Alignment of worm-like micelles at intermediate and high shear rates

Hypothesis: Rheology combined with Small-Angle Neutron Scattering (Rheo-SANS) can determine the local structural order in Worm-Like Micelle (WLM) solutions when the shear rate increases beyond the ending of the gradient shear banding. There, micelles are supposedly aligned, but viscosity reveals a transition regime as the shear rate increases. Experiments: The mixture of 3-[dimethyl(tetradecyl)azaniumyl]propane-1-sulfonate (TDPS), sodium dodecylsulfate (SDS) (R = [SDS]/[TDPS] = 0.55), and a water solution of NaCl (0.2 mol/L), was studied with mechanical rheology and Small-Angle Neutron Scattering (SANS) in the quiescent fluid and under flow. Findings: The system self-assembles in WLMs and presents gradient shear banding. SANS patterns of the bands formed during the shear banding were obtained in a Couette geometry along the 1-2 plane, as well as the orientation parameter along the gap. At very high shear rates, in the paranematic phase, we found an apparent transition on the flow curves with its corresponding change in the orientation parameter. The origin of this transition is unclear, but we present possible explanations of why we observe it. (C) 2019 Elsevier Inc. All rights reserved.