The supercooling and freezing of small water droplets falling in air and other gases

Abstract

The freezing of individual droplets of very pure supercooled water, ranging in diameter from 5 to 120 μ m, has been photographed and measured as they fall through a vertical tempera­ture gradient established in cold, purified helium, air and other gases. From measurements made on a few hundreds of droplets of the same size, it is possible to deduce the median (or mean) freezing temperature of the group and the statistical spread about the mean temperature. When the ‘observed’ freezing temperatures are corrected for the thermal inertia of the falling droplets and adjusted to a standard rate of cooling, they are in good agreement with earlier work in which the droplets were cooled in a liquid supporting medium. The observed variation of freezing temperature with droplet diameter may be satisfactorily interpreted in terms of the theory of homogeneous nucleation of supercooled water if the specific surface free energy of the crystal/liquid interface is taken to be 20 erg cm -2 between —35 and —40°C. The experimental results are also consistent with, and offer additional support for, the recent Némethy-Scheraga statistical thermodynamical treatment of the ‘flickering-cluster’ model of liquid water.