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Rheology of solid-stabilized emulsions
Emulsions are dispersions of one
immiscible liquid into another, the archetypical example being mixtures of oil
and water. They are not
thermodynamically stabile, and require some stabilizing agent to prevent the
drops from coalescence. Commonly used are amphiphilic
surfactants – molecules that have two distinct moieties, one preferring water, and other oil. Such surfactants
readily adsorb at the interface and reduce the surface tension between fluids,
making the dispersed phase effectively more miscible. They prevent coalescence
by keeping drops from touching, therefore stabilizing the emulsion.
Despite the lack of distinct
hydrophilic and lipophilic parts, homogenous
colloidal particles can do the same. If partly wetted by both fluids they can
adsorb at the interface as well, with energy gained from reduction of
interfacial area between fluid phases. Solid-stabilized, also known as “
Despite
being comprised solely of fluids, highly concentrated emulsions can posses
strong shear rigidity, like solids do. The nature of this elasticity is unusual
-- it exists only because the repulsive droplets are compressed by an external
osmotic pressure, and packed to a sufficiently large droplet volume fraction,
which permits the storage of interfacial elastic shear energy. When osmotic pressure approaches
This has
been studied for the case of surfactants. But how does the presence of solid
particles affect rheology?
We believe
the rigidity of particles, and more so their organization, leads to different
response to deformation. Particles interact through balance of electrostatic
and capillary forces, that tends to organize them at
the interface. So does their concentration, the necessity to cover drops and
protect from coalescing. The
interfaces are structured, able to store energy upon deformation. We study the rheology of this packing
of a packing – the interplay between 3D structure of emulsion
droplets and 2D structure of colloidal particles at their interfaces.
This
project is done with support and collaboration of Schlumberger-Doll Research.
Kosta Ladavac
McKay
Laboratory
kosta@deas.harvard.edu