Rheological properties of aqueous silica particle suspensions

The rheological properties of aqueous silica suspensions were investigated as a function of volume fraction of particles, particle size, and shear rate. Monodisperse particles of three sizes (0.1, 0.5, and 1 mu m) were synthesized according to the Stober process. In addition two fumed silica samples ( Aerosil 200 and Aerosil 380) were investigated. These particles form stable agglomerates consisting of primary particles in solution. The Stober particle suspensions showed a shear thinning behavior at volume fractions above 0.3. Furthermore, the viscosity increased with decreasing particle size. Depending on the particle size, the relative viscosity of the suspensions started to increase substantially at volume fractions between 0.3 and 0.4. Oscillation measurements confirmed a transition from liquid-like viscoelastic suspensions to solid-like viscoelastic suspensions in this range. The Aerosil suspensions displayed a shear thinning behavior at volume fractions above 0.04. The relative viscosity increased at this concentration as well. The differences between the Stober particle suspensions and Aerosil suspensions were attributed to agglomeration of primary particles in the latter case. However, the relative viscosity of all the suspensions were well fitted to the Krieger and Dougherty model.