Persistence of Birefringence in Sheared Solutions of Wormlike Micelles
mediaposted on 06.01.2009 by Bradley D. Frounfelker, Gokul C. Kalur, Bani H. Cipriano, Dganit Danino, Srinivasa R. Raghavan
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When aqueous solutions containing wormlike micelles (worms) are sheared, the micellar chains tend to align with the flow, which in turn leads to flow-birefringence. When shear is stopped, the worms rapidly revert to an isotropic state in typical samples, and the birefringence disappears. In this study, we present a system of cationic worms that shows a different behavior: not only do the samples become intensely birefringent when sheared but the birefringence also persists for hours (and even days) after shear is stopped. These results suggest that shear-aligned worms in the sample are trapped in their aligned state for long periods of time, an aspect that is confirmed by cryo-transmission electron microscopy (cryo-TEM). We seek to determine the origin of this unusual behavior. Our results show that the persistent birefringence is observed for cationic worms induced by hydroxy-naphthoate but not salicylate counterions. These observations suggest that the micellar alignment is stabilized by intermicellar attractive interactions (such as π−π and cation−π) that arise when large aromatic counterions are anchored within the micelles.