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| {{distinguish|Superfluidity|Subcooling}}
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| {{lead too long|date=February 2013}}
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| '''Supercooling''', also known as '''undercooling''',<ref>{{Cite journal | last = Rathz | first = Tom | title = Undercooling | url = http://science.nasa.gov/ssl/msad/dtf/under1.htm | accessdate = 2010-01-12 | archiveurl = http://www.webcitation.org/5mjJauR7S | archivedate = 2010-01-12 | postscript =. | publisher = [[NASA]] | ref = harv}}</ref> is the process of lowering the temperature of a [[liquid]] or a [[gas]] below its [[melting point|freezing point]] without it becoming a [[solid]].
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| A liquid below its standard freezing point will [[crystallization process|crystallize]] in the presence of a [[nucleation|seed crystal or nucleus]] around which a [[crystal]] structure can form creating a solid. However, lacking any such [[seed crystal|nuclei]], the liquid [[phase (matter)|phase]] can be maintained all the way down to the temperature at which [[nucleation#Homogeneous nucleation|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.
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| [[Water]] normally freezes at 273.15 [[Kelvin|K]] (0 °C or 32 °F) however it can also be "supercooled" at [[standard pressure]] down to its [[nucleation#Homogeneous nucleation|crystal homogeneous nucleation]] at almost 224.8 K (−48.3 °C/−55 °F).<ref>{{cite journal|last=Moore|first=Emily|coauthors=Valeria Molinero|title=structural transformation in supercooled water controls the crystallization rate of ice|journal=Nature|date=24 November 2011|volume=479|pages=506–508|doi=10.1038/nature10586|url=http://arxiv.org/abs/1107.1622|accessdate=24November2011|arxiv = 1107.1622 |bibcode = 2011Natur.479..506M }}</ref><ref name="debenedetti 42">{{harvnb|Debenedetti|Stanley|2003|p=42}}</ref> The process of supercooling requires that water be pure and free of [[nucleation]] sites, which can be achieved by processes like [[reverse osmosis]], but the cooling itself does not require any specialised technique. If water is cooled at a rate on the order of 10<sup>6</sup> K/s, the crystal nucleation can be avoided and water becomes a [[Glass#Physical properties|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).<ref>{{harvnb|Giovambattista|Angell|Sciortino|Stanley|2004|p=047801-1}}.</ref>
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| [[Amorphous ice|Glassy water]] can be heated up to approximately 150 K (−123 °C/−189.4 °F).<ref name="debenedetti 42"/>
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| 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.
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| Droplets of supercooled water often exist in [[Stratus cloud|stratiform]] and [[cumulus cloud|cumulus]] [[cloud]]s. [[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.
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| 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".<ref>"[http://jeb.biologists.org/cgi/reprint/26/1/57.pdf A new method of freezing-point determination for small quantities]", J. A. Ramsay, ''[[J. Exp. Biol.]]''.1949; 26</ref> It is, however, possible, at a given pressure to [[superheating|superheat]] a liquid above its [[boiling point]] without it becoming gaseous.
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| 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 [[Solution|solute]]; an example of this is the freezing point depression that occurs when [[sodium chloride]] is added to pure water.
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| == Constitutional supercooling ==
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| [[File:Constitutional supercooling - phase diagram, concentration, and temperature.png|thumb|Constitutional supercooling – phase diagram, concentration, and temperature]]
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| Constitutional supercooling, which 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.
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| Supercooled zones are observed when the [[liquidus]] temperature gradient at the interface is larger than the temperature gradient.
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| : <math>\left.\frac{\partial T_L}{\partial x}\right|_{x=0} > \frac{\partial T}{\partial x}</math>
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| or
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| : <math>m \left.\frac{\partial C_L}{\partial x}\right|_{x=0} > \frac{\partial T}{\partial x}</math>
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| The slope of the liquidus phase boundary on the phase diagram is <math>m = \partial T_L / \partial C_L</math>
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| The concentration gradient is related to points, <math>C^{LS}</math> and <math>C^{SL}</math>, on the phase diagram:
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| : <math>\left.\frac{\partial C_L}{\partial x}\right|_{x=0} = - \frac{C^{LS} - C^{SL}}{D/v}</math>
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| For steady-state growth <math>C^{SL}=C_0</math> and the partition function <math>k=\frac{C^{SL}}{C^{LS}}</math> can be assumed to be constant. Therefore the minimum thermal gradient necessary to create a stable solid front is as expressed below.
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| : <math>\frac{\partial T}{\partial x} < \frac{m C_0 (1-k) v}{kD}</math>
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| For more information, see the equation (3) of <ref>http://chem.wzu.edu.cn/UploadFile/20094315104316.pdf#page=99 page from 99~100</ref>
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| ==In plants==
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| 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 protein]]s that prevent ice nucleation.<ref>http://4e.plantphys.net/article.php?ch=25&id=254 Plant physiology Ice Formation in Higher-Plant Cells</ref>
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| ==In animals==
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| The osmotic concentration of the [[body fluid]]s 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 <!-- which? --> (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).<ref>
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| "Teleost fish have an osmotic concentration in their body fluids of about 300–400 [[Osmole (unit)|mosm]], and this corresponds to a freezing point of about −0.6 to −0.8 °C. Sea water in the polar regions often has a temperature of about −1.8 °C ... Do they have a lower freezing point than ordinary fish, or do they remain supercooled throughout life? The answer is that both possibilities seem to have been realized.", Knut Schmidt-Nielsen, ''Animal Physiology, Adaption and Environment'' (Cambridge University Press, 1975), p.279.
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| "In summer, the surface fish in the Hebron Fjord, Labrador, have no freezing problem. The fish that live deeper, however, where the water is at −1.73 °C, have a freezing point in their body fluids of −1.0 °C and must remain supercooled (Scholander ''et al.'', 1957)." (Ibid., p. 280)
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| </ref>
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| ==Applications==
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| One commercial application of supercooling is in [[refrigeration]]. For example, there are freezers that cool drinks to a supercooled level<ref>http://www.chillchamber.hussmann.com</ref> so that when they are opened, they form a slush. Another example is a product that can supercool the beverage in a conventional freezer.<ref>http://www.slush-it.com</ref> [[The Coca-Cola Company]] also briefly marketed special vending machines containing [[Sprite (soft drink)|Sprite]] in the UK, and Coke in Singapore, which stored the bottles in a supercooled state so that their content would turn to [[Slush (beverage)|slush]] upon opening.<ref>{{cite web|url=http://www.wired.com/gadgetlab/2007/09/coca-cola-plans/|title=Coca Cola Plans High Tech, Super Cool Sprite|author=Charlie Sorrel|work=[[Wired (magazine)|Wired]]|publisher=[[Condé Nast]]|date=2007-09-21|accessdate=2013-12-05}}</ref>
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| ==See also==
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| *[[Amorphous solid]]
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| *[[Pumpable ice technology]]
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| *[[Subcooling]]
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| *[[Ultracold atom]]
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| *[[Viscous liquid]]
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| *[[Freezing rain]]
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| ==References==
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| {{Reflist}}
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| ==Further reading==
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| *{{cite journal |first=P. G. |last=Debenedetti | last2 = Stanley | first2 = H. E. |title=Supercooled and Glassy Water |journal=[[Physics Today]] |volume=56 |issue=6 |pages=40–46 |year=2003 |url=http://polymer.bu.edu/hes/articles/ds03.pdf |doi=10.1063/1.1595053 |ref=harv|bibcode = 2003PhT....56f..40D }}
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| *{{cite journal |last=Giovambattista |first=N. | last2 = Angell | first2 = C. A. | last3 = Sciortino | first3 = F. | last4 = Stanley | first4 = H. E. |title=Glass-Transition Temperature of Water: A Simulation Study |journal=Physical Review Letters |volume=93 |issue=4 |date=July 2004 |url=http://polymer.bu.edu/hes/articles/gass04.pdf |doi=10.1103/PhysRevLett.93.047801 |pages=047801 |pmid=15323794 |ref=harv | bibcode=2004PhRvL..93d7801G|arxiv = cond-mat/0403133 }}
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| *{{cite journal |last=Rogerson |first=M. A. |coauthors=Cardoso, S. S. S. |title=Solidification in heat packs: III. Metallic trigger |journal=AIChE Journal |volume=49 |issue=2 |pages=522–529 |date=April 2004 |url=http://www3.interscience.wiley.com/cgi-bin/abstract/108065684/ABSTRACT |doi=10.1002/aic.690490222 |ref=harv}}
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| ==External links==
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| *[http://www.youtube.com/watch?v=YuPfsAdEG2E Video example]
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| *[http://www.youtube.com/watch?v=q_gfIiVJ5Dc Video example #2]
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| *[http://www.youtube.com/watch?v=pTdiTe3x0Bo Video example #3]
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| *[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=1&index1=130463 Supercooled liquids on arxiv.org]
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| {{Phase of matter}}
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| {{Glass science}}
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| [[Category:Thermodynamic processes]]
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| [[Category:Condensed matter physics]]
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| [[Category:Concepts in physics]]
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| [[Category:Glass physics]]
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