Rheo-Nmr and Numerical Study of the Nematodynamics in Cylindrical Couette Geometry

Abstract

A numerical study of the hydrodynamics of nematic liquid crystals in cylindrical Couette geometry is presented, based on Leslie-Ericksen theory. The aim of this study is to interpret the director dynamics observed in Rheo-NMR experiments, where the sample (lyotropic cholesteric system PBLG/m-cresol 14%) is subjected to viscous, elastic and magnetic torques. When analysing the macroscopic response of the liquid crystal polymer to the azimuthal Couette flow, competing with the effect of the strong magnetic field applied perpendicular to the shear plane in the NMR set-up, we observe the existence of a steady state characterised by a distribution of director orientations neither parallel to the field nor in the shear plane. We show that this behaviour requires the occurrence of an instability, like the director analogue of Taylor cells, associated to a secondary flow. A proper modelling of the velocity profile allows us to interpret satisfactorily the main features of the experimental data.

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

“…Nematodynamic equations describe the resulting director dynamics in a number of different system configurations of this type. [20][21][22][23][24][25][26][27][28][29] When a steady-state director orientation arises and the elastic torques are of minor significance, the shear-rate dependence of the steady-state angle, q, is given simply by: 30…”

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

“…It was shown previously that the director of PBLG can be changed by the application of shear forces at high concentrations. – This change is typically monitored by acquiring deuterium spectra of the deuterated solvent in the lyotropic mesophase. – While samples containing low amounts of PBLG (e.g., a 4% PBLG/CDCl 3 sample) did not show any deuterium splitting in accordance with the lack of alignment (Figure A), acquiring corresponding deuterium spectra on a 13% PBLG/CDCl 3 sample resulted in well-separated doublets with a quadrupolar splitting of |Δν Q | = 482 Hz (Figure B). Upon rotation at 48 Hz using the rheo-NMR apparatus introduced in the previous section, the quadrupolar splitting changed significantly to a value of 193 Hz (Figure C).…”