At their freezing points, liquids change into solids, but not if they are supercooled. Supercooling is the process of reducing the temperature of a liquid below its freezing point. When a liquid freezes to a solid, it forms a crystalline matrix, but that matrix needs a seed to start growing. In the absence of a seed, the liquid becomes supercooled. The properties of supercooled liquids make them useful for several applications.
Supercooling in Food Science
Supercooling has applications in improving the taste and texture of frozen foods. Freezing is a common way to preserve food, but the ice crystals that form in the cells of fruits, vegetables and meat burst the cells and change the texture of the food once it has thawed. Using pulsed electric and magnetic fields, it is possible to cool stored food below the freezing point, but keep the water from crystallizing and creating tissue damage. This technology is still in development at the time of publication, but it has the potential to produce frozen food that tastes more like fresh food.
Using Supercooling to Stay Warm
You can keep one supercooled liquid in your pocket. Reusable hand-warmers employ supercooling to store heat for later use. The hand-warmers are sealed in heat-safe plastic bags filled with a solution of sodium acetate and water. To charge the hand-warmer, you submerse it in boiling water. When you remove it from the water, the sodium acetate solution cools to room temperature, below its normal freezing point, but remains liquid. There is a small metal disk in the bag as well. By bending the disk, you create a seed for crystallization and the sodium acetate solidifies in an exothermic reaction, releasing heat.
When you think of liquids, you might not consider metals, but metals have a freezing point, too; it is just usually at very high temperatures, and is often called the melting point. It is possible to supercool metals hundreds of degrees below their freezing point, a property that is valuable in manufacturing semiconductors. Semiconductors share properties with electric conductors and insulators and are used in many consumer electronics. Supercooling alloys, combinations of metals, to create semiconductors creates a more effective crystalline structure.
Supercooling in Nature
Plants and animals use supercooling in frigid conditions to keep their body tissues from being damaged by ice crystal formation. Some species of insects have eliminated points in their physiology that could act as seeds for ice crystals. By eliminating points where ice may form and holding perfectly still, they avoid freezing during hibernation. Some species of plants also allow their tissues to supercool, although supercooled buds can dehydrate, losing their protection, so supercooling is generally combined with other strategies to avoid freezing and ice damage.
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