Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 343

Chapter 23
Shape and Interfacial Viscoelastic Response
of Emulsion Droplets in Shear Flow
Philipp Erni, Vishweshwara Herle, Erich J. Windhab and
Peter Fischer
INSTITUTE OF FOOD SCIENCE AND NUTRITION, ETH ZURICH,
CH-8092 ZURICH, SWITZERLAND
23.1 Introduction
Proteins adsorbed at fluid fluid interfaces are relevant to a number of phe-
nomena in colloidal systems, such as the stability and flow behaviour of food
foams and emulsions, 1-3 the mechanics of cell membranes 4 and enzymatic
catalysis at liquid interfaces. 5 Unlike small-molecule surfactants, proteins not
only reduce the interfacial tension of a lipid water or air water interface on
adsorption, but they also strongly modify the rheological properties of the
interface, under both shear and dilatational/compressional deformations. 6,7
Furthermore, the adsorption of surface-active proteins appears almost irrevers-
ible, as has been recently shown in sub-phase exchange experiments: 7-9 only
minor desorption and negligible changes in interfacial tension have been
observed after a protein solution is replaced with protein-free buffer surrounding
a protein-covered pendant drop. These effects have been described for a number
of surface-active proteins of both biological and technical relevance, such as
b-casein, 10-13 b-lactoglobulin, 13-18 bovine serum albumin 19-21 or lysozyme. 20
Emulsions, as well as immiscible polymer blends and phase-separated biopol-
ymer mixtures, develop flow-induced morphologies when stresses due to the
applied flow overcome the interfacial forces that favour the spherical drop
morphology at rest. 22,23 Drops can be subjected to deformation, breakup, and
coalescence; all of these processes are associated with characteristic light scat-
tering patterns. 24 Shape anisotropy of emulsion droplets in the micrometre size
range can be studied by rheometer-based small-angle light scattering (Rheo-
SALS). 25-27 Such measurements have been performed for immiscible polymer
blends, where absolute viscosities are very large, and for phase-separated bio-
polymer mixtures, where interfacial tensions between two different aqueous
phases are very small. 25 In both cases, the ratio of interfacial to hydrodynamic
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