Thermodynamic origin of surface melting on ice crystals

Significance Phase transitions of ice are a major source of a diverse set of natural phenomena on Earth. In particular, quasi-liquid layers (QLLs) resulting from surface melting are recognized to be key players involved in various natural phenomena spanning from making snowballs to electrification of thunderclouds. With the aid of in situ observations with our advanced optical microscopy combined with two-beam interferometry, we elucidate a thermodynamic origin of the formation of QLLs and their unique wetting behavior (pseudo-partial wetting and wetting transitions) on ice surfaces. We show that QLLs are a metastable transient state formed through vapor growth and sublimation of ice that are absent at equilibrium. 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.