Superkülmumine

Superkülmumine is a physical phenomenon in which a substance is cooled below its normal freezing point without forming a solid phase. In practice, it describes a metastable state in which liquids remain liquid despite temperatures at which crystallization would be thermodynamically favored. The degree of superkülmumine is often expressed as the amount of supercooling: the difference between the actual temperature and the equilibrium freezing point.

The effect occurs most readily in very pure liquids with few nucleation sites. Cooling rate, container surfaces, and lack of impurities or disturbances reduce the likelihood of crystallization, allowing the liquid to stay unfrozen to lower temperatures. Crystallization begins when a nucleation event occurs or when the liquid comes into contact with a suitable surface; the latent heat released during freezing can cause rapid, localized solidification and sometimes propagate through the liquid.

Common examples include distilled water cooled in clean conditions, which can remain liquid well below 0 °C, and supercooled cloud droplets in the atmosphere that remain liquid until they encounter ice nuclei or surfaces, at which point freezing can trigger rapidly. In laboratory and industrial contexts, controlled superkülmumine is relevant for cryopreservation, chemical processing, and material science, where avoiding or delaying ice formation can be advantageous. Conversely, unintended nucleation can lead to abrupt freezing, which may damage biological samples or equipment.

Limitations and considerations include the presence of impurities, rough container walls, vibrations, or mechanical agitation, all of which can provide nucleation sites and reduce the degree of supercooling achievable. Related concepts include nucleation, metastable liquids, and the glass transition in some liquids.

See also: supercooling, nucleation, metastable phase, cryopreservation.