Supercooling

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Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid.

A liquid below its melting point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the glass transition where the system is an amorphous — that is, non-crystalline — solid.

Water has a melting point of 273 K (0°C) but can be supercooled at ambient pressure down to its crystal homogeneous nucleation at almost 231 K (−42°C).^[1] If cooled at a rate of the order of 1 million K per second, the crystal nucleation can be avoided and water becomes a glass. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108°C).^[2] Glassy water can be heated up to approximately 150 K (−123°C).^[3] In the range of temperatures between 231 K (−42°C) and 150 K (−123°C) experiments find only crystal ice.

Droplets of supercooled water often exist in stratiform and cumulus clouds. They form into ice when they are struck by the wings of passing airplanes and abruptly crystallize.

An equivalent to supercooling for the process of melting solids does not exist: a solid will always melt at the same temperature for a given pressure. It is, however, possible to superheat a liquid above its boiling point without it becoming gaseous.

An application of supercooling is the heat pad ("HotBag", "Thermo-Pad", etc.) containing sodium acetate (CH₃COONa, also used as food additive E 262): the heat required for melting is retained when the temperature drops and the liquid gets below the melting temperature. The heat is released on solidification, which is triggered by flexing a (patented[4]) small flat disc of notched ferrous metal which releases very tiny adhered crystals of sodium acetate [5] into the solution which then act as nucleation sites for the recrystallization of the remainder of the salt solution.

References

^ P. G. Debenedetti and H. E. Stanley, "Supercooled and Glassy Water," Physics Today 56 [issue 6], 40-46 (2003) http://polymer.bu.edu/hes/articles/ds03.pdf

^ http://polymer.bu.edu/hes/articles/gass04.pdf