Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 345

used was from a Millipore Biocel water purification system. Experiments were
performed at 201C. The SDS solutions and buffers were stored at 51C. Protein
solutions were prepared on the days of the experiments and used immediately.
23.2.2 Emulsion Preparation and Characterization
Oil-in-water emulsions were produced in a two-step process. First, a parent
emulsion with relatively high oil volume fraction ( f ¼ 20 vol.%) was produced
with the aqueous solution (1 wt.%) of protein or surfactant in a laboratory-
scale rotor/stator homogenizer (Kinematica Polytron, Switzerland). The vol-
ume area mean droplet diameter d 32 was in the range 1-10 mm. The parent
emulsion was produced with excess emulsifier in the aqueous phase. In the
second step, appropriate aliquots of the parent emulsion were diluted with
the continuous phase to produce the final dilute emulsions of low phase volume
fraction ( f ¼ 0.1 vol.% for SALS and f ¼ 3-5 vol.% for rheometry) and a
sufficiently high continuous phase viscosity to induce detectable droplet defor-
mation. (This latter condition was necessary, since the hydrodynamic stresses
attainable at low shear-rates in a low-viscosity environment are several orders
of magnitude too weak to overcome the capillary pressure of the droplets in
rheometric flows, in particular at small droplet radii.) For the SDS system, the
continuous phase additionally contained excess surfactant to yield a final
surfactant concentration of 1 wt.% in the dilute emulsion used for the scatter-
ing experiments. For the protein system, no additional protein was added to the
continuous phase, since we assume that b-lactoglobulin does not significantly
desorb from the oil water interface upon dilution. 8,9
Particular care had to be taken to avoid inclusion of air bubbles upon mixing
of the parent emulsion with the higher viscosity continuous phase. For the
experiments shown here, the droplet-free matrices were weighed into centrifuge
vials and centrifuged at 5000 g for 5 min to remove air bubbles. Appropriate
amounts of the parent emulsion were then slowly added to the continuous
phase using a small helix stirrer within the same vials.
To vary the droplet concentration in the final sample, appropriate amounts
of buffer or surfactant solution were added to the parent emulsion before
mixing. Since the emulsions were prepared in two steps, it should be noted that
the final continuous phase ultimately contains water from the parent emulsion,
which must be taken into account for the continuous phase rheology. Use of
time/concentration superposition methods for the continuous phase moduli
and complex viscosity greatly facilitated the experiments when samples with
different water concentrations in the dextrin were studied.
Droplet-size distributions were measured by laser diffraction (LS-13320,
Beckman Coulter, USA).
23.2.3 Rheo-SALS and Rheology
The Rheo-SALS measurements were performed with the device described by
Herle et al. 35 The apparatus is based on the stress-controlled DSR rheometer