ISSN:
1435-1528
Keywords:
Suspension
;
Brownian motion
;
hydrodynamic interaction
;
shear-thinning
;
shear thickening
;
dynamic clustering
;
BKZ constitutive equation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00368130
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