Unterkühlungen

Unterkühlung, also known as supercooling, is a phenomenon in physics and chemistry where a substance is cooled below its freezing point without becoming solid. This state is metastable, meaning it is not the most stable state under the given conditions. The liquid remains in its liquid form even though the temperature is below its normal freezing point. This is possible because the formation of a solid crystal requires nucleation sites, which are imperfections or impurities in the liquid. If a liquid is free of such sites, or if nucleation is inhibited, it can remain liquid below its freezing point.

The process of unterkühlung occurs when a liquid is cooled slowly and undisturbed, preventing the formation of initial crystal structures. If a nucleation site is introduced, such as a small crystal, a sharp tap, or even a dust particle, the liquid can rapidly solidify. This rapid solidification is an exothermic process, releasing latent heat of fusion and potentially causing the temperature to rise back to the freezing point.

Examples of unterkühlung can be observed in everyday life. Water can be unterkühlt to a few degrees below 0°C (32°F). Alcoholic beverages, especially those with high purity, are also prone to this phenomenon. In industrial applications, unterkühlung is relevant in processes like ice production and the handling of cryogenic liquids. Understanding and controlling unterkühlung is important in various scientific and engineering fields.