Discovering the Molecular Origins of Solid-Liquid Friction at Ice-Ih / Water Interfaces

In this dissertation I present work on the modeling of hydrodynamic friction at the surface of ice. In the hydrodynamic regime, a slider traversing an ice surface is fully supported by a quasi-liquid layer (QLL), and the resulting friction is composed of three parts; solid-liquid friction between the ice crystal and the QLL, viscous shearing of the surface premelt, and the drag force due to capillary bridges forming between the QLL and the slider.

Using non-equilibrium molecular dynamics simulations, the basal {0001}, pris- matic {10 ̄10}, 14◦ pyramidal {20 ̄21}, and secondary prismatic {11 ̄20} facets of ice-Ih were drawn through liquid water with a momentum flux between the solid and liq- uid phases. The spatial transition between the ice and the liquid was quantified using structural and dynamic order parameters. Estimates of the interfacial width using structural measures gave widths of ∼ 6-10 ̊A. Dynamic interfacial widths were quantified and predicted a slightly broader interface, ∼ 10-15 ̊A wide. In addition, spatial decomposition of molecular orientational correlation functions indicated that the short- and longer-time decay components behave differently closer to the inter- face. In all cases the interface was observed to be stable under the presence of a shear.

Water sliding over the surface of ice is observed to be in the no-slip limit. A friction coefficient appropriate for negative slip boundary conditions is presented, and crystal facet dependent friction is observed. The computed friction of these interfaces is found to be invariant to the shear rate and direction of shear relative to the surface features. This trend is observed for two water potentials, and is shown to correlate strongly with the surface hydrogen-bond density. A momentum transmission model is proposed, which relates the observed friction with the density of solid to liquid hydrogen bonds, the shear viscosity of the liquid, and the width of the interface. Data collected from both water models agrees well with the proposed transmission model.

An investigation of the temperature dependence of the surface premelt at the basal and prismatic facets of an ice-Ih crystal is presented. Spatially resolved diffusion constants are computed normal to the ice surface, and estimates of the shear viscosity for the QLLs are obtained. A facet dependence is observed in the shear viscosity which agrees with the trend observed in the computed solid-liquid friction coefficients. Structural and dynamic properties of the QLLs are compared with simulations of supercooled liquid water. It is observed that the QLLs have characteristics distinctly different than the supercooled bulk liquid, attributed to the presence of the vapor interface.