Shear banding and the isotropic-to-nematic transition in wormlike micelles

Abstract

Using deuterium NMR spectroscopy in a Couette cell, we observe shear-induced nematic ordering in the concentrated wormlike-micelle system CTAB/D(2)O, and our results are qualitatively consistent with birefringence studies, and in exact quantitative agreement with the degree of order measured in neutron-diffraction measurements. The width of the nematic region depends on shear rate, as well as on the temperature proximity to the equilibrium isotropic-nematic transition. Comparison of the nematic order profiles with velocity profiles obtained under identical conditions shows quite clearly that the nematic state is not identifiable with a highly sheared, low viscosity layer, and we conclude that the process of shearing induces a nematic state of high viscosity, possibly associated with mesoscale ordering. We present a simple model in which transition from the high shear branch to the viscous nematic branch is counterbalanced by subsequent relaxation of nematic order.

An optical fiber based interferometer to measure velocity profiles in sheared complex fluids

“…Callaghan et al have used NMR to image the velocity field of sheared wormlike micelles in both cone-and-plate and Couette (e = 1 mm) geometries [8,9]. Results in the isotropic-nematic coexistence regime show a strong correlation between shear bands and bands of different structural order.…”

“…However, the main drawback of this technique is its poor temporal resolution: in Ref. [9] the authors report on a broad velocity distribution due to fluctuations of the flow field but are not able to resolve these fluctuations in time. Indeed, the accumulation time needed for one profile is about an hour.…”

“…Results in the isotropic-nematic coexistence regime show a strong correlation between shear bands and bands of different structural order. NMR can resolve details down to 20 µm [8,9] and appears as a very powerful tool for local measurements in complex fluids. However, the main drawback of this technique is its poor temporal resolution: in Ref.…”

“…For more than two decades, great experimental effort has been devoted to developping new methods for measuring local quantities of rheological interest such as the local stress, the velocity field, or the local velocity gradient [8,9,17,18]. Ideally, such a technique should be implemented on standard rheometers.…”

“…In micellar solutions, each "band" was shown to correspond to different fluid structures, isotropic or nematic, using flow birefringence [6] or NMR spectroscopy [8]. The correspondence between the branch flowing atγ 2 and the nematic state is still under debate [9].…”