unterkühlten

Unterkühlung, also known as supercooling or undercooling, is a phenomenon in thermodynamics where a substance is cooled below its freezing point without becoming solid. This occurs when the substance remains in a liquid state despite being at a temperature below its normal freezing point. For this to happen, nucleation sites, which are surfaces or particles where crystal formation can begin, must be absent or insufficient. The liquid is in a metastable state, meaning it is stable for a period but can transition to a more stable solid state if disturbed.

When an undercooled liquid is disturbed, such as by agitation, the introduction of a seed crystal, or even a significant temperature fluctuation, the nucleation process can be initiated. This often results in rapid and sometimes dramatic solidification, a process known as a phase transition. The release of latent heat during this solidification can cause the temperature of the substance to rise back to its normal freezing point.

Unterkühlung is observed in various contexts, from water in clouds forming ice crystals at temperatures below 0°C to the freezing of biological tissues. Understanding undercooling is important in fields like meteorology, materials science, and cryopreservation, as it influences processes like ice formation, crystallization of materials, and the preservation of biological samples. The degree of undercooling a substance can achieve depends on its purity, the presence of nucleation agents, and the rate of cooling.