Propagation of Freezing in Supercooled Water-In-Antifreeze-Oil Emulsions

Water-in-oil (W/O) emulsions, in which water droplets are separated by a continuous oil phase, are frequently used in food and cosmetic products. However, the freezing kinetics of W/O emulsions are not yet well understood. In this study, we find that freezing propagates to individual water droplets that are in direct contact with other frozen droplets. When droplets are not in contact, freezing does not propagate even when the emulsions are cooled to -18 °C. Two measures of the perimeter and the area of the frozen droplets in emulsions are defined to evaluate the propagation velocity of freezing using a simple mathematical model. The velocity is highest (4 × 10² μm s^-1) at -18 °C, which is lower than the freezing velocity of individual droplets at the same temperature (1.2 × 10³ μm s^-1). Water-oil interfaces, or a thin layer of oil between droplets, act as a barrier to propagation of freezing. The dependence of the freezing velocity on the degree of supercooling is consistent with results from a previous study; however, the absolute value of the freezing velocity is smaller by a factor of 10². The propagation velocity also depends on the degree of supercooling but its dependence is different from that of the freezing velocity. Features of freezing of water droplets immersed in antifreeze oil are discussed.