Rotation of a rebounding-coalescing droplet on a superhydrophobic surface

Droplet impact and droplet coalescence are two very common phenomena. When these two processes occur on a superhydrophobic surface in an appropriate sequence, an interesting but little-noticed phenomenon will occur with rotation of the rebounding-coalescing droplet. When a droplet impacts another stationary droplet on a superhydrophobic surface with an appropriate velocity and position, the reaction force produced by the impact and the moment arm formed by the liquid bridge produces a reversed torque. This reverse torque causes the droplet to rotate after rebounding. The liquid bridge in the early stage of the coalescence process is the key to the rotation, and the relative development speed of coalescence and rebound determines whether rotation can occur and its relative strength.