Ice crystallization kinetics in supercooled droplets from a molecular perspective

Ice nucleation and crystallization in supercooled water droplets are critical to processes in freeze desalination, cryopreservation, and atmospheric science. While previous studies focused on bulk or substrate-bound systems, this study uniquely investigates freely suspended droplets across nano- and micro-scales to isolate intrinsic freezing behavior, hypothesizing that size, salinity, and cooling rate critically influence nucleation thermodynamics and kinetics.

Experiments were conducted on freely suspended microliter-sized droplets in a subcooled environment, covering a salinity range of 0-70 g/L and volumes from 0.5 to 3.5 μL. Detailed MD simulations were performed on nanoscale droplets to evaluate ice nucleation behavior as a function of droplet size (5.04-9.54 nm), salinity (0-70 g/L), and cooling rate (0.1-10,000 K/ns).

Experiments revealed a two-stage freezing process involving rapid, surface-initiated dendritic freezing accompanied by recalescence, followed by slower, semi-isothermal volumetric crystal growth. In contrast, MD simulations showed single-stage freezing in nanodroplets driven by their high surface-area-to-volume ratio. Slower cooling increased nucleation rates by 50 %, while rapid cooling induced vitrification and suppressed nucleation. Salinity reduced recalescence by up to 44 % and nucleation rates by 39-56 %, though high ion concentrations potentially promoted nucleation due to ion-induced heterogeneities. Structural analysis revealed stacking-disordered ice with a stable cubic-to-hexagonal ratio, and saltwater droplets exhibited ∼26 % larger critical nuclei due to the reduced water activity. Importantly, our simulations revealed ion pairing and clustering at the droplet interface, which altered local water structuring and introduced spatial heterogeneity. These results advance mechanistic understanding of ice formation mechanisms in supercooled systems relevant to desalination and other applications by highlighting how interfacial ion distributions, ion clustering, and size-dependent confinement modulate nucleation pathways.