Paper Number
PG10 

Session
Polyelectrolytes, Self-assembling Systems & Gels

Title
Tracking wormlike micelle topology during steady and transient shear flows with Dielectric RheoSANS

Presentation Date and Time
October 17, 2018 (Wednesday) 10:15

Track / Room
Track 3 / Bellaire

Authors (Click on name to view author profile)

  1. Riley, John K. (National Institute of Standards and Technology, NCNR)
  2. Richards, Jeffrey J. (National Institute of Standards and Technology, NCNR)
  3. Wagner, Norman J. (University of Delaware)
  4. Butler, Paul D. (National Institute of Standards and Technology, NCNR)

Author and Affiliation Lines (in printed abstract book)
John K. Riley1, Jeffrey J. Richards1, Norman J. Wagner2, and Paul D. Butler1
1NCNR, National Institute of Standards and Technology, Gaithersburg, MD 20899; 2University of Delaware, Newark, DE

Speaker / Presenter 
Riley, John K.

Text of Abstract
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 evolution of conductivity, permittivity, stress, and segmental anisotropy of reverse WLMs. Reverse WLMs are 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. To isolate the effects of branching, Dielectric RheoSANS is performed on WLMs in n-decane, which exhibit a continuous branching transition for water-to-surfactant ratios above the maximum in zero-shear viscosity. 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 larger 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. We will discuss 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 reveals rich information on dynamic structure during periods of fast transient stress response.