Micro and nano bubbles on polystyrene film/water interface

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<ul><li><p>M</p><p>Da</p><p>b</p><p>h</p><p>a</p><p>ARRAA</p><p>KSCSA</p><p>1</p><p>pd</p><p>H</p><p>h0</p><p>Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 128135</p><p>Contents lists available at ScienceDirect</p><p>Colloids and Surfaces A: Physicochemical andEngineering Aspects</p><p>j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa</p><p>icro and nano bubbles on polystyrene film/water interface</p><p>ayong Lia,b,, Xuezeng Zhaoa,</p><p>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</p><p> i g h l i g h t s</p><p>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.</p><p>g r a p h i c a l a b s t r a c t</p><p> r t i c l e i n f o</p><p>rticle history:eceived 11 February 2014eceived in revised form 7 June 2014ccepted 11 June 2014vailable online 8 July 2014</p><p>eywords:urface nanobubblesontact angleize dependence</p><p>a b s t r a c t</p><p>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</p><p> Corresponding author at: School of Mechanical and Electrical Engineering,arbin Institute of Technology, Harbin 150001, China. Tel.: +86 13945170437.</p><p> Corresponding author.E-mail addresses: lidayong 78@163.com (D. Li), Zhaoxz@hit.edu.cn (X. Zhao).</p><p>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.</p><p>the year 2000, nanobubbles have been imaged and studied byatomic force microscope (AFM) [116] and other measuring tech-niques such as spectroscopy technique [17] (most recently throughdirect optical visualization [18,19]), rapid cryofixation technique[20] 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</p><p>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</p><p>dx.doi.org/10.1016/j.colsurfa.2014.06.022http://www.sciencedirect.com/science/journal/09277757http://www.elsevier.com/locate/colsurfahttp://crossmark.crossref.org/dialog/?doi=10.1016/j.colsurfa.2014.06.022&amp;domain=pdfmailto:lidayong_78@163.commailto:Zhaoxz@hit.edu.cndx.doi.org/10.1016/j.colsurfa.2014.06.022</p></li><li><p>D. Li, X. Zhao / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 128135 129</p><p>Table 1Summary of studies on the dependence of contact angle on the bubble size.</p><p>Substrate Gas type RMS roughness Bubble size Tip correction Ref.</p><p>HOPG Air 0.20.3 nm0.62.6 nm</p><p>Rc 250 nm Yes [7]</p><p>HOPG Air 0.7 nm Rc 2000 nm Yes [5]HOPG H2; air Rc 1800 nm No [35]Gold-ODT</p><p>Gold-MHDAAir Rc 1200 nm Yes [38]</p><p>Si-PFDCS Methane; nitrogen; oxygen 0.4 nm Rc 3000 nm Yes [37]Au (1 1 1) Air 0.2 nm r 100 nm Yes [36]Si-TMCS Air 2.7 nm 2r 800 nm No [31]</p><p>S thylchlorosilane; ODT, octadecanethiol; MHDA, 16-mercaptohexadecanoic acid.</p><p>td[</p><p>cHoabatnaa</p><p>cscttdscihcbcvaiLstcrsalibmnss</p><p>tbesb</p><p>ubstrate abbreviations: PS, polystyrene; PFDCS, 1H,1H,2H,2H-perfluorodecyl-dime</p><p>o each other can emerge into a big one [2831], (6) disappear inegassed water and reappear when the liquid is exposed to air5,6,32,33].</p><p>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</p><p> material property, the contact angle of macroscale bubbles shoulde substrate dependent. But AFM studies [7] 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</p><p>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. [7] 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 [38]. However, Van Lim-eek [37] 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 [39] validated the gas-dependency results ofimbeek. Secondly, for the dependence of contact angle on lateralize ((2r)), Kameda and Nakabayashi [36] 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. [31]. 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 [36]. 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.</p><p>The line tension is usually taken into account in the study ofhe relationship between the size and the contact angle of surface</p><p>ubbles [31,33,36,37]. The line tension was defined as the excessnergy per unit length of the three-phase contact line [40]. 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.</p><p>caused by the unsaturated molecular interactions in the transitionzone and also from the local interfacial deformations in this zonecaused by the surface forces [40]. 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[40], as can be seen in Fig. 1.</p><p>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</p><p>cos = cos Y </p><p>lgr(1)</p><p>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 [7], so cos Y &gt; 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 angle means the lower inner pressureof the surface bubbles with the same base radius and thus meansa longer lifetime. At the same time, the negative line tension cancontribute to the three phase contact line pinning by balancing thesurface tension. The pinned contact line which has been provedin experimental [30] and theoretical [42] studies can limit the gas</p><p>diffusion through the water and thus stabilize the surface bubbles.The value of line tension of surface bubbles calculated in most ofthe previous studies was negative [31,33,37]. Although Kameda andNakabayashi [36] calculated a positive line tension for bubbles with</p></li><li><p>130 D. Li, X. Zhao / Colloids and Surfaces A: Physicochem. Eng. Aspects 459 (2014) 128135</p><p>Table 2Root-mean-square (RMS) roughness of seven different substrates.</p><p>Substrate 1 2 3 4 5 6 7</p><p>RMS (nm) 0.58 0.72 1.14 1.73 2.67 3.01 3.16Scan area (m2) 5 5 5 5 5 5 5Speed (rpm) 5000 4000 3000 5000 4000 3000 3000Spin coater a a a b b b b</p><p>Sso</p><p>lpwfctscr</p><p>P3o3mtsideeg</p><p>2</p><p>2</p><p>bt4s(v(cBwgabQ5loi</p><p>2</p><p>Zciic</p><p>pin coater a (TA-280, China): the wafer was held on a vacuum chuck by a vacuumuction force when spin coating; Spin coater b (TB-610, China), the wafer was fixedn a holder with spring clamps when spin coating.</p><p>ateral size less than 20 nm, which is consistent with Lis theoreticalrediction [43], i.e., the line tension should be positive for a dropith the radius of contact line approaching to zero. When the sur-</p><p>ace bubbles have the lateral size larger than 20 nm, the line tensionalculated in Kamedas work changes to be negative. Consideringhe lateral size of surface bubbles imaged in most of the previoustudies were always less than 2 m. Therefore, it is also signifi-ant to investigate whether the line tension of surface bubbles canemain negative for bubbles in large size scale or not.</p><p>In this work, we investigated surface bubbles formed onolystyrene (PS) films with a roughness range from 0.58 nm to.36 nm, in water by using AFM. The big microbubbles were imagedn the rougher substrates with surface roughness of 3.11 nm and.36 nm. The possible nucleation reasons and features of the bigicrobubbles were investigated. We have also addressed the ques-</p><p>ion How does the contact angle depend on the surface bubbleize in a larger bubble size scale (200 nm to 13 m), includ-ng the dependence of contact angle on lateral size (2r), theependence of contact angle on height (h) and the depend-nce of contact angle on curvature radius (Rc). In addition, theffect of line tension on the imaged surface bubbles was investi-ated.</p><p>. Experimental</p><p>.1. Substrate/water</p><p>The silicon (1 0 0) substrates coated with PS film were preparedy spin coating PS (molecular weight 350000, SigmaAldrich) solu-ion with a concentration of 0.50% (weight) at a speed of 3000 rpm,000 rpm and 5000 rpm. To obtain the substrates with differenturface roughness, two spin coaters were used. For one spin coaterTA-280, China), the Si wafer was held on a vacuum chuck by aacuum suction force when spin coating. For the other spin coaterTB-610, China), the wafer was fixed on the holder with springlamps. Seven substrates were produced, as shown in Table 2.efore spin coating, the silicon wafers (dimension 1.0 cm 1.0 cm)ere cleaned in a sonication bath of strong sulfuric acid and hydro-en peroxide solution (weight ratio 3:1) for 30 min, followed bycetone for 30 min and purified water for 10 min and subsequentlylew to dry with nitrogen for use. Water was purified using a Milli-</p><p> A10 system with resistivity of 18.2 M cm, and before use, about0 mL of water was allowed to equilibrate in air for hours in a stain-ess steel container which had a volume of 100 mL, then a dropf water was injected into the liquid cell using a glass syringe formaging.</p><p>.2. Atomic force microscopy (AFM)</p><p>Tapping-mode AFM (NTEGRA platform, NT-MDT Company,elenograd, Moscow) was used to image the silicon substrate</p><p>oated with PS film in both air and purified water. When imag-ng in water, the substrate was immersed in the water, completely,n a fluid cell with a maximum volume of about 1 mL. Rectangularantilevers (CSG30 probe, NT-MDT Company) with a tip curvatureFig. 2. Height image of PS coated silicon wafer using tapping mode AFM in air.</p><p>radii Rt = 17 3 nm measured by SEM imaging, a typical springconstant k = 0.51 0.02 Nm1 obtained based on the model ofCleveland et al. [44]. The measured resonance frequencies of thecantilever, with a lock-in amplifier (SRS 830), were about 66 kHz inair and about 21 kHz in the experimental water. When the exper-iments were carried out, more than five different scan locationswere chosen, both the height and phase images were recordedsimultaneously with the amplitude set point of 95% and scan-ning rate of 0.5 Hz. The free amplitude A0(nA) was obtained fromamplitude phase distance curves recorded before and after eachcaptured height image [7,16,30]. It was recalculated into nanome-ters by multiplying the deflection sensitivities, and a typical value offree amplitude was A0 = 3.5 0.2 nm. The ambient temperature waskept at 25 C, and for each experiment, all samples were imaged inair before imaging in purified water. The height image of substrate-6 (refer to Table 2) on which the big microbubble was imaged firstlyis shown in Fig. 2. The PS film thickness of substrate-6 was deter-mined by AFM nanoshaving [3], and the average...</p></li></ul>

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