Proc. Natl Acad. Sci. USA 113, 1749–1753 (2016)

Ice surfaces are almost always covered with a thin layer of liquid water, even when the ice's temperature is below 0 °C. These quasi-liquid layers, as they are called, govern a range of surface-melting phenomena pertaining to various themes like meteorology, food conservation and winter sports. Little is known about the structure and the formation of the liquid layers, however. Harutoshi Asakawa and colleagues have now studied quasi-liquid layers by means of a combination of laser confocal microscopy and differential interference contrast microscopy.

The authors grew single crystals of ice on an AgI crystal, and put them in a chamber where the crystals' temperature and the partial pressure of the water vapour could be controlled. They found that two different types of morphologies for liquid layers occur: round, isolated drops (α-phase) and liquid films (β-phase). An investigation of the critical partial pressure above which the liquid phases developed, for various sample temperatures, revealed that supersaturation is required for the α- or β-phase to form. These phases are therefore thermodynamically metastable and do not form by melting of the ice below, but through condensation of the supersaturated gas phase above.