Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 347

Rheology (without light scattering) was performed at 201C in a cell CC16 of
concentric cylinder geometry with a Physica MCR300 rheometer (Anton Paar,
Germany). The deformation amplitude in the oscillatory tests was set to 20%.
23.2.4 Single Drop Deformation Experiments
To study the effects of an adsorbed protein layer on the deformation of single
drops suspended in a sheared continuous phase, we used an optical shear cell
with real-time control of the drop position. The device is a modification of
Taylor's classic band apparatus and is described in detail elsewhere. 36
For experiments with protein-covered interfaces, a drop was incubated in a
vial containing protein solution (c ¼ 0.1 wt.%) for a specific time interval
during which the surface-active protein adsorbed to the oil water interface. 34
As protein adsorption at liquid interfaces is effectively irreversible under the
conditions present here, the protein was assumed to behave as an insoluble
layer once it was adsorbed. 7-9 Subsequently, the protein-covered drop was
carefully immersed in washing buffer to remove unadsorbed protein and
transferred into the flow cell via a vial containing continuous phase only.
Using this procedure, it was possible to perform flow experiments on the
identical drop with and without adsorbed protein. The deformation parameter
D ¼ (L B)/(L + B) was used to evaluate the data from the experiments,
where L and B are the long and short axes of the drop projected into the plane
of velocity versus shear gradient.
23.2.5 Interfacial Characterization
The interfacial shear moduli were measured using a biconical disc interfacial
rheometer, as illustrated in Figure 2(a) and described in detail elsewhere. 37
A Physica MCR 300 rheometer (Anton Paar, Germany) was adapted for
interfacial rheometry. Interfacial tensions at the oil water interface were
measured using the pendant drop method. 38 For tensiometry and interfacial
rheometry, all components in contact with the fluids were consecutively cleaned
with hot water, ethanol and chloroform, before and after every measurement.
23.3 Results
A particular advantage of the interfacial rheological method used here is the
ease with which the complex measuring protocols could be automated. An
example is shown in Figure 2 for b-lactoglobulin adsorbed at the oil solution
interface (protein concentration 100 mg L 1 , pH 6.7, ionic strength 90 mM). To
study the time-dependent rheology of the globular protein films, short fre-
quency sweeps were repeated over several hours. For early film ages, when
changes in surface pressure and shear moduli were still occurring fast, the
lowest frequency was not tested, since the time necessary to gather sufficient
data (about 1/6 of a full oscillation cycle with the device used) was in the range