Freezing and shattering of supercooled water drops



Supercooled water drops of about 0·05 cm radius were frozen at temperatures between −4°C and −25°C under various conditions. Explosive shattering and ejected ice splinters were observed when supercooled drops were frozen under conditions of free convection in hydrogen and helium at 1·0 atm pressure, in air below 0·13 atm pressure and in carbon dioxide below 0·05 atm pressure. Drops did not fracture when ventilated at terminal velocity unless they were rotated about an axis normal to the airstream during freezing. Fracture was also observed when either the drops were not allowed sufficient time to attain thermal and solution equilibrium with the environment before nucleation, or when drops were frozen in carbon dioxide at a pressure above 0·3 atm.

A condition for drop fracture is the establishment of a strong shell of ice around the drop, which in turn depends on the distribution of heat transfer to the environment during freezing and on the effective thermal conductivity of the environment. The volume of gas dissolved in the drop is a secondary factor, except in the case of a very soluble gas such as carbon dioxide where splinters are produced by vigorous effervescence during freezing. In the atmosphere the shattering of an individual drop appears possible only if the drop rotates during freezing. This appears unlikely since any asymmetry in shape would tend to orient the drop as it fell.