Freezing of a Supercooled Water Drop after an Impact onto a Solid Wall

Supercooled water freezes as a result of ice nucleation, propagation along the substrate of a thin ice layer and subsequent expansion of a mushy region of ice dendrites. In this experimental study, the impact, spreading and solidification of the drop are observed in a cold wind tunnel using a high-speed video system. The statistics of the nucleation times after a supercooled water drop impacts onto a dry solid substrate are analyzed. The experiments demonstrate that the rate of the ice nucleation is enhanced significantly by drop impact and continuously reduces over time. The nucleation rate increases with higher impact velocity and is enhanced by the substrate roughness. This effect is explained by the presence of the small bubbles in the liquid drops, generated by drop impact and fast spreading. The surfaces of these bubbles serve as the additional triggers for ice nucleation. Moreover, the effect of the presence of the bubbles becomes even more significant when the wetted area reduces due to the drop receding. The average number of the nucleation sites in this case increases, since the number of bubbles does not reduce despite the reductions of the wetted area. These bubbles are probably captured by the receding contact line.