Pancake-shaped freezing of a droplet impacting a supercooled surface: Evidence for a threshold temperature

The residual diameter of a droplet at a temperature of 25 °C impacting a cold surface was investigated using high-speed photography. The temperature of the impacted surface was varied from -50 to 25 °C. A threshold temperature was found below which the impacting droplet freezes near the maximum spreading diameter, resulting in pancake-shaped freezing. However, above the threshold temperature, the lamella retracts before freezing. The maximum spreading diameter remains nearly unchanged for surfaces with various temperatures for the same Weber number. Variations of the threshold temperature required for pancake-shaped freezing as a function of the Weber number were determined. The threshold temperature decreases as the Weber number increases. A model for predicting the threshold temperature is proposed and compared with experimental observations. It is shown that it is the freezing time of the lamella tip that determines the threshold temperature.