9. Rheology
Introduction
Rheology is the science of the deformation and flow of matter, and its
study has contributed much towards clarifying ideas concerning the
nature of colloidal systems. It is a subject of tremendous and
increasing technological importance - in many industries, such as
rubber, plastics, food, paint and textiles, the suitability of the
products involved is to a large extent judged in terms of their
mechanical properties^118. In biology and medicine (particularly
haematology) rheological behaviour is also of major importance^119.
The most straightforward rheological behaviour is exhibited on the
one hand by Newtonian viscous fluids and on the other by Hookean
elastic solids. However, most materials, particularly those of a
colloidal nature, exhibit mechanical behaviour which is intermediate
between these two extremes, with both viscous and elastic character-
istics in evidence. Such materials are termed viscoelastic.
There are two general approaches to rheology, the first being to set
up mathematical expressions which describe rheological phenomena
without undue reference to their causes, and the second, with which
the following discussion is mainly concerned, is to correlate observed
mechanical behaviour with the detailed structure of the material in
question. This is not an easy task. The rheological behaviour of
colloidal dispersions depends mainly on the following factors:
- Viscosity of the dispersion medium.
- Particle concentration.
- Particle size and shape.
- Particle-particle and particle-dispersion medium interactions.
Because of the complications involved, this aspect of rheology is still
in many respects a mainly descriptive science. However, in recent