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clearly seen is that it does not flow between the plates, but slips. The
same figure also shows that the sample slips on both sides of the plates.
As such slip effects can also occur during manufacturing processes,
when a smooth surface such as a pipe is used, it is necessary to charac-
terise these by calculating the slip velocity and correcting the shear rate
(Barnes, 1995). To calculate slip velocity, the size of the used geom-
etry should be changed and obtained results should be extrapolated to
a very large size (see for example Yoshimura and Prud'homme, 1988;
Chakrabandhu and Singh, 2005; Ahuja and Singh, 2009).
Changing the physical or chemical characteristics of the walls, by
physically roughening or profiling the surface, can be a way of eliminat-
ing or minimising slip effects. Another alternative is to use a vane system
or a squeeze flow system (Barnes, 1995); both rheological systems are
described later in this chapter.
2.5
VISCOELASTICITY AND OSCILLATION
The previous section focused on flow curves for non-Newtonian ma-
terials under steady shear conditions. Every food has a unique flow
curve, and these data are critical to a large number of industrial applica-
tions. Clearly, from an engineering point of view, the steady flow curve
is the most valuable way to characterise the rheological behaviour of
foods. Steady shear viscosity is a property of all materials, irrespective
of whether they do or do not demonstrate elastic behaviour. However,
many phenomena cannot be described by the viscosity definition alone,
and elastic behaviour must be taken into consideration (Steffe, 1996).
Elastic flow is reversible; i.e. by removing the stress, the deformed
body recovers its original shape, and the applied work is mostly recov-
ered. However viscoelastic materials, such as dough, cereal extrudates
and cheese, show both flow and elasticity and, in terms of modelling,
viscoelastic materials provide special challenges.
Transient experiments, such as oscillatory and creep tests, generate
data, which can quantify viscoelasticity. In the oscillatory technique, a
sample is subjected to harmonically varying (usually sinusoidal) small-
amplitude deformations in a simple shear field. This is a non-destructive
test, which is able to characterise viscoelastic behaviour. Oscillation can
also be used to determine structural changes occurring in the sample. It
is also a useful tool for measuring rheologically complex samples such
as semi-solids and food pastes (for example, see Ferry, 1980; Tsardaka,
1990; Rao and Steffe, 1992; Steffe, 1996; Reilly, 1997).
These types of experimental testing lead to the simultaneous mea-
surement of numerous viscoelastic properties such as complex viscosity
(
η
∗
), dynamic viscosity (
η
), phase (out-of-phase) angle (
δ
), complex
