Chemistry Reference
In-Depth Information
in fair agreement with the value determined using pair-potential curves. Here, the
Casson model has been used to partially linearise a pseudoplastic system rather
than a system with a true yield stress.
The Casson model and that of Michaels and Bolger form a class of solutions
for describing shear thinning. These two models are particularly attractive in
that they relate to the colloid interaction forces, which can be ascertained
through careful characterisation. They also include rate constants controlling
the breakdown and formation of aggregates in a shear field. Establishing these
terms provides the greatest diculty. Whilst these models lack the rigorous
detail embodied in linear viscoelastic theory they preserve the notion of
structural breakdown and reformation in the shear field. The temporal changes
of the system become increasingly important as the systems become strongly
aggregated. The distortion of the floc structure playing an important role in
energy dissipation mechanisms. The inherent thixotropy and structural irrever-
sibility as interaction strengths increase means that strongly aggregated systems
require a different description of their behaviour.
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6.3.5 Strongly Aggregated and Coagulated Systems
Colloidal particles are essentially attractive, so without the presence of a
repulsive barrier to prevent their close approach they will coagulate. The
interaction between the particles is typically dominated by the van der Waals'
interactions, although strong aggregation can be induced by the attraction of
dissimilar charges. A good example of strong aggregation is that shown by
clays. These are formed from anisometric particles, typically plates with a layer
structure. This means that the edges and faces are chemically distinct and can
have different isoelectric points. Over certain ranges of pH they can possess
different signs to their charges and as a consequence of the mutual electrostatic
attraction as well as the van der Waals' forces, they aggregate. Clays can be
used to throw pots and then it is essential that they posses a yield stress to
enable shapes to be formed correctly. Coagulated systems usually show plastic
flow at moderate concentrations. As coagulation is rapid, fairly open networks
of particles often form. These are space filling and have large sediment volumes.
Establishing the linear viscoelastic regime is often very dicult as typically very
low strains and stresses are required and these can be outside instrumental
ranges. The simplest model for the yield stress is to assume that it arises from a
two-body interaction. The yield stress required to separate the particles is
proportional to the colloid interaction force between the particles divided by
the area of interaction.
s
Y
/
F
a
2
j
2
ð
6
:
56
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Russel et al.
16
have pointed out that this approach is oversimplistic in not
allowing for microstructural changes in the network as the concentration
increases. However, the nature of the aggregate structure is path dependent,
so the way in which the system is prepared is often critical in determining its
