generally has a negligible yield stress. Some rheological properties at a time
scale of about 1 s would be very roughly
Dynamic shear modulus G^0 ¼10–20 kPa
Loss tangent tand¼1–0.5
Apparent viscosity Za¼10 kPa?sThe systems are markedly strain rate thinning, meaning thatZais much
higher at lower strain rates.
If the dough is made of wheat flour, it shows markedstrain hardening,
i.e., an increase in modulus with increasing strain (despite its strainrate
thinning character). The same applies, though to a much lesser extent, to a
dough made of rye flour. The importance of these properties was discussed
in Section 17.1.3.
Fluid Gels. The name seems like a contradiction in terms. What is
meant is a concentrated dispersion of gel particles in a liquid. These systems
can be made in various ways, for instance by making a hot solution of a
suitable polymer, which is then cooled in a scraped-surface heat exchanger.
The cooling causes gel formation and the vigorous stirring breaks it into
fragments. In this way a polydisperse and quite concentrated suspension of
anisometric gel particles is formed; see Figure 17.26c. It turns out that
anisometry of the particles is an essential property.
The effect of the volume fractionjon the modulus of such systems is
illustrated in Figure 17.28. The value ofjcan be varied by diluting with
water or by centrifuging the dispersion and removing the supernatant.
Curve (a) refers to almostperfect monodiperse spheres.Ifjis below the
critical value for close packingjcr, the system will be liquid; the viscosity
tends to go to infinity asjapproachesjcr. At that concentration, the system
obtains a stiffness: a modulus can be measured. A very slight increase inj
gives a large increase in modulus, the increase leveling off at slightly higher
j, where the stress becomes large enough to deform the particles (the
particle modulus was about 40 kPa in this case).
The other curves relate toanisometric polydisperse systems.Atcrel¼1,
the systems do contain some free liquid, since an agar gel tends to show
some syneresis upon fracturing. The main difference with curve (a) concerns
the significant shear modulus at lower concentrations. This is because the
anisometric particles get in each other’s way as soon as the system is being
sheared. (It may be noted that the relation between Young’s modulus and
concentration will probably be different.) It is also seen (curve b) that