J. G. Dash

The premelting of ice and its environmental consequences

Several mechanisms can extend the equilibrium domain of a liquid phase into the solid region of the normal phase diagram. The causes of premelting, which include surface melting, interface curvature and substrate disorder, occur in all types of substances, including H2O. In the case of H2O, premelting can have important environmental consequences, among which are the heaving of frozen ground, breakdown of rock and concrete, sintering of snow, flow of glaciers, scavenging of atmospheric trace gases by snow and ice, and the electrification of thunderclouds. The article reviews the basic mechanisms of premelting and discusses their roles in the environmental phenomena. The principal results of numerous studies are reviewed, and trends in current research are outlined.

Optical study of surface melting on ice

We have used an optical reflection technique to measure the thickness of water films existing on the surfaces of isolated single H2O ice crystals, at temperatures below O°C. The crystals were grown in situ from pure water vapor. When investigated under these conditions the water films were found to be of finite thickness at all T up to and including the triple point, indicating incomplete wetting of water on ice, or incomplete surface melting. The maximum film thickness detected was approximately 200 A. At TTR water drops appeared on the ice facets, observed by interference microscopy. Their contact angles were consistent with the above findings. The addition of air to the atmosphere caused the film thickness to diverge on some orientations of the surface, i.e. it caused complete wetting or complete surface melting.

Premelting of ice in porous silica glass

Abstract The liquid water thickness at an ice/silica interface in frozen porous silica has been measured with pulsed NMR in a temperature range of −30 to 0°C. The liquid layer exists still at −30°C, ∼ 10 A thick, grows with temperature, and diverges at a depressed melting point due to the pore curvature. The temperature dependence of the thickness is 90(T m − T) −0.60 A , which does not follow the standard theory of surface melting for a van der Waals solid. The discrepancy is attributed to the special system of H 2 O SiO 2 with a large curvature and a strong interaction.

Thermomolecular Pressure in Surface Melting: Motivation for Frost Heave

A thermomolecular pressure is associated with surface melting, and it can drive mass flow along an interface under a lateral temperature gradient. The pressure is a universal thermodynamic function in the limit of thick films. It may be responsible for frost heave in frozen ground.

Charge transfer in thunderstorms and the surface melting of ice

Abstract A mechanism is proposed for the electrification of thunderclouds, whereby charge separation in ice-hail collisions is a consequence of mass transfer between the particles due to size and growth effects in surface melting. The theoretical trends and magnitudes are consistent with laboratory observations of charging.

Theory of ice premelting in monosized powders

Abstract The gradual freezing of water in inert porous media is considered to be a consequence of interfacial and grain boundary melting combined with curvature-induced depression of the melting point. The unfrozen fraction is calculated for monosized powders of spherical particles. It is found that the unfrozen fraction contains separate terms associated with interfacial and curvature melting, with distinctive dependencies on temperature and particle size. The calculated fraction is in excellent agreement with measurements on pure water in graphitized carbon black and polystyrene micron-sized spheres. The introduction of dissolved impurities adds a third term, with an intermediate functional dependence.

Theory of charge and mass transfer in ice‐ice collisions

A new model describes charge and mass transfer in ice-ice collisions in terms of fundamental molecular physics. Drawing on clues from results of recent and earlier experiments, the theory treats the collisions as interlinked events acting in three stages: before collision, during contact, and withdrawal. The theory provides quantitative descriptions of charge and mass transfer and their dependence on growth rate, temperature, and impact energy. Application of the theory to experimental simulations of thunderstorm charging explains general trends in terms of basic microscopic processes.

Giant Facets at Ice Grain Boundary Grooves

The energy barrier for nucleation on the basal plane of ice has a striking manifestation when the basal orientation is present in a grain boundary groove. As the ice-water interface containing the groove advances, large planar facets grow out of the grain boundary. The facets exhibit dramatic hysteresis, and they can be up to 30 times larger than predicted by equilibrium models. Measurements of the growth direction of the facets yield insight into the nature of the nucleation process. The facets also provide a way to study the relaxation of twist along grain boundaries.

Dynamics of Faceted Grain Boundary Grooves

The dynamics of dislocation-free crystal facets is examined in the context of grain boundary grooves at the junction between two crystallites of a solid and the liquid phase. The geometry and thermal conditions of grain boundary grooves allow a detailed analysis of facet morphology during solidification in terms of the nucleation and spreading rates of elementary crystal planes. Observations on the freezing of water in a two-dimensional cell reveal several dynamical features which are treated by the theory. Additional observations provide indications for the stiffness and premelting of grain boundaries.

Classical rotational inertia of solid 4He.

The observation of reduced rotational inertia in a cell containing solid 4He has been interpreted as evidence for superfluidity of the solid. We propose an alternative explanation: slippage of the solid, due to grain boundary premelting between the solid and dense adsorbed layers at the container wall. We calculate the range of film thickness, and determine the viscosity that will account for the missing rotational inertia. Grain boundary premelting also explains inertial anomalies in an earlier study of solid helium in porous glass and indicates that the liquid is partially superfluid.