What is undercooling?
Supercooling, or also known as undercooling, is the state in which a material remains liquid at a temperature below its freezing temperature or solidification point.
This state is often referred to as "metastable", which is a state in which the reactions evolve very slowly, but at the slightest disturbance, they can accelerate extremely rapidly to reach a more stable state.
Why does matter remain in a liquid state?
To be able to experience the phenomenon of supercooling, the liquid must be sufficiently pure. This phenomenon is related to the surface tension of the molecules at the solid-liquid interface.
This can be explained at different levels:
- Initially, the liquid is pure and therefore contains very few impurities. These small particles will, below 0°C (at 1atm), start to freeze. However, due to the thermal agitation, they will immediately be reheated and therefore melt and delay the formation of ice.
- In a second step, if during the formation of ice crystals, the energy released is lower than that used to form the solid-liquid interface, the medium will remain liquid without solidification and therefore without ice formation.
What triggers the transition from liquid to solid state?
The supercooled state is relatively unstable, and a slight change or destabilization can lead to a direct and sudden transition to the solid state. There are several possible disturbances that act as initiators to start the solidification reaction: contact with ice cubes, vibrations, presence of impurities, shock, etc. This reaction often happens "in chain": an impurity arrives which leads to the solidification of one and very quickly, follows with a solidification of the rest of the material.
The example of water supercooling
We learn from a very young age that water freezes at 0°C (at 1atm) and then changes state to become ice. However, water is a concrete case that illustrates the principle of supercooling. There are many videos illustrating this phenomenon. The freezing of water does not start spontaneously but needs a disturbance to initiate. The water can then remain liquid at record temperatures such as down to -40°C by remaining in its liquid state. Then, at the slightest impurity or disturbance, it suddenly turns to ice.
Heaters, a direct application of supercooling
The heaters use this principle with the use of sodium acetate which is supercooled and crystallises following a shock (opening of the pellet) and then releases energy in the form of heat, the reaction is exothermic!