Hierarchical structured sol-gel coating by laser textured template imprinting for surface superhydrophobicity

This paper reports a fabrication method generating biomimetic hierarchical structures simulating the lotus leaf's surface resulting in superhydrophobicity and self-cleaning functions. The surface micropatterns were imprinted on a hard sol-gel protective coating by a template made by the laser direct ablation technique. Bump structures were precisely controlled. The nano-structures were superimposed by self-assembled surface modified silica nanoparticles, forming two scale hierarchical structures. The water contact angle of the micropatterned surface after imprinting was 138 degrees, which was further increased to 160.3 degrees with hysteresis of 0.9 degrees by superimposing nanoparticles. The superhydrophobicity is comparable to the natural lotus leaf and the hierarchical structure is an optimal combination of micro- and nano-structures, which mimics the lotus leaf's surface. A comprehensive theoretical model which combines Wenzel and Cassie-Baxter states including the transition mode and the roughness factor predicted the air-trapping fraction as being 83% on the optimum structure. This fabrication method is fast, cost effective and reproducible for large areas. This paper also provides a design guideline for predetermined experimental parameters to achieve the lotus leaf effect.