Director reorientation processes in a monodomain thin nematic liquid crystal film: A deuterium NMR spectroscopy study

Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the director dynamics in the nematic liquid crystal, 4-pentyl-4'-cyanobiphenyl (5CB), confined between two glass plates and subject to magnetic and electric fields. The nematic cell was held in the NMR probe head so that the electric field, whose direction is normal to the substrate surface, makes an angle of about 45degrees with the magnetic field. This experimental geometry avoids the degeneracy in the field-induced alignment pathway for the director found for larger angles. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of time. When the electric field, whose intensity is controlled so that the director makes an angle with the magnetic field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to the magnetic field because Delta(epsilon) over tilde and Delta(chi) over tilde are both positive for 5CB. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-on and the turn-off alignment processes as a function of time. The realignment pathway of the director was monitored by measuring Delta(nu) over tilde, the deuterium quadrupolar splitting. We have studied the time dependence of the director orientation for the turn-on and turn-off processes at different temperatures in the nematic phase. The temperature independence of Delta(chi) over tilde/Delta(epsilon) over tilde is also discussed on the basis of experiment and theory. The diamagnetic anisotropy and the rotational viscosity coefficient were also determined as a function of temperature. The deuterium NMR spectra corresponding to the field-induced director dynamics were predicted by an analysis based on hydrodynamic theory. (C) 2002 American Institute of Physics.