92168.pdf

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