Wenzel to Cassie Transition in Superhydrophobic Randomly Rough Surfaces

The possibility of inducing a Wenzel to Cassie transition by vibration on randomly rough surfaces is considered in order to test the robustness of their superhydrophobic character, i.e., the thermodynamic stability of the water-surface Cassie state. By using a self-masked plasma etching process surfaces with a different Wenzel roughness factor r _w, have been tailored. This parameter, given the Young angle, has been theoretically demonstrated to play a key role in determining superhydrophobicity robustness. When initially placed in Wenzel state and excited by vertical vibration these surfaces show a sensitively different behavior. A Wenzel to Cassie (or de-wetting) transition is observed near the first resonant mode only for the superhydrophobic surface predicted to be robust according the r _w based criterion. This surface, at relatively low displacement amplitude, gives rise to a complete detachment and bouncing of the drop.