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Direct Observation of Internal Fluidity in a Water Droplet during Sliding on Hydrophobic Surfaces Munetoshi Sakai,† Jeong-Hwan Song,† Naoya Yoshida,†,‡ Shunsuke Suzuki,†,§ Yoshikazu Kameshima,†,§ and Akira Nakajima*,†,§ Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, Research Center for AdVanced Science and Technology, The UniVersity of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904 Japan, and 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 ReceiVed February 2, 2006. In Final Form: March 15, 2006 In the current study, we used a high-speed camera system with particle image velocimetry to observe the internal fluidity of water droplets during sliding. The droplets’ velocity during sliding was controlled by slipping and rolling motions. On the superhydrophobic surface, with a contact angle of 150°, the droplet fell at high velocity by slipping. However, on a normal hydrophobic surface whose water contact angle was around 100°, both slipping and rolling controlled the droplet’s velocity during sliding. In addition, the advancing velocity might be large when the slip velocity is large and the contact area is small.
I. Introduction Technologies related to hydrophobic coatings are important for suppressing chemical reactions and bonding formations between water and solid surfaces. Such coatings have been adopted for various industrial uses, including anti-wetting, antisnow (or ice) adherence, anti-rusting, and reduction of friction resistance.1-4 The contact water angle is widely used as a criterion for evaluating surface hydrophobicity. However, recognition of the importance of dynamic hydrophobicity is growing in various industries, such as those of glass, automobiles, and electronics. To date, the sliding angle (the critical angle at which a water droplet with a certain weight begins to slide down an inclined plate) and contact-angle hysteresis (the difference between the receding contact angle and the advancing contact angle) have been commonly employed as criteria for assessing the dynamic hydrophobicity of solid surfaces. The sliding angle value includes no information on watershedding kinetics such as sliding acceleration or velocity. Recently, information related to the speed at which the droplet is removable from the surface at a certain tilt angle is becoming more important than that at the lowest tilt angle at which the droplet slides down.5 A hydrophobic surface with a low sliding angle does not always exhibit high sliding acceleration or velocity for a water droplet. Yoshida et al. evaluated the sliding acceleration of water droplets on several hydrophobic polymer coatings; those results revealed that the order of sliding acceleration does not coincide with that of the sliding angle.6 Miwa et al. revealed that a water droplet slides down a superhydrophobic surface by a * Corresponding author. E-mail:
[email protected]. Tel: +813-5734-2525. Fax: +81-3-5734-3355. † Kanagawa Academy of Science and Technology. ‡ The University of Tokyo. § Tokyo Institute of Technology. (1) Zisman, W. A. Ind. Eng. Chem. 1963, 55, 19. (2) Baier, R. E.; Meyer, P. E. CHEMTECH 1986, 16, 178. (3) Nostro, P. L. AdV. Colloid Interface Sci. 1995, 56, 245. (4) Yamaguchi, G.; Takai, K.; Saito, H. IEICE Trans. Electron. 2000, E83-C, 1139. (5) Nakajima, A. J. Ceram. Soc. Jpn. 2004, 112, 533. (6) Yoshida, N.; Abe, Y.; Shigeta, H.; Nakajima, A.; Ohsaki, H.; Hashimoto, K.; Watanabe, T. J. Am. Chem. Soc. 2006, 128, 743.
Figure 1. Schematic illustration of the sliding behavior evaluation system for a water droplet.
constant acceleration motion.7 In contrast, Quere et al. reported the constant velocity of a sliding glycerol droplet with higher viscosity than water.8 However, studies of the relationship between the properties of solid surfaces and these kinetic properties remain limited.9-11 Recently, it has been reported that the dominant sliding mode for a water droplet changes from slipping to rolling on a superhydrophobic surface.12 This result implies that sliding acceleration depends on the sliding mode. Direct observation of the fluidity of a droplet during sliding provides important information about the relationship between a solid surface and a sliding droplet. An effective approach to the direct observation of fluidity is particle tracing velocimetry (PTV). In this study, we evaluated the internal fluidity in the downfall of water droplets using PTV with a high-speed camera system on three typical hydrophobic surfaces. Then the ratio of slipping and rolling during sliding was examined in reference to the interaction between the solid and water at the interface. (7) Miwa, M.; Fujishima, A.; Nakajima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754. (8) Richard, D.; Quere, D. Europhys. Lett. 1999, 48, 286. (9) Durbin, P. A. J. Fluid Mech. 1988, 197, 157. (10) Carre, A.; Shanahan, M. E. R. J. Adhes. 1995, 49, 177. (11) Yoshida, N.; Abe, Y.; Shigeta, H.; Takami, K.; Osaki, H.; Watanabe, T.; Hashimoto, K. J. Sol.-Gel Sci. Technol. 2004, 31, 195. (12) Gogte, S.; Vorobieff, P.; Tresdell, R.; Mammoli, A.; van Swol, F.; Shah, P.; Brinker, C. J. Phys. Fluids 2005, 17, 51701.
10.1021/la060323u CCC: $33.50 © 2006 American Chemical Society Published on Web 04/28/2006
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Figure 2. Sequential photographs in the downfall of the droplet. (a) Superhydrophobic coating. Image capture conditions: frame rate, 1000 [frame/s]; shutter speed, 1/2500 [s]. (b) ODS coating. Image capture conditions: frame rate, 250 [frame/s]; shutter speed, 1/2000 [s]. (c) FAS-3 coating. Image capture conditions are identical to those for the ODS coating. The shortest distance between divisions in the rule is 1 mm.
II. Experimental Section Sample Preparation. This study evaluated three hydrophobic coatings: a superhydrophobic coating, a fluoroalkylsilane coating, and an alkylsilane coating. A Si substrate (n-type Si (100); Aki Corp., Japan) that had been cut into 15 mm × 40 mm pieces was sonicated in ethanol and dried in a N2 flow. Then it was cleaned photochemically using vacuum ultraviolet (VUV, λ ) 172 nm, a Xe excimer lamp, UER20-172; Ushio Inc., Tokyo, Japan) treatment under an air atmosphere for 15 min. All coatings were applied onto the cleaned Si wafer. A superhydrophobic coating was prepared by spraying a commercial paint (HIREC450; NTT AT Corp., Tokyo, Japan) onto the Si wafer. Silane coatings were applied using chemical vapor deposition (CVD). Octa-decyl-tri-methoxy-silane (ODS, CH3(CH2)17Si(OCH3)3; Sigma-Aldrich Corp., Milwaukee, WI) and tri-fluoro-propyl-tri-methoxy-silane (FAS-3, CF3CH2CH2Si(OCH3)3, KBM 7103; Shinetsu Chemical Co., Japan) were used as waterrepellent agents. A glass container with a cleaned Si substrate and 0.2 mL of a silane was heated in a furnace at 423 K (for ODS) or 448 K (for FAS) for 1 h. After coating, the substrate was rinsed with methylene chloride, ethanol, and water. Evaluation of Coatings. Surface roughness (Ra) was evaluated in a 5-µm square area using atomic force microscopy (AFM, JSPM5200; JEOL, Tokyo, Japan) with a Si cantilever. The sessile drop method, using a contact-angle meter (Dropmaster 500; Kyowa Interface Science Co. Ltd., Saitama, Japan), was used to measure the contact angles. The droplet mass for contact-angle measurement was 3 mg. The contact angles were measured at five different points for each coating. The surface was blown with ionized air (Winstat BF-Z; Shishido Electrostatic Ltd., Tokyo, Japan) to eliminate static electricity on the surface before each measurement. A sliding-angle measurement system (SA-20; Kyowa Interface Science Co. Ltd.) recorded the sliding angles of a 30-mg water droplet. The sliding acceleration of the water droplet was evaluated by direct observation. The sample was tilted at 35°. The alignment of
the evaluation system of the sliding acceleration is depicted in Figure 1. Commercial polystyrene spherical particles with a density similar to that of water were used as indicator particles (725A; Duke Scientific Corp., Palo Alto, CA; diameter: 222 µm; density: 1.05 cm3/g). A 30-µL water droplet with 1.0 wt % indicator particles was placed on a micropipet tip (PPC1-1000; Iwashita Engineering, Inc., Tokyo, Japan). Preliminary experiments revealed that this amount of indicator particle caused little difference in the water viscosity. The droplet was then placed calmly on the inclined sample surface. Sequential photographs of the sliding action of the water droplets on the surface were taken every 0.5 or 4 ms using a high-speed digital camera system (512 PCI; Photron Ltd., Tokyo, Japan). The sliding acceleration was estimated by measuring the sliding distance of the front or rear edge of the contact line between the droplet and sample surface from the initial starting point (Dipp Macro; Ditect Co. Ltd., Tokyo, Japan). The internal fluidity was evaluated by measuring the coordinate of the indicator particle in the downfall droplet (Dipp Flow; Ditect Co. Ltd., Tokyo, Japan).
III. Results and Discussion The respective contact angles, sliding angles, and surface roughness for the superhydrophobic coating, ODS coating, and FAS-3 coating were 150, 100, and 79.5° for the contact angle; 5, 10, and 13° for the contact-angle hysteresis; and 2580, 0.19, and 0.1 nm for surface roughness. Figure 2 shows captured images of a water droplet during sliding using the high-speed camera system. Because the camera was also inclined at the same angle (35°) of the sample, the direction of the droplet’s motion in the image is horizontal (xdirection). These photographs of the water droplet with the indicator particles depict the internal fluidity during sliding. On the superhydrophobic coating, an indicator moves with the droplet. However, its position in the droplet does not change. In contrast,
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Figure 4. Total velocity (solid line), indicator velocity (dotted line), and rolling velocity, as calculated by the fitting function in Figure 3. (a) Superhydrophobic coating; (b) ODS coating; and (c) FAS-3 coating.
Figure 3. Sliding behavior and internal fluidity of a water droplet during downfall. The correlation coefficient, R2, was rounded off to three decimal places. (a) Superhydrophobic coating; (b) ODS coating; and (c) FAS-3 coating.
the position of indicators in the droplets on the ODS coating and FAS-3 coating changes when these droplets move. This result suggests that the water droplet fluidity on the superhydrophobic coating is almost entirely a slipping motion; that on ODS and FAS-3 coatings includes rolling with slipping. Figure 3 shows the sliding behavior of water droplets on each coating. A water droplet on the superhydrophobic coating exhibited almost constant accelerated motion (Figure 3a). Although a slight difference exists between an advancing measure point and a receding one, the difference is small. The sliding acceleration was greater than that of gravity (9810 × sin 35° ) 5627 mm/s2; the result obtained was 6334 mm/s2 for a receding measuring point and 7763 mm/s2 for an advancing measure point).
Because a water droplet does not adhere to the declined superhydrophobic coating, the droplet was ejected from a pipet by slight air pressure. Its contribution was included in these acceleration values. A water droplet on the ODS coating seems to slide down by constant accelerated motion in the early stage of sliding. However, its velocity becomes constant after 0.08 s (Figure 3b). The initial time delay (0.055 s) for indicator particles is the time required for identifying an indicator particle at the ideal position for measurement. The inflection point (around 0.10 s) indicates the arrival of the indicator particle to the ODS coating surface. Therefore, the directions of moving distance for the droplet itself and the indicator particle are identical after this inflection point (see Figure 2b,c). Once the particle reaches the coating surface, the indicator particle stops moving if the internal fluidity is completely rolling. However, the velocity exists after reaching the coating surface, and the linear function provided the best fit of the relationship between the distance of the indicator particle and the time for the fitting interval from 0.10 to 0.14 s. This result further implies that the sliding includes not only rolling but also slipping: the velocity in this period is attributable to the apparent velocity by slipping. The advancing or receding
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measurement point of the droplet was faster than that for the indicator particle in this time interval. That fact also suggests that the advancing or receding velocity measurement points include both slipping and rolling effects. The droplet on the FAS-3 coating exhibited a similar trend (Figure 3c), but the sliding velocity is much less than that on the ODS coating. On a hydrophobic surface, the rolling velocity (Vr) in the water droplet during sliding can be described by its advancing velocity (Vad) minus the indicator particle velocity (Vi) on the contact line (|Vr| ) Vad - Vi). Figure 4 portrays the relationship of these velocities. On the superhydrophobic coating, most of the advance velocity was slipping because that for rolling was almost zero (Figure 4a). During sliding of the water droplet, the shear stress was produced by the interaction of the solid and water.13 Because the practical contact area between the solid and water is small on the superhydrophobic coating,14,15 the amount of the three phase (solid-liquid-air) contact line is also small, and its length is short. Therefore, the effect of the interaction and resultant shear stress during droplet motion is almost negligible. Therefore, water droplets on the superhydrophobic coating slid down by slipping without rolling. This result was consistent with the analysis of Gogte et al.12 On the normal hydrophobic coating, a certain amount of contact area and a three-phase contact line exist between the solid and water. Consequently, we infer that the shear stress was produced by the interaction between the solid and water during sliding. Figure 3 shows unequivocally that the slip velocity was larger and that rolling was faster on the ODS coating than on the FAS-3 coating. A plausible explanation of this result is the low contact angle, the strong interaction between the water and the fluorocarbon surface,16,17 and the molecular rigidity.18 On the basis of the sliding distance during the time interval from 0.10 (13) Hino, M. Ryuutai Rikigaku; Asakura Shoten Press: Tokyo, Japan, 1994; pp 261-274. (14) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (15) Ou, J.; Perot, B.; Rothestein, J. P. Phys. Fluids 2004, 16, 4634. (16) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (17) Umeyama, H.; Morokuma, K. J. Am. Chem. Soc. 1977, 99, 1316.
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to 0.14 s for the ODS coating and that from 0.40 to 0.54 s for the FAS-3 coating, the apparent velocity ratios, |Vr/Vad|, by rolling on the ODS coating and on the FAS-3 coating are respectively calculated as 52% (ODS: 103.1 - 49.1 ) 54 mm/s f 54/103.1 ) 52%) and 63% (FAS-3: 24.8 - 9.19 ) 15.6 mm/s f 15.6/ 24.8 ) 63%) (Figure 4b,c). In other words, another element, 1 - |Vr/Vad|, of the advanced velocity was composed by slipping, 48% (ODS) and 37% (FAS-3). Therefore, the advance velocity might be large when both the rolling velocity and the ratio |Vr/ Vad| are small. Material factors that influence the rolling/slipping ratio are surface roughness, surface homogeneity, chemical composition, and the surface energy distribution. If the measurement period is increased somewhat, rolling mode may appear, even on the superhydrophobic coating. The result of the rolling/slipping ratio and their values in this study is for the early stage of sliding. They depend not only on the material’s surface, but also on the measurement period. Detailed analyses on the effect of these parameters are subjects for future work.
IV. Conclusion This study measured the internal fluidity in the downfall of water. Almost no rolling mode in the sliding of water droplets was observed on the superhydrophobic surface. On the hydrophobic surface with constant velocity (no acceleration) motion, rolling of indicator particles, similar to a caterpillar motion, was observed during sliding. The rolling velocity in the water droplet during sliding can be described by its advancing velocity minus the indicator particle velocity on the contact line. The advancing velocity of the water droplet might be large when the slipping is large and the contact area is small. Note Added after ASAP Publication. This manuscript was originally published on the Web April 28, 2006. The manuscript was reposted May 5, 2006 with a revised Figure 2. LA060323U (18) Murase, H. Proceedings of the SCJ 5th Interface Meeting, Tokyo, Japan, 1998; Science Council of Japan: Tokyo, 1998; pp 9-18 (in Japanese).
