Nonlinear rheology of a concentrated spherical silica suspension:

Abstract

Time-dependent nonlinear flow behavior was investigated for a model hard-sphere suspension, a 50 wt% suspension of spherical silica particles (radius = 40 nm; effective volume fraction = 0.53) in a 2.27/1 (wt/wt) ethylene glycol/glycerol mixture. The suspension had two stress components, the Brownian stress σ^B and the hydrodynamic stress σ^H After start-up of flow at various shear rates $\dot \gamma$, the viscosity growth function η₊ (t, $\dot \gamma$) was measured with time t until it reached the steady state. The viscosity decay function η₋ (t, $\dot \gamma$) was measured after cessation of flow from the steady as well as transient states. At low $\dot \gamma$ where the steady state viscosity η ($\dot \gamma$) exhibited the shear-thinning, the η₋ (t, $\dot \gamma$) and η⁺ (t, $\dot \gamma$) data were quantitatively described with a BKZ constitutive equation utilizing data for nonlinear relaxation moduli G (t, γ). This result enabled us to attribute the thinning behavior to the decrease of the Brownian contribution η^B = σ^B/$\dot \gamma$ (considered in the BKZ equation through damping of G (t, γ)). On the other hand, at high $\dot \gamma$ where η ($\dot \gamma$) exhibited the thickening, the BKZ prediction largely deviated from the η₊ (t, $\dot \gamma$) and η₊ (t, $\dot \gamma$) data, the latter obtained after cessation of steady flow. This result suggested that the thickening was due to an enhancement of the hydrodynamic contribution η^H = σ^H/$\dot \gamma$ (not considered in the BKZ equation). However, when the flow was stopped at the transient state and only a small strain (<0.2) was applied, η^H was hardly enhanced and the η₋ (t, $\dot \gamma$) data agreed with the BKZ prediction. Correspondingly, the onset of thickening of η₊ (t, $\dot \gamma$) was characterized with a $\dot \gamma$-insensitive strain (≌ 0.2). On the basis of these results, the enhancement of η^H (thickening mechanism) was related to dynamic clustering of the particles that took place only when the strain applied through the fast flow was larger than a characteristic strain necessary for close approach/collision of the particles.