Comparative Molecular Dynamics Study of Vapor-Exposed Basal, Prismatic, and Pyramidal Surfaces of Ice

We present the results of molecular dynamics simulations in which ice I-h slabs with free basal, prismatic, 28 degrees pyramidal, and 14 degrees pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within similar to 10 ns and QLL stratification into outer (epsilon(1)) and inner (epsilon(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14 degrees pyramidal and basal facets (similar to 230 Pa), while significantly lower values are obtained for the prismatic and 28 degrees pyramidal facets (similar to 200 Pa). The geometric thickness of the QLL also varies between facets, with the 14 degrees pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28 degrees pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.