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When a substance is dried via normal methods of applying heat and pressure at a finite rate, the substance passes through the liquid-gas barrier, where the amount of capillary stress changes, causing the substance to deflate. This drying process affects the overall surface tension of the substance, causing delicate structures to break or degenerate. To avoid this problem, there is supercritical drying, which dries a substance via high heat and pressure, and goes around the liquid-gas boundary instead of moving through it. The density of the liquid and gas are the same and, molecularly, there is no difference between the two. Supercritical drying can be used with supercritical fluids, and there are several different drying methods.
The normal drying process involves using medium heat or pressure and is fine when applied to substances such as water, which are not easily broken. Some substances or devices — such as microelectromechanical devices that have tiny machinery — experience an imbalance during this drying process because, when the surface tension of the liquid changes to a gas, it pulls against the structure of the substance. In delicate structures, this pulling can create problems.
To get around this surface-tension issue, supercritical drying is one method that skirts the liquid-gas boundary and does not affect the substance’s capillary stress. Capillary stress is the space between the substance’s pores and, when the liquid becomes a gas by normal means, the capillary stress causes the substance to collapse. To do this, a supercritical fluid is required. These fluids look like liquids but are able to expand and compress like gases; they also are able to dissolve other substances. Preparing these fluids involves saturating the pores with an organic solvent.
There are several ways of performing supercritical drying. In the high-pressure and high-temperature method, a pressure chamber is filled with the supercritical fluid and the organic solvent in which the supercritical fluid was immersed. The substance is then quickly exposed to heat and pressure that goes beyond its critical limit, causing the fluid to change into a gas where capillary stress is maintained.
While the high-pressure and high-temperature method is the most common way of performing supercritical drying, there is a low-temperature method; this method is safer, because the other one can be explosive, and some substances cannot handle the high pressure and heat. Instead of an organic solvent, carbon dioxide is used, because it is supercritically extracted at a low temperature. Supercritical drying with this method is not always successful, because some fluids will react with the carbon dioxide to create metal carbonates.
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