Pancake bouncing: simulations and theory and experimental verification.

Drops impacting superhydrophobic surfaces normally spread, retract, and leave the surface in an approximately spherical shape, with little loss of energy. Recently, however, it was shown that drops can leave the substrate before retracting while still in an extended pancake-like form. We use mesoscale simulations and theoretical arguments, compared to experimental data, to show that such "pancake bouncing" occurs when impacting fluid that enters the surface is slowed and then expelled by capillary forces. For the drop to bounce as a pancake, two criteria must be satisfied: the fluid must return to the surface at the appropriate time, and it must do so with sufficient kinetic energy to lift the drop. We argue that this will occur for superhydrophobic surfaces with topological features having dimensions of ∼200 µm, larger than those normally considered. The contact time of pancake bouncing events is reduced by up to 5-fold compared to that of conventional bouncing, suggesting relevance to drop shedding and anti-icing applications.

Pancake bouncing on superhydrophobic surfaces.

Engineering surfaces that promote rapid drop detachment1,2 is of importance to a wide range of applications including anti-icing3-5, dropwise condensation6, and self-cleaning7-9. Here we show how superhydrophobic surfaces patterned with lattices of submillimetre-scale posts decorated with nano-textures can generate a counter-intuitive bouncing regime: drops spread on impact and then leave the surface in a flattened, pancake shape without retracting. This allows for a four-fold reduction in contact time compared to conventional complete rebound1,10-13. We demonstrate that the pancake bouncing results from the rectification of capillary energy stored in the penetrated liquid into upward motion adequate to lift the drop. Moreover, the timescales for lateral drop spreading over the surface and for vertical motion must be comparable. In particular, by designing surfaces with tapered micro/nanotextures which behave as harmonic springs, the timescales become independent of the impact velocity, allowing the occurrence of pancake bouncing and rapid drop detachment over a wide range of impact velocities.