Superhydrophobicity of hierarchical ZnO nanowire coatings

Hierarchical superhydrophobic surfaces were constructed by growing various lengths of ZnO nanowires on micro-scale Si pyramids produced by chemical etching. The nano-size effect on wettability of nano/micro complex structures has been investigated by adjusting the ZnO nanowire length. As the nanowire length on the Si pyramid surface increases, a transition from the Wenzel state to the Cassie state occurs. A water contact angle as high as 169.1 degrees, with a contact angle hysteresis less than 1.3 degrees, was formed by growing ZnO nanowires of a suitable length on Si micro-pyramids. Compression and impact experiments further demonstrate the robust superhydrophobicity of ZnO nanowire-Si pyramid hierarchical structures.