Rheo-NMR studies of a nematic worm-like micelle system in a high-shear-rate regime

Abstract

Deuterium NMR spectroscopy has been used to study the director dynamics of the nematic liquid-crystal system cetyl trimethylammonium bromide (CTAB)/D₂O under the action of applied viscous torques. Shear forces were applied using a custom-built Couette cell that was introduced into an NMR superconducting magnet so that its rotational axis was parallel to the magnetic field direction, along which the liquid-crystal director originally aligned. Subsequently, a series of experiments was undertaken in which the inner cylinder of the cell was rotated continuously, at different rates, using a stepper motor. Above a threshold shear rate it was found that the director of the entire sample reoriented until a steady-state orientation was reached, the value of which depended on the average applied shear rate. These experiments revealed the presence of two distinct regimes. At the lower shear rates used, the observed behaviour was successfully captured by modelling the competition between the applied viscous torque and magnetic terms, and, upon cessation of shear the resultant spectra evolved during the relaxation of the director orientation in a way that simply mirrored those observed after the initiation of shearing. However, upon increasing the shear rate further, another regime was found in which the time evolution of the spectra towards the steady state was clearly not the simple reverse of that observed during relaxation. It is hypothesised that at these faster shear rates the relevant timescale of induced reorientation becomes short compared with the characteristic timescale of the thermally-activated scission and recombination of the micelles. As such, shear forces in this regime not only reorient micelles but also provide a novel mechanism for micelles to reach the steady state reorientation via the “disassembly” of micelles. The time evolution of the angular distribution function of the director orientation was extracted in these cases via fitting of the complex experimental spectra using a genetic algorithm and indeed shows that while a proportion of micelles appear to simply reorient as intact entities, as seen at lower rates, a distinct population circumvent this route, appearing more rapidly at the steady state orientation.