Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 351

Figure 4 Comparison of the frequency-dependent storage modulus G 0 ( o ) for emulsions
stabilized with either SDS or b -lactoglobulin ( b -LG) (droplet phase fraction f
¼ 4.5 vol.%). Note the absence of a pronounced relaxation shoulder for the
protein system. The insets show contour plots of Rheo-SALS patterns obtained
for the same emulsions, but with a lower droplet phase volume fraction of
f ¼ 0.1 vol.%. The SALS images were taken at the maximum deformation rate
within an oscillation cycle (maximum in d g (t)/dt). The innermost ring of the
scattering patterns is from the beam stop

detected. In the latter work, the authors attributed the 'fast' relaxation process
to shape relaxation, and the slow one to an interfacial viscoelastic effect. The
difference from our results shown here is that in the present case the combined
effects of small droplets (d 32 B 2-3 mm) and interfacial rheology of the

adsorbed protein do not allow us to detect whether such in-plane interfacial
relaxation timescales are present, as they completely suppress deformation of
the droplets. We note that, in similar experiments with larger drops in steady
shear flow we did find flow-induced anisotropy even for protein-stabilized
emulsions; compared to surfactant-stabilized emulsions, these systems showed
reduced anisotropy at identical Capillary numbers. 43
Figure 5(a) shows deformation experiments performed with millimetre-sized
drops in the parallel band flow cell. The deformation parameter D is plotted as
a function of Capillary number for both a clean drop and a protein-covered
drop. The value of Ca is calculated from the static interfacial tension; for the
protein layers, we have s ¼ 11.3mNm 1 , as measured by pendant-drop
tensiometry with a 1-hour-old drop. Although the interfacial tension is lower
than the clean interface, the presence of the b-lactoglobulin adsorption layer
restricts the ease of deformation, rather than facilitating it. The results are
similar to those with drops covered with lysozyme, as reported elsewhere, 34
with the exception that only very minor shape fluctuations (wobbling) have been