254 Rheology
Many colloidal systems exhibit so-called Bingham flow, which is
characterised by the equationa - o-B = rjD (9.7)Plasticity is due to a continuous structural network which imparts
rigidity to the sample and which must be broken before flow can
occur. It is often difficult to distinguish between plastic and ordinary
shear-thinning behaviour. Modelling clay, drilling muds and certain
pigment dispersions are examples of plastic dispersions. Suspensions
of carbon black in hydrocarbon oil often acquire a yield value on
standing and become conducting, owing to the contact between the
carbon particles which is developed throughout the system^217 ,
Shearing reduces this conductivity, and the addition of peptising
agents reduces both the conductivity and the yield value.Shear-thickeningShear-thickening is characterised by an increase in apparent viscosity
with increasing rate of deformation.
Shear-thickening is shown in particular, as a dilatant effect, by
pastes of densely packed peptised particles in which there is only
snfficient liquid to fill the voids. As the shear rate is increased, this
dense packing must be broken down to permit the particles to flow
past one another. The resulting expansion leaves insufficient liquid to
fill the voids and is opposed by surface tension forces. This explains
why wet sand apparently becomes dry and firm when walked upon.
Time-dependent phenomena
ThixotropyThixotropy is the time-dependent analogue of shear-thinning and
plastic behaviour, and arises from somewhat similar causes. If a
thixotropic system is allowed to stand and is then sheared at
a constant rate, the apparent viscosity decreases with time until a
balance between structural breakdown and structure re-formation is
reached. If the sheared system is then allowed to stand, it eventually
regains its original structure. A thixotropic hysteresis loop (Figure