Underafkøling

Underafkøling, also known as supercooling, is a phenomenon where a liquid is cooled below its freezing point without becoming solid. This state is metastable, meaning it is stable under normal conditions but can transition to a solid state if disturbed.

The process of underafkøling occurs when a liquid does not have sufficient nucleation sites. Nucleation sites are surfaces or particles that facilitate the formation of crystals. In pure liquids, these sites are often absent, preventing the liquid from freezing at its normal freezing point. Instead, it remains in a liquid state even at temperatures below freezing.

When underafcooled liquid is disturbed, such as by shaking, stirring, or introducing a nucleation site, the process of crystallization can be triggered. The liquid rapidly solidifies, releasing latent heat of fusion and returning to its equilibrium freezing point. This rapid solidification can sometimes be quite dramatic.

Common examples of underafkøling can be observed with pure water. If carefully handled, water can be cooled to temperatures below 0°C and remain liquid. Another common occurrence is with beverages purchased from a freezer that are not yet solid but are extremely cold. If handled gently, these can be poured into a glass and then rapidly solidify upon pouring.

Understanding underafkøling is important in various fields, including meteorology, where it plays a role in the formation of hail and ice crystals in clouds, and in food science, for techniques like flash freezing.