Successive Rebounds of Impinging Water Droplets on\nSuperhydrophobic Surfaces

Abstract

When\na water droplet strikes a superhydrophobic surface, there\nmay be several to a few tens of rebounds before it comes to rest.\nAlthough this intriguing multiphase flow phenomenon has received a\ngreat deal of attention from interfacial scientists and engineers,\nthe underlying dynamics have not yet been completely resolved. In\nthis paper, we report on an experimental investigation into the bouncing\nbehavior of water droplets impinging on macroscopically flat superhydrophobic\nsurfaces. We show that the restitution coefficient, which quantifies\nthe energy consumed during impact and rebound, exhibits a nonmonotonic\ndependence on the Weber number. It is the droplet–surface friction\nthat restricts the rebound height of the impinging droplet, so its\nrestitution coefficient increases with the Weber number when the impact\nvelocity is below a critical value. Above this value, the viscous\nfriction within a thin liquid layer close to the superhydrophobic\nsurface becomes dominant, and thus, the restitution coefficient decreases\nsharply. On the basis of energy analyses, semiempirical formulas are\nproposed to describe the restitution coefficient, and these can be\nemployed to predict the number of successive rebounds of impinging\ndroplets on superhydrophobic surfaces.