micro and nano bubbles on polystyrene film/water interface
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Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 128135
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Colloids and Surfaces A: Physicochemical andEngineering Aspects
j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa
icro and nano bubbles on polystyrene film/water interface
ayong Lia,b,, Xuezeng Zhaoa,
School of Mechanical and Electrical Engineering, Harbin Institute of Technology, Harbin 150001, ChinaSchool of Mechanical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, China
i g h l i g h t s
Big micro surface bubbles wereimaged on PS film with AFM.The influence of surface roughness onsurface bubbles was studied.Size dependence of the contact anglewas investigated in a larger size scale.The effect of line tension on surfacebubbles was analyzed.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 11 February 2014eceived in revised form 7 June 2014ccepted 11 June 2014vailable online 8 July 2014
eywords:urface nanobubblesontact angleize dependence
a b s t r a c t
Surface bubbles at polystyrene (PS) film/water interface were imaged using the atomic force microscope(AFM), the surface roughness ranged from 0.58 nm to 3.36 nm in scan area of 5 m2. Big microbubble witha lateral size up to 13 m and a height up to 400 nm was reported. The possible reasons for nucleationof big microbubbles were investigated and found that surface roughness and surface properties play asignificant role. Further, we focused on the problem how does the contact angle (measured through air)rely on the bubble size in a lateral size of 200 nm to 13 m, which is the largest size scale for surfacebubbles found so far. It was found that the dependence of contact angle on lateral size (2r) and height(h) is linear for bubbles on smooth substrates, but nonlinear and even keep constant with the increase ofbubble size for bubbles on rough substrates. While studying the dependence of contact angle on curvaturetomic force microscope (AFM) radius (Rc), an inversion in direction between the bubbles in different size scale was found. The resultsobtained were in close resemblance with the results of other studies. The line tension of surface bubbleson the seven PS substrates in our experiments was calculated and all of the seven line tension values arenegative (the average line tension in this study was 1.07 nN), which should be responsible for theanomalous low contact angle and the size-dependence of the surface bubbles.. IntroductionIn recent decade, one of significant discoveries in interfacialhysics is nanobubbles, which are micro/nano-scopic gaseousomains that form at the interface between solid and liquid. From
Corresponding author at: School of Mechanical and Electrical Engineering,arbin Institute of Technology, Harbin 150001, China. Tel.: +86 13945170437.
Corresponding author.E-mail addresses: lidayong email@example.com (D. Li), Zhaoxz@hit.edu.cn (X. Zhao).
ttp://dx.doi.org/10.1016/j.colsurfa.2014.06.022927-7757/ 2014 Elsevier B.V. All rights reserved. 2014 Elsevier B.V. All rights reserved.
the year 2000, nanobubbles have been imaged and studied byatomic force microscope (AFM)  and other measuring tech-niques such as spectroscopy technique  (most recently throughdirect optical visualization [18,19]), rapid cryofixation technique and quartz crystal microbalances technique [21,22]. Studiesshow that surface bubbles appear with the following features:(1) typical spherical cap shaped [7,12,20,23], (2) typical heights
and curvature radii of 10100 nm and 1002000 nm [5,12,23]respectively, (3) the contact angle (measured through air) is muchsmaller than that of macroscopic bubbles [7,2427], (4) abnormallongevity for several days [1,2,17], (5) two or more bubbles close
D. Li, X. Zhao / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 128135 129
Table 1Summary of studies on the dependence of contact angle on the bubble size.
Substrate Gas type RMS roughness Bubble size Tip correction Ref.
HOPG Air 0.20.3 nm0.62.6 nm
Rc 250 nm Yes 
HOPG Air 0.7 nm Rc 2000 nm Yes HOPG H2; air Rc 1800 nm No Gold-ODT
Gold-MHDAAir Rc 1200 nm Yes 
Si-PFDCS Methane; nitrogen; oxygen 0.4 nm Rc 3000 nm Yes Au (1 1 1) Air 0.2 nm r 100 nm Yes Si-TMCS Air 2.7 nm 2r 800 nm No 
S thylchlorosilane; ODT, octadecanethiol; MHDA, 16-mercaptohexadecanoic acid.
ubstrate abbreviations: PS, polystyrene; PFDCS, 1H,1H,2H,2H-perfluorodecyl-dime
o each other can emerge into a big one , (6) disappear inegassed water and reappear when the liquid is exposed to air5,6,32,33].
The studies on the properties, influence factors and appli-ations of nanobubbles have been developed deeply [9,10,34].owever, the stability (anomalous longevity) and the contact anglef nanobubbles are still open questions. In the conventional view, as
material property, the contact angle of macroscale bubbles shoulde substrate dependent. But AFM studies  show that the contactngle of nanoscale bubbles (measured through air) is much lowerhan that of macroscale bubbles. Thus one would expect that theanoscopic contact angle will be size-dependent, i.e., the contactngle of nanobubbles will increase with the increase of bubble sizend will approach the macroscopic one for large enough bubbles.So far, many efforts have been made to study the dependence of
ontact angle on the size of surface bubbles [5,7,31,3538]. Table 1hows a brief summary of studies regarding the dependence ofontact angle on the bubble size. First, for the dependence of con-act angle on curvature radius ((Rc)), Borkent et al.  concludedhat contact angle (measured through water, before tip correction)ecreases with an increase of the curvature radius of bubbles. Butuch a dependence changes dramatically after tip correction: theontact angle keeps constant within the experimental error. Sim-larly, other studies have reported that the contact angle on theydrophobic surfaces does not change with radius [5,35,38] whilehanges slightly on the hydrophilic surface . However, Van Lim-eek  investigated 7 different types of gas, and found that theontact angle (measured through water) increased with the cur-ature radius of nanobubbles for all gas types they studied, whichre opposite to the results of Borkent. In addition, a recent numer-cal study of Grosfils  validated the gas-dependency results ofimbeek. Secondly, for the dependence of contact angle on lateralize ((2r)), Kameda and Nakabayashi  found that the con-act angle increased with the increase in the radius of three-phaseontact line (when lateral size 100 nm), which agrees with theesults of Yang et al. . In contrast, for the bubbles in smallerize range (lateral size 20 nm), an adverse trend was obtained,nd a probable reason for this was thought to be the effect ofine tension . The previous studies discussed above show thatt is still undefined whether the contact angle of surface bub-les is size-dependent or not. In addition, it should be noted thatost of the previous works focused on the surface bubbles withanoscale (Table 1). So it is significant to investigate the relation-hip between the contact angle and the bubble size in large sizecale.
The line tension is usually taken into account in the study ofhe relationship between the size and the contact angle of surface
ubbles [31,33,36,37]. The line tension was defined as the excessnergy per unit length of the three-phase contact line . Theources of the excess energy of the contact line were thought toe originated from both the changes in the local interfacial tensionFig. 1. The sketch of the effect of line tension on the contact angle of surface bubbles.
caused by the unsaturated molecular interactions in the transitionzone and also from the local interfacial deformations in this zonecaused by the surface forces . The effect of line tension willlead to a reduced contact angle of surface bubbles in nanoscale ascompared with the macroscopic contact angle in the transition zone, as can be seen in Fig. 1.
The difference between the nanoscopic and macroscopic con-tact angle which is linked to the effect of line tension can also beexplained by the modified Youngs equation [31,41], that is
cos = cos Y
where Y is Young contact angle, lg is the liquidgas surface ten-sion, r is contact line radius which is equal to the reciprocal of thegeodesic curvature and is the line tension. The range of Youngcontact angle is less than 90 measured through air for the bubblesformed on the hydrophobic substrates , so cos Y > 0. If the signof is negative, together with positive surface tension lg and con-tact line radius r, then the contact angle calculated by Eq. (1) willbe a reduced one as compared to the Young contact angle Y. Thismeans that the negative line tension should be responsible for theanomalously low contact angle. On the basis of the LaplaceYoungsequation, the smaller contact ang