Alteration of freezing paradigms of an impact water droplet on different cold surfaces

Abstract

The influence of surface properties on the dynamics and freezing of an impact water droplet is investigated under various impact velocities and surface temperatures. Three distinct icing paradigms are found, namely the widely observed Central-Pointy Icing paradigm (CPI), the annular paradigm with Exterior Freezing front First (EFF), and the annular paradigm with Interior Freezing front First (IFF). High impact velocities and low surface temperatures are beneficial to the IFF icing paradigm. Upon decreasing the impact velocity and/or increasing the surface temperature, the IFF paradigm transits to EFF and ultimately to CPI. The interior freezing front occurring prior to the exterior one is mainly attributed to the small difference of the thickness between the central and peripheral liquid, and the large temperature difference between the droplet and liquid-substrate interface. Raising the impact velocity and surface wettability enhances droplet spreading, resulting in a small thickness difference. The liquid-substrate interface temperature is collectively determined by surface temperature, density, thermal conductivity and specific heat capacity of the solid surface. A freezing paradigm map described by the dimensionless temperature and the maximum spreading diameter is proposed to evaluate the conditions under which one of the three distinct freezing paradigms appears. A large spreading area and a low liquid-substrate interface temperature are prerequisites for the emergence of IFF paradigm.