A complete kinetic model of freezing processes in supercooled water microdrops

The freezing of supercooled liquid water microdroplets is a complex process with multiple stages. The earliest stages (ice nucleation and growth) are well studied, and there is a renewed interest in understanding the latest stages (drop cracking and shattering) because they may impact how water clouds become ice clouds. The middle stages such as spicule growth are less well characterized, and obtaining a complete description of freezing is difficult, because the freezing of single drops is stochastic. Here we report a seven-stage kinetic model that quantifies all freezing stages, including the middle stages of out-to-inside freezing and spicule growth. The model is for 40-µm diameter drops freezing in a vacuum chamber after homogeneous ice nucleation near -39 °C. To develop the model, we used freezing stage probabilities and drop shattering probabilities from tens of thousands of drops. The parameters of the model, such as transformation times and probabilities, were determined though a complex fitting procedure against a stochastic computational model of droplet freezing. This model is the first of its type and suggests that it will be possible to develop similar models for important cases such as drop freezing in clouds.