ARTICLE pubs.acs.org/Langmuir
Fabrication of Surfaces with Extremely High Contact Angle Hysteresis from Polyelectrolyte Multilayer Liming Wang, Jingjing Wei, and Zhaohui Su* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, and Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China
bS Supporting Information ABSTRACT: High contact angle hysteresis on polyelectrolyte multilayers (PEMs) ionpaired with hydrophobic perfluorooctanoate anions is reported. Both the bilayer number of PEMs and the ionic strength of deposition solutions have significant influence on contact angle hysteresis: higher ionic strength and greater bilayer number cause increased contact angle hysteresis values. The hysteresis values of ∼100° were observed on smooth PEMs and pinning of the receding contact line on hydrophilic defects is implicated as the cause of hysteresis. Surface roughness can be used to further tune the contact angle hysteresis on the PEMs. A surface with extremely high contact angle hysteresis of 156° was fabricated when a PEM was deposited on a rough substrate coated with submicrometer scale silica spheres. It was demonstrated that this extremely high value of contact angle hysteresis resulted from the penetration of water into the rough asperities on the substrate. The same substrate hydrophobized by chemical vapor deposition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane exhibits high advancing contact angle and low hysteresis.
’ INTRODUCTION Wettability plays a central and fundamental role in numerous practical applications, such as cleaning, painting, coating, drying, and adhesion.1,2 Contact angle analysis is thus valuable in characterizing surfaces both because of its convenience and its high sensitivity to details of interfacial structure at the angstrom scale. For an “ideal” surface that is flat, inert, and chemically homogeneous, an equilibrium liquid contact angle can be uniquely defined by Young’s equation.3 The real surfaces are neither perfectly flat nor chemically homogeneous, and contact angles observed differ from the Young’s angle under the effect of free energy barriers introduced by roughness and/or chemical heterogeneity. As a result, observed static contact angles fall between two extreme values: the advancing contact angle (θA) and receding contact angle (θR).4 θA and θR together are characteristic of the surface chemistry and topography, and the difference between them is referred to as contact angle hysteresis (Δθ = θA θR). Contact angle hysteresis plays a decisive role in the motion of liquid droplets on solid surfaces.57 The relationship between contact angle hysteresis and surface hydrophobicity was reported by Furmidge8 and reveals the minimum tilt angle (θslide) at which a liquid droplet will spontaneously slide down upon the effect of its own gravity overcoming the surface tension force holding it onto the surface, as shown by the equation mg sin θslide ¼ kwγlv ðcos θR cos θA Þ where k is a constant, g is the acceleration of the gravity, m and w are the mass and contact diameter of the droplet, and γlv is the surface tension of the liquid. The equation suggests that contact angle hysteresis is highly relevant to surface adhesion and friction, r 2011 American Chemical Society
and the surface becomes more adhesive to a liquid droplet as the contact angle hysteresis increases. Usually, a liquid droplet on a surface with low contact angle hysteresis can move easily under even little perturbation, while surfaces with high contact angle hysteresis are very adhesive to liquid droplets. Contact angle hysteresis mainly results from topographic roughness7,911 and chemical heterogeneity.4,1214 Extrand11,15 and McCarthy,1620 in particular, have demonstrated that events occurring at the three-phase contact line during advancing and receding of the liquid droplet are crucial to contact angle hysteresis, such as the formation of microcapillary bridges during dewetting as the contact line recedes.10,1418 When a liquid droplet wets a rough hydrophobic surface, one of two states of wetting is typically present: the homogeneous wetting (Wenzel) or the composite wetting (Cassie) state. In the Wenzel state, the liquid fully penetrates into surface asperities, which pins the contact line of the liquid droplet and this pinning leads to high contact angle hysteresis as the contact line is continuous and stable. Thus, the contact angle and contact angle hysteresis on a rough hydrophobic surface increase with surface roughness. On the other hand, when the wetting is in the Cassie mode, air remains trapped in the cavities of the rough surface; the liquid droplet sits partially on air when deposited on the surface, and the contact angles would follow the Cassie equation.19 In this case, the three-phase contact line is discontinuous and unstable, which thus causes a low contact angle hysteresis, and the contact angle tends to increase with surface roughness while the hysteresis decreases.7,2022 Received: October 8, 2011 Revised: October 31, 2011 Published: November 01, 2011 15299
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Langmuir For flat but chemically heterogeneous surfaces, high contact angle hysteresis mainly stems from the fact that the receding line is pinned by high surface energy components, while the advancing line is pinned by low surface energy components.13,23 How sparse and dense defects affect hysteresis has been modeled and analyzed theoretically.24,25 Superhydrophobic surfaces have attracted tremendous interest in the past decade.11,21,2632 Usually, the term superhydrophobic indicates surfaces that display very high water contact angle, which often exhibit low contact angle hysteresis as well and on which a water droplet can roll off easily, exhibiting antiadhesion behavior. However, McCarthy and co-workers also discovered a different kind of surface that exhibits water contact angle hysteresis as high as 161°.7 Recently, more attention has been paid to similar surfaces that exhibit both high water contact angle and high adhesion to water droplets.3339 For example, inspired by high adhesive force of gecko’s feet and rose petals, Jiang and co-workers reported several methods to fabricate different kinds of superhydrophobic surfaces with high adhesive force3335 and further demonstrated their application in no-loss transfer of liquid droplets.40 Balu et al. reported a sticky superhydrophobic surface with a contact angle hysteresis of 79° by coating cellulose paper with a thin fluorocarbon film.37 Sheng et al. demonstrated that when smearing hydrophobic molecules onto an extended Teflon film, the surface showed a θA of about 140° and a contact angle hysteresis greater than 60°.41 Other groups have reported that condensation on superhydrophobic surfaces can lead to a dramatic increase in contact angle hysteresis to greater than 100°, which results from severely limiting droplet mobility due to pinning of the contact line between surface asperities as the wetting is in Wenzel state.5,27,42 Despite this progress in fabrication of sticky superhydrophobic surfaces, a systematic study of the effects of chemical heterogeneity and topographic roughness on contact angle hysteresis is still highly desirable. Recently, we demonstrated that the surface of a typical polyelectrolyte multilayer (PEM) can easily be hydrophobized by ion exchange chemistry;43,44 the flat surfaces thus obtained exhibit high contact angle hysteresis due to hydrophilic defects inherent in the multilayers.23 The protocol can be applied to rough substrates without significantly altering surface topology.45 In the present study, we systematically investigate the relationship between contact angle hysteresis and chemical defects and surface topographic features for PEMs and compare them with wetting characteristics of surfaces with identical topologies but that were hydrophobized by a chemical vapor deposition (CVD) method and thus are almost free of hydrophilic defects. We report flat and rough surfaces with high contact angle hysteresis and show that the presence of surface defects is the predominant factor leading to high contact angle hysteresis.
’ EXPERIMENTAL SECTION Materials. Poly(diallydimethylammonium chloride) (PDDA, Mw = 200 000350 000), poly(sodium 4-styrenesulfonate) (PSS, Mw = 70 000), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS), perfluorooctanoic acid (CF3(CF2)6COOH), silicon tetrachloride (SiCl4), and aqueous sodium silicate were all purchased from Sigma-Aldrich. Sodium chloride and sodium hydroxide (99.5+%) were purchased from Sinopharm Chemical Reagent Co., Ltd., and used as received. Sodium perfluorooctanoate (PFO) (0.10 M) was prepared by reacting 0.010 mol of the corresponding acid with NaOH in water, and the volume of the solution was increased to 100.0 mL. An alcohol suspension of silica
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submicrometer spheres with a concentration of about 2.0 wt % containing ∼90% silica spheres of about 600 nm and ∼10% silica spheres of about 220 nm was kindly provided by Prof. Junqi Sun of Jilin University. N-silicon (100) wafers were purchased from Wafer Works Corp. (Shanghai, China). Water (18.2 MΩ cm) was purified with a Millipore Simplicity system and used for all the experiments. Substrate Treatment. Silicon wafers were cleaned in a hot piranha solution (H2SO4/H2O2, 7:3 mixture) at 80 °C for 30 min, then washed sequentially with copious amounts of acetone, ethanol, and water, and dried with a N2 flow. Caution: piranha solution reacts violently with organic materials and should be handled with great care. Preparation of the Nanostructured Substrate. A clean silicon wafer was first immersed into a PDDA aqueous solution (1.0 mg/mL) for 15 min, followed by rinsing with water for 1 min and drying with N2 flow, and then the substrate was immersed into an aqueous solution of sodium silicate (100 mM, pH 11.5) for 10 min, rinsed with water for 1 min, and dried with N2. This cycle was repeated to yield a (PDDA/sodium silicate)6 film with nanostructure on the Si substrate.46,47 Preparation of the Microstructured Substrate. The microstructured substrate was fabricated by deposition of submicrometer scale silica spheres onto a silicon wafer according to a previous report.46 An alcoholic silica suspension was first sonicated for 10 min to uniformly disperse the silica spheres. A clean silicon wafer was immersed into the suspension for 10 s at room temperature and then withdrawn from the suspension at a rate of ∼1.5 mm/s. After the alcoholic solvent sufficiently volatilized in a few seconds, silica spheres were successfully deposited onto the substrate surface. The deposition process was repeated for three times to produce a rough surface. In order to make the surface structure more robust, a cross-linking reaction was carried out to stabilize the silica spheres. Specifically, a silica-sphere-coated substrate was dipped into a toluene solution of SiCl4 (1 wt %) and triethylamine (0.6 wt %) for 30 min and then washed with toluene several times, hydrolyzed in water, and dried with a flow of nitrogen. PEM Fabrication and Counterion Exchange. PEMs were assembled at room temperature by alternate dipping of a substrate in PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) aqueous solutions for 15 min each with water rinsing and N2 drying in between until a desired number of layers was obtained. NaCl of various concentrations was maintained in the polyelectrolyte solutions. All PEMs were capped with a PDDA outermost layer, and the Cl counterion in the PEMs was exchanged by immersing the PEMs in an aqueous PFO solution (0.10 M) for 1 min, followed by rinsing with water and drying with N2. In this work, the contact angles and contact angle hysteresis were always measured after the PEM surfaces were hydrophobized by counterion exchange with PFO anions. Chemical Modification of the Substrate. POTS was used to modify substrate surfaces by the chemical vapor deposition (CVD) method. A sealed vessel containing the substrate and several drops of POTS was heated in an oven at about 120 °C for 3 h to enable the reaction between the OH groups on the substrate surfaces and the POTS and then maintained at about 150 °C for 1.5 h to remove the unreacted POTS molecules. Characterization. Microstructures of the nanoscale asperities and the microstructured surface coated by submicroscale silica spheres were observed on a field emission scanning electron microscope (FESEM, Micro FEI Philips XL-30-ESEM-FEG) operating at 20 kV. Topography and roughness of the PEMs were assessed with a tapping mode atomic force microscope (AFM, SPA-300HV, with a SPI3800N Probe Station, Seiko Instruments Inc.). Probes with a resonant frequency of 60150 kHz and a spring constant of 3 N/m were used. Root-mean-square (rms) roughness was calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N ðzi zav Þ2 rms ¼ N i¼1
∑
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Table 1. Wettability of the Surfaces of PEMs and POTS Deposited on Different Substrates
Figure 1. Advancing and receding contact angles and contact angle hysteresis of PDDA(PSS/PDDA)n with PFO counterion assembled on flat substrate at 0.1 M NaCl, as functions of the number of bilayers, n. where zi is the z value of a specific pixel, zav is the average value of the z values in the scan area, and N is the number of pixels in the same area. Water contact angles were measured using a Kr€uss DSA10-MK2 drop shape analyzer at room temperature using water as the probe fluid (4 μL). Each contact angle value reported was an average of at least five independent measurements.
’ RESULTS AND DISCUSSION Between the two factors which primarily contribute to surface contact angle hysteresis, roughness is believed to have a greater influence on apparent contact angles and thus contact angle hysteresis compared to chemical defects (heterogeneities).10 Accordingly, the effect of roughness or topographic features on contact angles has attracted persistent attention,9,10,20,2629,3437,48 whereas the studies of chemical defects on surface wettability are rare.4,1214 Surfaces of PEMs assembled by the layer-by-layer (LbL) technique are known to be heterogeneous due to interpenetration of constituent layers and desorption of assembled molecules. Recently, in a preliminary study we reported a method to reveal and quantify surface heterogeneities (surface defects) in PEMs and demonstrated that contact angle hysteresis may be correlated to area fraction of the defects.23 Thus, PEMs are suitable targets for studying the effect of defects on wettability. All PEMs used in this work were assembled from PSS and PDDA with a PDDAcapped layer and then hydrophobized with PFO anion via counterion exchange.4345 The surfaces of these PEMs are mostly hydrophobic PFO units, with small fractions of areas where PFO units are absent, hydrophilic defects.23 Because ionic strength in polyelectrolyte solutions49,50 and number of bilayers deposited5052 are crucial to the PEM buildup and properties, we first probed the effects of these two factors on the contact angle hysteresis. Figure 1 displays contact angles and hysteresis for the PEMs deposited on flat substrates from solutions containing 0.1 M NaCl with number of bilayers n ranging from 0 to 8. While the receding contact angle remained low at about 20°, both the advancing contact angle and the hysteresis rose with n rapidly and reached plateau values of ∼120° and ∼100°, respectively. The surfaces of these PEMs were flat and featureless with rms roughness values of ∼1 nm (Supporting Information). Therefore, the increases of both advancing contact angle and hysteresis with n result from gradual decrease of area fraction of hydrophilic
defects on the hydrophobized PEMs as demonstrated previously23 and not from surface roughness increases. Specifically, hydrophilic defect domains on PEMs show a strong affinity to chemisorbed water surrounding them, which pin the three-phase contact line of the receding water droplet and result in extremely low receding contact angles; advancing contact angle is more sensitive than receding contact angle to the fraction of hydrophilic defects, which decreases with the increase of n of the PEMs.23 The results are also in accord with the findings for a liquid droplet on a flat but chemically heterogeneous surface based on the Cassie model.53 In addition, contact angle data for PEMs assembled at salt free and 1.0 M NaCl exhibited similar trends, but plateaus were reached at lower n for higher salt condition (Supporting Information). This demonstrates that salt can significantly decrease the amount of surface defects in PEMs, which is consistent with the fact that higher salt concentrations in deposition solutions lead to thicker and more coherent films.44,54,55 By analogy, we can expect that for a hydrophilic surface bearing hydrophobic defects contact angle hysteresis would also vary as a function of the defect fraction but in a reversed manner; i.e., more hydrophobic defects in a hydrophilic surface would elevate the hysteresis. On the basis of the above results, we can conclude that for these PEMs contact angle hysteresis is dominated by surface defects, and surface roughness contributes partially only when it is high. For subsequent experiments, PEMs assembled at 1.0 M NaCl with n = 3, denoted PEM-3, were used. CVD is a widely used classic method to prepare homogeneous hydrophobic surfaces.11,46 The small molecules in the vapor phase can readily react with the substrate to form a uniform coverage on the surface. The wetting behavior of a flat silicon wafer that had been hydrophobized with POTS molecules by the CVD method was studied. The surface remains smooth with a rms value of ∼1.0 nm after POTS deposition. The static and advancing contact angles on the flat POTS surface are ∼110° and ∼119°, respectively, similar to that for PEM-3. However, the receding contact angle is as high as ∼99°, which is prominently different from the low receding contact angles on the PEMs (Table 1). As a result, the flat POTS surface exhibits a contact angle hysteresis value of 20°, much lower than that for the flat PEMs. 15301
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Figure 2. SEM image of the nanoscale asperities on the silicon substrate (a). AFM height image and 3D profile showing the nanoscale topographic features on the substrates (b, c).
Figure 3. SEM micrographs of (a) silica-sphere-coated substrate after PEM-3 deposition (inset shows a water droplet pinned to the surface turned upside down) and (b) silica spheres deposited on the substrate. (c) is a magnification of (a), showing that the spheres are cross-linked, and (d) is the side view of (a).
These results further demonstrate that the presence of defect domains on the PEM surface is responsible for the high contact angle hysteresis. Since surface topography plays an important role in wetting of liquid droplets on solid substrates, substrate roughness was introduced to further tune the wetting behavior of both. We prepared a nanostructured substrate covered with nanoscale asperities for PEM and POTS deposition and explored the effect of the topographic features on the contact angle hysteresis on both the PEM and POTS surfaces. It has been reported that LbL assembly of polyamines and sodium silicate can lead to surface asperities with the size of several tens of nanometers by manipulating the number of deposition cycles.47 PDDA and sodium silicate were used here to prepare the nanopapillae on flat Si substrates. After deposition of six bilayers, asperities with size of 2050 nm were formed and randomly distributed on the substrate, as revealed by SEM and AFM measurements (Figure 2). Then a PEM-3 film was deposited onto the substrate. PEM-3 exhibits a high contact angle hysteresis on a flat substrate, as
shown in the previous section, and has a rather small thickness of ∼27 nm (Supporting Information) so that the topography of the structured substrate should be preserved. One may expect that this surface would exhibit a higher advancing contact angle and contact angle hysteresis compared to the PEM-3 on a flat substrate as the substrate roughness has significantly increased.22,34 To our surprise, no apparent change in water contact angles stemming from the surface nanopapillae was observed. The advancing and receding contact angles are still around 116° and 20°, respectively (Table 1). This indicates that nanoscale asperities do not change the wetting behavior of the PEM surface. On the other hand, the same nanostructured substrate coated with POTS via CVD is more hydrophobic than the POTS surface on a flat substrate. The static contact angle can be as high as ∼119°, and the advancing/receding contact angles are ∼127° and ∼90°, respectively, presenting a contact angle hysteresis value of 37°, which is 17° higher than the counterpart for the flat substrate (Table 1). These results suggest that the hydrophilic defect domains at the three-phase contact line on the PEM 15302
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Langmuir surface diminish the influence of the nanoscale asperities on the movements of water droplets, which is probably because the sizes of asperities and the chemical defects are of the same order of magnitude. To further examine the effects of surface topography on the contact angle hysteresis of the PEM surface, a much rougher substrate was prepared by coating a flat silicon wafer with submicrometer-scaled silica spheres.46 A PEM-3 was then deposited onto this substrate. Figure 3 illustrates the SEM images of the microstructured substrate after PEM-3 deposition. It can be clearly seen that several layers of silica spheres are built on the substrate (Figure 3a,b), the bottom layer has a large coverage, and there are still many micrometer-scaled unoccupied regions randomly distributed on the substrate. The cross-linking reaction (Experimental Section) after deposition of the silica sphere is important to prevent the silica spheres from being removed from the substrate by setting up links among the spheres and the substrate, which can be clearly seen in Figure 3c. Figure 3d is the side view of the rough surface structures. Unlike the hexagonal packing of the spheres in the bottom layer (Figure 3c), particles in top layers are sparsely distributed over the bottom layers, which further promotes surface roughness. The PEM-3 coated on this substrate shows sticky superhydrophobic behavior: the advancing contact angle is as high as 156°, while the receding contact angle is ∼0°, and the static water contact angle ∼152° (Table 1). The great difference between the advancing and receding contact angles indicates an extremely high contact angle hysteresis value of greater than 150°. High contact angle hysteresis values usually mean strong adhesive forces of the surface to the liquid droplets; in fact, a water droplet remains pinned to the surface even when the substrate is turned upside down, as shown in Figure 3a (inset). It is clear that this is an adhesive superhydrophobic surface.33 In contrast, the POTS deposited on the microstructured substrate exhibits high static/advancing/receding contact angles of 156°, 160°, and 150°, respectively (Table 1). The small contact angle hysteresis of 10° is much lower compared to the PEM3 surface on the same substrate. Consequently, unlike the adhesive PEM surface, the rough POTS surface is antiadhesive against water droplets; water on this surface rolls off easily when it is slightly tilted. The discontinuity of the contact line of the water droplets on the rough POTS surface due to the existence of air bubbles beneath the droplet accounts for the low contact angle hysteresis value, and the droplet is in the low adhesive Cassie state (Figure 4, left). The striking difference in contact angle hysteresis between these two sets of surfaces, formed by deposition of PEM and POTS via LbL and CVD, respectively, on substrates of same topographies, indicates that surface defects can greatly impact wetting properties. The defect domains randomly distributed on the PEM surface strongly associate with water molecules as discussed above, which results in the penetration of the water droplet between the surface asperities on the microstructured substrate. In this case, the substrate/water contact area at the three-phase contact line greatly increases and so does the length of the contact line. Consequently, the pinning of the water droplet by the substrate sharply increases at the contact line because the contact line must negotiate higher energy barriers between metastable states,5 which eventually leads to extremely low receding contact angles and thus extremely high contact angle hysteresis. Compared to the flat or nanostructured substrates, the surface features on the silica-sphere-coated surface
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Figure 4. Schematic illustrations of a water droplet on the microstructured substrate coated with POTS (left) and the PEM (right).The hydrophilic defect regions on the PEM surface could lead to collapse of the liquid droplet into the surface asperities, which explains the transition from the low adhesive Cassie state (left) to the high adhesive Wenzel state (right).
largely enhance both the static and advancing contact angles while greatly reduce the receding contact angle. Since it is generally believed that an advancing liquid preferentially samples hydrophobic components while a receding liquid preferentially samples hydrophilic components,13 these results show that the surface roughness enhances both hydrophobicity of hydrophobic regions which have a great influence on the static and advancing contact angles and hydrophilicity of the hydrophilic defect regions which mainly affect the receding contact angle on the same surface. The high values of contact angle hysteresis (especially the extremely low receding contact angle) indicate that the wetting is in the Wenzel state (Figure 4, right). We further deposited the nanopapillae onto the silica-sphere-coated surface to prepare a hierarchical surface with higher roughness and then assembled the PEM-3 onto this substrate; however, no increase of the contact angle hysteresis was observed compared to the rough silica-sphere-coated surface (Supporting Information). Although surface defects are always existent with PEMs, the PEM surfaces do not always exhibit high contact angle hysteresis. Topographic roughness of the substrate is a decisive factor for contact angle hysteresis for these PEM surfaces with defects. When the PEM was deposited on a rough substrate coated with microscale forestlike gold clusters, the contact angle hysteresis on the surface was largely diminished, and a self-cleaning surface can be obtained.45 A reasonable explanation is that the extremely large surface roughness decreases the possibility for defects on a PEM surface to contact with water droplets. Therefore, combining hydrophobized PEMs with moderate rough surface features is an effective way to take full advantage of the hydrophilic surface defects of the PEMs toward surfaces with high contact angle hysteresis.
’ SUMMARY In this paper, we have studied the effects of surface defects and roughness on contact angle hysteresis and demonstrated surfaces with high contact angle hysteresis values on hydrophobic PEM surfaces in the presence of hydrophilic defects. Compared to POTS surface formed via a CVD method, which is uniform and hydrophobic, a PEM surface hydrophobized with PFO counterion exhibits similar static and advancing contact angle but has numerous hydrophilic defects. The PEM surfaces exhibit much larger contact angle hysteresis compared to the POTS surfaces on substrates of same topographies, indicating that hydrophilic defects are crucial to high contact angle hysteresis. The effects of roughness on contact angle hysteresis for surfaces with defects are more complicated. On flat substrates with low roughness, the 15303
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Langmuir PEM surface exhibits large contact angle hysteresis (as high as 100°), and similar wetting behavior is found on nanostructured substrates with asperities of about 2050 nm size. However, on microstructured substrates the PEM surface exhibits sticky superhydrophobic behavior with an extremely high contact angle hysteresis of 156°. Obviously, the high contact angle hysteresis of the PEM surfaces is due to pinning of the contact line of the receding liquid by the hydrophilic defects, the effects of which are enhanced on the microstructured substrate where the hydrophilic defects can cause penetration of the water droplet between the surface asperities on the rough substrate, resulting in extremely high values of the contact angle hysteresis. This work provides fundamental insights into the contact angle hysteresis phenomenon as well as a convenient method for fabrication of sticky superhydrophobic surfaces, surfaces with high contact angle hysteresis.
’ ASSOCIATED CONTENT
bS
Supporting Information. More contact angle and rms roughness data for PEMs deposited on flat substrates, thickness measurements of PEM-3, and SEM images of hierarchical surface of nanoscale asperities on the silica-sphere-coated substrate. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel (+86)431-85262854; Fax (+86)431-85262126; e-mail zhsu@ ciac.jl.cn.
’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21174145). Z.S. thanks the NSFC Fund for Creative Research Groups (50921062) for support. ’ REFERENCES (1) Mittal, K. L. Contact Angle, Wettability and Adhesion; VSP: Utrecht, 1993. (2) Chen, X. X.; Gao, J.; Song, B.; Smet, M.; Zhang, X. Langmuir 2010, 26, 104. (3) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (4) Johnson, R. E.; Dettre, R. H. J. Phys. Chem. 1964, 68, 1744. (5) Wier, K. A.; McCarthy, T. J. Langmuir 2006, 22, 2433. (6) Kawasaki, K. J. J. Colloid Sci. 1960, 15, 402. (7) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395. (8) Furmidge, C. G. L. J. Colloid Sci. 1962, 17, 309. (9) Bartell, F. E.; Shepard, J. W. J. Phys. Chem. 1953, 57, 211. (10) Extrand, C. W. Langmuir 2002, 18, 7991. (11) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (12) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (13) Priest, C.; Sedev, R.; Ralston, J. Phys. Rev. Lett. 2007, 99. (14) Extrand, C. W. Langmuir 2003, 19, 3793. (15) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 6234. (16) Gao, L. C.; McCarthy, T. J. Langmuir 2007, 23, 3762. (17) Krumpfer, J. W.; McCarthy, T. J. Faraday Discuss. 2010, 146, 103. (18) Krumpfer, J. W.; Bian, P.; Zheng, P. W.; Gao, L. C.; McCarthy, T. J. Langmuir 2011, 27, 2166. (19) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 0546.
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