Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 43

system is a gel, or simply a very viscous fluid. Finally, oscillatory measurements can

also be made in tension/compression, leading to alternative parameters, such as the
storage (E 0 )andloss(E 00 ) Young

s moduli, with all other corresponding relationships
still holding. For gels and networks, these measurements are relatively uncommon,
but they do have the major advantage that this geometry helps to reduce slip, often a
real problem with low-concentration ionic gels of polysaccharides ( Chapter 5 ).
Although shear mode oscillation is more frequently used, it sometimes suffers from
slippage. The longitudinal vibration method is free from slippage. It can be used only
for self-supporting gels for which the strain from its own weight is almost negligible.
When the gel is surrounded by oil, the temperature dependence of the complex
Young

'

s modulus can be easily determined. We do not discuss these further here,
although a number of such measurements are discussed in Chapter 5 and other
chapters.

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2.5.1.2 Controlled strain versus controlled stress

Nowadays the majority of modern instruments are of the controlled-stress type.
However, they usually still generate results in controlled-strain form, that is, as the
modulus components G 0 and G 00 . Strictly speaking, since stress is applied and the
strain is measured, then results should be reported as the components of the complex
compliance J 0 and J 00 . However, most instruments circumvent this by applying a stress
and measuring the strain, but in a servo or feedback mode, so that it appears that they
are indeed controlling the strain. For many applications and systems this is accept-
able, but for systems very close to gelation, it is certainly not ideal. This is because
there is no sure way of controlling the feedback when the system just changes from
solution (sol) to gel, and yet at the same time guaranteeing that the strain remains very
low. For such systems there is a further advantage in a genuine controlled-strain
technique, in that the mechanical driving head and the measurement transducer are
completely separate assemblies - the only link between them is the test sample and

geometry.
At this point it is important to point out the potential (and real) problems that can be
caused by slippage of the sample, particularly for those where syneresis ( of the

gel, i.e. loss of solvent at the gel surface) occurs. Much of the data collected for such
materials is unfortunately

' weeping '

flawed by slippage at the geometry surface. For example,
certain carrageenan gels ( Chapter 5 ) are particularly susceptible, as was shown convinc-
ingly by Richardson and Goycoolea ( 1994 ). They assembled a special punctured cylin-
drical fixture designed to eliminate slippage, and compared measurements made with this

and with a standard concentric cylinder. Results for gelatin (not susceptible to slip) were
identical; for the carrageenan samples very different results were obtained. The use of
roughened or serrated surfaces for the standard geometries is an alternative solution for
avoiding slippage. The oscillating tension/compression experiment mentioned above
also tends to reduce this problem. Non-standard geometries, such as rotating vane
geometries, have been widely used in complex

fluid rheology such as food formulations
(Barnes and Nguyen, 2001 ), in particular under shearing conditions, allowing the
determination of a yield stress.