Layered contact line freezing phenomenon for droplet impact on a cold inclined substrate

The present study experimentally investigates the freezing dynamics of an impacted water droplet on the cold substrates with inclinations ranging from 0° to 60° and surface temperature maintained between −10 and −30 °C. Upon impact, the droplet exhibits inertia-driven oscillations before settling for low substrate inclination, whereas sliding and rolling motions are observed at high substrate inclinations. For high substrate inclinations, first an initial contact line is formed, followed by a droplet liquid intermittent jump over the contact line to further establish fresh contact downstream due to capillary pressure-driven oscillations. Such multiple rewetting events are observed for high-impact Weber numbers and are proposed as layered contact line freezing in the present study. The frozen droplet footprint increases linearly with the tangential impact Weber number for inclined substrates and with the normal Weber number for horizontal substrates. Furthermore, it exhibits weak dependence on the substrate temperature at low impact Weber numbers. The preferential sliding and rolling of droplet liquid in the downstream direction leads to significant enhancement in droplet footprint in the downstream of its initial contact point with the substrate, whereas its spreading toward upstream of the contact point is suppressed by nearly an order of magnitude compared to the downward spreading. A theoretical model has also been developed to predict the freezing time of the droplet incorporating its morphological details and shows good agreement with the experimental measurements for low substrate inclinations.