The Key Physics of Ice Premelting

TL;DR

The article reframes ice premelting as a competition between short-range structural forces and long-range van der Waals interactions within a wetting-framework. By combining toy and continuum models, DLP theory, and state-of-the-art simulations, it explains why ice surfaces develop nanometer-thick premelting films that do not fully wet the surface, while still allowing observable surface steps and complex morphologies. It also introduces a kinetic, out-of-equilibrium perspective to account for condensation and growth near the triple point, predicting a rich phase diagram with quasi-stationary films, kinetic condensation, and spinodal lines that connect to the Nakaya diagram. The work highlights how a multi-scale, multi-method approach reconciles disparate experimental findings and offers a path toward predictive control of ice crystal habits in natural and industrial contexts.

Abstract

A disordered quasi-liquid layer of water is thought to cover the ice surface, but many issues, such as its onset temperature, its thickness, or its actual relation to bulk liquid water have been a matter of unsettled controversy for more than a century. In this perspective article, current computer simulations and experimental results are discussed under the light of a suitable theoretical framework. It is found that using a combination of wetting physics, the theory of intermolecular forces, statistical mechanics and out of equilibrium physics a large number of conflicting results can be reconciled and collected into a consistent description of the ice surface. This helps understand the crucial role of surface properties in a range of important applications, from the enigmatic structure of snow crystals to the slipperiness of ice.

Figures (15)

• Figure 1: Diagram of snow crystal morphology, a descendant of the early Nakaya diagram due to Kobayashi.kobayashi57 The figure displays the habit of ice crystals grown in the atmosphere as a function of temperature and excess water vapor density over ice saturation. The origin of the complex shapes that appear as saturation increases are relatively well understood. The reason why simple hexagonal prisms grown at low vapor saturation change from plates, to columns and back again to plates as temperature decreases has remained a mystery ever since Nakaya first published his results. Reproduced from https://doi.org/10.1063/1.282508160 70 (2007),furukawa07 with the permission of AIP publishing .
• Figure 2: Extent of ice premelting. A selection of all non-invasive experimental results since 1990 for temperatures up to -1 $^{\circ}$C lead to premelting film thicknesses that agree within an order of magnitude. Experimental results from Elbaum et al.elbaum93 (ELD), Dosch et al.dosch95 (DLB), Bluhm et al.bluhm02 (BOFHS), Sadtchenko and Ewing (SE),sadtchenko02 and Mitsui and Aoki,mitsui19 (MA). Representative results from computer simulations are provided for the TIP4P/Ice model from Llombart et al.llombart20b (LNM).
• Figure 3: Snapshot of an ice-vapor interface as obtained in computer simulations for the TIP4P/Ice model. Red particles correspond to water molecules in a liquid-like state. Blue molecules are in a solid like state as determined using a suitable order parameter. The interfacial structure may be characterized by two fluctuating surfaces which bound the premelting film from the ice and vapor bulk phases. Reproduced from Llombart et al.llombart20b Sci. Adv. 6 eaay9322 (2020). Distributed under a Creative Commons Attribution License CC-BY.
• Figure 4: Phase diagram of water in the neighborhood of the triple point. Full black lines illustrate the equilibrium phase boundaries, including the sublimation line separating ice from vapor and the condensation line, separating liquid water from vapor. The black dashed line is the metastable prolongation of the condensation line. Equilibrium premelting occurs for a path along the sublimation line, as indicated by the blue arrow. The equilibrium thickness of the premelting film is dictated by the distance of the sublimation line to the metastable prolongation of the condensation line, as indicated by the green segment. The red line indicates a non-equilibrium path, with three different regimes as described in Section IX. In region a, below the kinetic condensation line, shown in dashed blue, ice grows but the premelting film remains in a steady state of constant thickness. In region b, above the kinetic condensation line but below the kinetic spinodal line shown in dashed red, the premelting film remains in steady state but vapor can condense and form droplets atop. In region c, above the kinetic spinodal line, condensation occurs faster than freezing, the premelting film diverges and ice freezes in a wet mode.
• Figure 5: Toy model of surface premelting. (a) Free energy of a two-phase system approaching the triple point. At high under-cooling, the free energy exhibits two minima of equal height, corresponding to coexistence of the two phases. Along the sublimation line, a metastable minimum gradually emerges as under-cooling decreases. At the triple point, all three minima reach the same free energy. (b) Corresponding order parameter profiles, showing the gradual enrichment of the metastable phase as under-cooling decreases. Lines from violet to red indicate system behavior in order of decreasing under-cooling, as highlighted by the black arrows.