[in Japanese]

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[in Japanese]

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  Wettability control of a solid against a liquid is widely investigated in industry for application to daily life because it can produce various physical and chemical phenomena related to solids and liquids, achieving beneficial purposes. Recent researches revealed that static and dynamic wettability are affected by nano-scale surface characteristics on the solids, and the importance of the viewpoint from materials science is gradually increasing in this topic. This paper presents a short review of the studies on the superhydrophilicity of TiO₂ photocatalyst, processing and properties of superhydrophobic materials, and control of dynamic hydrophobicity, mainly of the past decade of our group.

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  Physics of wettability and water repellency of fractal-structure surfaces has been reviewed. Wetting behavior of the fractal surface is predicted on the basis of the theory of wettability of a flat and a rough surface. Since the fractal surface has a very large surface-area density, a hydrophobic (hydrophilic) fractal surface can be super water repellent (superwettable). An expression for the contact angle of a liquid droplet placed on a fractal surface is expressed as a function of the fractal dimension, the range of fractal behavior, and the contacting ratio of the surface. Experimental results are also mentioned as to a super-water-repellent fractal surface made of alkylketene dimer; a water droplet on this surface shows a contact angle as large as 174°.

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  This short review summarizes a preparation method for superhydrophobic surface by spray-coating technique. Superhydrophobic coatings can be prepared on a variety of substrate such as paper, metal, and cotton by spraying nanoparticle suspensions. As nanoparticles, hydrophobic silica nanoparticles, nano-pigments, and carbon nanotubes are applicable. In order to provide superhydrophobicity for the nanoparticle coatings, several factors (i.e., size of nanoparticles, surface energy of nanoparticles, types of solvent, and the amount of the coatings) played crucial roles. The spray-coating technique is a simple, safe and low-cost method to form superhydrophobic surface.

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