ARTICLE pubs.acs.org/Langmuir
Sliding of Water Droplets on Hydrophobic Surfaces with Various Hydrophilic Region Sizes Tsutomu Furuta,† Munetoshi Sakai,‡ Toshihiro Isobe,† Sachiko Matsushita,† 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 ‡ Kanagawa Academy of Science and Technology, 308 East, Kanagawa Science Park, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan
bS Supporting Information ABSTRACT: Four patterned surfaces with hydrophilic areas of different sizes were prepared using photolithography with a smooth octadecyltrimethoxysilane (ODS) hydrophobic coating. The hydrophilic area in the surfaces was aligned hexagonally with a constant area fraction. The sliding angle and contact angle hysteresis of the water droplets increased concomitantly with increasing pattern size. The increase of the contact line distortion between defects at the receding side plays an important role in this trend. The droplet sliding velocity also increased concomitantly with increasing pattern size. This trend was simulated by a simple flow model. The contribution of the interface between the ODS region and the hydrophilic area was deduced from this trend. This study demonstrated the different size dependency of the chemical surface defects for sliding behavior between the critical moment at which a droplet slides down and the period when a droplet is sliding.
I. INTRODUCTION Recently, hydrophobic coatings have attracted much attention for eventual use in various industries. Hydrophobicity of solid surfaces can be evaluated from static wettability and dynamic wettability.1 The water contact angle is commonly used as a criterion for evaluating static wettability. Although the sliding angle (the tilt angle at which the water droplet of a certain volume starts to slide down) and the contact angle hysteresis (the absolute value of the difference of cosines between advancing (θa) and receding (θr) contact angles) reflect the motion or deformation of droplets, it is not an index of dynamic hydrophobicity, such as sliding acceleration or velocity, because these are thermodynamic properties and are not a function of time. However, the sliding velocity has played an important role when practical sliding behavior is discussed. Studies of the relation between surface properties and sliding velocity or acceleration have been increasing gradually.24 That surface defects affect the motion of water droplets and increase sliding angle and the contact angle hysteresis is wellknown.5 This phenomenon is commonly designated as the “pinning effect”.6 The pinning effect is separable into two categories:7,8 the physical pinning effect, which originates from surface roughness, and the chemical pinning effect, which results from chemical heterogeneity on the surface. To date, various studies of the pinning effect have been conducted.919 Very r 2011 American Chemical Society
recently, we demonstrated that the sliding behavior of water droplets is affected by the surface roughness, even of scale around 10 nm, and demonstrated that highly physical homogeneity (average surface roughness (Ra) less than 1.0 nm) is necessary to avoid the effect of physical pinning completely.4 However, few experimental studies have examined the chemical pinning effect with avoidance of physical roughness. Moreover, no reports have described the effect of surface chemical defect sizes on the contact angle hysteresis, sliding angle, or sliding velocity with sustained defect shape and density in the surface. Patterned coatings of various types have been prepared using self-assembled monolayers (SAMs) and photolithography.2023 Sugimura et al. reported the preparation of chemically patterned SAM coatings formed by photolithography with vacuum ultraviolet (VUV) irradiation.20 The wettability on such surfaces has been investigated in recent years.2123 The stickslip motion of the sliding droplet on the horizontal stripe-patterned coating has been reported. For this study, we prepared four hexagonally patterned surfaces, which included homothetic hydrophilic areas in different sizes, using a photolithography process with a hydrophobic SAM coating of a silane coupling agent. Then, Received: January 29, 2011 Revised: March 25, 2011 Published: April 28, 2011 7307
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Figure 1. Schematic illustration of the hexagonal chemical pattern structure.
Figure 2. LMS images of (a) pattern 1, (b) pattern 2, (c) pattern 3, and (d) pattern 4 with applied water based ink.
Table 1. Values of the Hole Radius (b0) and Pattern Distance (p0) for Hexagonally Patterned Photomasks of Four Types
Water-based ink was put on each chemically patterned coating. A hydrophilic area obtained using the decomposition of ODS coating was observed using a laser microscope (LMS, 1LM21W; Nikon Corp., Tokyo, Japan). The surface roughness of the obtained coatings was evaluated in a 50 μm square area using atomic force microscopy (AFM, JSPM-4200; JEOL, Tokyo, Japan) with a Si cantilever (NSC36-c; μ-masch, Narva Mtn., Estonia). 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. Five points were measured for each coating. Then average and standard deviation values were obtained. The sliding angle of a 30 μL water droplet on the chemically patterned coatings was measured using an automatic measurement system (SA-20; Kyowa Interface Science Co. Ltd.). The contact angle hysteresis value was also calculated from the advancing contact angle (θa) and the receding contact angle (θr) simultaneously when the sliding angles were measured. Three points were measured for each coating. Then the average and standard deviation values were estimated. The sliding velocity for a 30 μL water droplet was measured on the chemically patterned coating inclined at 35° using a high-speed camera (1024PCI; Photron Ltd., Tokyo, Japan). The sliding velocity of the advancing contact line was evaluated from the sliding distance. This distance was measured at the advancing edge of the sliding droplets using commercial software (DIP-Macro; Ditect Co. Ltd., Tokyo, Japan; KAST, Kanagawa, Japan). Three points were measured for each coating. Then the average and standard deviation values were calculated. It is noteworthy that the size of the water droplets (30 μL) is sufficiently large in comparison with the size of the chemical pattern on the coating.
pattern no. 1
2
3
4
b0 [μm]
1.0
2.0
5.0
10.0
F0 [μm]
6.0
12.0
30.0
60.0
the effects of the size-dependence of the chemical surface defects for contact-angle hysteresis, sliding angle, and sliding velocity were investigated.
II. EXPERIMENTAL SECTION We prepared smooth hydrophobic SAM coatings using octadecyltrimethoxysilane (ODS, CH3(CH2)17Si(OCH3)3; Aldrich Chemical Co. Inc., Milwaukee, WI) for the chemically patterned coating. The SAM was prepared using chemical vapor deposition (CVD). The surface of a Si(100) wafer (Aki Corp., Miyagi, Japan) was washed using ethanol and acetone each for 7 min. It was then precleaned using vacuum ultraviolet (VUV) light (λ = 172 nm with power density of around 7 mW cm2, UEM20-172; Ushio Inc., Tokyo, Japan) irradiation for 15 min. Smooth hydrophobic SAM coatings were obtained by heating the Si wafer with 20 μL of ODS in a Petri dish by flowing N2 at 150 °C for 60 min. After the coating was rinsed with toluene, acetone, and water, it was dried. Four chemically patterned coatings were prepared from the ODS coating using similar photomasks where holes (light-exposed area) were arranged in the regular hexagonal form. Figure 1 shows the pattern structure, and Table 1 shows values of the hole radius (b0) and pattern distance (p0) at each photomask. The ratio of the light-exposed area to the total surface was 10%; it was constant for each photomask. The photomask was put on the ODS coating, and the photoetching treatment was conducted by VUV light irradiation in vacuum conditions (100 Pa) for 10 min. The ODS placed under the light-exposed area of the photomask decomposed and became hydrophilic. The chemically patterned coating was obtained by rinsing with toluene and by subsequent drying. Values of the hole radius and pattern distance in each photomask are described as b0 and p0. Also, b and p represent practical values of the hole radius and pattern distance in each chemically patterned coating prepared using photolithography.
III. RESULTS AND DISCUSSION Figure 2 portrays LMS images of obtained chemically patterned coatings with applied water-based ink. Results show that circular chemical defects (hydrophilic areas) were formed uniformly with regular hexagonal alignment. Anisotropy of the aspect ratio was almost negligible. Figure 3 depicts AFM micrographs of obtained coating. No particulate agglomerates were observed on these coatings. The surface roughness values (Ra) were as follows: (a) pattern 1, 0.3 nm; (b) pattern 2, 0.7 nm; (c) pattern 3, 0.3 nm; and (d) pattern 4, 0.4 nm. These coatings sustained nanolevel smoothness with suppressing physical roughness. The ODS chain length is a few nanometers. It is expected that ODS molecules are not standing vertically but are instead lying or declining against 7308
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Table 2. Contact Angle Values (θ*), Area Fractions of Decomposed Area (fUV), Calculated Values of Hole Radius (bcal), and Ratios between the Hole Radius of the Photomask (b0) and the Calculated Value (bcal/b0) on Each Chemically Patterned Coating pattern no.
Figure 3. AFM micrographs of (a) pattern 1, (b) pattern 2, (c) pattern 3, and (d) pattern 4.
substrate on the surface after chemical vapor deposition treatment. It is further expected that part of the decomposed area should be included in the scanning area on this size relation. However, measurements at several different points provided almost identical results. Therefore, we consider that the obtained coatings sustained nanolevel smoothness with suppressing physical roughness even if ODS molecules were decomposed by photolithography. The surface of the chemically patterned coating is definable as the compound surface of the ODS coating area (hydrophobic) and the decomposed area (hydrophilic). From Cassie’s model,24 the contact angle on the compound surface of ODS coating and hydrophilic region by UV illumination (θ*) can be described as cos θ ¼ fUv cos θUV þ fODS cos θODS ¼ fUV cos θUV þ ð1 fUV Þcos θODS
ð1Þ
where fUV and fODS (= 1 fUV), respectively, signify the area fractions (
