When a water drop bounces back from a hydrophobic surface, its initial, spherical shape is usually restored. Now, experiments with a specially engineered superhydrophobic surface made from micrometre-sized tapered pillars covered with copper oxide ‘nanoflowers’ show that droplets can bounce back with a flat, pancake-like shape. Engineering surfaces that promote rapid drop detachment1,2 is of importance to a wide range of applications including anti-icing3,4,5, dropwise condensation6 and self-cleaning7,8,9. Here we show how superhydrophobic surfaces patterned with lattices of submillimetre-scale posts decorated with nanotextures 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 a fourfold reduction in contact time compared with conventional complete rebound 1,10,11,12,13. 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 that 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.

Surfaces directing fluid flows: Although surfaces can be made to attract or repel liquids using coatings, surface textures with specific curvature can also be used to achieve the same effect. However, fluid transport is usually limited by the specific pattern that only drives flow at the surface itself. Feng et al . created a dual-reentrant surface that has an asymmetric profile so that fluids spread out at the surface and subsurface layers. Furthermore, these surfaces can be designed so that different liquids will naturally steer in opposing directions simply because of their specific interactions with the surface. —MSL

The impact of liquid drops on solid surfaces is ubiquitous in nature, and of practical importance in many industrial processes. A drop hitting a flat surface retains a circular symmetry throughout the impact process. Here we show that a drop impinging on Echevaria leaves exhibits asymmetric bouncing dynamics with distinct spreading and retraction along two perpendicular directions. This is a direct consequence of the cylindrical leaves that have a convex/concave architecture of size comparable to the drop. Systematic experimental investigations on mimetic surfaces and lattice Boltzmann simulations reveal that this novel phenomenon results from an asymmetric momentum and mass distribution that allows for preferential fluid pumping around the drop rim. The asymmetry of the bouncing leads to ∼40% reduction in contact time.

Droplet impacting on solid or liquid interfaces is a ubiquitous phenomenon in nature. Although complete rebound of droplets is widely observed on superhydrophobic surfaces, the bouncing of droplets on liquid is usually vulnerable due to easy collapse of entrapped air pocket underneath the impinging droplet. Here, we report a superhydrophobic-like bouncing regime on thin liquid film, characterized by the contact time, the spreading dynamics, and the restitution coefficient independent of underlying liquid film. Through experimental exploration and theoretical analysis, we demonstrate that the manifestation of such a superhydrophobic-like bouncing necessitates an intricate interplay between the Weber number, the thickness and viscosity of liquid film. Such insights allow us to tune the droplet behaviours in a well-controlled fashion. We anticipate that the combination of superhydrophobic-like bouncing with inherent advantages of emerging slippery liquid interfaces will find a wide range of applications.