many workers (although hardly ever published), the correlation between the
test result and coalescence in practice tends to be quite poor.
Droplet Size. From the above, it may be clear that, other things
being equal, an emulsion with larger drops will generally be less stable to
coalescence. This is because the film between larger drops will be larger, even
if We<1, and it will be much larger if We>1. Moreover, larger drops will
more readily sediment, which will (a) increase the magnitude of We, (b) lead
to a smallerhvalue, and (c) very much increase the time that a thin layer
between drops exists. It has often been observed that an emulsion showing
coalescence while standing remained stable if sedimentation was prevented
by slowly rotating the vessel end over end. Finally, the larger the drops, the
smaller their number, and the smaller the number of coalescence events
needed to produce a visible change, e.g., the formation of a continuous layer
of the liquid making up the disperse phase.
Effects of Flow. As long as We remains smaller than unity, stirring
of an emulsion has little effect on coalescence rate. At high We numbers,
however, the drainage of the film between the droplets to a thickness where
coalescence can occur is an important variable. In studies on coalescence of
emulsions during agitation, it is often observed that the coalescence rate at
first increases with increasing velocity gradient, because the encounter rate is
proportional toC. For still higherCvalues coalescence rate often decreases,
because the duration of an encounter, and hence the time available for
drainage, is inversely proportional toC. The change from increasing to
decreasing rate occurs at a lowerCvalue for larger drops, since they make a
larger film, and hence need a longer drainage time. Such relations have been
observed, for instance, in stirred W–O emulsions to be used for margarine
making.
Oil-in-Water Emulsions. An example of the coalescence of
protein-stabilized dropletswith a planar interface is shown in Figure 13.16.
It is seen that droplet diameter has a strong effect, presumably because a
larger drop gives a larger effective film radius. However, from the protein
concentration and the aging time, a value of the surface loadGof about
0.4 mg?m^2 is calculated, i.e., far below the plateau value of about 3, and
even considerably below the value of about 1 mg?m^2 for extended peptide
chains. For conditions where the plateau value was reached, no coa-
lescence was observed. This is a general observation: protein-stabilized
O–W emulsions, with droplet size of a few mm or less and a plateau
surface load, are very stable against coalescence. If part of the adsorbed
protein is wholly or partly displaced by a small-molecule amphiphile