Freezing Patterns of Supercooled Binary Droplets on Cold Hydrophobic Surfaces

Freezing of binary droplets is ubiquitous in industrial processes, playing a critical role in aerospace engineering and advanced manufacturing. In this work, the freezing behaviors of water-ethanol binary droplets on cold hydrophobic surfaces were explored. Employing high-speed and infrared imaging, along with visualization in a Hele-Shaw cell, four distinct freezing patterns were identified, termed as "incomplete freezing", "localized bulge", "dispersed bulge", and "no bulge", respectively. Besides, the phase diagram is developed to unravel the dependence of freezing patterns on ethanol concentration and subcooling degree. These freezing patterns arise from the competition between the migration velocity of ethanol (<i>V</i>_m) near the freezing front and the propagation velocity of the freezing front (<i>V</i>_f). Moreover, a sustained depression of the freezing point is observed at the freezing front due to the accumulation of ethanol. A theoretical model based on the Stefan problem well predicts the freezing time, which is dependent on ethanol concentration.