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{{redirect|Supercool|the fictional drug from Upright Citizens Brigade|Upright Citizens Brigade#Supercool}}
'''Supercooling''' is the process of chilling a [[liquid]] below its [[melting point|freezing point]], without it becoming [[solid]].

A liquid below its freezing point will [[crystallization process|crystallize]] in the presence of a [[nucleation|seed crystal or nucleus]] around which a [[crystal]] structure can form. However, lacking any such nucleus, the liquid [[phase (matter)|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 temperature|glass transition]] where the system is an amorphous—that is, non-crystalline—solid.

Water has a freezing point of 273.15 K (0 °C or 32 °F) but can be supercooled at standard pressure down to its [[nucleation#Homogeneous_nucleation|crystal homogeneous nucleation]] at almost 231 K (−42 °C).<ref name="debenedetti 42">[[#debenedetti|Debenedetti, p. 42.]]</ref> If 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]]. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108 °C).<ref>[[#giovambattista|Giovambattista, p. 047801-1.]]</ref>
Glassy water can be heated up to approximately 150 K (−123 °C).<ref name="debenedetti 42"/>
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 [[Stratus cloud|stratiform]] and [[cumulus cloud|cumulus]] [[cloud]]s.  They form into [[ice]] when they are struck by the wings of passing [[fixed-wing aircraft|airplane]]s and abruptly crystallize. (This causes problems with lift, so aircraft that are expected to fly in such conditions are equipped with a [[deicing]] 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]]. It is, however, possible to [[superheating|superheat]] a liquid above its [[boiling point]] without it becoming gaseous.

==Constitutional supercooling==

[[Image:Constitutional_supercooling_-_phase_diagram%2C_concentration%2C_and_temperature.png|right]]

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.

:<math>\frac{\partial T_L}{\partial x} |_{x=0} > \frac{\partial T}{\partial x}</math>

or

:<math>m \frac{\partial C_L}{\partial x} |_{x=0} > \frac{\partial T}{\partial x}</math>

The slope of the liquidus phase boundary on the phase diagram is <math>m = \partial T_L / \partial C_L</math>

The concentration gradient is related to points, <math>C^{LS}</math> and <math>C^{SL}</math>, on the phase diagram:

:<math>\frac{\partial C_L}{\partial x} |_{x=0} = - \frac{C^{LS} - C^{SL}}{D/v}</math>

The minimum thermal gradient necessary to create a stable solid front is as expressed below.

:<math>\frac{\partial T}{\partial x} < \frac{m C_0 (1-k) v}{kD}</math>

==See also==
* [[Diamond dust]], an Antarctic optical phenomenon
* [[Flash freezing]]
* [[Nucleation]]
* [[Supersaturation]]
* [[Vitrification]]

==References==
{{Refbegin}}
<ul>
<li id="debenedetti">{{cite journal |first=P. G. |last=Debenedetti |coauthors=Stanley, H. E. |title=Supercooled and Glassy Water |journal=[[Physics Today]] |volume=56 |issue=6 |pages=40&ndash;46 |year=2003 |url=http://polymer.bu.edu/hes/articles/ds03.pdf |format=PDF}}
<li id="giovambattista">{{cite journal |last=Giovambattista |first=N. |coauthors=Angell, C. A.; Sciortino, F.; Stanley, H. E. |title=Glass-Transition Temperature of Water: A Simulation Study |journal=Physical Review Letters |volume=93 |issue=4 |month=July |year=2004 |url=http://polymer.bu.edu/hes/articles/gass04.pdf |format=PDF}}
<li id="rogerson">{{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&ndash;529 |month=April |year=2004 |url=http://www3.interscience.wiley.com/cgi-bin/abstract/108065684/ABSTRACT}}
</ul>
{{Refend}}
{{Reflist}}
<!--[http://www.sfu.ca/physics/ugrad/courses/teaching_resources/demoindex/thermal/th4c/patentheatpack.html Patent No. 4,007,390].-->

==External links==
*[http://f0rked.com/articles/supercooling Supercooled Water] Matt Sparks's blog entry about his experience with a cold garage and supercooling.  Includes videos of supercooled water freezing after being agitated.
*[http://www.youtube.com/watch?v=YuPfsAdEG2E Video example]
*[http://www.youtube.com/watch?v=q_gfIiVJ5Dc Video example #2]
[[Category:Thermodynamic processes]]
[[Category:Condensed matter physics]]
[[Category:Fundamental physics concepts]]
[[Category:Glass engineering and science]]

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