Premelting
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2007-01-10T13:35:14Z
Stepanovas
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'''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]].
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) [http://www.physicstoday.org/vol-58/iss-12/p50.shtml] (requires subscription)
* ''Explaining Ice: The Answers Are Slippery'' (Kenneth Chang, ''The New York Times'', February 21 2006) [http://www.nytimes.com/2006/02/21/science/21ice.html] (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.
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[[Category:Phases of matter]]