Water Nanofilms Facilitate Ice Crystal Growth across Droplets

A novel mechanism that underlies the peculiar cascading freezing of multiple supercooled droplets on surfaces is reported. The initial ice crystal growth in large droplets is communicated via connected water nanofilms to smaller droplets. Using high-speed imaging, we show that the presence of a water nanofilm with thickness higher than a critical value around 4.5 nm facilitates a freezing front propagation from one droplet to its neighbors on a hydrophilic surface. The freezing propagation rate through water nanofilms is approximately 40% of the recalescence front growth rate within a droplet, both of which are well described by kinetic crystal growth theory.