Supercooling, also known as undercooling,^[1] is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid.

A liquid below its standard freezing 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. Homogeneous nucleation can occur above the glass transition temperature, but if homogenous nucleation has not occurred above that temperature an amorphous (non-crystalline) solid will form.

Water normally freezes at 273.15 K (0 °C or 32 °F) but it can also be "supercooled" at standard pressure down to its crystal homogeneous nucleation at almost 224.8 K (−48.3 °C/−55 °F).^[2][3] If, however, it is cooled at a rate on the order of 10⁶ K/s, 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/−162.4 °F).^[4] Glassy water can be heated up to approximately 150 K (−123 °C/−189.4 °F).^[3] In the range of temperatures between 231 K (−42 °C/−43.6 °F) and 150 K (−123 °C/−189.4 °F) experiments find only crystal ice.

Droplets of supercooled water often exist in stratiform and cumulus clouds. Aircraft flying through these clouds seed an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft's wings or blockage of its instruments and probes, unless the aircraft are equipped with an appropriate de-icing system. Freezing rain is also caused by supercooled droplets.

The process opposite to supercooling, the melting of a solid above the freezing point, is much more difficult, and a solid will almost always melt at the same temperature for a given pressure. For this reason, it is the melting point which is usually identified, using melting point apparatus; even when the subject of a paper is "freezing-point determination", the actual methodology is "the principle of observing the disappearance rather than the formation of ice".^[5] It is, however, possible, at a given pressure to superheat a liquid above its boiling point without it becoming gaseous.

Supercooling is often confused with freezing-point depression. Supercooling is the cooling of a liquid below its freezing point without it becoming solid. Freezing point depression is when a solution can be cooled below the freezing point of the corresponding pure liquid due to the presence of the solute; an example of this is the freezing point depression that occurs when sodium chloride is added to pure water.

Constitutional supercooling[link]

Constitutional supercooling – phase diagram, concentration, and temperature

Constitutional supercooling occurs during solidification, is due to compositional changes, and results in cooling a liquid below the freezing point ahead of the solid–liquid interface. When solidifying a liquid, the interface is often unstable, and the velocity of the solid–liquid interface must be small in order to avoid constitutional supercooling.

Supercooled zones are observed when the liquidus temperature gradient at the interface is larger than the temperature gradient.

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or

Failed to parse (Missing texvc executable; please see math/README to configure.): m \left.\frac{\partial C_L}{\partial x}\right|_{x=0} > \frac{\partial T}{\partial x}

The slope of the liquidus phase boundary on the phase diagram is Failed to parse (Missing texvc executable; please see math/README to configure.): m = \partial T_L / \partial C_L

The concentration gradient is related to points, Failed to parse (Missing texvc executable; please see math/README to configure.): C^{LS}

and Failed to parse (Missing texvc executable; please see math/README to configure.): C^{SL}

, on the phase diagram:

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For steady-state growth Failed to parse (Missing texvc executable; please see math/README to configure.): C^{SL}=C_0

and the partition function Failed to parse (Missing texvc executable; please see math/README to configure.): k=\frac{C^{SL}}{C^{LS}}
can be assumed to be constant. Therefore the minimum thermal gradient necessary to create a stable solid front is as expressed below.

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{\partial T}{\partial x} < \frac{m C_0 (1-k) v}{kD}

For more information, see the equation (3) of ^[6]

In plants[link]

Some plants are able to supercool the fluid in their cells cytosol and vacuole and thereby survive temperatures down to −40 °C. This is partly achieved through the synthesis of antifreeze proteins that prevent ice nucleation.^[7]

In fish[link]

The osmotic concentration of the body fluids of fish is lower than the osmotic concentration of sea water. Therefore the freezing point of fish can be above the temperature of sea water. The freezing point can be lowered by anti-freeze agents, but there are some fish (within the teleostei infraclass) whose freezing point is higher than the temperature of the surrounding sea water, and therefore the body fluids of these fish are supercooled. These fish must live well below the water surface, because they must not come into contact with ice nuclei (otherwise they would freeze immediately).^[8]

Applications[link]

One commercial application of supercooling is in refrigeration. For example, there are freezers that cool drinks to a supercooled level^[9] so that when it is opened it slushes over. Another example is a product that can supercool the beverage in a conventional freezer.^[10] The Coca-Cola Company also briefly marketed special vending machines containing Sprite in the UK, and Coke in Singapore, which stored the bottles in a supercooled state so that their content would turn to slush upon opening.

See also[link]

• Amorphous solid
• Pumpable ice technology
• Subcooling
• Viscous liquid

References[link]

Further reading[link]

• Debenedetti, P. G.; Stanley, H. E. (2003). "Supercooled and Glassy Water". Physics Today 56 (6): 40–46. Bibcode 2003PhT....56f..40D. DOI:10.1063/1.1595053. http://polymer.bu.edu/hes/articles/ds03.pdf. 
• Giovambattista, N.; Angell, C. A.; Sciortino, F.; Stanley, H. E. (July 2004). "Glass-Transition Temperature of Water: A Simulation Study". Physical Review Letters 93 (4): 047801. arXiv:cond-mat/0403133. Bibcode 2004PhRvL..93d7801G. DOI:10.1103/PhysRevLett.93.047801. PMID 15323794. http://polymer.bu.edu/hes/articles/gass04.pdf. 
• Rogerson, M. A.; Cardoso, S. S. S. (April 2004). "Solidification in heat packs: III. Metallic trigger". AIChE Journal 49 (2): 522–529. DOI:10.1002/aic.690490222. http://www3.interscience.wiley.com/cgi-bin/abstract/108065684/ABSTRACT. 

External links[link]

• Video example
• Video example #2
• Video example #3
• Supercooled liquids on arxiv.org

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