Normal impact of supercooled water drops onto a smooth ice surface: experiments and modelling

The present study is devoted to the experimental investigation and theoretical modelling of the interaction between fluid flow and solidification during the impact of supercooled water drops onto an ice surface. Using a high-speed video system, the impact process is captured with a high spatial and temporal resolution in a side view. The lamella thinning and the residual ice layer thickness in the centre of impact are determined from the high-speed videos for varying drop and surface temperatures, and impact velocities. It is shown that the temperature of the impact surface has a negligible influence and the drop temperature has a dominating influence on the lamella thinning and the final ice layer thickness. For decreasing drop temperatures, higher freezing rates cause a decreased rate of lamella thinning and a larger thickness of the resulting ice layer. On the other hand, a higher impact velocity causes an increasing speed of lamella thinning and a smaller thickness of the resulting ice layer. Based on a postulated flow in the spreading lamella and considering the ice layer growth and the developing viscous boundary layer, the upper limit for the resulting ice layer thickness is theoretically modelled. The theory shows very good agreement with the experimental results for all impact conditions. Based on the derived theoretical scaling, a semi-empirical equation is obtained which allows an a priori prediction of the final ice layer thickness resulting from a single drop impact, knowing the impact conditions. This capability is important for the improvement of existing ice accretion models.