Bouncing droplets of water

Understanding the mechanisms behind a drop of water spinning after bouncing on a hydrophobic surface with a hydrophilic pattern can be of interest for various applications, e.g., in engineering or biology. We studied this phenomenon both theoretically and experimentally. On the theoretical side, we analyzed the relationship between the droplet shape and its rotation speed under surface tension, assuming certain symmetries of the shape: a flattened cylinder, an oblate spheroid, and a rudimentary derivation for a general body of revolution. Our experimental setup consisted of soot atop glass slides for the hydrophobic surface and hydrophilic pattern. Despite a lack of accurate control of both pattern reproducibility and the experimental environment, we were able to experimentally corroborate our theory that uses surface tension to predict the relation between the shape and angular velocity of the droplet after bouncing off a hydrophobic surface with a given hydrophilic pattern.