Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact

In this paper, we present static and dynamic wetting interactions of water droplets on a variety of superhydrophobic surfaces. For sessile droplets, wetting states were determined by measuring contact angles and comparing them to that obtained from equilibrium Cassie and Wenzel states. Surprisingly, we find that roll-off angles are minimized on surfaces expected to induce Wenzel-like wetting in equilibrium. We argue that droplets on these surfaces arc metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the capillary pressure resulting from the energy barrier required to completely wet the posts. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets using a high-speed camera and specific surface textures that can delineate various wetting regimes show very good agreement with this simple pressure-balance model. These studies show that hierarchical micro-nano surfaces are optimum for droplet impact resistance.