Introduction - Physical Gels from Biological and Synthetic Polymers - page 5

least because it is both cheap and easy. Slightly more sophisticated methods, which
involve capillary tubes or falling ball methods, have subsequently been proposed.
However, except for quick screening of systems, these approaches are little exploited
nowadays, and the science of rheology - now understood as the study of the mechanical

behaviour of liquids and solids has been exploited instead.

In fact there have been a number of attempts to de

-

ne the nature of a gel in more formal

' terms, including by the late John Ferry ( 1980 ) and more recently by

Burchard and Ross-Murphy ( 1990 ), Almdal et al.( 1993 ) and Nishinari ( 2009 ). All of
these de

rheological '

nitions are in terms of the mechanical (rheological) properties of the materials in
question, and the response of a gel to deformation (for example shear) and time, as
detailed in later chapters.
However, the easiest of these defnitions to explain is that of Ferry, who requires that a
gel is not able to sustain a steady-state

flow. In other words, when it is subjected to a

steady flow-rate experiment, for example stirring at a constant rate, it will tend to fracture

or rupture, as we would expect for a solid, rather than to

flow like a liquid. Unfortunately

this de nition excludes some systems of interest to us, and so appears to be too narrow.

Two later approaches (Burchard and Ross-Murphy, 1990 ; Almdal et al., 1993 ) try to
represent gel characteristics in terms of response of the material over time.
The recent article by Nishinari ( 2009 ) re-addresses the rheological de nition of a gel

and its various arguments and counter-arguments. In the end he cites te Nijenhuis ( 1997 ),
who was obliged to conclude, along the lines of Jordan Lloyd, that ' a gel is a gel, as long

as one cannot prove that it is not a gel ' . Nevertheless he makes the important point that a

gel can be de ned both by its mechanical behaviour and by its structural features.

Succeeding sections will try to maintain this structure

- property linkage.

1.2.2 Multidisciplinary nature of gel studies

The implicit structural complexity in all these de nitions should already have established

to the reader is satisfaction that the study of gels is a highly multidisciplinary activity.

Indeed we can enumerate the disciplines of direct relevance to the study of physical gels
and of their formation, gelation, as follows:

'

* Gels are an aspect of macromolecular science: polymeric chains are needed to create a
network.
* Gels are an aspect of colloid science: aggregation and association of small particles (or
macromolecules such as globular proteins) can lead to physical and colloidal gels.
* Mathematical treatments of networks are needed to describe their topology. The
network itself may become a mathematical abstraction (like self-avoiding walks),
while issues of criticality (gelation and the gel point) are related to other problems in
statistical physics.
* Mechanical behaviour: rubber elasticity is expected for networks from

flexible chains.
When the network includes rod-like chain elements or more rigid connecting elements,
things become more complicated. Although the matrix is a

fluid, we may consider an

analogy with composite materials, where the network is the filler.