distances. The physical building blocks of such a system may be called
structural elements, i.e., regions that are bounded by a closed surface, where
at least some of the properties within such a region are different from those
in the rest of the system. Structural elements can be particles, such as air
bubbles, oil droplets, crystals, starch granules, cells, etc. If these particles are
separate from each other, the system is called adispersion.Figure9.2
illustrates a dispersion with various structural elements. Also the continuous
material surrounding the particles in a dispersion is a structural element. A
structural element can be heterogeneous itself, containing further structural
elements. Think of starch granules in a cell of a potato, or of fat crystals in
the oil droplets of an emulsion (Fig. 9.2, E). Structural elements can also be
(nearly) space filling, like parenchyma cells in the soft tissue of a fruit or
myofibrils in a muscle.
In many cases, internal interaction forces act between structural
elements. By internal we mean in this context that the forces have their
origin in the (properties of the) materials making up the structural elements.
This excludes external forces, e.g., caused by gravity, by flow, or by an
electric field. The interaction forces can be attractive or repulsive, and the
FIGURE9.2 A liquid dispersion with various structural elements. (A) Gas bubble.
(B) Emulsion droplets. (C) Polymer molecule. (D) Solid particles (amorphous). (E)
Oil droplet with fat crystals. (F) Crystal. (G) Floc or aggregate of particles. (H)
Fibers. L, Continuous phase. Highly schematic and not to scale (for instance, F is
likely to be orders of magnitude larger than C).