Chemistry Reference
In-Depth Information
true rheological material properties from the subjective (empirical and
generally instrument-dependent) material characterisations. Rheometry
can be broadly divided into two categories:
(i)
Steady shear characterisation.
This provides viscosity data of fluid
materials and/or simulates a process shear rate. It provides infor-
mation about the material's response to varying flow rate regimes
by measuring its viscosity, which is usually shear-rate dependent.
(ii)
Dynamic or oscillatory shear.
This provides structural informa-
tion, time dependency and temperature stability/dependency of the
material. It facilitates the distinction between elastic and viscous
contributions to a measured stress as a function of frequency by
measuring storage and loss moduli. These tests can also be non-
destructive to the structure of the samples, as the amount of strain
applied to the sample is very small.
2.3.4
Applied rheology
Applied rheology is concerned with deformation and flow of com-
plex material in geometries of practical interest. Many problems of this
kind occur in chemical and process engineering, such as flow of non-
Newtonian fluids in channels of various geometries, flow through gra-
nular beds, mixing of non-Newtonian fluids and extrusion (Tabilo-
Munizaga and Barbosa-Canovas, 2005).
2.4
STEADY-STATE SHEAR FLOW BEHAVIOUR: VISCOSITY
By definition, rheology is the deformation and flow of matter, and rhe-
ological properties are based on the flow and deformation responses of
a substance when subjected to stress (Barnes, 1999; Rao, 1999). Defor-
mation is the relative displacement of points of a body. This deformation
may be viscous flow, elastic deformation or a combination of the two.
Viscous flow is an irreversible deformation, which means that when
the stress is removed the material does not return to its original form;
i.e. work is converted to heat. Fig. 2.1 shows common types of flow
curves, plotted as shear stress against shear rate. Shear-thinning and
shear-thickening flows are shown by curves and a Newtonian flow by
a straight line. Any yield stress is shown by interception on the stress
axis. The yield stress is the stress which must be exceeded before flow
starts.
The existence of yield stresses is controversial as some authors con-
sider them to be artefacts resulting from high Newtonian viscosity at
low shear rates (De Kee and Chan, 1983; Barnes and Walters, 1985;
