Premelting

Premelting (also surface melting) refers to a quasi-liquid film that can occur on the surface of a solid even below melting point (${\displaystyle T_{m}}$). The thickness of the film is temperature (${\displaystyle T}$) dependent. This effect is common for all crystalline materials. Premelting shows its effects in frost heave, and, taking grain boundary interfaces into account, maybe even in the movement of glaciers.

Considering a solid-vapour interface, complete and incomplete premelting can be distinguished. During a temperature rise from below to above ${\displaystyle T_{m}}$, in the case of complete premelting, the solid melts homogeneously from the outside to the inside; in the case of incomplete premelting, the liquid film stays very thin during the beginning of the melting process, but droplets start to form on the interface. In either case, the solid always melts from the outside inwards, never from the inside.