Superkülmutus

Superkülmutus, or supercooling, is a physical phenomenon in which a liquid is cooled below its normal freezing point without forming a solid phase. The state is metastable; it persists only because nucleation of the solid phase has not yet occurred. Superkülmutus depends on high purity, the absence of nucleation sites, and controlled cooling. In practical terms, many liquids can be cooled tens of degrees below their freezing point before crystallization begins; disturbance, impurities, or contact with a seed crystal typically triggers rapid solidification.

Two main pathways are recognized: homogeneous nucleation, where crystal formation emerges spontaneously in the liquid, and heterogeneous nucleation, where impurities or container walls catalyze solidification. The degree of supercooling is defined as the difference between the freezing point and the actual temperature.

Water is a well-known example; in clean, undisturbed conditions, microdroplets can remain liquid below 0°C, with clouds hosting supercooled water droplets down to temperatures around −40°C, freezing suddenly when ice nuclei are encountered. Many organic solvents and cryogenic liquids can also be supercooled under suitable conditions. Applications include the study of crystallization kinetics, cryopreservation research, and certain food and materials processing techniques where avoiding ice formation or controlling crystallization is desirable. However, the metastable state is fragile and long-term stability is limited.

Etymology: in Finnish scientific usage, külmutus means freezing, and superkülmutus denotes this metastable cooling phenomenon. See also: cryogenics, nucleation, phase transition.