In the initial stage of gel drying, the free water part (4) is mainly evaporated. After the water evaporates, a meniscus pointing inward is generated on the surface of the gel network mesopores. If the mesopores are approximately regarded as cylinders [3], the water evaporation process begins to gradually penetrate into the material. At this time, the drying enters the deceleration stage. The process is manifested as the migration and vaporization of water in the capillary. The evaporation rate mainly depends on the position of the evaporation surface and the partial pressure of water vapor on the evaporation curved liquid surface Pv. According to Kelvin's law, in a capillary with a radius of r and a wetting angle of θ, the relationship between the partial pressure of vapor on the curved liquid surface Pv and the corresponding free water surface and saturated vapor pressure P0 at the same temperature is:
Pv = P0 exp[-﹙2σ/RTρ﹚×﹙cosθ/r﹚] (1)
Where σ is the surface tension of the liquid, R is the universal gas constant, and ρ is the density. Evaporation rate in this stage:
ΔP=P0 exp[-﹙2σ/RTρ﹚×﹙cosθ/r﹚]－PA （2）
(2) shows that in this drying stage, not only the humidity in the shell making room affects the evaporation rate of the gel, but also the diameter of the capillary tube cannot be ignored. Moreover, when the temperature and humidity in the shell making room are constant, the drying rate of the gel is determined by the size of the capillary radius. The vapor partial pressure Pv on the capillary curved liquid surface in the gel is proportional to the capillary radius r. The smaller the capillary radius r, the lower the vapor partial pressure Pv on the curved liquid surface, the smaller the mass transfer driving force, and the slower the water evaporation rate.
During this process, the size of the capillary pore size is also constantly changing, and the capillary tension is:
P = 2σcosθ /r （3）[3]
The capillary force will cause the capillary pore size to become smaller. As the capillary pore size further decreases, the capillary tension will further increase, causing the particles to further contact, squeeze, and aggregate, and the gel skeleton will shrink.