Thermodynamic origin of surface melting on ice crystals

Abstract

Since the pioneering prediction of surface melting by Michael Faraday, it has been widely accepted that thin water layers, called quasi-liquid layers (QLLs), homogeneously and completely wet ice surfaces. Contrary to this conventional wisdom, here we both theoretically and experimentally demonstrate that QLLs have more than two wetting states and that there is a first-order wetting transition between them. Furthermore, we find that QLLs are born not only under supersaturated conditions, as recently reported, but also at undersaturation, but QLLs are absent at equilibrium. This means that QLLs are a metastable transient state formed through vapor growth and sublimation of ice, casting a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice-vapor equilibrium. We propose a simple but general physical model that consistently explains these aspects of surface melting and QLLs. Our model shows that a unique interfacial potential solely controls both the wetting and thermodynamic behavior of QLLs.surface melting | quasi-liquid layer | advanced optical microscopy | pseudo-partial wetting | wetting transition I n general, surfaces and interfaces yield unique phase transitions absent in the bulk (1-5). Surface melting (or premelting) of ice (3, 4) is one typical and classical example that has been known since the first prediction by Michael Faraday in 1842 (6). He hypothesized that thin water layers, now called quasi-liquid layers (QLLs), wet ice crystal surfaces even at a temperature below the melting point. Since then, this phenomenon has attracted considerable attention not only because of its importance in the fundamental understanding of melting (a solid-toliquid transition) itself but also as a link to a diverse set of natural phenomena in subzero environments: making snowballs, slippage on ice surfaces, frost heave, recrystallization and coarsening of ice grains, morphological change of snow crystals, electrification of thunderclouds, and ozone-depleting reactions (3, 4, 7). Furthermore, it is now recognized that surface melting is not specific to ice but rather is universally seen in a wide range of crystalline surfaces such as metals, semiconductors, ceramics, rare gases, and organic and colloidal systems (8-12). Its underlying physics is therefore also inseparable from material science and technology.Although the origin of surface melting, including the nature of QLLs themselves, is still far from completely understood and a matter of active debate (13-18), it is at least phenomenologically believed that surface melting is driven by the reduction of the surface free energy by the presence of intervening liquid between the solid and gas phases (3,4,13,19). More sophisticated approaches have also been proposed in terms of surface phase transitions (1,3,4,20). In contrast to such theoretical speculations, however, the direct observation and the accurate characterization of QLLs by experiments are still highly challenging because of their thinness, assumed to be less t...

Closer look at the surface of ice

“…These states are characterized by either a droplet of liquid forming directly on the surface of ice, as would be favored by the long-ranged forces, or a droplet of liquid forming on top of a thin liquid film, as would be favored by the short-ranged forces. Clear observations of both are provided in the study by Murata et al (2), and, importantly, hysteresis is found in transitioning between these two states. This hysteresis is postulated to occur due to the metastability afforded by a first-order phase transition.…”

“…Such droplets were visualized earlier by Elbaum et al (11) in the water vapor pressure, with simple physical modeling rooted in classical wetting theory, to propose a thermodynamic origin for these observations. Building on previous work by Elbaum and Schick (12), Murata et al (2) posit that droplets form due to long-ranged attractive interactions between ice and vapor. For thick liquid layers, longranged interactions in the form of dispersion forces dominate over molecular interactions and ultimately determine the wetting behavior of the premelted layer.…”

“…This attraction pins the premelting layer thickness to a finite value, rendering surface melting incomplete as the triple point is approached. In the study by Murata et al (2), incomplete melting is clearly signaled by the protruding micrometersized droplets they are able to optically image. These droplets fail to wet the surface of ice and consequently have finite contact angles.…”

“…Although experimental and theoretical work has confirmed the existence of a liquid-like layer atop ice surfaces, its molecular origins and physical properties are still actively debated. In PNAS, using high-resolution optical interferometry, Murata et al (2) propose that the liquid layer atop ice can adopt two different wetting states with a first-order phase transition between them.…”

“…Murata et al (2) succeed by making direct in situ observations of the surface of ice using an advanced optical microscope. In doing so, they challenge the conventional understanding of premelting on ice as a simple uniform liquid layer.…”