(592b) The Effect of Branching on the Nonlinear Rheology of Wormlike Micelles (WLMs) Using Spatiotemporally-Resolved Small Angle Neutron Scattering (SANS)

The nonlinear rheology and segmental alignment of a model series of WLMs is studied to determine the effect of branching on shear flow phenomena. Branching in the mixed cationic/anionic surfactant (CTAT/SDBS) is controlled via the addition of the hydrotropic salt sodium tosylate. The shear-induced segmental alignment of the micelles is measured for local gap positions and time under large amplitude oscillatory shear (LAOS) in the flow-gradient plane using novel flow-SANS instrumentation [1] as well as temporally in conjunction with rheology in the flow-vorticity plane [2]. Our ability to resolve the microstructure via scattering projections in multiple planes of flow provides new insights into the effect of flow on the WLM microstructure under time-dependent shear flow. Local segmental orientation and alignment in the flow-gradient plane is found to be a complex function of the branching level, radial position, and time during the oscillation. Spatiotemporal SANS demonstrates that LAOS can elicit simultaneous elastic and viscous material responses depending on gap position, thereby corroborating recent theoretical predictions with experiment for the first time [3]. Furthermore, the maximum alignment in the flow-gradient plane under LAOS is found to be significantly higher than that observed in steady shear at the same instantaneous shear rate. This nontrivial hyperalignment under dynamic deformation is interpreted in terms of the dynamics of the microstructural rearrangement during LAOS. This research quantitatively links micellar microstructure and topology to the measured nonlinear shear rheology of WLM solutions, providing a more complete data set to enable interpretation of the rheology and rigorous testing of microstructure-based constitutive equations.

[1] A.K. Gurnon, et al., J. Visual Exp. 84 (2014).
[2] C.R. Lopez-Barron, et al., Phys. Rev. Lett. 108 (2012).
[3] J.J. Stickel et al., Journal of Rheology 57 (2013).