Investigating the Molecular-Scale Mechanism of Deposition Ice Nucleation on Silver Iodide Surfaces

Heterogeneous ice nucleation is a ubiquitous process in the natural and built environment. Deposition ice nucleation, according to the traditional view,, occurs in a subsaturated water vapor environment without the presence of supercooled water on the solid, ice-forming surface. This process is notably significant among the various ice formation mechanisms in high-altitude cirrus and mixed-phase clouds. Despite its significance, our understanding of the microscopic mechanism of deposition ice nucleation remains quite limited. This study introduces an adsorption-based mechanism for deposition ice nucleation through results from a combination of atomistic simulations, experiments and theoretical modeling. Silver iodide (AgI) particles prove highly efficient as ice-nucleating particles (INPs), commonly employed in rain seeding, and stand as one of the most potent laboratory surrogates for ice nucleation. In this study, AgI is used as a substrate for the simulations. The study involves a combination of grand canonical Monte Carlo and molecular dynamics (GCMC/MD) techniques to investigate deposition ice nucleation on AgI. We find that water initially adsorbs in clusters which merge and grow over time to form layers of supercooled water. Ice nucleation on silver iodide requires at minimum the adsorption of 4 molecular layers of water. Guided by the simulations we propose the following fundamental freezing steps: 1) Water molecules adsorb on the surface, forming nanodroplets. 2) The supercooled water nanodroplets merge into a continuous multilayer when they grow to about 3 molecular layers thick. 3) The layer continues to grow until the critical thickness for freezing is reached. 4) The critical ice cluster continues to grow.