How micropatterns and air pressure affect splashing on surfaces.

We experimentally investigate the splashing mechanism of a millimeter-sized ethanol drop impinging on a structured solid surface, composed of micropillars, through side-view and top-view high-speed imaging. By increasing the impact velocity, we can tune the impact outcome from a gentle deposition to a violent splash, at which tiny droplets are emitted as the liquid sheet spreads laterally. We measure the splashing threshold for different micropatterns and find that the arrangement of the pillars significantly affects the splashing outcome. In particular, directional splashing in the direction in which air flows through the pattern is possible. Our top-view observations of impact dynamics reveal that entrapped air is responsible for the splashing. Indeed, by lowering the pressure of the surrounding air we show that we can suppress the splashing in the explored parameter regime.