The rheology of charge stabilized silica suspensions

Abstract

The flow properties of aqueous suspensions of silica particles with diameters ranging from 117 to 780 nm were measured as a function of volume fraction, ionic strength, and continuous phase composition. Dense, aqueous suspensions of all sizes displayed similar behavior with a yield stress in the limit of low shear rate, shear thinning as the stress was raised and, above a critical volume fraction, shear thickening. Master flow curves which are weakly dependent on particle size, volume fraction, surface potential, magnitude of the decay length for the electrostatic forces, or particle size distribution are produced when stress is scaled on the suspension’s elastic modulus, G, and shear rate on G/ηc. Here ηc is the continuous phase viscosity. For suspensions which ordered at rest and of sufficiently high volume faction, shear thickening occurred as a discontinuous decrease in shear rate as the stress increased. This behavior was not observed in suspensions altered so that no order occurred at rest. When the continuous phase viscosity was increased by suspending the particles in a glycerin solution, no evidence for a yield stress or ordering were observed. These suspensions thinned with increasing stress to a minimum viscosity where thickening occurred as a smooth transition. The elastic scaling that successfully produced master curves for the aqueous suspension failed for the glycerin suspensions.