Ice crystallization kinetics in supercooled droplets from a molecular perspective

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.