Influence of pH, high salinity and particle concentration on stability and rheological properties of aqueous suspensions of fumed silica

Multiphase light scattering and transient-dynamic rheometry were used in combination in order to control the stability, rheology and gelation of aqueous Aerosil 200 suspensions with regard to the influence of pH, particle and electrolyte concentrations. Effects of these parameters were studied by obtaining the critical concentration of particles for transition from Newtonian to shear thinning flow in each system. This was attributed to microstructural changes due to the interparticle forces. Further confirmation of results was obtained by measuring apparent viscosities, yield stresses, time dependent transient rheometry and light scattering from the samples. All results were described by extended-DLVO theory. Suspensions with acidic pH or at the isoelectric point had high stability even in presence of high concentration of NaCl due to the short-range repulsive hydration forces. On the other hand, suspensions with the highest salt concentration and basic pH showed time dependant rheology, largest yield values and viscosities, smallest critical particle concentration for flow regime transition and high capability of gel formation. Verification of gelation kinetics and the gel strength with regard to particle and salt concentration were implemented by three typical tests: time sweep, strain amplitude and frequency amplitude sweep in the dynamic oscillation mode at pH 8.5. Strength of the gelled networks was found to be greater at either higher salt concentration or higher concentration of particles. Strong-link regime of gel was found to govern the system regarding to increase either in particle volume fraction or in electrolyte. The scaling theory was tested for the system at 0.6 M NaCl and power-law exponent of the volume fraction dependence of storage modulus was found to be 3.8. In addition, applying the fractal model for strong gels calculated fractal dimension of flocs df around 1.85. Based on the exponent value of 3.78 for G′ and the fractal flocs dimension of 1.85, it was proposed that cluster-diffusion-limited cluster aggregation (DLCA) was mainly governing the gel formation mechanism.