Page 264 - Introduction to Colloid and Surface Chemistry

Rheology 253

If particle aggregation occurs in a colloidal system, then an increase
in the shear rate will tend to break down the aggregates, which will
result, among other things, in a reduction of the amount of solvent
immobilised by the particles, thus lowering the apparent viscosity of
the system.
Shear-thinning is particularly common to systems containing
asymmetric particles. Asymmetric particles disturb the flow lines to a
greater extent when they are randomly orientated at low-velocity
gradients than when they have been aligned at high-velocity
gradients. In addition, particle interaction and solvent immobilisation
are favoured when conditions of random orientation prevail.
The apparent viscosity of a system which thins on shearing is most
susceptible to changes in the shear rate in the intermediate range
where there is a balance between randomness and alignment, and
between aggregation and dispersion.

Plasticity and yield value

Plasticity is similar to shear-thinning, except that the system does not
flow noticeably until the shearing stress exceeds a certain minimum
value. The applied stress corresponding to a small but arbitrarily
chosen rate of deformation is termed the yield value.

Shear-thickening
JB
' * Shear-thinning
Plastic
no ~
Yield value

Shear stress, o

Figure 9.4 Steady-state forms of non-Newtonian flow