Reversible Photoinduced Wettability Transition of Hierarchical ZnO Structures

Control over the wettability of solids as well as the manufacturing of functional surfaces with special wetting properties has aroused much interest because of great advantages given to various applications in daily life, industry, and agriculture. We report here the dynamic optical control of the wetting behavior of liquids on hierarchically structured ZnO surfaces produced by irradiating silicon (Si) wafers with femtosecond laser pulses and subsequently coating them with ZnO by pulsed laser deposition. The final surface exhibits roughness at two length scales, comprising micrometer-sized Si spikes decorated with ZnO nanoprotrusions. It is shown that a liquid droplet on these surfaces can be rapidly and reversibly switched between hydrophobicity and superhydrophilicity by alternating UV illumination and dark storage or thermal heating. By studying the magnitude and the rate of the photoinduced transitions, we investigated the contribution of roughness at different scales in the framework of two theoretical wettability models.