premelting

Premelting is a phenomenon in which a solid exhibits liquid-like properties at its interfaces while the bulk material remains solid below its bulk melting point. It is most readily observed at surfaces and grain boundaries, as well as in confined regions such as pores, where interfacial energies favor the formation of a thin liquid-like layer, often called a premelt film. The layer can form gradually as temperature approaches the melting point and its thickness depends on temperature, impurities, curvature, and stress.

The underlying mechanism is thermodynamic: as temperature rises toward the melting point, the interfacial free energy can be reduced by creating a disordered or liquid-like film along interfaces. This premelt layer may be stable over a range of temperatures and can transition to more extensive surface melting or even facilitate liquid-like flow along the interface. Impurities, pressure, and curvature can modify the onset temperature and the film’s thickness.

Premelting occurs in a variety of solids. Ice is the most studied example, where a quasi-liquid layer forms on exposed surfaces and grows in thickness as temperature increases toward 0°C. In metals and salts, surface and grain boundary premelting has been observed, affecting diffusion, wetting, and sintering. Premelting can influence mechanical properties near the melting point, including creep, fracture, and grain growth, and plays a role in processes such as corrosion and crystallization.

Study of premelting relies on techniques spanning microscopy, spectroscopy, and calorimetry under controlled temperatures, aiming to characterize the film thickness, structure, and its impact on material behavior.