Critical point drying
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Critical point drying is a drying technique used to prevent damage to delicate items, such as scientific samples. During the phase change from liquid to gas, the surface tension that occurs between the gas-liquid interface can cause damage. The surface tension is avoided by drying with a solution that is taken from subcritical to supercritical fluid, which avoids a gas-liquid interface because the densities of the gas and liquid are equivalent. Water is not used because its critical point is very high, so the water is displaced with alcohol or acetone. More than one substitution may have to be made because the solution that displaces the water must be miscible with the supercritical solution. After dehydration, the specimen is placed in a subcritical solution. Carbon dioxide is the most frequently used substance. The temperature and pressure are increased until the solution is supercritical and the liquid changes to gas without damaging the sample.
