Dynamics of wormlike micelles in elongational flows

Wormlike micelles provide an opportunity to study the behavior of semiflexible macromolecules in elongational flows. We constructed a microfluidic cross-flow device coupled with fluorescence microscopy to image individual wormlike micelles and measure their dynamics in planar elongational flow. These polymer micelles prove stable in elongational flow and exhibit a sharp transition between regimes where Brownian motion dominates the micellar dynamics and where the micelles stretch with the flow. The coil-stretch transition and micellar relaxation time were identified by examining a distribution of micelle lengths at various flow rates. The relationship between micellar relaxation time and length is consistent with hydrodynamic theory. At higher Weissenberg number, micelle stretching is nearly as rapid as the rate of stretching of the surrounding fluid, yet also results more frequently in sharply folded conformations. In contrast to DNA in extensional flow, these relatively more stiff macromolecules exhibit fewer alignment modes.