First, let's understand vaporization and sublimation. The process of a substance changing from liquid phase to gas phase is called vaporization. For water, if the ambient temperature is greater than 0 degrees and the water vapor partial pressure is less than the saturated vapor pressure of water, then vaporization can proceed smoothly. At present, most drying methods work according to this principle. The process of a substance changing from solid phase to gas phase is called sublimation. If the ambient temperature is less than 0 degrees, water exists in the form of solid ice, and the water vapor partial pressure in the environment is less than the saturated vapor pressure of water on the ice surface, then sublimation can proceed smoothly. If the water vapor partial pressure in the environment is greater than the saturated vapor pressure of water on the ice surface, the water vapor condenses into ice or frost. We call this condensation, which is the opposite process of sublimation.
What is the freeze-drying phenomenon? It is generally believed that after washing clothes, clothes should be dried in a high temperature, ventilated and dry environment. The higher the ambient temperature and the higher the dryness of the air, the faster the clothes will dry. However, in cold areas, people have found that washed clothes can be dried even if they are left in the sun where the ice and snow never melt. In addition, people have also found that unpackaged food placed in cold storage and refrigerator freezers is frozen hard, but the surface is dry. This is the freeze-drying phenomenon.
Next, let's understand the principle of freeze-drying. We said earlier that when the temperature is 0 degrees, water is just at the freezing point. At this time, water is in the three states of liquid, gas and solid. We call this point the triple point of water. When the temperature rises, water appears in the form of vapor and liquid; when the temperature falls, water appears in the form of vapor and solid. When water freezes into ice, we know from previous studies that as long as the water vapor partial pressure in the environment is less than the saturated vapor pressure of ice at the corresponding temperature, the ice will directly change from solid to vapor and undergo a drying process. We call this process sublimation drying, and the phenomenon of ice directly changing from solid to gas is called sublimation. We dry the material in a frozen state. During the drying process, the ice does not melt. The process of ice directly vaporizing from solid to dry is called freeze drying (abbreviated as freeze drying).
