Surface energy and surface topography are two key factors in the wettability of solid substrates. The surface energy determines the contact angle (CA) of a liquid on a flat substrate and the geometrical factor enhances the wetting property for a hydrophilic surface (or non-wetting for a hydrophobic surface). Applying external stimuli is a valuable approach for rendering the change in surface chemistry and/or topography, and for driving the wettability transition of smart surfaces. This review describes the current state-of-the-art research on the reversibly switchable wettability of surface brought about by external stimuli, including surface conversion between superhydrophobicity and superhydrophilicity prepared from inorganic oxides or/and photoactive organic molecules, movement of liquid droplets driven by molecular machines, and light-driven switching of superhydrophobic adhesion.

Contents
1 Introduction
1.1 Basic theory of surface wettability
1.2 Photoresponsive materials
2 Inorganic-oxide-based photoresponsive surfaces
3 Organic-compound-based photoresponsive surfaces
3.1 Non-azobenzene compound photoresponsive surfaces
3.2 Azobenzene compound photoresponsive surfaces
4 Conclusion