"Lock-and-key" geometry effect of patterned surfaces: wettability and switching of adhesive force.

A rough surface can be a regular (engineered surface), a random (irregular rough surface), or an intermediate case (hierarchical rough surface). Whichever case is used for wettability, a truly superhydrophobic surface exhibits not only a high contact angle (>150 8) but also a low-contact-angle hysteresis (sliding angle). Quéré et al. theoretically described how contact-angle hysteresis generates an adhesive force and the contact angle and hysteresis can be controlled by tailoring the surface topography of the solid substrate. Researchers have since attempted to capture these properties in synthetic materials with nanoscale surface features or changes of surface topography. For flexibility in adapting to rough surfaces, poly (dimethylsiloxane) (PDMS) elastomer, an ideal elastic material in terms of its stress–strain response, has attracted great attention. Particular interest has focused on introducing nanoscale structures onto microscale surfaces using surface treatments to reach a hydrophobic state, such as mechanically assembled monolayers, CO2 pulsed-laser etching, [8–10] UV/ ozone surface treatments, SF6 plasmas, [12] oxygen plasma and chemical surface treatments, and laser etching. Besides these, Hang et al. created an artificial lotus leaf by