Deuterium NMR investigation of field-induced director dynamics: the role of backflow

Deuterium NMR spectroscopy has been used to investigate the director dynami cs in the nematic liquid crystal, 4-pentyl-4 ' -cyanobiphenyl, confined bet ween two glass plates and subject to orthogonal magnetic and pulsed electri c fields. When the pulsed electric field, whose intensity is strong enough to make the director align normal to the magnetic field, is applied to the nematic film, the director moves from being parallel to the magnetic field to being parallel to the electric field. After the pulsed electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-off realignment process as a function of time. With this experimental geometry the director alignment is not unique as it can rotate equally probably clockwise or cou nterclockwise in this realignment process. That is, the realignment pathway for the director is degenerate, which establishes a director flow pattern. We have studied the time dependence of the director orientation and distri bution for the turn-off process. The deuterium NMR spectra corresponding to the director dynamics in the realignment pathways were predicted by a cont inuum theory analysis including a time dependent viscous torque with an eff ective rotational viscosity. (C) 2001 Elsevier Science B.V. All rights rese rved.