We note in passing that the Vrij theory, as modified by introducing the
Gibbs elasticity, is an extension of Gibbs’ explanation of film stability
(Section 10.7).
Some Other Factors. If the modified Vrij theory predicts stability,
the film is indeed nearly always stable. However, some films predicted to be
unstable resist rupture for a very long time. Several mechanisms have been
suggested to explain this, but some of them do not apply, or are quite
uncertain, for the surfactants commonly used in foods.
Surface active polymers, such as proteins, can give very stable films.
The main explanation will be a not very smallgvalue and a strong repulsion
acting at a relatively large distance, but there seem to be other factors
involved. In some cases, a correlation between film stability and the
apparent surface shear viscosityZSSa of A–W or O–W surfactant layers has
been observed, but there are exceptions as well. Molecular size or the
thickness of the surface layers may be involved and probably also the layer
coherence. Film rupture would also need a kind of disruption of the
adsorption layers; presumably, this will readily occur for most surfactants,
but a layer of protein molecules that are somehow cross-linked may resist
disruption.
Finally, the discussion here has been restricted to static films. In
dynamicsituations as during foam formation, where films are periodically
stretched, other factors come into play, as briefly explained in Section
11.2.3.
13.4.2 Emulsions
Figure 13.9 illustrates the two essential steps in coalescence. The droplets
have first to encounter each other—by aggregation or in a sediment layer—
and come close before the film between them can rupture, leading to their
merging into one drop. Nearly always, the first step is much faster than the
second. This means that coalescence tends to be afirst-order process, unlike
the aggregation that often precedes it. Only if the droplets immediately
coalesce upon encountering each other—which may happen if the surfactant
concentration is very small—will the second order aggregation rate be
determinant.
Weber Number. When droplets come close, they will either keep
their spherical shape or be pressed together, with formation of a flat film
between them, as depicted in Figure 13.15a. What will happen is determined
by theratio of the external stressthat forces the drops togetherover the