Chemistry Reference
In-Depth Information
shear rate /s
-1
Figure 6.4
A schematic of a thixotropic loop.
So, at long experimental times the loop disappears and equilibrium has been
achieved. The use of nonlinear viscoelastic models has much to recommend it in
that, as in the linear theory, they can be used to predict experimental responses.
It provides justification for more empirical models of a seemingly arbitrary
nature. However, they often contain a wealth of parameters that are dicult to
predict but can be obtained experimental fitting.
6.2.3 Yield Stress Sedimentation, and Linearity
The nonlinear response of plastic materials is more challenging in many respects
than pseudoplastic materials. While some yield phenomena, such as that seen in
clay dispersions of montmorilinite, can be catastrophic in nature and recover
very rapidly, others such as polymer/particle blends can yield slowly. Not all
clay structures catastrophically thin. Clay platelets forming an elastic structure
can be deformed by a finite strain such that they align with the deforming field.
When the strain is released they do not necessarily recover to the initial
structure, the microstructure of the material becoming ''permanently'' changed.
As a consequence their yield response and the nature of the viscoelasticity
changes. There is significant hysteresis. Modelling these changes with a phe-
nomenological model has to account for what is effectively a rheologically
induced phase change. Distinguishing between plastic and pseudo-
plastic samples can provide the rheologist with a severe challenge. One area
where this is of great value is in the control of sedimentation in colloidal
systems. If we consider a dispersion of particles denser than the surrounding
medium, the particles will experience a force due to the action of gravity. For an
isolated particle the sedimentation velocity is given by balancing the frictional
drag of the solvent with the gravitational force. For spherical particles of radius
a in a solvent with a viscosity Z
o
we obtain
Dr
4
3
pa
3
g
¼
v
0
6pZ
o
a
ð
6
:
26
Þ
