Directed rebounding of droplets by microscale surface roughness gradients

Impact dynamics of water droplets on superhydrophobic surfaces with different textures are known to vary dramatically, from total rebounding to complete sticking. Here we show that droplet rebounding on textured surfaces can be significantly influenced by the uniformity of the surface roughness. By engineering nonuniform textures (i.e., roughness gradients) on the surface, we are able to not only manipulate the axial rebound of the droplet, but also introduce a prescribed lateral component to the rebound trajectory. The measured directed rebounding is shown to fit a simple model balancing droplet inertia against the Young's force imbalance from side to side.