- Particle shape. In emulsions the particles tend to be almost
perfectly spherical, but in other food dispersions spherical
particles are the exception rather than the rule. In shear flow
particles rotate (see Figure 5.3). This means that they sweep out a
bigger volume during rotation than does a sphere of the same net
volume. In other words, the effective volume fraction of particles
is increased. This is more strongly so for prolate (cigar-shaped)
ellipsoids or rods, than for oblate (disc-shaped) ellipsoids or
platelets. The effect increases with increasing anisometry. Particles
with an irregular or dented surface also exhibit a difference
between effective and net volume. - Colloidal interaction forces between particles. If particles repel each
other, the viscosity is always increased, except at very smallj; this
means thatZis generally affected but½Zis not. Weak mutual
attraction also tends to enhanceZsomewhat, but if the attraction
is strong enough to cause aggregation of particles, ½Zcan be
greatly enhanced: the aggregates tend to enclose a lot of solvent,
which means that effectivejis greatly increased. - Swelling. Figure 5.3 schematically depicts a coiled polymer
molecule and illustrates that such a ‘‘particle’’ encloses a lot of
solvent (although the enclosed solvent is not completely immobi-
lized: see Section 6.2.2). This is comparable to the entrapment of
solvent in aggregates mentioned above. Protein molecules always
contain some water, i.e., are ‘‘swollen.’’ - Particle size. For anisometric particles, their size has an effect.
Small particles show rotational diffusion, and this is more rapid
for a smaller particle. This affects the average orientation of the
particles, hence the increase in viscosity due to anisometry.
Smaller particles would thus give a higher viscosity. Also Factor 2
can come into play: the smaller the particles at a given value ofj,
the smaller the interparticle distance and the larger the effect of
repulsive interaction can be. On the other hand, attractive forces
tend to have a smaller effect on smaller particles.
The magnitude of most of the effects mentioned also
depends on flow type. In elongational flow, for instance, rodlike
particles may obtain a parallel orientation in the direction of flow,
which tends to decrease the viscosity increase caused by the
particles. In turbulent flow, the relations are more complicated.
Strain Rate Thinning. For a Newtonian liquid,Zis independent of
the magnitude of the stress, or of the velocity gradient, applied. In many
cases, however, this is not the case, and the viscosity depends on the velocity