Microstructure of shear-banding wormlike micellar solutions

COLL 288

Norman J. Wagner, wagnernj@udel.edu1, Florian Nettesheim, netteshe@che.udel.edu1, Matthew E. Helgeson1, and Lionel Porcar, lionel.porcar@nist.gov2. (1) Department of Chemical Engineering, University of Delaware, Center for Molecular and Engineering Thermodynamics, Colburn Laboratory, Newark, DE 19716, (2) The NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, MS 8562, Gaithersburg, MD 20899-8562
Fundamental questions surround the mechanism by which wormlike micelles (WLM) shear band. Rheological models have been proposed to explain the stress-plateau and hysteresis characteristic of shear banding based on macroscopic constitutive models. However, the underlying mechanism that leads to shear banding may be related to the self-assembled microstructure and thermodynamic state of the WLM. To explore this hypothesis, we develop and validate a 1-2 plane shear cell for small angle neutron scattering investigations of the microstructure. Further, the cell enables separate measurements of the microstructure during flow in both the high and low shear bands. Three model systems of ionic surfactant WLM are measured, where each has a very different underlying thermodynamic phase behavior. Significant differences are observed for the flow-induced structure in the high shear band, pointing to the influence of the underlying equilibrium microstructure and phase behavior on the high shear band microstructure and flow behavior. Rheological results and associated modeling connects the measured structure to the measured rheology in the different shear bands. The results demonstrate that the underlying microstructures accompany shear banding in WLMs depend on the surfactant system.
 

Surfactant Self-Assembly
6:00 PM-8:00 PM, Monday, August 20, 2007 BCEC -- 153B, Oral

Division of Colloid & Surface Chemistry

The 234th ACS National Meeting, Boston, MA, August 19-23, 2007