Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 344

stresses is in a range in which drop sizes, timescales and shear stresses are
experimentally accessible for rheological and rheo-optical measurements. The
flow and interfacial properties of the system can be combined into a dimension-
less group, the Capillary number, defined as Ca ¼ t R/ s , i.e., the ratio of
hydrodynamic stress ( t ) to interfacial stress ( s /R), where s is the inter-
facial tension and R is the radius of the undeformed droplet. When
Ca exceeds a so-called critical Capillary number Ca crit , droplet break up occurs;
values for Ca crit as a function of the ratio of the viscosities of dispersed and
continuous phases have been collected for various flow types and materials. 28-31
The breakup behaviour of protein-covered emulsion droplets has been shown to
be influenced by the rheological properties of the adsorption layer. 32,33 Recently,
the effect of an adsorbed protein layer on the flow-induced deformation of
millimetre-sized drops has been demonstrated in an optical flow cell. 34 It was
shown that, despite a considerable decrease in interfacial tension, a protein-
covered drop is less deformed under identical hydrodynamic stresses due to the
viscoelastic properties of the macromolecular adsorption layer.
In this contribution, anisotropy in dilute emulsions under flow is studied by
Rheo-SALS and the effects of adsorbed protein layers with interfacial rheology
are assessed. Emulsions were prepared with either excess surfactant, sodium
dodecyl sulfate (SDS), or a surface-active globular protein, b-lactoglobulin. The
SDS was used far above its critical micelle concentration (cmc), and hence the
interfacial stress condition of the droplets can be approximated by a pseudo-
equilibrium interfacial tension. That is, shear and dilatational interfacial
stresses, including interfacial concentration gradients, are absent, or they are
balanced on a timescale much faster than our experimental observation time. In
contrast, the deformation of emulsion droplets stabilized with b-lactoglobulin is
expected to be governed by the solid-like behaviour of the adsorbed protein
layer, the properties of which are studied with interfacial rheometry.
23.2 Experimental
23.2.1 Materials
The b-lactoglobulin (80% HPCE, mixture of genetic variants A and B), sodium
chloride, sodium dihydrogen phosphate, chloroform (all analysis grade) and SDS
(Ultra grade, Mr 288.38, Prod. No. 71725) were purchased from Sigma Chemical
(St. Louis, MO) and were used as received. Protein solutions were prepared in
phosphate buffer (ionic strength 90 mM, pH 6.7, protein concentration 1 wt.%).
For the oil phase, (+)-carvene (Sigma, dynamic viscosity 0.9 mPa s) was used,
and the density was adjusted with sucrose acetate isobutyrate (SAIB, Eastman
Chemical, USA) at a concentration of 60 wt.%, and giving a dispersed phase
viscosity of 32 mPa s. The continuous phase in the single drop experiments was
polyethylene glycol (PEG 35000S, Clariant) in aqueous solution at a concentra-
tion of 40 wt.% with a viscosity of 2.5 Pa s at 201C. For the emulsion
experiments, the continuous phase was high-viscosity dextrin (BCsweet 01146,
Cerestar). Both the dispersed and continuous phases were Newtonian. All water