(232l) Using Rheo-SALS to Study Shear Induced Phase Separation (SIPS) in Aqueous Solutions of Cationic Surfactant and Salt
Cationic surfactants in the presence of salt can self - assemble into long, flexible chains, known as ?thread? or ?worm like micelles? (WLM). In semi - dilute concentration range, the entanglement of these worm like micelles give rise to transient networks, forming viscoelastic solutions. These solutions are shown to exhibit very well defined ?Maxwellian? linear rheology with a single relaxation time, but an unusual flow behavior under shear. Specifically, there have been recent studies elucidating the formation of shear banding, i.e., the splitting of a viscometric flow into bands of different local shear rates. In the present study, we investigate further the nonlinear shear rheology of cationic surfactant solutions in conjunction with small angle light scattering (SALS): specifically near the miscibility gap of 40mM Erucyl bis(hydroxyethyl) methylammonium chloride (EHAC) and sodium salicylate (NaSal), by using a rheo-SALS instrument (TA instruments) developed in our group at the University of Delaware.
Isotropic solutions of a high NaSal concentration (200-800mM), show shear-banding that manifests rheologically as a stress plateau between 25 to 45°C. Interestingly, this stress plateau is observed to be very weakly dependent on the temperature, but the critical shear rates vary systematically with temperature. The SALS pattern in flow-vorticity plane show no scattering below the critical shear rate and a ?butterfly? pattern with enhanced scattering in the flow direction. This is indicative of a shear induced phase separation (SIPS) with large structures aligned along the vorticity direction. The isotropic solutions with low NaSal (<20mM) do not show any evidence of shear-banding or SIPS. We observe a maximum in zero shear viscosity for which the NaSal concentration is independent of the temperature. This behavior is unlike that of cationic WLM surfactant solutions that exhibit shear banding in the vicinity of an isotropic-nematic phase transition, pointing to the possibility of different underlying mechanisms driving shear banding in highly branched WLM solutions.