Statistical analysis of freezing stages in supercooled water microdroplets

The solidification of metastable liquids is a relatively unexplored process. An important example is the freezing of supercooled water microdroplets, which is relevant to atmospheric processes. We investigated freezing in 40 µm water droplets that were supercooled by evaporation in a vacuum chamber, a process that has many similarities to the freezing of water in clouds. Individual droplets froze after different times of flight due to the randomness of ice nucleation. A large number of drops were imaged optically to capture the dendritic growth of ice crystals and subsequent solidification processes up to the cracking and shattering of drops. More than thirteen thousand droplet images, recorded at several times of flight, were analyzed and classified into eight image types that correspond to different stages of freezing. As the time of flight increased, the distribution of image types evolved from primarily liquid drops to primarily shattered drops. Despite a substantial broadening caused by ice nucleation statistics, the change in distributions with time could be used to determine what physical freezing stages correspond to the observed image types, their temporal order, and their approximate duration. The homogeneous ice nucleation rate was estimated and found to be consistent with previous measurements.