(244e) Flow-Induced Microstructure of Linear and Branched Wormlike Micelles

Under appropriate conditions of surfactant and salt concentration, aqueous solutions of amphiphilic molecules form self-assembled high aspect ratio threadlike micellar structures known as wormlike micelles (WLMs). These structures are highly dynamic, constantly breaking and reforming in equilibrium, and can lead to non-Newtonian rheological properties and complex flow behavior such as shear-thinning, viscoelasticity, and shear banding. In nonlinear flow regimes, shear-thinning rheology of WLM dispersions is typically accompanied by alignment of micellar threads in the flow direction. There has been much success in probing and characterizing these flow-induced microstructural changes using small-angle neutron scattering techniques (flow-SANS) and with techniques that combine simultaneous rheology and neutron scattering measurements (rheo-SANS). While dispersions of linear WLMs have been studied extensively with this approach, the effect of WLM branching on flow-induced structure and rheology remains largely uncharacterized. In this talk, we will discuss recent rheo-SANS and 1-2 shear plane flow-SANS experiments comparing linear and branched WLM dispersions in nonlinear flow regimes. WLM branching is induced at high salt concentrations in a dispersion of sodium laureth-1 sulfate (SLE1), an anionic surfactant used as a detergent and a foaming agent in the consumer products industry. Rheo-SANS measurements in the flow-vorticity (1-3) plane show that while both linear and branched WLM systems exhibit shear-thinning rheology, flow-induced ordering is observed only for branched WLMs. These results are in contrast to previous reports on a similar euryl bis(hydroxyethyl) methylammonium chloride (EHAC) surfactant system, which suggest that WLM branching hinders shear alignment. By examining spatially-resolved flow-SANS measurements in the 1-2 shear plane, as well as by fitting scattering data to appropriate models to extract structural parameters, we explore possible mechanisms for the differences between the shear-induced structure of linear and branched WLMs and the disparities between the behavior of SLE1 and EHAC surfactant systems.