Dispersion visualization of model fluids in a transparent Couette flow cell

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Abstract
Dispersion mechanisms in model fluid systems of different viscosity ratios were studied in a transparent Couette flow cell. The counter-rotating concentric cylinders were driven by two independent dc motors. Drops of the minor phase were then maintained at a constant position by fixing the inner and outer cylinders’ rotational speeds. The advantage of this new setup is that visualization can be made at high shear rates without any secondary flow effects, usually observed with cone-plate or parallel plates geometry. Constant viscosityviscoelasticdrops (Boger fluid) and Newtonian drops [high viscositypolydimethylsiloxane,(PDMS)],deformed at low shear rates in a Newtonian matrix (low viscosityPDMS), oriented along the flow field and dropdeformation increased with shear rate, as expected. However, when a critical shear rate (characteristic of the fluid system used) was reached, the deformeddrops began to contract in the flow direction. When increasing the shear rate over this critical value, drop contraction was followed by elongation perpendicular to the flow direction, i.e., parallel to the vorticity axis. This elongation increased with shear rate until the final breakup occurred. These deformation and breakup mechanisms were attributed to elastic normal forces present at high deformation rates.