For more than a century, scientists have debated whether a thin liquid water film always coats ice below its melting point and, if so, how thick that film might be under different conditions. The idea dates back to Michael Faraday, who proposed that solid ice could support a microscopically thin liquid layer across its surface, but experimental efforts since then have produced conflicting measurements of the film thickness and even its existence.
Luis G. MacDowell of Universidad Complutense de Madrid set out to resolve this controversy by revisiting the phase diagram of water and ice and examining how all three phases, solid, liquid, and vapor, coexist. In the phase diagram there is a specific combination of temperature and pressure known as the triple point, where ice, liquid water, and water vapor are equally stable and can remain in perfect equilibrium.
Using computer simulations to follow the motion of molecules at the ice surface under these conditions, MacDowell observed that a liquid-like film only a few nanometers thick appears at the triple point. Many laboratory measurements, however, have reported much thicker films, leading to a long standing mismatch between theory and experiment that demanded an explanation.
MacDowell argues that much of this disagreement can be traced to the extreme sensitivity of the system to departures from the exact equilibrium point. "Equilibrium is a point," he said. "You are as close as you can be, but never just there. Just a tiny deviation can become sufficiently out of equilibrium, making it very difficult to measure these things."
According to the work, the liquid film remains confined to a limited thickness in the vicinity of the triple point because of the unusual density properties of water, in which the solid phase is energetically favored over the liquid. This balance restricts how far the liquid-like layer can grow before the system reverts to solid ice.
By combining concepts from different areas of physics, MacDowell extends the analysis beyond film thickness to account for observations of liquid droplets that condense on top of the premelting film, indicating partial wetting of the surface. The same framework allows him to propose a mechanism that links surface phase behavior to the way ice crystals grow into very different shapes as environmental conditions change.
"This sequence of transitions in the shape of the snow crystals is related to changes in premelting film thickness that occur at the surface of ice," MacDowell said. "It exhibits surface phase transitions, and at each transition, you have a sudden change of the properties and of the growth rate of the faces." When different faces of a crystal grow at different rates, the overall morphology can shift from compact to plate like or columnar forms.
Because the basal face and prism faces of a crystal can advance at distinct rates depending on the local surface phase, the same material can generate a wide variety of crystal habits over a relatively narrow temperature range. MacDowell suggests that this sensitivity makes the premelting film and its transitions relevant to several open problems, including how ice particles interact in clouds and how the microscopic structure of the surface controls friction and sliding.
The study points to potential applications in atmospheric physics, where accurate descriptions of snow crystal growth feed into models of cloud microphysics and precipitation, and in tribology and sports engineering, where the slipperiness of ice surfaces remains only partially understood. MacDowell hopes that the theoretical tools developed in this work will help clarify how frictional heating and impurities such as dissolved salts or dust particles modify the premelting film.
The problem is not yet fully resolved, and the author plans to extend the simulations to explore how shear forces and contaminants interact with the liquid-like layer to alter its thickness and dynamics. Such efforts could help to connect the equilibrium picture developed in this study with real world conditions, ranging from natural snowpacks and glaciers to ice rinks and engineered icy surfaces.
Research Report:The key physics of ice premelting
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