supercooling

Supercooling is the process of cooling a liquid below its normal freezing point while it remains in a liquid state. The resulting condition is metastable: given a sufficient perturbation or a nucleation site, the liquid can rapidly crystallize and freeze.

Achieving supercooling typically requires very clean, smooth containers, minimal impurities, and gradual, uniform cooling to avoid triggering nucleation. In pure water, for example, it is possible to cool below 0°C without ice formation if no seed crystals are present. In practice, small amounts of dust, scratches, or contact with ice can trigger crystallization.

Two nucleation mechanisms are involved. Homogeneous nucleation occurs in the bulk of the liquid when it is cooled far below its freezing point, while heterogeneous nucleation occurs on surfaces or impurities at higher temperatures. The presence and type of nucleation sites strongly influence the likelihood and speed of freezing.

Supercooling has natural and technological relevance. In the atmosphere, supercooled water droplets can exist at temperatures well below freezing and freeze upon contact with ice nuclei, affecting precipitation. In cryopreservation and vitrification, controlled supercooling or rapid transitions to a glassy state help avoid damaging ice crystals. In the food industry, controlling freezing can influence texture and quality.

Forcing freezing can be achieved by introducing a seed crystal, scratching a surface, or altering conditions to promote nucleation. Supercooling is distinct from freezing-point depression, which lowers the equilibrium freezing point due to solutes, whereas supercooling concerns kinetic arrest of crystallization.