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Chapter 2

2.3.2. Emulsion evolution

Due to the high interfacial area and surface free energy, oil and water in
emulsions will separate to form two continuous phases. Thus the properties of the
emulsion (drop size distribution, mean drop size and other properties) will change
with time. The stability of emulsion is characterized by the time-dependent
behavior of its basic parameters.

Emulsions may degrade via a number of different mechanisms. Figure 2.3
schematically illustrates several physical instabilities that lead to phase separation
in emulsions. Sedimentation or creaming takes place when the two liquids exhibit
different densities due to the gravity. Aggregation occurs when droplets stay very
close to one another and form flocs. Coalescence takes place when the thin film
of continuous phase between two drops breaks and they fuse rapidly to form a
single droplet. Ostwald ripening occurs due to the difference of solubility of drops
with different drop size, which does not require the droplets to be close each
other.

In the following discussion, theory on sedimentation and creaming,
aggregation and coalescence is presented. The effect Ofsurfactants and solids on
the emulsions will also be mentioned in next section.

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