Freezing of micrometer-sized liquid droplets of pure water evaporatively cooled in a vacuum

Freezing processes are reported for pure-water droplets generated in a vacuum in the size range of 49-71 μm in diameter. The process is characterized for each size by measurement of a freezing curve, where the fraction of frozen droplets is evaluated as a function of time. The 49 μm droplet was found to freeze at a time between 7.0 and 7.9 ms after being generated at room temperature, where the fraction of frozen droplets increased from 5% to 95%; the freezing time was thus distributed statistically within 1 ms. The freezing time was retarded by about 3 ms as the size increases from 49 to 71 μm, while the rise time of the freezing curve was almost unchanged. Numerical simulation of a cooling curve, i.e., the temperature of a droplet as a function of time, revealed that the droplets in the present size range are frozen at almost the same temperature between 233 and 236 K. The freezing curves measured in the experiment were well reproduced by numerical simulation based on the simulated cooling curves combined with the temperature dependence of the volume-based homogeneous ice nucleation rates of pure water reported previously. It was also found that a droplet is disintegrated into a few fragments upon freezing, which suggests formation of a frozen shell in the outer region of a droplet.