Why is the Surface of Ice Wet?

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), completely wet ice surfaces. Contrary to this conventional wisdom, we demonstrate that QLLs have 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 but also at undersaturation, but QLLs are absent at equilibrium. Here we propose a simple physical model that consistently explains these new aspects of surface melting of ice. We show that QLLs are a metastable transient state formed through vapour growth and sublimation of ice, which casts a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice-vapor equilibrium.