Supercritical drying bypasses the liquid-gas boundary, avoiding damage to delicate structures caused by capillary stress. Supercritical fluids are used and there are various methods, including a safer low-temperature method using carbon dioxide.
When a substance is dried via the 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 crack or degenerate. To avoid this problem, there is supercritical drying, which dries a substance using high heat and pressure and bypasses the liquid-gas boundary instead of crossing it. The density of liquid and gas is the same, and from a molecular point of view, there is no difference between the two. Supercritical drying can be used with supercritical fluids and there are several drying methods.
The normal drying process involves the use of medium heat or pressure and is fine when applied to substances such as water, which do not break down easily. Some substances or devices — such as microelectromechanical devices that have tiny machinery — experience an imbalance during this drying process because, as the liquid’s surface tension turns into a gas, it pulls against the structure of the substance. In delicate structures, this traction can create problems.
To circumvent this surface tension problem, supercritical drying is a method that bypasses the liquid-gas boundary and does not affect the capillary stress of the substance. Capillary stress is the space between the pores of the substance, and when the liquid becomes a gas by normal means, 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 are also capable of dissolving other substances. The preparation of these fluids involves saturating the pores with an organic solvent.
There are several ways to perform supercritical drying. In the high-pressure, high-temperature method, a pressure chamber is filled with the supercritical fluid and the organic solvent in which the supercritical fluid has been immersed. The substance is then rapidly exposed to heat and pressure exceeding its critical limit, causing the fluid to transform into a gas in which capillary stress is maintained.
While the high-pressure, high-temperature method is the most common way to perform supercritical drying, there is a low-temperature method; this method is safer, because the other may be explosive, and some substances can’t stand the high pressure and heat. Carbon dioxide is used instead of an organic solvent, because it is extracted supercritically at a low temperature. Supercritical drying by this method is not always successful, because some fluids will react with carbon dioxide to create metal carbonates.
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