theoretical resultspresented above, it can be confidently stated that nanobubbles doexist. The variety of experimental techniques used to detectnanobubbles shows that the existence is real, and not an artifact ofthe experimental system. Rejections of the existence of nanobubblesby other researchers are based on experimental systems in whichnanobubble formation are not favorable. Additionally, theoreticalobjects on the stability of nanobubbles are based on macroscalethermodynamics (Young's equation for example), which are notcomprehensive at the nanoscale.

From the research in this review a number of issues have beenresolved and many points learnt:

1) The importance of surface preparation: Surface preparation andexperimental implementation are very important if systematicstudies of nanobubbles and the NBCF are to be completed. It is theauthors' view that the majority of previous works on nanobubblesare experimentally flawed because surface preparation techniquesthat influence the results were not considered, such as simplywashing with ethanol. It is these faults (which were unknown atthe time) that resulted in a great deal of confusion in the study ofhydrophobic forces. It was only until the works by Parker et al. [2],who suggested nanobubbles, and later Zhang et al. [53], whodiscovered the solvent-exchange method, that discrepanciesbetween groups studying hydrophobic forces could be understood.Previous researchers cannot be blamed for these errors, but anyfurther research that does not take into consideration theseproblems will encounter difficulties.

2) The NBCF is not a hydrophobic force: The NBCF must be separatedfrom further discussion on hydrophobic forces. The force due tonanobubble interactions is the combination of two main mechan-isms, the force resulting in the rupture of the film between twonanobubbles (or a nanobubble and a hydrophobic solid) and thecapillary force of the resulting gas bridge. The force that rupturesthe film is a true hydrophobic force, but the resulting long rangeattraction is a capillary force. Further comments linking nanobub-bles and hydrophobic forces will continue to cause confusion ontrue hydrophobic forces.

3) Classical thermodynamics is not enough: Much of the theoreticalbacklash against nanobubble stability is based on macroscopicthermodynamics which is not all encompassing at the nanoscale.At the nanoscale other phenomena come into play such as linetension, surface forces and possibly water density depletion layersat the hydrophobic surface. The research in this review indicatesthat any attempts to understand nanobubbles using macroscaletheories are fraught with danger. Further insight into nanoscalephenomena is required. Also, nanobubbles may not be inequilibrium, thus using thermodynamics to understand nanobub-ble stability is not appropriate.

4) What defines hydrophobicity: A unified concept of the termhydrophobicity and hydrophilicity of a solid surface is required.Flotation states a solid is hydrophobic if it can float effectively,others say the macroscopic receding water contact angle must beover 90°. But what does this mean at the nanoscale? Even mica,


which is generally completely wetted by water, can supportnanobubbles at its surface [55]. In this thesis, the 1-octanolesterified silica surfaces used are considered as hydrophilic bymost, but at the nanoscale they are super-hydrophobic (contactangle over 150°). The author feels that the Young contact angleshould not be regarded as a degree of hydrophobicity as it is afunction of size, roughness and heterogeneity. Instead, the wettingbehavior of a solid surface as a function of these parameters shouldbe a more commonly used concept.

An understanding of the issues raised will facilitate furtherdevelopment of nanobubble theory and the engineering of nanobub-bles. Once a greater understanding of the formation and stability ofnanobubbles is achieved, the manipulation of nanobubbles canincrease efficiency and develop new technology is in areas suchparticle processing, friction, biology, surface de-fouling, coagulation,electrolysis, microfluidics, NEMS and MEMS.

Acknowledgement

The authors gratefully acknowledge the Australian ResearchCouncil for financial support through a Discovery Project grant.

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