Freezing behaviors of an impacting droplet on subcooled hydrophobic surfaces

The solidification of impacting water droplets on cold substrates wildly exists in nature and engineering devices, which would significantly affect their performances. The freezing mechanisms of impacting water droplets on hydrophobic surfaces are not well understood. In this work, we explore the freezing behaviors of impacting water droplets on a cold hydrophobic surface in the regime where the interaction between the droplet impact dynamics and solidification is strong. We observe six different freezing morphologies on hydrophobic surfaces via tuning the substrate temperature and impact velocity. The diverse freezing morphologies are resulted from the competition of timescales associated with the solidification and droplet impact dynamics. The final freezing area on surfaces shows a non-monotonic variation with impact height when T_s = -38° C. We thus developed a model to predict the final freezing area, which agrees well with the experimental results.