Effect of surface temperature on splashing of droplets impacting on a cold superhydrophobic surface

Abstract

Droplet splashing on a solid surface is ubiquitous in many industrial applications. While previous studies mainly focus on the effect of viscosity or surface tension by employing different liquids, the combined effect of viscosity and surface tension induced by lowering surface temperature is still unclear. In this paper, the effect of surface temperature (from −9.8 °C to −35.3 °C) on prompt splashing and receding breakup of a water droplet impacting a superhydrophobic aluminum surface has been experimentally investigated at various Weber numbers. The droplet spreads smoothly for all surface temperatures in the relatively low Weber range ($<$100). In the intermediate Weber range (100–150), the critical Weber number for the formation of receding breakup increases with decreasing surface temperature. The droplet splashes regardless of the surface temperature value in the relatively high Weber range ($>$150). Compared with prompt splashing, the influence of surface temperature on receding breakup is more significant because the time for receding breakup is about 4 times longer than that for prompt splashing. The prompt splashing in the high-$We$ region and the receding breakup in the medium-$We$ region were studied qualitatively based on the unbalanced forces acting on the lamella of the spreading or receding liquid layer. A regime map described by the dimensionless temperature and capillary number is proposed to reveal the conditions under which a specific dynamic behavior dominates. Finally, empirical correlations linking droplet spreading/receding breakup threshold and droplet receding breakup/prompt splashing threshold are proposed.