At the beginning of the silica sol coating gel, the skeleton is very soft and the capillary tension in the system is also small. As the water evaporates and the gel skeleton shrinks, the mechanical strength of the silicon oxygen bond skeleton gradually increases. When the rigid strength of the silicon oxygen bond in the skeleton makes the skeleton unable to continue to shrink, the structure of the gel is fixed. If the strength of the gel skeleton is not enough to resist the capillary pressure at this time, the gel will break or crack.
In the initial stage of the drying process, the capillary force will cause the capillary pore size to become smaller. As the capillary pore size gradually decreases, the evaporation liquid surface in the capillary gap will continue to move upward. For a period of time, the evaporation surface will stay on the surface of the gel, but when the evaporation of the liquid no longer shrinks the skeleton, the evaporation process begins to penetrate into the system, and the capillary tension will increase rapidly.
This point is called the critical point, and the liquid-gas interface begins to enter the gap of the gel. Theoretically, most of the microcracks of the shell itself are generated at this stage, which is the result of the interaction between the capillary force and the strength of the gel skeleton. Therefore, this critical point is the most critical point that needs to be controlled in the shell making process.