Shape and Interfacial Viscoelastic Response of Emulsion Droplets in Shear Flow - Food Colloids: Self-Assembly and Material Science - page 353

the deformation. This result suggests that the strength of the globular protein
layer relevant to the drop deformation is determined at early interface ages,
whereas the network properties developed during ageing are of secondary
importance. Since the ageing behaviour of a deformed drop under shear hardly
correlates with the long-time interfacial shear moduli or viscosity, the role of
the dilatational properties should be clarified as well.
The results presented above demonstrate that viscoelastic protein layers
adsorbed at the oil water interface restrict the deformation of emulsion
droplets under flow, and that Rheo-SALS is a suitable method to study such
effects for droplets in the micrometre size range. At identical Capillary num-
bers, the flow-induced anisotropy is significantly smaller for protein-covered
droplets as compared to drops with an adsorbed small-molecule surfactant.
A somewhat unspectacular, but nonetheless important, conclusion can be
drawn from Figure 4: for protein-stabilized emulsions with practically relevant
droplet sizes (Ca o 0.1), scattering will be isotropic even at elevated shear
stresses and the flow regime in which deformation occurs will hardly ever be
reached. Therefore, protein-covered emulsion droplets, once broken down to
their final sizes in the micrometre range, do not deform anymore, especially if
the continuous fluid is of low viscosity. Consequently, these droplets should be
considered as 'solid' dispersed spheres covered with a charged polymer adsorp-
tion layer. Whereas the main contribution to this behaviour is due to the
capillary pressure, which is inversely proportional to the radius, Figure 4 shows
that interfacial rheological properties enhance the 'effective capillary pressure'
and therefore stabilize the droplets at small deformations. We should also keep
in mind that all of the above results are valid for small deformations, and
consequently for sub-critical Capillary numbers (Ca o Ca crit ). The fact that a
protein-covered droplet is stabilized in the small-deformation limit does not
necessarily mean that it will be more stable against break up at higher defor-
mations. As demonstrated for single drops 32 and later confirmed for emul-
sions, 33 the role of protein layers in the break up behaviour can even be quite
contrary to what we have found for the small-deformation behaviour: Williams
et al. 32 found that, whereas b-casein or b-lactoglobulin at low concentrations
stabilizes the droplets, higher b-lactoglobulin concentrations result in droplet
breakup behaviour essentially controlled by the rigidity of the globular protein
network, minimizing the importance of the bulk viscosity. Rigid protein layers
can therefore enhance droplet break up, even though they stabilize the same
droplet at smaller deformations.
Acknowledgements
We thank Bruno Pfister, Rok Gunde, Dani Kiechl, Jan Corsano, Christoph
Eschbach and Martina Haug at ETH, and Patrick Heyer, Jo¨ rg La¨ uger and
Victor Kusnezov at Anton Paar/Physica. Parts of this project were supported
by the Swiss National Science Foundation (SNF project no. 200020-108052)
and by ETH Zu ¨ rich.