Coalescence. This is caused byrupture of the filmbetween two
emulsion drops or two foam bubbles. The driving force is the decrease in
free energy resulting when the total surface area is decreased, as occurs after
film rupture. The Laplace equation (Section 10.5.1) plays a key role.
Aggregation. The interactions involved are treated in Chapter 12.
It follows that the main cause is oftenvan der Waals attraction(Section
12.2), as given by the Hamaker equations. Another important cause is
depletion interaction, where the driving force is increase in mixing entropy of
polymer molecules or other small species present in solution (Section
12.3.3).
Partial Coalescence. This is a complicated phenomenon. It can
occur in O–W emulsions if part of the oil in the droplets has crystallized.
The ultimate driving force is, again, a decrease in interfacial free energy, but
the relations given by Hamaker, Laplace, and Young (Section 10.6.1) all are
involved.
Sedimentation. This can either be settling, i.e., downward
sedimentation, or creaming, i.e., upward (negative) sedimentation. The
driving force is a difference in buoyancy, or in other words the decrease in
potential energy of the particles that will occur upon sedimentation.
Figure 13.1 also indicates whatkind of particlescan be involved in the
various changes; solid particles are generally not spherical. It is further
shown (by backward arrows) whether the change is readilyreversible, e.g.,
by changing the physicochemical conditions or by mild stirring. The
indicated effects of agitation will be discussed later. It may further be noted
that (a) and (b) involve transport of moleculesthrough the continuous
phase, generally by diffusion. In (d), (e), and (f) theparticlesare transported:
by Brownian motion (d), or because of a field force (d–f). In (c),flow of the
particle fluidoccurs, and this is also involved in (e). Many of the changes in
dispersity can be arrested by immobilization of the particles.
Consequences. The resulting changes in the dispersion can be of
various kinds. In (a–e)particle sizechanges, whereas in (f) the position of
the particles becomes less random, leading todemixing. Changes (a–c) cause
a change in particle size only, whereas (d) and (e) also cause a change in
particle shape. The latter may lead to the formation of large structures,
enclosing part of the continuous phase, and ultimately to a space-filling
network (Section 13.2.3). Changes (a–c) can only cause coarsening of the
dispersion, and the particles will ultimately become visible by eye.