Chemistry Reference
In-Depth Information
Bingham plastic
Shear thinning
Newtonian
Shear thickening
Shear rate
Fig. 2.1
Flow curves (shear stress vs shear rate) for different types of flow behaviour.
Evans, 1992). Barnes and Walters (1985) insisted that the yield stress
is a myth, because 'if a material flows at high stresses it will also flow,
however slowly, at low stresses'. Nonetheless, this viscosity in many
dispersed systems is so high that the material would take years to flow.
Therefore, in practice, yield stress is an engineering reality (Spaans and
Williams, 1995), and this parameter can be quite useful in characterising
materials, within the range of shear rates and time scales encountered
in commercial processes (Papanastasiou, 1987).
2.4.1
Rheological models for shear flow
Modelling offers a means of representing a large quantity of rheological
data in terms of a simple mathematical expression. Flow models are
frequently encountered in the literature. They are useful for the treat-
ment and summarising rheological data. However, none of the available
models could possibly fit the rheological behaviour of a material under
a wide range of conditions, for example, when using a wide range of
shear rates.
2
.
4
.
1
.
1
Time-independent flow models
The flow of most materials is independent of time. For a Newtonian
liquid, viscosity is equal to shear stress divided by shear rate, and it
is independent of both time and shear rate; however, it may vary with
temperature and pressure.
