Bio-Inspired Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact

We present fluid dynamic videos of droplet impact on three types of superhydrophobic surfaces to explicitly show the effect of surface texture on droplet impact resistance and recoil efficacy. We predict that droplets impacting on superhydrophobic surfaces can completely recoil when the surface-dependent capillary pressure PC resists the dynamic impact pressures such as droplet-dependent water hammer pressure PWH and Bernoulli pressure PB. We perform droplet impact experiments on three types of surfaces: (1) sparse microtextured hydrophobic surface with PC < PB < PWH that leads to extensive texture wetting (2) dense microtextured hydrophobic surface with PB < PC < PWH that leads to partial texture wetting (3) hierarchical surface with nanodendrites on microposts (mimics the lotus leaf structure) exhibits complete droplet recoil as the capillary pressure exceeds the water hammer and Bernoulli pressures (PB < PWH < PC). This fundamental understanding can aid surface design for droplet impact resistance for a variety of applications (for e.g., anti ice surfaces for aircraft engine applications)