Shear-induced structures formed during thixotropic loops in dilute worm-micelle solutions

The shear stress response of the system CTAB/NaSal/water is studied using strain thixotropic loops in the low concentration regime and [NaSal]/[CTAB]=1[NaSal]/[CTAB]=1. Stress response during up-shear curves depends on the ramping rate. However, stress response during down-shear curves collapses in one curve, no matter the rate used during the ramping down process; it does not depend on the history of the system. In the process of ramping up during the thixotropic loops, the system forms shear-induced structures. We were able to observe them through the scattered light produced by those structures, when the fluid is under shear in the gap of a transparent Couette cell. During the ramping down, the shear-induced structures survive until the shear rate vanishes. Models that describe the form of the up-shear and down-shear curves were presented, as well as, how thixotropic loops can give information about this kind of systems. In particular, for down-shear curves, we can define a decay shear rate constant that follows an Arrhenius temperature dependence.

Strain thixotropic loops and images of light scattered from the fluid showing the formation and decay of shear-induced structures in solutions of dilute wormlike micelles.Figure optionsDownload as PowerPoint slide