Kinematic and thermodynamic studies on water micro-droplets supercooled in a vacuum

Evaporation of water droplets in a vacuum induces rapid evaporative cooling that leads to a supercooled state of water. Observation of supercooled water provides valuable insights into ice nucleation and subsequent freezing processes. Here we introduce 40-μm water droplets into a vacuum to study their cooling and freezing dynamics by several experimental techniques. High-speed imaging is employed to observe oscillatory distortion that reflects surface tension and viscosity of the supercooled droplets and to capture fragmentation of freezing droplets. Observation of whispering gallery modes in the OH stretching Raman band enables precise measurement of the droplet size in the course of evaporation. Furthermore, a freezing curve, i.e., a fraction of frozen droplets as a function of time, is measured by capturing laser-scattering images to discriminate between frozen and unfrozen droplets. The experimental approaches to the evaporation rate and subsequent freezing time, along with thermodynamics simulation based on the Knudsen theory, allow us to discuss homogeneous ice nucleation rates between 232 and 235 K.