Paper Number
PO101 

Session
Poster Session

Title
Long-time transient responses of branched wormlike micelles under nonlinear shear flows

Presentation Date and Time
October 8, 2014 (Wednesday) 6:05

Track / Room
Poster Session / Poster

Authors (Click on name to view author profile)

1. Calabrese, Michelle A. (University of Delaware, Chemical and Biomolecular Engineering)
2. Rogers, Simon A. (University of Delaware, Chemical and Biomolecular Engineering)
3. Wagner, Norman J. (University of Delaware, Department of Chemical and Biomolecular Engineering)

Author and Affiliation Lines (in printed abstract book)
Michelle A. Calabrese, Simon A. Rogers, and Norman J. Wagner
Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716

Speaker / Presenter 
Calabrese, Michelle A.

Text of Abstract
The nonlinear rheology and microstructure of a model series of wormlike micelles (WLMs) are measured by rheology and flow small angle neutron scattering (SANS) [1,2] to ascertain whether transient material responses are affected by micellar branching. The degree of branching in the mixed cationic/anionic surfactant (CTAT/SDBS) system is controlled by adding sodium tosylate. With increasing branching, we observe differences in the development of the shear stress and shear-induced segmental alignment of the micelles under the startup of shear and large amplitude oscillatory shear (LAOS) in the flow-gradient and flow-vorticity planes. The evolution of the macroscopic stress during shear startup for a nearly linear WLM system exhibits an overshoot and a short transient response (<60s) in the shear banding regime. The segmental alignment of the WLMs in the 1-3 plane, however, evolves very slowly (400s) to steady state. This discrepancy between rheology and alignment factor corresponds with shear band formation and disappears at lower shear rates where shear banding is not present. For highly branched WLMs where banding is not observed, both alignment factor and shear stress evolve toward steady state rapidly. A different behavior is observed under LAOS, where the stress response of nearly linear WLMs reaches steady alternance within several cycles and remains stable. In branched samples at similar De and Wi numbers, the measured stress overshoot and segmental alignment factor evolve very slowly over the course of five hours. This highly repeatable and unique aging under LAOS and recovery upon cessation of shear is interpreted as a stress relief mechanism driven by branch rearrangement. This research quantitatively links micellar microstructure and topology to the measured transient and steady shear rheology of WLM solutions, providing insight into the formation of flow instabilities and shear induced microstructures.

[1] A. Kate Gurnon et al. JoVe, (2014).
[2] C. Lopez-Barron et al. PRL, (2012).