Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 352

Figure 5 Single drop deformation experiment. (a) Deformation D ¼ f(Ca) of a oil drop
before and after adsorption of b -lactoglobulin (continuous phase viscosity 940
mPa s, drop/continuous phase viscosity ratio 0.096, incubation time of drop in
protein solution 60min). (b) Deformation behaviour for different residence
times of the drop in the protein solution. The value of Ca is based on the
transient values of the interfacial tension after 1, 10 and 60 min, respectively.
(c) Images of the same drop (radius 0.74 mm) with and without the adsorbed
protein layer: I, no flow; II, uncovered drop deforming at shear rate of 6.8 s 1 ;
III, same drop at same shear rate, but after incubation in the protein solution for
10 min
observed here. This is likely to be due to the higher continuous phase viscosity
in our experiments. In Figure 5(a) the mean deformation values, averaged over
at least a second, are reported. We note that, in the static interfacial tension
case at the given viscosity ratio, the experimental data collapse onto one line
with a slope of approximately one, whereas the values found for the protein-
covered interfaces are much lower. All interfacial rheological data measured
with b-lactoglobulin showed a strong time dependence (see above).
An obvious question for protein-covered drops is how the interface age
influences the small-deformation behaviour. This can be studied by varying
the incubation time of the drop in the protein solution. Figure 5(b) shows
deformation responses for drops with different interface ages of 1, 10 and
60 min. The magnitude of Ca was calculated from transient values of the
interfacial tension measured in the pendant-drop tensiometer at different times:
s ¼ 19, 16 and 11.3 mN m 1 for 1, 10 and 60 min, respectively. In contrast to
the uncovered drop, all values for D are lower, but we find that the differ-
ences for the individual interface ages are hardly detectable by our method:
even at the shortest age of 1 min (which is the shortest realistic timescale
accessible with a well-trained experimenter), the protein layers strongly restrict