Viscosity of Sols before Aggregation Begins

This is of relatively minor importance in the overall view of silica behavior, yet must be considered in interpreting viscosity data obtained during early stages of polymerization. 

When silica first polymerizes to very small discrete particles at low pH, the increase in viscosity can be appreciable because so much water is associated with the surface of the particles. As polymerization progresses with increase in particle size, the viscosity decreases, which may seem surprising. Thereafter the viscosity increases again when the particles begin to aggregate. This initial viscosity of the sol of discrete particles must be taken into account in calculating the development of the gel phase, which has already been discussed. 

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