Rheo-NMR studies of the behavior of a nematic liquid crystal in a low-shear-rate regime: The transition from director alignment to reorientation

Abstract

Deuterium NMR spectroscopy has been used to study the director dynamics of the nematic liquid-crystal system cetyl trimethylammonium bromide/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, the inner cylinder of the cell was rotated continuously at different rates using a stepper motor. The resulting time evolution and ultimate steady-state orientation of the director, governed by the competition between the applied viscous torque with elastic and magnetic terms, was measured via observed changes in the deuterium spectrum. Using a simple gearbox allowed unprecedented access to a low-shear-rate regime in which, above a threshold shear rate, the director of part of the sample was observed to reorient, while the remaining part still aligned with the magnetic field. Subsequent increases in the applied rotational rate were found to increase the relative proportion of the orienting fraction. Spatially resolved NMR spectra showed that the orienting and field-aligned fractions formed separated bands across the gap of the Couette cell, with director reorientation being initiated at the moving inner wall. The behavior was found to be consistent with the often ignored variation in velocity gradient manifest across the gap of a cylindrical cell, so that as the angular frequency of the inner cylinder was increased the radial location of the critical shear rate required for reorientation traversed the gap. Once the applied rotational rate was sufficient to reorient the director of the entire sample, the dependence of the exhibited steady-state orientation on the average applied shear rate was measured. These results could be fitted to an analytical solution of the force-balance equation, made tractable by the assumption that the elasticity term was of minor significance and could be ignored. Additionally, the use of a numerical solution of the full force-balance equation, which explicitly includes elasticity and secondary flow and additionally allows the time evolution of the director orientation to be calculated, was investigated.

Rheo-NMR at the Phase Transition of Liquid Crystalline Poly-γ-benzyl-l-glutamate: Phase Kinetics and a Valuable Tool for the Measurement of Residual Dipolar Couplings

“…35,78 However, we could not find comprehensive NMR studies at or below its phase transition. 36,104,105 While a change of alignment of PBLG and other LC phases in general has been shown previously under shear conditions, 4,10,13,14,49,80,88,92 we could show that spinning with rates above 20 Hz induces alignment even in PBLG/CDCl 3 samples at concentrations far below the LC transition. While dilution of the polymer component in the LC system seems to lead to a breakdown of intermolecular interactions in the static case, shearing of the sample seems to provide the statistical basis for close intermolecular contacts that are necessary for an aligned phase.…”

Validation of a low field Rheo-NMR instrument and application to shear-induced migration of suspended non-colloidal particles in Couette flow

“…The usual Rheo-NMR experiment lacks a direct measure of shear stress although custom systems have been proposed that combine NMR and torque sensors [19,20], albeit still based on superconducting magnets with limited industrial application. Rheo-NMR has been applied to the study of many complex samples, including shear-banding materials [21][22][23], polymer solutions [24,25], worm-like micelles [26,27], liquid crystalline materials [19,[28][29][30][31], and colloidal particle suspensions [32,33]. Shear-induced migration of non-colloidal particles has been studied by NMR imaging in Couette [34][35][36] and pipe flow [37,38].…”

Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation

“…The former technique characterizes the system in static (or rotating) conditions; Rheo-NMR is a powerful tool to investigate shear-induced deformations. It has been used by some of us, and its intrinsic potentialities are well acquainted. − Self-diffusion anisotropy, finally, indicates whether the nematic fluids preferentially orient in magnetic fields. The above methods jointly allow characterizing in detail some properties of NT-based nematic fluids.…”

“…It has been used by some of us, and its intrinsic potentialities are well acquainted. 23 NMR provides useful information on local order and dynamics. In particular, water self-diffusion gives information on obstruction effects.…”