Heterogeneous ice nucleation at solid surfaces is important in many physical systems including the Earth's atmosphere. AgI is one of the best ice nucleating agents known; however, why AgI is such an effective ice nucleus is unclear. Using molecular dynamics simulations, we show that a good lattice match between ice and a AgI surface is insufficient to predict the ice nucleation ability of the surface. Seven faces modeled to represent surfaces of both β-AgI and γ-AgI, each having a good lattice match with hexagonal and/or cubic ice, are considered, but ice nucleation is observed for only three. Our model simulations clearly show that the detailed atomistic structure of the surface is of crucial importance for ice nucleation. For example, when AgI is cleaved along certain crystal planes two faces result, one with silver ions and the other with iodide ions exposed as the outermost layer. Both faces have identical lattice matches with ice, but in our simulations ice nucleation occurred only at silver exposed surfaces. Moreover, although hexagonal ice is often the only polymorph of ice considered in discussions of heterogeneous ice nucleation, cubic ice was frequently observed in our simulations. We demonstrate that one possible mechanism of ice nucleation by AgI consists of particular AgI surfaces imposing a structure in the adjacent water layer that closely resembles a layer that exists in bulk ice (hexagonal or cubic). Ice nucleates at these surfaces and grows almost layer-by-layer into the bulk.

Nucleation of ice by airborne particles is a process vital to weather and climate, yet our understanding of the mechanisms underlying this process is limited. Kaolinite is a clay that is a significant component of airborne particles and is an effective ice nucleus. Despite receiving considerable attention, the microscopic mechanism(s) by which kaolinite nucleates ice is not known. We report molecular dynamics simulations of heterogeneous ice nucleation by kaolinite (001) surfaces. Both the Al-surface and the Si-surface nucleate ice. For the Al-surface, reorientation of the surface hydroxyl groups is essential for ice nucleation. This flexibility allows the Al-surface to adopt a structure which is compatible with hexagonal ice, Ih, at the atomic level. On the rigid Si-surface, ice nucleates via an unusual structure that consists of an ordered arrangement of hexagonal and cubic ice layers, joined at their basal planes where the interfacial energy cost is low. This ice structure provides a good match to the atomistic structure of the Si-surface. This example is important and may have far-reaching implications because it demonstrates that potential ice nuclei need not be good atomic-level matches to particular planes of ice Ih or cubic ice, Ic. It suggests that surfaces can act as effective ice nuclei by matching one of the much larger set of planes that can be constructed by regular arrangements of hexagonal and cubic ice.

Silver iodide is one of the most effective ice nuclei known. We use molecular dynamics simulations to investigate ice nucleation by AgI disks and plates with radii ranging from 1.15 to 2.99 nm. It is shown that disks and plates in this size range are effective ice nuclei, nucleating bulk ice at temperatures as warm as 14 K below the equilibrium freezing temperature, on simulation time scales (up to a few hundred nanoseconds). Ice nucleated on the Ag exposed surface of AgI disks and plates. Shortly after supercooling an ice cluster forms on the AgI surface. The AgI-stabilized ice cluster fluctuates in size as time progresses, but, once formed, it is constantly present. Eventually, depending on the disk or plate size and the degree of supercooling, a cluster fluctuation achieves critical size, and ice nucleates and rapidly grows to fill the simulation cell. Larger AgI disks and plates support larger ice clusters and hence can nucleate ice at warmer temperatures. This work may be useful for understanding the mechanism of ice nucleation on nanoparticles and active sites of larger atmospheric particles.

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