Effect of Dew Condensation on the Wettability of Rough Hydrophobic

Jul 14, 2010 - Dew condensation effects on the wettability of rough and smooth coatings of two fluoroalkylsilanes (FAS3 and. FAS17) were investigated ...
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Effect of Dew Condensation on the Wettability of Rough Hydrophobic Surfaces Coated with Two Different Silanes Tsutomu Furuta,†,‡ Munetoshi Sakai,‡ Toshihiro Isobe,† and Akira Nakajima*,†,‡ †

Department of Metallurgy and Ceramic Science Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan, and ‡Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan Received April 26, 2010. Revised Manuscript Received June 29, 2010

Dew condensation effects on the wettability of rough and smooth coatings of two fluoroalkylsilanes (FAS3 and FAS17) were investigated by controlling the temperature. Contact angles of the coatings decreased concomitantly with decreasing surface temperature. Inflection points in the temperature dependence of contact angles were observed at the dew point. They were attributable to the change of the interfacial free energy of the solid-gas interface by water adsorption. The contact angle decrease suggested a mode transition from Cassie to Wenzel on the rough surface, and resulted from the surface wettability change and the increase of the condensation amount of water. The contact angle change by increasing temperature from -6 °C revealed that the Wenzel mode is more stable than Cassie’s mode.

I. Introduction Surfaces with a water contact angle greater than 150°, which have attracted great interest, are commonly designated as superhydrophobic surfaces. The small contact area between a superhydrophobic surface and water limits chemical reactions and bond formation through water. Various phenomena such as snow adhesion, oxidation, and electrical conduction are therefore mitigated on the surface.1 Superhydrophobic surfaces have been prepared for earlier studies to provide both rough solid surfaces and lower surface energy. Wenzel modified Young’s equation and described the contact angle θ0 on a rough surface as follows:2 cos θ0 ¼

rðγSV - γSL Þ ¼ r cos θ γLV

ð1Þ

Therein, γSL, γSV, and γLV, respectively, denote the interfacial free energies per unit area of solid-liquid, solid-gas, and liquid-gas interfaces. In addition, θ is the contact angle on the smooth surface. In that equation, r signifies the roughness factor, which is defined as the ratio of the actual area of a rough surface to the geometrically projected area. Because r is always greater than unity in this equation, the surface roughness enhances hydrophobicity of the hydrophobic surface. Cassie proposed an equation for a hydrophobic surface with large roughness. With increasing surface roughness, air intrudes into the hydrophobic solid-liquid interface. It is assumed that the interface comprises solid and air. When a unit area of the surface has a wetted solid surface area fraction f with a water contact angle θ, the contact angle on the surface can be expressed as the following equation, assuming 180° water contact angle for air:3 cos θ0 ¼ f cos θ þ ð1 - f Þcos 180° ¼ f cos θ þ f - 1

ð2Þ

*Corresponding author. Akira Nakajima. Tel.: þ81-3-5734-2525. Fax: þ81-3-5734-3355. E-mail: [email protected]. (1) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (2) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (3) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

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Reportedly, the contribution of Cassie’s mode is dominant for a superhydrophobic surface with a high water-shedding property.4 Numerous methods combining both appropriate surface roughness and low surface energy for the processing of superhydrophobic surfaces have been reported.5-18 For the preparation of a superhydrophobic surface using inorganic materials, surface treatment to the lower surface energy using organic compounds such as silane coupling agents is usually necessary. These two wettability modes, Wenzel and Cassie, do not always appear as the most stable state. Cassie’s mode appears as a metastable state even when Wenzel’s mode is the most stable.19,20 Moreover, in the special case, Cassie’s mode is attainable on the surface whose water contact angle for the smooth one is less than 90°.22 Recently, studies of the droplet wettability mode (4) Miwa, M.; Nakajima, A; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (5) Washo, B. D. Org. Coat. Appl. Polym. Sci. Proc. 1982, 47, 69. (6) Morra, M.; Occhiello, E.; Garbassi, F. Langmuir 1989, 5, 872. (7) Ogawa, K.; Soga, M.; Takada, Y.; Nakayama, I. Jpn. J. Appl. Phys. 1993, 32, L614. (8) Yamauchi, G.; Miller, J. D.; Saito, H.; Takai, K.; Ueda, T.; Takazawa, H.; Yamamoto, H.; Nishi, S. Colloids Surf. A 1996, 116, 125. (9) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (10) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (11) Tadanaga, K.; Katata, N.; Minami, T. J. Am. Ceram. Soc. 1997, 80, 3213. (12) Hozumi, A.; Takai, O. Thin Solid Films 1997, 303, 222. (13) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800. (14) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (15) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (16) Erbil, H. Y.; Demirel, A. L.; Avcı, Y.; Mert, O. Science 2003, 299, 1377. (17) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (18) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (19) Ishino, C.; Okumura, K.; Quere, D. Europhys. Lett. 2004, 68, 419. (20) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633. (21) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999. (22) McHale, G.; Aqil, S.; Shirtcliffe, N. J.; Newton, M. I.; Erbil, H. Y. Langmuir 2005, 21, 11053.

Published on Web 07/14/2010

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transition from Cassie to Wenzel are increasing.19-35 Quere and Lafuma observed this transition by sandwiching a water droplet between two superhydrophobic surfaces with application of pressure.31,32 Barbieri et al. examined the wetting mode transition of the surface with flat-topped pillars from theoretical and experimental aspects, reporting that the transition was a function of pillar parameters that was determined geometrically from the surface roughness.26 Patanka24 and Marmur29 showed that the more stable configuration is always that with the lowest contact angle. Dew condensation is commonly observed in nature and in daily life. When the air temperature drops below the dew point, water vapor in the air becomes liquid and condensation occurs. Recently, several studies investigating dew condensation on superhydrophobic surfaces with orderly structures have been reported.30-38 However, only a few studies have been reported of the random structure, which is more important from a practical viewpoint.34,35 Moreover, no comparative investigation of the effect of dew condensation on the wettability of hydrophobic surfaces obtained by coating two silane agents on inorganic solid surfaces with and without surface roughness by controlling temperature has been reported to date. In a previous report, the authors described processing of a transparent superhydrophobic coating with nanoscale roughness using boehmite and a sublimation agent.14 For the present study, we used this method to prepare inorganic material surfaces having arbitrary roughness. Subsequently, one of two silane agents was coated, yielding highly hydrophobic surfaces. Using these samples, effects of dew condensation on the hydrophobicity of the surfaces were investigated using a Peltier cooling system. Additionally, we prepared smooth coatings using these silanes and evaluated the effect of dew condensation on the surface wettability as a comparison.

II. Experimental Section A commercial boehmite powder (AlOOH, DISPAL 18N4; Contec Co. Ltd., Hamburg, Germany) and reagent-grade acetylacetonate (AACA, Al(C5H7O2)3; Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan) were mixed with ethanol. The weight ratios of boehmite and AACA to ethanol were, respectively, 0.002 and 0.037. The suspensions were sonicated for 40 min. During sonication, AACA was dissolved into ethanol. A Si (100) wafer (Aki Corp., Miyagi, Japan) was cut into plates (50  50 mm2). The sonicated suspensions were coated onto Si plates (1 mm thickness) by spin-coating at 1000 rpm for 10 s. The coated wafers were dried at room temperature for a few minutes. During drying, the plates became opaque. Heating of the plates was then conducted on a hotplate heated at 460 °C for 20 s. The opaque coatings produced white smoke; the plates became clearly visible again during this heat treatment. These coating and (23) Bico, J.; Marzolin, C.; Quere, D. Europhys. Lett. 1999, 47, 220. (24) Patankar, N. A. Langmuir 2004, 20, 7097. (25) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (26) Barbieri, L.; Wagner, E.; Hoffmann, P. Langmuir 2007, 23, 1723. (27) Bormashenko, E.; Pogreb, R.; Whyman, G.; Erlich, M. Langmuir 2007, 23, 6501. (28) Moulinet, S.; Bartolo, D. Eur. Phys. J. E 2007, 24, 251. (29) Marmur, A. Langmuir 2003, 19, 8343. (30) Dorrer, C.; Rυ.he, J. Langmuir 2007, 23, 3820. (31) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (32) Quere, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109. (33) Wier, K. A.; McCarthy, T. J. Langmuir 2006, 22, 2433. (34) Cheng, Y. T.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 144101. (35) Xiao, X. C.; Cheng, Y. T.; Sheldon, B. W.; Rankin, J. J. Mater. Res. 2008, 23, 2174. (36) Narhe, R. D.; Beysens, D. A. Phys. Rev. Lett. 2004, 93, 76103. (37) Narhe, R. D.; Beysens, D. A. Europhys. Lett. 2006, 75, 98. (38) Beysens, D. C. R. Phys. 2006, 7, 1082.

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calcination procedures were repeated five times for suspensions to coat the plates completely. In our experiments, 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17, CF3(CF2)7(CH2)2Si(OCH3)3, TSL8233; GE Toshiba Silicones, Tokyo, Japan) and trifluoropropyltrimethoxysilane (FAS-3, CF3(CH2)2Si(OCH3)3, KBM-7103; Shin-Etsu Chemical Co. Ltd., Tokyo, Japan) were used as water-repellent agents. The plates coated with the roughened boehmite thin films were cleaned of organic contaminants using vacuum ultraviolet illumination (VUV, 172 nm wavelength, UER-20; Ushio Inc., Tokyo, Japan) for 15 min in air at room temperature. The precleaned plates were coated with organosilanes using chemical vapor deposition (CVD) by heating together with 0.02 cm3 of either FAS-17 or FAS-3 in a Petri dish at 150 °C for 60 min (FAS17) or 100 °C for 90 min (FAS-3) with flowing N2. Then, before drying, the sample surfaces were rinsed using acetone, toluene, and distilled water. These coating procedures and CVD conditions used for the preparation of highly homogeneous smooth coatings were conducted using these silanes.39-41 Hereinafter, we designate these samples, respectively, as rough FAS3 or rough FAS17 coating. Under the same conditions, the surfaces coated with FAS3 or FAS17 onto the smooth Si plate (without boehmite coating) were also prepared. Hereinafter, we designate these samples, respectively, as smooth FAS3 and smooth FAS17 coating. Surface roughness (Ra) was evaluated in a 5-μm-square area using an atomic force microscope (AFM, JSPM-4200; JEOL, Tokyo, Japan) with a Si cantilever (NSC36-c; μ-masch, Narva Mtn., Estonia). Surface roughness (Ra) and the roughness factor (r) were obtained using surface analysis software (WinSPM Data Processing 2.15; JEOL, Japan) of the AFM. The sessile drop method, using a contact-angle meter (Dropmaster DM-500; Kyowa Interface Science Co. Ltd., Saitama, Japan), was used to measure the contact angles. The measured water droplet was 3.0 μL. The surface was blown with ionized air before measurement to eliminate any electrostatic charge. The sliding angle of a 10 μL water droplet on the sample surfaces was measured using an automatic measurement system (SA-20; Kyowa Interface Science Co. Ltd.). Five points (contact angle) or three points (sliding angle) were measured for each sample, then averaged. For measurement of the contact angles or sliding angles, a Peltier cooling system (10021; Japan High Tech Co. Ltd., Fukuoka, Japan) was attached to the sample stage on the devices described above. First, the plate surface temperature was set to 22.5, 12.0, 5.0, 2.0, -1.0, -3.0, and -6.0 °C. The static contact angles were measured. The measurement was performed 3 min after the placement of water droplets on each surface. Second, the surface temperature of the plate was set to -6.0 °C and a water droplet (3.0 μL) was placed. Then, the temperature was increased to 25 °C at the heating rate of 10 °C/min, and the contact angle change was evaluated during the removal of dew condensation. In this description, the temperature is the controlled temperature of Peltier cooling system. Atmospheric conditions for these measurements were 22.5 °C and 30% relative humidity. The dew point was around 3 °C under these conditions.

III. Results and Discussion Figure 1a,b portrays AFM micrographs of rough FAS3 and FAS17 coatings. An SEM micrograph of the rough coating is shown in the Supporting Information. Surface roughness (Ra) and roughness factor (r) values of these samples were, respectively, 48 nm and 1.41 for rough FAS3 coating and 46 nm and 1.47 for rough FAS17 coating. These samples possess almost (39) Suzuki, S.; Nakajima, A.; Yoshida, N.; Sakai, M.; Hashimoto, A.; Kameshima, Y.; Okada, K. Langmuir 2007, 23, 8674. (40) Suzuki, S.; Nakajima, A.; Yoshida, N.; Sakai, M.; Hashimoto, A.; Kameshima, Y.; Okada, K. Chem. Phys. Lett. 2007, 445, 37. (41) Furuta, T.; Sakai, M.; Isobe, T.; Nakajima, A. Langmuir 2009, 25, 11998.

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Figure 1. AFM micrographs of (a) rough FAS3, (b) rough FAS17, (c) smooth FAS3, and (d) smooth FAS17 coatings.

Figure 2. Contact angle changes of (a) smooth FAS17 (0) and FAS3 (O) coatings, and (b) rough FAS17 (9) and FAS3 (b) coatings according to changes in the surface temperature.

equivalent random roughness. The static contact angles were, respectively, 146° (average) ( 2° (standard deviation) (FAS3) and 160° ( 2° (FAS17). The sliding angle for the sample coated with FAS17 was 4° ( 2°; the droplet did not slide down on the rough FAS3 coating even when the tilt angle was 90°. Neither heterogeneous defects nor dust particles were observed on the surface of smooth FAS3 and FAS17 coatings. The Ra values for these coatings were 0.3 nm for each (Figure 1c,d). The static contact angles were, respectively, 78° ( 1° (smooth FAS3) and 104° ( 1° (smooth FAS17). Although we tried to prepare the smooth boehmite coating by changing various process parameters, the coatings always retained roughness on the scale of a few nanometers. For that reason, we were unable to prepare it. Therefore, we used Si substrate as the smooth coating samples for this study. The chemical compositions of boehmite and the Si substrate differed. The FAS molecules would therefore be coated onto them with a different state. However, these surfaces were nearly amorphous. (Note: The Si substrate surface became amorphous oxide because of the VUV treatment, and pseudoboehmite, which is lowcrystallinity boehmite, was used in this study. Even after the heat Langmuir 2010, 26(16), 13305–13309

treatment at 460 °C, some of the boehmite transforms into transition alumina with low crystallinity.) Consequently, the material dependence of the FAS deposition state was less remarkable than in the cases using crystalline Si, boehmite, or sapphire. The boehmite coating with a few nanometers scale roughness and Si substrate both exhibited superhydrophilicity after VUV treatment, suggesting that they are amorphous oxide or hydroxide surfaces with many OH groups. Therefore, although some approximation is included for the deposition state of FAS molecules between boehmite and Si surfaces, the comparison of contact angle values on these coatings is feasible to some degree. Figure 2 portrays contact angles on (a) the smooth FAS3 or FAS17 coatings and (b) the rough FAS3 or FAS17 coatings at each temperature. Contact angles were decreased gradually concomitantly with decreasing temperature. Moreover, clear inflection points were observed at the dew point (dashed line shown around 3 °C), except for the rough FAS17 coating. Previous studies of smooth surfaces coated with FAS3 or FAS17 revealed that freezing of the droplets does not occur at the temperatures used for this study.39,40 Water should be supercooled when the temperature is less than 0 °C. The surface energy DOI: 10.1021/la101663a

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Figure 3. Photographs of droplets approaching surfaces with respective temperatures of (a) 22.5 °C and (b) 12 °C (sample: rough FAS17 coating).

of water increases concomitantly with decreasing temperature.42 Therefore, the surface energy change of water provides no explanation for the temperature dependence of the contact angle in Figure 2. Additionally, results strongly suggest that the effect of the change of the interfacial free energy of the solid-gas interface should be more significant than that of interfacial free energy of the solid-liquid interface on the trend change in the temperature dependence of the contact angle at the inflection point because the point corresponds to the dew point. The interfacial free energy of the solid-gas interface is increased by adsorption of water; the solid surface became more hydrophilic. If the interfacial free energy of the solid-liquid interface plays an important role, then the contact angle values can be expected to be decreased continuously, irrespective of the dew point. It is noteworthy that contact angle values decreased concomitantly with decreasing surface temperature, even at temperatures higher than the dew point. Figure 3 presents photographs of droplets approaching rough FAS17 coatings of 22.5 and 12 °C. For the sample temperature of 12 °C, dew condensation occurred on the surface with the approaching droplet surface. This phenomenon was observed similarly for other coatings, but not in the case of 22.5 °C. This dew condensation was probably attributable to the partial increase of humidity by approaching liquid phase with curvature to the solid, and the resultant decrease of dew point of the atmosphere near the solid surface. Consequently, it is inferred that the atmosphere near the solid surface partially reaches the dew point even when its surface temperature is higher than 3 °C. Miwa et al. constructed an equation combining Cassie’s mode with Wenzel’s as follows:4 cos θ0 ¼ r 3 f cos θ þ ð1 - f Þcos 180° ¼ r 3 f cos θ þ f - 1 ð3Þ where f signifies the area fraction of solid, θ denotes a water contact angle, and r signifies the roughness factor. They derived eq 3 with consideration of a specific surface structure: a series of uniform needles. They prepared various highly hydrophobic surfaces using the same experimental procedure as that used in this study and then examined the relation between contact angles and sliding angles on the surfaces using this equation. Therefore, we also used it for comparison of surface hydrophobic state among these surfaces in this study. On the basis of this equation and the contact angle values presented in Figure 2a,b, it is feasible to estimate the area fraction in the rough coatings as follows: f ¼

cos θ0 þ 1 r 3 cos θ þ 1

(42) Trinh, E. H.; Ohsaka, K. Int. J. Thermophys. 1995, 16, 545.

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ð4Þ

Figure 4. Area fraction change of the rough FAS17 (9) and FAS3 (b) coatings produced by changing the surface temperature.

Figure 4 portrays calculated area fraction values ( f ) on the rough FAS3 or FAS17 coating at each temperature. With decreasing air amount at the solid-liquid interface, the area fraction value approaches unity, and then the wetting mode changes from Cassie to Wenzel. The calculation results suggest that the area fraction value was nearly constant for the rough FAS17 coating in the temperatures of dew point and -6 °C. On the other hand, for the rough FAS3 coating, the area fraction increases, suggesting that the air amount decreases concomitantly with decreasing temperature in the same region. From Figure 2a, contact angle values were sustained more than 90° on the smooth FAS17 coating, even if the surface temperature was -6.0 °C. However, on the smooth FAS3 coating, contact angle values were decreased to less than 70°. The result of the contact angle values on each smooth coating show that FAS3 is more hydrophilic than FAS17 is. Therefore, it is expected that water spreading on the solid surface and the increased Wenzel’s ratio will be more remarkable than those of FAS17 at the region lower than the dew point. The constant area fraction for the rough FAS17 coating was attributable to the low hydrophilicity. Similar temperature dependence of the wettability shown in Figure 2 was also obtained when a droplet was set on the solid surface at room temperature and subsequently cooled to -6 °C by 10 °C/min (see Supporting Information). On the basis of these results, it is conceivable that the contact angle decreased because of the increase of the condensation amount of water and the surface wettability change to hydrophilic. The contact angle values for the Wenzel’s mode calculated from both the roughness factors and the contact angles for smooth surface were, respectively, 110° (FAS17) and 73° (FAS3). On the basis of these contact angle values and discussion by Ishino et al.,19 it is expected that the Wenzel’s mode was energetically Langmuir 2010, 26(16), 13305–13309

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Figure 5. Contact angle change from -6 to 25 °C on (a) smooth coatings and (b) rough coatings by heating 10 °C/min: squares, contact angle of FAS17 coating (left axis); circles, contact angle of FAS3 coating (left axis); dashed lines, surface temperature of FAS17 (right axis); solid lines, surface temperature of FAS3 (right axis).

more stable than Cassie’s mode. The surface of the rough FAS3 coating is intrinsically hydrophilic. The Cassie mode can probably be maintained solely by the influence of surface topography. However, these calculated contact angle values were smaller than the practical values obtained from this study, suggesting that Cassie’s mode remained when the temperature was -6 °C, which is consistent with the result presented in Figure 4. The degree of remaining Cassie’s mode will depend on solid surface characteristics and experimental conditions. The sliding angle of a 10 μL water droplet C on the sample surface coated with FAS17 was 47° ( 4°. It was 4° ( 2° at room temperature; this difference implies that the water shedding property had deteriorated considerably. This trend is consistent with results of earlier studies, which revealed a contact angle hysteresis increase or droplet mobility decrease resulting from dew condensation.31-34 We placed water droplets on sample surfaces cooled at -6 °C. Then, the surface temperature was increased to 25 °C by 10 °C/ min. The contact angles during this temperature change on (a) smooth coatings and (b) rough coatings are portrayed in Figure 5a and b. Contact angle values of the smooth coatings increased concomitantly with increasing surface temperature. On the other hand, contact angle values did not recover to initial values on the rough coatings, suggesting that Wenzel’s mode is thermodynamically more stable than Cassie’s. In this study, the effect of evaporation is negligible at temperatures less than the dew point because water vapor saturated around the solid surface. In practice, the droplet mass was confirmed as constant under that condition. However, a slight decrease of the contact angle after removing condensation by increasing the surface temperature will result from water evaporation.22,41 This study demonstrated that the stability of the Cassie mode on a random rough surface depends on the top silane coating. Because of the nanometer-scale surface roughness of these surfaces, direct observation of the nucleation and growth of dew condensation was not feasible. The silanes used for this study differ only by the length of fluorocarbon moiety. However, the results of this study will be affected not only by the silane length, but also by its surface density (excluded volume) and homogeneity. The excluded volumes of FAS3 and FAS17 are expected to be different because of molecular length differences. Therefore, it is reasonable to infer that a difference exists in the surface coverage ratio between these two silanes. We examined various CVD

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conditions on Si substrate for FAS3 and FAS17, but the contact angles of smooth FAS3 and FAS17 coatings became saturated near the values shown in this study. Contact angle values of FAS3 coating on rough surface did not increase either even if the coating period is increased. Moreover, results of additional experiments on the wet coating by soaking (not reported herein) suggest that FAS3 molecules have a more remarkable trend to condense or polymerize each other than FAS17 molecules have. Differences in the chemical properties of FAS3 and FAS17 do exist. Therefore, the conditions for wet coating of these two differed similarly as for CVD. Additionally, the freezing of supercooled water on the smooth FAS3 coating occurs at higher temperatures than it does on FAS17.40 On the basis of these results, it is expected that the FAS3 coating possesses a more hydrophilic portion (such as the exposed part of Si surface or unbounded OH group in the silane molecules) than FAS17 does. These properties are expected to be reflected also on the boehmite surface. Therefore, it is deduced that the difference in contact angle values on the FAS17 and FAS3 coating originated from the differences of molecular length and chemical features of these molecules, and that the resultant surface coverage ratio and homogeneity will be different. Detailed analysis of the contribution of these differences to wettability of the entire surface shall be addressed in future studies.

IV. Conclusion This study examined effects of dew condensation on the surface wettability for rough and smooth coatings of two different fluoroalkylsilanes (FAS3 and FAS17). The rough coatings possessed almost identical roughness. Contact angles of the FAS3 or FAS17 coatings decreased concomitantly with decreasing surface temperature. At the dew point, clear inflection points were observed in the temperature dependence of contact angles, suggesting the change of the interfacial free energy of the solid-gas interface by water adsorption. The contact angle decrease implies a mode transition from Cassie to Wenzel. The decrease was attributed to the surface wettability change and the increase of the condensation amount of water. The contact angle change attributable to heating revealed that the Wenzel mode is more stable than the Cassie mode. Supporting Information Available: Additional figures as described. This material is available free of charge via the Internet at http://pubs.acs.org.

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