Dispersion visualization of model fluids in a transparent Couette flow cell

Dispersion mechanisms in model fluid systems of different viscosity ratios were studied in a transparent Couette Row cell. The counter-rotating concen tric cylinders were driven by two independent de 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 Row ef fects, usually observed with cone-plate or parallel plates geometry. Consta nt viscosity viscoelastic drops (Boger fluid) and Newtonian drops [high vis cosity polydimethylsiloxane, (PDMS)], deformed at low shear rates in a Newt onian matrix (low viscosity PDMS), oriented along the Row field and drop de formation increased with shear rate, as expected. However, when a critical shear rate (characteristic of the fluid system used) was reached, the defor med drops began to contract in the Row 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 norma l forces present at high deformation rates. (C) 2001 The Society of Rheolog y.