Shape variation and unique tip formation of a sessile water droplet during freezing

Theoretical and experimental investigations are conducted to study the freezing process of a sessile water droplet. A novel model that considers the effects of both supercooling and gravity and assumes the freezing front (ice-water interface) to be a spherical surface is developed to simulate the droplet freezing behaviors. Two correlations describing the evolution of triple-phase line height and that of contact angle of unfrozen ice-water mixture on frozen ice are developed to close the model, with their concrete expressions being determined by combining the theory and experiment. Experiments are conducted on different sizes of water droplets deposited on cold aluminum plates having different contact angles and temperatures. The droplet freezing behaviors including the triple-phase line movements and droplet shape evolutions are observed and recorded using the image recognition technology. Calculations are implemented to simulate the evolution of droplet freezing shape, the movement of freezing front, the formation of pointy tip, and the variations of droplet volume and expansion rate. Comparisons are made between the calculation and experiment, and good agreements are obtained, with the calculated freezing times agreeing with the measured ones within 2%.