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 super-hydrophobicity 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.