Persistence of Birefringence in Sheared Solutions of Wormlike Micelles

Abstract

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 pi-pi and cation-pi) that arise when large aromatic counterions are anchored within the micelles.

Microstructure and rheology of a flow-induced structured phase in wormlike micellar solutions

“…The relaxation time of the SIS was found to range from several seconds to a couple of hours (26). Similarly, others observed birefringent SIS formation in a solution of surfactant and salt under shear and subsequent disappearance of the birefringent structures after the flow was stopped (24,37). Based on time-dependent SALS studies under a simple shear flow, Kadoma and van Egmond (12) suggested that CTAB/NaSal-based (0.03/ 0.24 M) SISs consist of highly elongated and locally concentrated micellar strings.…”

Rheological characterizations of wormlike micellar solutions containing cationic surfactant and anionic hydrotropic salt

“…With SHNC as counter ions, domains of p-p ions were paired and cationÀp groups were formed between HNC À ions and adjacent hydrophobic CTAB headgroups, inducing more viscoelastic and stable micellar networks compared with that of the CTAB/NaSal system. By focusing on 75 mM CTAB/SHNC solution, we observed a rich variety of nonlinear rheological behaviors under different deformation histories with temperatures ranging from 20 to 40 C. Since the naphthalene ring can easily form stable noncovalent bonds that arrest micelles under simple shear flows and small deformations [Frounfelker et al (2008)], when the deformation time is short compared with micelle relaxation time s r at 20 C, the viscous dissipation is dominant at higher deformation. At 25 C where the two time scales are comparable, yielding like behavior occurs and is accompanied by short range interactions between micelle head groups.…”

“…Accompanied by the results from linear and transient rheology procedures, LAOS illustrates a comprehensive picture of how wormlike micelles are affected by the cyclic switching on and off of the flow. As discussed earlier, the naphthalene ring can easily form stable noncovalent bonds that "arrest" micelles under simple shear flows and small deformations [Frounfelker et al (2008)]. We envision that noncovalent bonds in CTAB/SHNC wormlike micellar solutions can be potentially decomposed under large deformations and thermal agitations.…”

“…In addition, mixtures of cationic surfactant and excess amount of SHNC have shown pronounced birefringence phenomena lasting from milliseconds up to even hours after the flow stoppage [Frounfelker et al (2008); Mishra et al (1993a)]. Recent studies have used light scattering, flow birefringence, electron microscopy, and rheometric techniques to investigate the material properties of SHNC and cationic surfactant mixtures.…”

Scattering from polymer-like micelles

“…For the shear-induced states, details of the chemistry of the system need to be considered as very different behaviors are observed in systems that show similar behavior and structure at equilibrium. Vorticity banding is reported in one study, along with a triggered SANS technique that allows transient structured to be probed [31], the isotropic-to-nematic transition in another system [30••] and a recent report of persistent shear-induced structures [32]. Two types of polymer-like micelle systems were characterized in capillary flows using SAXS.…”