Wettability of fractal surfaces has been studied both theoretically and experimentally. 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. The result shows that fractal surfaces can be super water repellent (superwettable) when the surfaces are composed of hydrophobic (hydrophilic) materials. We also demonstrate a super-water-repellent fractal surface made of alkylketene dimer; a water droplet on this surface has a contact angle as large as 174°.

Super water-repellent surfaces showing a contact angle of 174° for water droplets have been made of alkylketene dimer (AKD). Water droplets roll around without attachment on the super water-repellent surfaces when tilted slightly. The AKD is a kind of wax and forms spontaneously a fractal structure in its surfaces by solidification from the melt. The fractal surfaces of AKD repel a water droplet completely and show a contact angle larger than 170° without any fluorination treatments. Theoretical prediction of the wettability of the fractal surfaces has been given in the previous paper. The relationship between the contact angle of the flat surface θ and that of the fractal surface θf is expressed by the equation cos θf = (L/l) D -2 cos θ where (L/l) D -2 is the surface area magnification factor. The fractal dimension of the solid AKD surface was determined to be D ≈ 2.3 applying the box-counting method to the SEM images of the AKD cross section. L and l, which are the largest and the smallest size limits of the fractal behavior of the surface, are also estimated from the box-counting method. The contact angles of some water/1,4-dioxane mixtures on the fractal and the flat AKD surfaces were determined, and the values of cos θf were plotted against cos θ. The plot of cos θf against cos θ agrees well with the theoretical prediction. It has been demonstrated by this work that the fractal concept is a powerful tool to develop some novel functional materials.

Surfaces that repel both water and liquid oils completely are very useful in daily life and in industry. Although surfaces with outstanding water-repellent properties have been developed and intensively studied recently," -31 surfaces with excellent oil-repellent properties, however, have not yet been reported in spite of their crucial importance. Wettability of a solid surface with a liquid is enhanced by the surface in particular by the presence of fractal structures. When the contact angle between a liquid and a flat solid surface is larger than 90", the surface repels the liquid completely if the surface rough enough.For a fractal rough structure the relationship between the contact angle of the flat surface Q and that of the rough surface 0, is expressed by the Equation (a).[2.3. In Equation (a),( L / I ) D -2 is the surface area magnification factor, L and I are the upper and lower limits of the fractal behavior of the surface, and D the fractal dimension. This idea has been realized recently for super water-repellent surfaces having the contact angle of 174", on using fractal We have applied the same concept in this work to the completely novel, super oil-repellent surfaces. The greatest difficulty in obtaining the super oil-repellent surface is to make a flat surface that has a contact angle greater than 9 0 for oils. Here the surface tension of the solid needed to achieve this condition is estimated. The contact angle 0 on the flat surface is determined by Young's equation [Eq. (b),where y s , yl,, and ysL are the surface tensions of the solid, the liquid, and the solid/liquid interfacial tension, respectively]. When the contact angle is 90", ys must be equal to ysL. The interfacial tension ysL can be approximated by Equation (c) (c) when the interaction force between the two materials is same kind. [61 Combining Equations (b) and (c), we obtain ys = yL/4 as condition for 0 = 90".Typical surface tensions of oils are 20-30 m N m -', and the value of y s must be on the order of several mNm-'. Such a small surface tension for a solid can probably be provided only by the trifluoromethyl group.['] Accordingly, our strategy for making super oil-repellent surfaces was to obtain a surface of enough roughness and then to cover the rough surface with trifluoromethyl groups by treating it with fluorinated compounds. Figure 1 shows a scanning electron microscopic photograph of the anodically oxidized rough surface of aluminum plate. This surface was analyzed by the box counting method1233J and found to be a fractal (dimension 2.16). This untreated surface is superwettable towards liquids, but can be converted into a super r-;'sr = ;'s + 71, -2b ;'s 7,.

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Responsive polymer gels that contain bilayer membranes have been synthesized and characterized as a novel hybrid material. The hybrid gels show some unique properties which cannot be obtained from the individual polymer gels and the bilayer membranes. Iridescent color resulting from the Bragg diffraction of visible light by the periodic structure of bilayer membranes can be changed by controlling the swelling degree of the gel. Introduction of bilayer membranes strongly affects the volume phase transition behavior of the gel. By the polymerization of a gel while shear flow is imposed upon the solution to be gelled, an anisotropic gel has been synthesized. The hybrid gels show more than a seven times larger elastic modulus and can be extended more than three times before fracture than the simple polymer gel.