Fabrication of biomimetic multi-scale surface of rice leaf and anisotropic superhydrophobic properties

Abstract

With the continuous exploration of the special morphologies and properties of skin of some animals and plant leaf surfaces in the nature, new hydrophobic surfaces are developed through imitation of the observed surface effects. The aluminum alloy surface which is similar to multi-scale hierarchical structure surface of biomimetic rice leaf can be obtained by combining high speed wire electrical discharge machining (WEDM) method and electric brush-plating technique. The morphological feature was analyzed by scanning electron microscopy (SEM). The surface wettability of the sample was measured at ambient temperature using a contact angle measurement instrument and the SCA20 software by placing a 4 μ L pure distilled water droplet onto the surface of the sample. Three dimensional (3D) profile of the sample surface was obtained through confocal laser scanning microscopy (LSCM). The elemental composition was determined by X-ray diffraction (XRD) analysis in order to determine the phase composition on the sample surface. The regular submillimeter groove ridge structures were processed on the aluminum alloy surfaces by the method of high speed wire electrical discharge machining. And the hierarchical cauliflower-like structures which were consisted of micro-protrusions and submicro-grains were formed on the sample surfaces by electric brush-plating technique. The anisotropic functional surfaces and the superhydrophobic surfaces were achieved by the combination of the regular submillimeter groove ridge structures and hierarchical cauliflower-like structures. The research showed that the regular submillimeter groove ridge structures are similar to the surface morphology on rice leaf, and the hierarchical cauliflower-like structures are similar to the surface morphology on lotus leaf. The results showed that double scale (micro-submicro) microscopic structure obtained by electric brush-plating can play a key role on the hydrophobicity of the surface. The regular submillimeter groove ridge structure obtained by high speed wire electrical discharge machining further enlarged the hydrophobic properties of the direction of parallel groove. The sample surface showed excellent superhydrophobic properties, and the contact angle of the direction of parallel groove could reach up to 151°. The sample surface also showed different hydrophobic properties in the direction of the parallel groove and the direction ofthe vertical groove. Due to the existence of the edge structure make the two directions, and the two directions have different static wettability, which also led to the surface anisotropy. With the changing of electric brush-plating time, the trend of double and multi scale structure surface contact angle had large difference on the sample surface. The sample surface is fabricated without any chemical modification by combining high speed wire electrical discharge machining method with electric brush-plating technique. Although the application of the surface anisotropy and superhydrophobicity were not showed in other places, it is important for animals and plants in the nature. With the deepening of the study, this research will rise to certain guiding significance on the applications of combining the anisotropic functional materials with the superhydrophobic materials.