Environmental Engineering Reference
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Kaolin
ξ=0.8
ξ=0.6
ξ=0.4
ξ=0.2
Zhou's model, equation [18]
equation [21]
100
10
0.2
0.3
0.4
0.5
0.6
0.7
0.8
f t
Figure 1.11. Variation in the bulk yield stress. The variation in the yield stress for a kaolin
suspension is reported as a function of the solid concentration (φ t coincides with the fine
fraction). The thin solid line represents the expectation of a decreasing bulk yield stress with
increasing coarse concentration (see equation [1.1]). The symbols represent the experimental
data obtained by varying the ratio ξ of large and small beads. After Ancey and Jorrot
[ANC 01]
- Adding a small amount of coarse particles leads to a decrease in the bulk yield
stress (for total solid concentrations as high as 0.55).
- Interestingly enough, in contrast with the authors' expectation, the bulk yield
stress starts diverging when the total solid concentration comes closer to the maximum
solid concentration.
- A striking feature of this abrupt rise is that the increase rate is very close to the
value measured for a pure kaolin dispersion. This could mean that coarse particles
surrounded by colloidal particles may very well behave in turn as colloidal particles
(this statement is naturally wrong).
- At low and moderate concentrations of coarse particles, the bulk yield stress
was independent of the particle size (when equal size distributions were tested), but it
increased significantly with increasing relative fractions of large particles.
- On the contrary, at high concentrations, the finer the distribution, the larger is the
yield stress.
The main and unexpected result of this experimental study is that bulk yield stress may
be significantly affected by the concentration of coarse particles, but its features (such
as the growth rate with a solid concentration) are still governed by the fine colloidal
fraction.
When the bulk is made up of a continuous distribution of particle distributions, the
bimodal-suspension approximation is no longer valid. Given substantial experimental
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