Nonlinear rheology of a concentrated spherical silica suspension: |
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Authors: | Dr Hiroshi Watanabe Ming-Long Yao Kunihiro Osaki Toshiyuki Shikata Hirokazu Niwa Yotaro Morishima |
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Institution: | (1) Institute for Chemical Research Kyoto University, Uji, 611 rKyoto-fu, Japan;(2) Rheometric Scientific, F.E., 2-19-6 Yanagibashi Taito-ku, 111 Tokyo, Japan;(3) Department of Macromolecular Science, Osaka University, Toyonaka, 560 Osaka, Japan |
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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
B
and the hydrodynamic stress
H
After start-up of flow at various shear rates
, the viscosity growth function + (t,
) was measured with time t until it reached the steady state. The viscosity decay function – (t,
) was measured after cessation of flow from the steady as well as transient states. At low
where the steady state viscosity (
) exhibited the shear-thinning, the – (t,
) and + (t,
) data were quantitatively described with a BKZ constitutive equation utilizing data for nonlinear relaxation moduli G (t, ). This result enabled us to attribute the thinning behavior to the decrease of the Brownian contribution
B
=
B
/
(considered in the BKZ equation through damping of G (t, )). On the other hand, at high
where (
) exhibited the thickening, the BKZ prediction largely deviated from the + (t,
) and + (t,
) data, the latter obtained after cessation of steady flow. This result suggested that the thickening was due to an enhancement of the hydrodynamic contribution
H
=
H
/
(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,
H
was hardly enhanced and the – (t,
) data agreed with the BKZ prediction. Correspondingly, the onset of thickening of + (t,
) was characterized with a
-insensitive strain ( 0.2). On the basis of these results, the enhancement of
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. |
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Keywords: | Suspension Brownian motion hydrodynamic interaction shear-thinning shear thickening dynamic clustering BKZ constitutive equation |
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