  Textured hydrophobic surfaces that repel liquid droplets unidirectionally are
found in nature such as butterfly wings and ryegrass leaves and are also
essential in technological processes such as self-cleaning and anti-icing.
However, droplet impact on such surfaces is not fully understood. Here, we
study, using a high-speed camera, droplet impact on surfaces with inclined
micropillars. We observed directional rebound at high impact speeds on surfaces
with dense arrays of pillars. We attribute this asymmetry to the difference in
wetting behavior of the structure sidewalls, causing slower retraction of the
contact line in the direction against the inclination compared to with the
inclination. The experimental observations are complemented with numerical
simulations to elucidate the detailed movement of the drops over the pillars.
These insights improve our understanding of droplet impact on hydrophobic
microstructures and may be a useful for designing structured surfaces for
controlling droplet mobility.
