Rheological properties of fumed silica suspensions in the presence of potassium chloride

Changes in rheological properties of fumed silica particles dispersed in aqueous KOH solution at pH 11 by the addition of KCl have been studied as functions of the silica and KCl concentrations. Since the fumed silica particles are stable in the state of aggregates of the primary particles, coagulation of such aggregates by addition of KCl should cause a gel. Thus, the resulting gel can be regarded as the weak-link gel according to the fractal gel model. States of the fumed silica suspensions in the presence of KCl were roughly classified into four phases, such as sol (S), pre-gelled (PG), gelled (G), and two-phase separated (TP) silica suspension from visual observation. The PG and G silica suspensions were conducted by strain and frequency sweep measurements of dynamic moduli, whereas the S silica suspensions were mainly examined by steady-state viscosity measurements. The critical strains obtained from the strain sweep measurements of storage modulus G' were almost independent of the silica volume fraction phi at lower KCl concentrations than 0.4 M, whereas they increased with the positive power-law of phi at higher KCl concentrations than 0.5 M. The positive power-law exponent is in agreement with the prediction for the weak-link gel. The PG and G silica suspensions show that the G' values in the linear response regions indicate little frequency dependence. The values of G' at the fixed KCl concentration exhibit a power-law behavior and the resulting power-law exponent increases with an increase in KCl concentration.