372938 Flow-Induced Microstructure of Linear and Branched Wormlike Micelles

Tuesday, November 18, 2014: 9:30 AM
M304 (Marriott Marquis Atlanta)
Jason Rich, Neutron Sciences, Soft Matter and Biology Division, Oak Ridge National Laboratory, Oak Ridge, TN, Michael Weaver, The Procter and Gamble Co., Kathleen Weigandt, National Institute of Standards and Technology, NIST Center for Neutron Research and Gregory Smith, Neutron Sciences, Soft Matter and Biology Division, Oak Ridge National Laboratory

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.

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See more of this Session: Complex Fluids I: Polymers and Macromolecules
See more of this Group/Topical: Engineering Sciences and Fundamentals