(503d) Branching and Alignment in Reverse Worm-like Micelles Studied with Simultaneous Dielectric Spectroscopy and Rheosans

Topology and branching play an important but poorly understood role in controlling the mechanical and flow properties of worm-like micelles (WLMs). To address the challenge of characterizing branching during flow of WLMs, dielectric spectroscopy, rheology, and small-angle neutron scattering (Dielectric RheoSANS) experiments are performed simultaneously to measure the concurrent evolution of conductivity, permittivity, stress, and segmental anisotropy of reverse WLMs. Reverse WLMs are microemulsions comprised of the phospholipid surfactant lecithin dispersed in oil with water solubilized in the micelle core. Their electrical properties are independently sensitive to the WLM topology and dynamics. To isolate the effects of branching, Dielectric RheoSANS is performed on WLMs in n-decane, which show fast breakage times and exhibit a continuous branching transition for water-to-surfactant ratios above the corresponding maximum in zero-shear viscosity. The unbranched WLMs in n-decane exhibit only subtle decreases in their electrical properties under flow that are driven by chain alignment and structural anisotropy in the plane perpendicular to the electric field and incident neutron beam. These results are in qualitative agreement with additional measurements on a purely linear WLM system in cyclohexane despite differences in breakage kinetics and a stronger tendency for the latter to shear band. In contrast, the branched micelles in n-decane (higher water content) undergo non-monotonic changes in permittivity and more pronounced decreases in conductivity under flow, revealing that branch-breaking plays a critical role in relieving stress in the early stages of shear thinning. Our approach provides the first direct signatures of changes in branching and connectivity during flow of WLMs. In this talk, we will discuss recent Dielectric RheoSANS experiments on WLMs undergoing steady-shear and large amplitude oscillatory shear (LAOS) flows. In the latter, structure-dielectric relationships allow us to track anisotropy through the electrical properties, which in turn reveals rich information on dynamic structural evolution during periods of fast transient stress response in order one Deborah number flows.