Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 310

filler effects of oil droplets on the viscoelastic properties of emulsion gels by small-
deformation mechanical measurements. They found that gels made from a

fine emul-

sion (containing smaller oil droplets) exhibited higher G 0

and G" values than the

corresponding gels from a coarse emulsion.
Work on this topic has recently been reviewed by Dickinson himself (Dickinson,
2012 ). His work is noteworthy since it combines experimental and simulation work. For
model systems of variable oil content and containing various proteins, almost all of
which are food-relevant, he describes general trends of behaviour at small and large
deformations, much of which is consistent with the work described above. Experimental
measures such as G 0 and

are considered in relation to reinforcement
effects for both active (reinforcing) and inactive

' strain to fracture '

filler particles. The former behave quite

similarly to the glass- lled systems above, while the latter behave almost like voids.

Brownian dynamics simulations ( Chapter 2 ) of aggregated particle networks containing
bonded and non-bonded particles are also described.
Of course if a polymeric viscous solution is prepared and then filled with particulates to

a high enough degree, it will tend to develop an apparent yield stress, i.e. no Newtonian
low shear rate plateau, an indication of solid-like behaviour, and so, by some de

nitions,

it becomes a filled gel. Work by Rayment and co-workers (Rayment et al., 1995 , 2000 )

examined viscous solutions of guar galactomannan (see below) filled with non-

gelatinized (small polyhedral shaped) rice starch and rod-like MCC particles. The
apparent yield stress could be calculated by simply adding a stress term to the usual
Cross equation (Cross, 1965 ) for

fluids showing a Newtonian plateau. ϕ m for the rice

starch was found to be around 0.46, signi cantly below the value for spheres, re ecting

the polydispersity of size and shape; the MCC rods gave ϕ m ~ 0.28.

Work by Kitano et al.( 1981 ) has shown that there is a good (negative) linear
correlation between the average aspect (length to diameter) ratio of rods and ϕ m .Of

course the area of reinforced polymer melts is a huge one, and outside the scope of the
present volume. Nevertheless its importance in the area of modern structural materials
remains paramount.

10.6 Gels involving molecular ( ' synergistic ' ) interactions

Perhaps driven by the interests of the food and pharmaceutical industries, many of the
systems involved in published mixed-gel work, particularly the so-called synergistic
gels, involve mixtures of speci c plant or microbial polysaccharides. This has been a

very active field for researchers, with a large number of publications. For this reason we

devote the rest of this chapter to these systems. We also introduce some polymers
appearing for the

first time in this chapter.

10.6.1 Polysaccharide systems

Some of these, including gellan, the carrageenans, xanthan and the galactomannans, have
already been introduced ( Chapters 5 and 8 ). However, several other non-gelling plant