A self-driven motion induced by freezing in free supercooled water drops

The freezing of liquid droplets can induce counterintuitive dynamics such as spontaneous trampolining on hydrophobic surfaces. Here we investigate how the freezing of free supercooled water drops in vacuum can induce a sudden motion of the drops. We propose that the motion is caused by a transient and asymmetric increase in the vapor pressure across the drop surface, as the drop heats to the melting temperature (recalesces) during freezing. For the limiting case where freezing is initiated at the surface of the drop, the magnitude of the induced drop velocity is maximal, and can be described by an analytical formula that does not depend on the drop size. The observed velocities imparted by freezing in two different experimental studies are somewhat smaller than the maximal velocity predicted by our formula, but comparable to it. We will discuss the reasons for observing motions close to the maximal velocity in the experimental studies, and how this self-driven motion could affect studies of supercooled water drops and of their freezing.