Ylijäähtyminen

Ylijäähtyminen, also known as supercooling, is a phenomenon where a liquid is cooled below its freezing point without becoming solid. This occurs when a liquid is kept sufficiently still and pure, preventing the formation of nucleation sites. Nucleation sites are imperfections or impurities where ice crystals can begin to form. In the absence of these sites, the molecules in the liquid remain in a disordered state even at temperatures below their normal freezing point.

This state is metastable, meaning it is stable under certain conditions but can readily transition to a more stable state, which in this case is solid ice. A slight disturbance, such as shaking the container, introducing a seed crystal, or even a sudden impact, can trigger rapid crystallization. The process is exothermic, meaning it releases heat, and the temperature of the liquid may briefly rise back to its freezing point as it solidifies.

Ylijäähtyminen can occur with various liquids, including water, although it is more commonly observed and dramatic with pure substances. Understanding ylijäähtyminen has applications in meteorology, where supercooled water droplets in clouds can lead to freezing rain when they come into contact with surfaces below freezing. It also has implications in food science and cryopreservation, where controlling the freezing process is crucial.