Trilevel-structured superhydrophobic pillar arrays with tunable optical functions

Water-repelling surfaces inspired by lotus leaves have been developed for their commercial needs in superhydrophobic and self-cleaning coatings on glasses and windows. The extraordinary properties originate from their multiscale structures with waxy materials. To obtain high transparency as well as superhydrophobicity, microhair arrays are designed with large spacing to reduce optical scattering effects caused by microstructures, but with a trilevel hierarchical structure to compensate for the loss of superhydrophobicity. In this study, a soft molding technique on wet pastes consisting of nanoparticles (NPs) is proposed to create a multilevel hierarchical structure of sub-100 nm nanoparticles, which demonstrates excellent water repellency. Additionally, full advantage is taken of the TiO₂ NP mesoporous structure for UV protection and for its ability to attach to various kinds of functional (for example, photoresponsive) dyes. Furthermore, the stability of fluorinated surfaces against UV light is enhanced by the passivation of the TiO₂ surface with a thin silica coating.