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 micro-patterns 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°, which was further increased to 160.3° with hysteresis of 0.9° 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.