Super Oil‐Repellent Surfaces

Abstract

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,.

Recent developments in polymeric superoleophobic surfaces

“…[25][26][27] However, superoleophobic surfaces are difficult to achieve due to the low surface tension of organic liquids (Table 1). [28][29][30][31][32] Superoleophobic surfaces are of great significance not only for fundamental research but also for various economically important applications in oil-repellent coatings, marine antifouling, antioil treatment of oil pipelines, and antibioadhesion. These demands emphasize the need for more research on superoleophobic surfaces.…”

“…Superoleophobic surfaces were first reported by Tsujii et al in 1997. 28 They fabricated a fluorinated monoalkylphosphate-modified fractal anodically oxidized aluminum surface, which showed a contact angle of 150 for rapeseed oil droplets.…”

“…28 They fabricated a fluorinated monoalkylphosphate-modified fractal anodically oxidized aluminum surface, which showed a contact angle of 150 for rapeseed oil droplets. They concluded the basic strategy of design as superoleophobic surfaces is treating a surface of enough roughness with trifluoromethyl groups of low enough surface tension.…”

Characterization and structural investigation of fractal porous-silica over an extremely wide scale range of pore size

“…12 (b) shows a comparison with the experimental and theoretical results assuming L = 2 m on the dependences of cumulative pore volume and pore volume on pore size in Eqs. (13) - (15). The cumulative pore volume in the size range from 2 m to 0.125 m in the experimental result can be reproduced by the sponge model as shown in Fig.…”

“…However, it has been clarified recently that the fractal is also fruitful in material designs. For instance, fractal surface structures emerge super-water repellency [8][9][10][11][12][13][14] and super oil-repellency [15]. The super water-repellent fractal structure is spontaneously formed on the surfaces of alkylketene dimer (AKD) and triglycerides waxes [8][9][10][12][13][14].…”