The emergence of drop-type and thin-layer-type quasi-liquid layers on ice crystal surfaces and their thermodynamic origin

Abstract

Ice crystals play crucially important roles in wide variety of natural phenomena on the earth’s surface. However, in-situ optical observation of quasi-liquid layers (QLLs), which governs properties of ice crystal surfaces at temperature close to 0 °C, has been an experimental challenge. In this review, we show our recent studies on the in-situ observation of QLLs formed by the surface melting of ice crystals at temperatures below 0 °C. We first show our advanced optical microscopy that can directly visualize individual elementary steps (0.4 nm in thickness) on ice crystal surfaces. Then we next demonstrate the emergence of two kinds of QLLs with different morphologies (droplets and thin layers) on basal and prism faces of ice crystals. We also determined the surface tension-to-shear viscosity ratio (the so-called characteristic velocity) of both QLLs, indicating that the drop-type and thin-layer-type QLLs are 20- and 200-times less fluidic than bulk water, respectively. In addition, we measured the contact angles of drop-type QLLs on ice crystal surfaces and found that the formation of the drop-type and thin-layer-type QLLs can be explained by the wetting transition of QLLs on ice crystal surfaces. Furthermore, we determined the water vapor pressure–temperature ranges, in which both QLLs can present, and found that both QLLs are formed kinetically only in supersaturated or undersaturated water vapor. Our findings show a novel picture of surface melting that were significantly different from the conventional picture in which one QLL phase appear uniformly on ice crystal surfaces.