We study how strong mechanical
shear is used to create emulsions,
dispersions of droplets of one liquid (e.g. oil) in another immiscible
liquid (e.g. water). These droplets are stabilized against coalescence
(i.e. fusion) by a small quantity of surfactant. Our research focuses on how
the non-Newtonian flow properties of concentrated emulsions having a
high droplet volume fraction affect the ruptured droplet size
distribution. We use an oscillatory controlled-strain plane-Couette
shear cell with time-resolved small angle light scattering to study the
droplet structure during the process of droplet stretching and
rupturing.
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This images shows a small angle light scattering pattern from a concentrated emulsion that has been created using the plane couette. The presence of a ring and spots in the scattering pattern indicates that the droplets are uniform in size and have become ordered by the shear. | ![]() |
During
the process of emulsification, the droplets are ruptured to
smaller and smaller sizes, thereby increasing the Laplace pressure
scale, given by the surface tension divided by the average droplet
radius. Since the Laplace pressure scale is directly proportional to
the shear modulus, G', of the concentrated emulsion, the emulsion
becomes increasingly more elastic-- the emulsion irreversibly
'elastifies' as a result of the shear. This is possible because the
total interfacial area of the droplets has increased and energy has
effectively been stored in the droplet structures. We have invented a
method, called Sinusoidal Amplitude Variation (SAV) Rheometry, to probe the
process of shear-induced elastification in any type of complex fluid,
including concentrated emulsions. Discreet oscillations of a large
amplitude sinusoidal shear are applied and then a frequency sweep (FS)
is performed to track how the linear viscoelasticity of the complex
fluid changes as a result of the shear. |
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