Everything about Supercooling totally explained
Supercooling is the process of chilling a
liquid below its
freezing point, without it becoming
solid.
A liquid below its freezing point will
crystallize in the presence of a
seed crystal or nucleus around which a
crystal structure can form. However, lacking any such nucleus, the liquid
phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the
glass transition where the system is an amorphous—that is, non-crystalline—solid.
Water has a freezing point of 273.15 K (0 °C or 32 °F) but can be supercooled at standard pressure down to its
crystal homogeneous nucleation at almost 231 K (−42 °C). If cooled at a rate on the order of 10
6 K/s, the crystal nucleation can be avoided and water becomes a
glass. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108 °C).
Glassy water can be heated up to approximately 150 K (−123 °C).
In the range of temperatures between 231 K (−42 °C) and 150 K (−123 °C) experiments find only crystal ice.
Droplets of supercooled water often exist in
stratiform and
cumulus clouds. They form into
ice when they're struck by the wings of passing
airplanes and abruptly crystallize. (This causes problems with lift, so aircraft that are expected to fly in such conditions are equipped with a
deicing system.)
Freezing rain is also caused by supercooled droplets.
The process opposite to supercooling, the melting of a solid above the freezing point, is much more difficult, and a solid will almost always melt at the same
temperature for a given
pressure. It is, however, possible to
superheat a liquid above its
boiling point without it becoming gaseous.
Constitutional supercooling
Constitutional supercooling occurs during solidification, is due to compositional changes, and results in cooling a liquid below the freezing point ahead of the solid-liquid interface. When solidifying a liquid, the interface is often unstable, and the velocity of the solid-liquid interface must be small in order to avoid constitutional supercooling.
Supercooled zones are observed when the liquidus temperature gradient at the interface is larger than the temperature gradient.
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