Regelation

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Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced. Many textbooks and websites claim that regelation can be demonstrated by looping a fine wire around a block of ice and attaching a heavy weight to it. The pressure exerted on the ice slowly melts it locally, permitting the wire to pass through the entire block. The wire's track will refill as soon as pressure is relieved, so the ice block will remain solid even after wire passes completely through. However, no one has shown how this is possible if a pressure of 500 atm is needed for ice to melt at –4 °C. This experiment is possible for ice at –10 °C or cooler. One cannot explain what is responsible for the above phenomenon if the pressure applied is much less than required.

Regelation was discovered by Michael Faraday. Regelation occurs only for substances, such as ice, that have the property of expanding upon freezing, for the melting points of those substances decrease with increasing external pressure. The melting point of ice falls by 0.0072 °C for each additional atm of pressure applied. For example, a pressure of 500 atmosphere (unit) is needed for ice to melt at –4 °C. ^[1]

[edit] Surface Melting

For a normal crystalline ice far below its melting point, there will be some relaxation of the atoms near the surface. Simulations of ice near to its melting point show that there is significant melting of the surface layers rather than a symmetric relaxation of atom positions. Nuclear magnetic resonance provided evidence for a liquid layer on the surface of ice. In 1998, using atomic force microscopy, Astrid Doppenschmidt and Hans Jurgen Butt, measured the thickness of the liquid-like layer on ice to be between 12 nm at –24 °C and 70 nm at –0.7 °C. Surface melting was found to begin at temperatures as low as –33 °C. ^[2]

Melting curve of water

Molecular structure of ice near the surface

The surface melting can account for the following:

Low coefficient of friction of ice, as experienced by skaters.
Ease of compaction of ice
High adhesion of ice surfaces

[edit] Examples of Regelation

A glacier can exert a sufficient amount of pressure on its lower surface to lower the melting point of its ice. The melting of the ice at the glacier's base allows it to move from a higher elevation to a lower elevation. Liquid water may flow from the base of a glacier at lower elevations when the temperature of the air is above the freezing point of water.