Frost halo rings beneath freezing droplets on cryogenic surfaces

We experimentally investigate the frosting phenomena beneath freezing droplets on cryogenic surfaces and reveal the emergence of previously unreported dual frost halo rings. We report that droplets impacting from low heights produce dual frost halo rings beneath the freezing droplet, whereas higher impact heights result in a single halo ring. The formation of dual halos originates from the raised bimodal morphology of the freezing droplet, which is shaped by the interplay of hydrodynamic spreading, freezing kinetics, and mechanical peeling dynamics. The locally thicker region of the raised thickness profile cools more slowly than the surrounding edge, maintaining a higher temperature that promotes localized sublimation at the freezing droplet base and subsequent vapor desublimation onto the adjacent cryogenic substrate, thereby resulting in an internal frost halo ring. Through capillary wave propagation analysis, we identify the morphology transition of different frost halo rings characterized by a power-exponent relationship between the dimensionless droplet falling height and radius. These findings provide insights into the complex interplay between capillary hydrodynamics and freezing kinetics under cryogenic conditions.