Chemistry Reference
In-Depth Information
between their systems and a polymer-stabilised system discussed above is
presumably due to the structure of the film between the drops and its
viscoelastic characteristics. Changing the temperature can cause changes in
elasticity, relaxation times and interfacial film properties. It can also lead to
phase inversion with a W/O becoming O/W. This change leads to a marked
difference in viscosity thus measuring the viscosity at a fixed shear rate as the
temperature is changed can be used to indicate the phase-inversion tempera-
ture. As the concentration of the emulsion is reduced below the maximum
packing fraction the elasticity is lost and the material becomes pseudoplastic.
For water in oil systems the behaviour observed is dictated by the extent of
aggregation, the surface layer and polydispersity. Large size polydispersity seen
in many emulsion systems tends to cause an emulsion to shear thin less rapidly
with applied stress than hard-sphere systems. Polyelectrolyte stabilised systems
can show both high- and low-shear-rate plateaus which are less commonly
detected with W/O emulsions. The high-shear viscosity can conform to a
Krieger-Dougherty model normally with a slightly lower intrinsic viscosity
to allow for internal circulation within the drop. At present our understanding
of emulsion behaviour is not as well developed as that of particulate or polymer
systems. Part of the di
culty in correlating the rheology lies in the high level of
characterisation required in order to differentiate between systems as well as the
greater diculty in preparing monodisperse model emulsions than rigid
particulate systems. However, this is understandable since emulsion character-
isation can be formidable.
REFERENCES
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