17.4 CLOSELY PACKED SYSTEMS
Several types of food systems may be considered as highly concentrated
dispersions of soft solid particles that touch each other; the volume fraction
of the particles generally exceeds 0.8. To a first approximation, the modulus
of such a system would equal the modulus of the particles; in practice, it will
often be substantially smaller.
Attractive forces may act between the particles, or the continuous
phase may be a weak gel, but this is not needed to obtain a solid consistency.
Consider a vacuum-sealed pack of ground coffee. Although the packing
material is flexible, the pack behaves as a hard solid. This is because the
particles are solid and are pressed together by atmospheric pressure and
cannot move relative to each other. Punching a hole in the pack leads to
influx of air and the contents change into a free flowing powder. It thus is
thevolume restrictionthat makes a closely packed system a (soft) solid.
In this section we will describe some closely packed systems and give
some relations for rheological properties in ideal cases. Some of the
structures occurring are illustrated in Figure 17.26.
Foams and Emulsions. Most polyhedral foams are soft solids, as
are some emulsions, like mayonnaise. The structure is illustrated in Figure
17.26a. The value of the volume fraction must be above the critical value for
close packing,jcr. For dispersions of perfect spheres that are not excessively
polydisperse,jcrtends to be about 0.71.
In Section 11.2.1 it is discussed how a foam is formed; see especially
Figure 11.1. Due to gravity the bubbles start to deform each other, and
FIGURE17.26 Structure of various closely packed systems. (a) Polyhedral foam or
emulsion. (b) Concentrated potato starch gel. (c) Concentrated dispersion of
anisometric gel particles (‘‘fluid gel’’). (d) Dispersion of fragments of a particle gel
(different fragments differ in grayness). Highly schematic. The scales can vary
considerably.