Linking Shear Banding and Orientational Order In Wormlike Micellar Solutions
Matthew H. Helgeson, University of Delaware, Newark, DE 19711, Matthew D. Reichert, Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19711, Norman J. Wagner, Department of Chemical Engineering, University of Delaware, 150 Academy St., Newark, DE 19716, and Eric Kaler, Stony Brook University, Stony Brook, NY 11794.
Shear banding has been observed in a variety of complex fluids, including polymer solutions, colloidal suspensions and, most prominently, wormlike micelles (WLMs). However, accurate modeling and engineering of shear banding fluids remains a challenge due to an inability to identify the microstructural mechanisms leading to shear banding. Here, we demonstrate the ability to measure spatially-resolved microstructure under shear using flow-small angle neutron scattering (flow-SANS) measurements in the 1-2 plane. Using a novel approach that combines rheometry, velocimetry, and flow-SANS in the 1-2 plane, we present the first complete study of local rheology and microstructure through the shear banding transition for a model WLM solution of cetyltrimethylammonium bromide (CTAB) micelles near the isotropic-nematic phase transition. The rheology of the fluid is well-characterized by the Giesekus constitutive equation with incorporated stress diffusion, which provides a selective criterion for banding and non-banding fluids, selection of the stress plateau, and direct coupling of local rheology to local micellar orientation and alignment. Flow-SANS measurements in the 1-2 plane are in excellent agreement with these predictions, which exhibit discontinuity in segmental alignment at the shear banding interface. Furthermore, comparison of the local microstructure and local rheology shows a critical segmental alignment, followed by a transition to nematic ordering. Using this understanding, we demonstrate the ability to rationally engineer shear banding (or lack thereof) in micellar solutions by manipulating the underlying structure and phase behavior of the fluid.