Premelting

Premelting, also known as surface melting, is the existence of liquid-like thin film coating the surface of a crystalline solid below its melting point. Also can occur in crystal volume and enhance diffusion.

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First postulated by prominent British physicist Michael Faraday in 1859, a premelting layer is the reason why ice is slippery. The commonly given explanation of pressure-induced lowering of melting point of ice is incorrect: although this does happen, this effect is too small to explain slippery ice. It is a combination of premelting and friction melting that enables us to enjoy wintertime activities such as skating and skiing. Surface melting of ice is also a key process in electrification of rain droplets leading to formation of thunderclouds as well as frost heave - a process powerful enough to move around large boulders and form giant "cracks" in the ground.

Premelting is observed in most materials including dielectric crystals (such as Argon), semiconductors and metals. However, some closely packed facets of metals (such as Au[111]) do not premelt, while all of the facets of dielectrics show premelting. This anomalous behavior of metals is attributed to surface layering which has been recently observed in liquid metals, but not in dielectric liquids.

An opposite (extremely rare and exotic) phenomenon called surface freezing can form in alkane chain liquids and liquid crystals. In surface frozen liquids, the surfaces show long-range in-plane ordering while bulk is liquid (disordered).

References

• Why is Ice Slippery? (Robert Rosenberg, Physics Today, December 2005) [1] (requires subscription)

• Explaining Ice: The Answers Are Slippery (Kenneth Chang, The New York Times, February 21 2006) [2] (requires subscription)

• A. Dimanov, J. Ingrin. Premilting and high-temperature diffusion of Ca in synthetic diopside: An increase of cation mobility. Phis Chem Minerals. 22, 1995 437-442.