A wind tunnel investigation of freezing of small water drops falling at terminal velocity in air

Abstract

AbstractA method utilizing a vertical wind tunnel is described, by which supercooled water drops and ice particles can be stably suspended at temperatures between 0 and −30°C. Using this method, we studied: (i) the freezing temperature of water drops of various purity and volume, (ii) the freezing temperature of drops nucleated by contact with dry clay particles and the freezing temperature of drops nucleated by clay particles which were suspended in them, (iii) the crystallographic characteristics of frozen drops, and (iv) the free fall motion of supercooled drops and frozen drops freely suspended in air. The first study showed that the median freezing temperature of a population of equal size water drops is a linear function of the logarithm of the drop volume, in agreement with the law found by Bigg for water drops in oil. The second study showed that the temperature at which a drop froze when nucleated by contact with dry clay particles was over 10°C warmer than the temperature at which a drop of the same size froze when nucleated by clay particles suspended within the drop. The third study showed that frozen drops, recovered from the wind tunnel airstream and investigated under a polarization microscope, were single crystalline or polycrystalline depending on the drop volume, the temperature of freezing, and the heat dissipative capability of the medium surrounding the drop. The lowest temperature for single crystalline freezing is significantly colder for drops which freeze while completely surrounded by air than for drops which freeze on contact with an ice crystal. The fourth study showed that a supercooled drop falls with a constant velocity and no horizontal movement, while a frozen drop often spins and tumbles, and falls along a helical path with a terminal velocity which is 6 to 7 per cent smaller than that of the supercooled drop before it froze. This means the life time of a cloud particle lengthens as a result of freezing.