When this water droplet falls and smacks the surface below, it doesn’t just bounce back up. It pirouettes in the air. The droplet gyrates likes this because of the four slits on the surface. Aided by surface tension, these slits guide the droplet to expand into four directions upon impact instead of expanding into all directions evenly.
When the droplet contracts as it bounces back up, it follows the curves of the slits, producing the spinning motion. The labs of Xi-Qiao Feng at Tsinghua University and Yanlin Song at the Chinese Academy of Sciences collaborated to research this type of patterned surface, which they made by coating a plate with a super water-repellent polymer 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane and then etching in the four slits with ultraviolet light. The researchers hope this surface’s ability to convert translational motion to rotational motion could be used in inkjet printing or potentially to harvest energy from falling raindrops.