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Supercooling is the process of cooling a liquid or gas below its freezing point without crystallizing. Pure water can be supercooled to -43.6°F, but impurities cause nucleation points and ice crystals to form. Supercooling is used in various applications, including the production of semiconductor nanostructures and hand warmers. It occurs in nature and is used by organisms to survive extreme cold. Pentagonal clusters prevent crystallization, and this effect is intensified in nanowire growth on silicon.
Supercooling is the process by which a liquid or gas is cooled beyond its freezing temperature without crystallizing into a solid. Under normal freezing conditions, such substances would freeze into crystals around a seed nucleus, a process known as heterogeneous nucleation. When there is no seed crystal or nucleus, the substance can remain liquid up to the point of homogeneous nucleation, which occurs at a much lower temperature.
Pure water freezes at 32°F (0°C) but can be supercooled to -43.6°F (-42°C). It must be distilled for subcooling to occur, because the presence of impurities would create nucleation points and allow ice crystals to form. Supercooled water turns to ice or slush very quickly when it encounters a substance on which it can form crystals. It is also possible to subcool water past the point of homogeneous nucleation, in which case it eventually solidifies into a type of glass.
Many different types of substances and solutions can be subcooled, making the process useful in a variety of applications. For example, supercooled metal alloys are used in the production of semiconductor nanostructures. Instant hand warmers, a popular commercial product, produce heat from the rapid crystallization of a supercooled sodium acetate solution.
The supersaturated sodium acetate solution used in hand warmers is created by heating water so that more sodium acetate can be dissolved than normal. This mixture is then supercooled to room temperature, remaining liquid when it would normally crystallize. When the hand warmer is squeezed, the disturbance lowers the energy barrier to crystallization and heat is released as crystals form.
Subcooling can be widely observed in nature. It can happen to water under glaciers, resulting in changes in sediment transport and glacial dynamics. Supercooled water droplets often form in high-altitude clouds and crystallize into ice when they encounter a solid object. This phenomenon is responsible for the formation of ice on aircraft wings.
Many living organisms also use this process. Tree and insect species that live in cold climates rely on it to lower the freezing point of their internal fluids. This produces tolerance to freezing conditions and allows these organisms to survive extremely cold temperatures.
Research has shown that supercooling occurs when the structure of a substance consists of atoms in pentagonal clusters. Pentagons cannot be arranged geometrically in such a way that they completely fill a crystalline space, so crystallization does not occur. The intensified effect of this was demonstrated during studies on the growth of nanowire structures on silicon.
