Tribology of Non-Adhesive Surfaces: Superhydrophobicity and Wetting Regime Transitions
Michael Nosonovsky, Bharat Bhushan
Non-adhesive and water-repellent surfaces are required for many tribological applications. Wetting of patterned superhydrophobic Si surfaces is studied as a multiscale process involving the macroscale (water droplet size), microscale (surface texture size), and nanoscale (molecular size). We study fundamental mechanisms of wetting, including the transition between various wetting regimes during microdroplet evaporation in environmental scanning electron microscope (ESEM), contact angle and contact angle hysteresis. We propose a generalized formulation of the Wenzel and Cassie equations, which is consistent with the broad range of experimental data. We show that the contact angle hysteresis involves two different mechanism and how transition from metastable partially wetted (Cassie) states to homogeneously wetted (Wenzel) stated depend upon droplet size and surface pattern parameters.