(247c) Rheo-Optics of Equilibrium Polymer Solutions: Wormlike Micelles in Planar Elongation

The dynamics of wormlike micelle solutions has been characterized rheo-optically. Two aqueous solutions were studied: one of cetyl pyridinium chloride (CPyCl) and cetyl trimethyl ammonium bromide (CTAB), each with co-solute sodium salicylate. The principal dynamic timescales of these solutions were determined by linear rheometry, and the flow-induced birefringence was determined microscopically in planar elongational flow generated in a microfluidic cross-slot. The stress optic rule was obeyed at low elongation rates. Moreover, the kinetics of alignment and relaxation were determined as fluid entered and exited from the extensional zone. As known previously, alignment requires accumulation of sufficient strain, so that peak birefringence is focused in a band along the line of stretching that intersects the stagnation point. As the flow rate increases, deviations from the stress optic rule occur. These become significant as the birefringence band changes shape and micellar alignment approaches saturation, at Weissenberg number near unity. The stress optic rule must be applied with care, because these deviations are obscured somewhat when average birefringence is measured by sampling an area near the stagnation point. Finally, at slightly higher flow rate, symmetric planar elongation is unstable: micelles (and indeed the stretching axis) tilt away from the outflow axis, when the flow symmetry splits and inlet streams are directed predominantly to one or the other outlet channels.