Supercooled water droplet impacting-freezing behaviors on cold superhydrophobic spheres

Abstract

The impacting-freezing behaviors of supercooled water droplets on superhydrophobic spheres are investigated. A numerical model using the VOF (Volume of Fluid) method and enthalpy–porosity technology is established and validated by comparing the droplet profiles and spreading area factors by simulations and experiments. The effects of Weber number, supercooling degree and sphere-to-droplet diameter ratio (D*) on the impacting-freezing dynamics, including the droplet profile, spreading area factor and final rebound/adhesion, are studied. The results indicate that the supercooling rarely influences the spreading stage but significantly affects the receding stage with a larger stable spreading area factor obtained at a greater supercooling degree. With the decreasing of diameter ratio, the droplet generates a larger maximum spreading arc angle, a higher receding speed and thus a shorter contact time. The maximum spreading area factor greatly increases with reducing diameter ratio for D*<10 but it remains almost unchanged for D*≥10. Three different final morphologies of full rebound, partial rebound and adhesion are obtained, revealing the competition between the fluid flow and phase change in the droplet impacting-freezing process. The morphology map of rebound and adhesion indicates that the boundaries for droplet rebound and adhesion initially move to a smaller supercooling degree and then revert to the previous value as the diameter ratio becomes smaller. The findings in this research may deepen our understanding of the mechanism of supercooled water droplet impacting and freezing on a superhydrophobic curved surface, and contribute to the design of anti-icing/frosting surface.