When the particles cannot get close

When the particles cannot get closer, Stage II begins and no further shrinkage is possible because the solid particles form a rigid skeleton, which cannot be further deformed under the effect of the capillarity forces. The only way to achieve further densification is through modification of the particle shape which is not possible in the case of SiO2 because of the reduced atom mobility in the viscous gel, the low solubility of the material and the low driving force to shape modification in the absence of external pressure. Further drying due to polycondensation might cause additional shrinkage of the gel, which can be accommodated only through the formation of pores [28].