Superkylning

Superkylning, or supercooling, is the process of cooling a liquid below its normal freezing point without it becoming solid. In this metastable state, the liquid remains liquid despite temperatures at which it would normally crystallize, until a disturbance or nucleation event triggers rapid freezing.

The phenomenon arises from the absence of effective nucleation sites or impurities that would typically initiate crystal formation. Liquids with high purity and controlled conditions can be cooled well below their freezing point, but they are susceptible to sudden freezing if shaken, touched, or otherwise disturbed. There are two nucleation pathways: homogeneous nucleation, which occurs spontaneously in a pure liquid, and heterogeneous nucleation, which is facilitated by surfaces or particles. Water is the most well-known example: pure water can be supercooled to around −40 °C, while in atmospheric clouds, supercooled droplets can exist at temperatures as low as −40 °C or lower but remain liquid until freezing is triggered.

Methods to achieve superkylning involve careful control of cooling rates, purification, and minimization of agitation, as well as sometimes the use of stabilizing agents or containers that reduce nucleation catalysts. In practice, superkylning is used or observed in diverse fields, including meteorology (where supercooled water droplets contribute to cloud physics and icing events), food technology (to preserve texture by delaying ice formation), and research on cryopreservation and materials science.

Limitations of superkylning include its inherently unstable nature and the small likelihood that a given droplet or sample will remain supercooled under changing conditions. A slight disturbance can initiate rapid freezing, which may have practical consequences for processes that rely on precise control of phase transitions.