Supercooling

Supercooling 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. The homogeneous nucleation can occur above the glass transition where the system is an amorphous (non-crystalline) solid.

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 231 K (−42 °C).^[1] If 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).^[2] Glassy water can be heated up to approximately 150 K (−123 °C).^[1] 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. (This causes problems with lift, so aircraft that are expected to fly in such conditions are equipped with a de-icing system.) Freezing rain is also caused by supercooled droplets.

The process opposite to supercooling, the melting of a solid below 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".^[3] It is, however, possible, at a given pressure to superheat a liquid above its boiling point without it becoming gaseous.

Constitutional supercooling

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.

or

The slope of the liquidus phase boundary on the phase diagram is

The concentration gradient is related to points, C^LS and C^SL, on the phase diagram:

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

In plants

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. ^[4]

Commercialization of supercooling

• Recently, several companies have started selling products that use supercooling principles. Supercooled Sprite from Coca-Cola was briefly marketed in the UK^[5].

• Other uses of supercooling include commercial hand warmers. The hand warmers contain Sodium Acetate in a supercooled state in a sealed plastic pouch. There is a disk in the pouch that is clicked to stimulate the liquid to turn into a solid. The freezing temperature of the liquid is higher then room temperature, so when the liquid turns into a solid, it releases enough energy to raise its temperature towards its freezing point, which heats the hands. To supercool the liquid again, it must first be heated to the melting temperature again and allowed to cool.

See also

• Amorphous solid
• Slush (beverage)

References

• supercooled liquids on arxiv.org

External links

• Video example
• Video example #2