Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces Masashi Miwa, † Akira Nakajima, † Akira Fujishima, ‡ Kazuhito Hashimoto,* ,† and Toshiya Watanabe* ,† Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan, and Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received December 20, 1999. In Final Form: March 22, 2000 Various superhydrophobic films having different surface roughnesses were prepared, and the relationships between the sliding angle, the contact angle, and the surface structure were investigated. In the highly hydrophobic region, the sliding angles of water droplets decreased with increasing contact angles. Microstructural observation revealed that surface structures that can trap air are important for the preparation of low-sliding-angle surfaces. We have also derived an equation that describes the relationship between sliding angles and contact angles on superhydrophobic surfaces with roughness. The results calculated on the basis of this equation agreed well with the experimental ones. Moreover, we have successfully prepared a transparent superhydrophobic film whose sliding angle is ∼1° for a 7 mg water droplet. On this film, there was almost no resistance to the sliding of water droplets. The film obtained satisfies the requirements of superhydrophobicity, transparency, and a low water sliding angle. Introduction Wettability is one of the important properties of solid surfaces from both fundamental and practical aspects. Among various factors, surface energy and surface rough- ness are the dominant factors for the wettability. When the surface energy is lowered, the hydrophobicity is enhanced. However, even a material with the lowest surface energy (6.7 mJ/m 2 for a surface with regularly aligned closest-hexagonal-packed -CF 3 groups) gives a water contact angle of only around 120°. 1 For higher hydrophobicity, providing a proper surface roughness is required. 2 In fact, surfaces with a water contact angle of more than 150° were developed by introducing proper roughness on materials having low surface energies. 3-23 While the contact angle of water has been commonly used as a criterion for the evaluation of hydrophobicity of the surface, this alone is insufficient for the evaluation of the sliding properties of water droplets on surfaces. 21,23-26 A surface with a high contact angle does not always show a low sliding angle, which is defined as the critical angle where a water droplet with a certain weight begins to slide down the inclined plate. For example, Murase et al. demonstrated that a fluoropolymer with a water contact angle of 117° shows a higher sliding angle than a poly- (dimethylsiloxane) with a water contact angle of 102°. 24-26 They have proposed that this phenomenon is due to a negative excess entropy caused by the rigidity of fluo- ropolymer segments and the enhancement of an icelike molecular arrangement in water. 25,26 Therefore, when we discuss hydrophobicity, the sliding property of water droplets should be evaluated separately from the contact angle. Bikerman investigated sliding angles on stainless steel plates with different finishes, having the contact angles around 90°, and proposed that the surface roughness provides resistance for the sliding of water droplets. 27 Johnson and Dettre theoretically simulated the effect of the surface roughness on the difference between the * Corresponding authors. 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