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We have used deuterium NMR spectroscopy to investigate the director dynamics and equilibrium behaviour in nematic liquid crystals (4‐pentyl‐ and 4‐octyl‐4′‐cyanobiphenyl, (5CB and 8CB), both specifically deuteriated) when subject to magnetic and a.c. electric fields. The angle between the magnetic and electric fields can be varied between 0 and 90° and the most common geometry we have used is for an angle of about 45°. For 5CB and 8CB (with positive and ) the director orientation was measured using time‐resolved NMR both when the electric field is applied and when it is turned off. In all cases it was found that the director alignment was uniform and the director relaxation follows closely the predictions of the torque‐balance equation given by the Leslie–Ericksen theory. In all these experiments we have employed a 10 kHz electric field; at such a relatively high frequency the director experiences an effectively constant value of the electric field. We have now investigated the behaviour of the nematic director for the two liquid crystals at much lower frequencies of the electric field: several Hz to about 1000 Hz. As before, the director orientation was measured using time‐resolved deuterium NMR spectroscopy. We have employed two geometries. In one, the electric and magnetic fields were inclined at ∼⃒50°. We found that the director oscillates between two extreme orientations (determined by the frequency and the field strength) in a plane formed by the magnetic and electric fields. The oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The director was found to remain uniformly aligned. The two extreme director orientations can also be determined from the NMR spectrum time‐averaged over many thousands of cycles of oscillations. At low frequencies (several Hz) these limiting angles are essentially independent of frequency but as the frequency increases so the two angles approach each other and become equal at high frequencies. More recently, we have used a geometry with the angle between the fields of ∼⃒90°. A threshold behaviour is observed in this geometry for the director orientation as a function of the applied voltage. The time‐averaged spectra at low frequencies and at certain voltages showed unusual powder‐like features. Time‐resolved NMR measurements at 40 Hz and different voltages near the threshold value were carried out to understand the oscillatory behaviour which was also simulated. Turn‐on and turn‐off dynamics at high frequency were conducted revealing intriguing differences between the two pathways for the field‐induced relaxation. These results will be discussed and interpreted in terms of the torque‐balance equation with a time dependent electric field. [ABSTRACT FROM AUTHOR]

We have studied the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium nuclear magnetic resonance (NMR) spectroscopy and continuum theory. A nematic liquid crystal, 4-pentyl-d[sub 2]-4[sup ′]-cyanobiphenyl (5CB-d[sub 2]) deuteriated in the α position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method. A series of deuterium NMR spectra, obtained using a quadrupolar echo sequence, was acquired as a function of the applied electric field, which can be used to explore the director deformation. The deuterium NMR spectra predicted by continuum theory involving the magnetic, electric, elastic, and unified surface anchoring energies are found to be in good agreement with experiment. The investigation also reveals that profiles of the director distribution for the case with a magnetic field have a top-hat shape different to the bowler-hat shape, which is well known as the director profile for the case with no magnetic field. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies of the field-induced alignment of the SmA phase using deuterium NMR spectroscopy have revealed a complex pattern of behaviour when the director is initially orthogonal to the aligning field. Here we report the electric field-induced alignment of the SmA director using time-resolved deuterium NMR when the aligning electric field E is at an angle with the magnetic field B of the spectrometer which is considerably smaller than 90°; here the director is initially aligned parallel to the magnetic field. The dynamics of the electric field-induced alignment of the director for the smectic phase of 4-α,α-d 2 -octyl-4′-cyanobiphenyl (8CB-d 2 ) was investigated at two angles between B and E of roughly 45° and 54.5° the so-called magic angle, at different electric field strengths and also at two temperatures, 305.1 K and 302.6 K. The dynamics of the SmA director alignment of 8CB-d 2 was monitored by measuring the deuterium NMR spectrum as a function of time. The results for the 45° and 54.5° geometries revealed, in contrast to the complex dynamics of alignment of theSmA director observed for the 90° geometry, a much simpler pattern of relaxation. Here the director appeared to be aligned almost as a monodomain. In general, for either of the two geometries employed here, the ultimate angle of alignment the director achieves relative to E depends on the electric field strength. Lowering the temperature by just 2.5°C from 305.1 K to 302.6 K has a dramatic effect on the rate of director relaxation presumably because of the large increase in the combined rotational viscosity and the elastic energy effectsof the SmA phase with decreasing temperature. Furthermore for the 45° geometry at 302.6 K, the SmA sample separates on relaxation ultimately into two domains with different alignment angles. [ABSTRACT FROM AUTHOR]

The dynamic alignment of the nematic director by near-orthogonal electric and magnetic fields has been investigated. The intermediate states during the relaxation process were found, with the aid of time-resolved deuterium NMR spectroscopy, to be markedly nonuniform. The macroscopic order was perturbed, although the initial and final states of the director appear to be essentially uniform. However, the initial state does have a profound influence on the uniformity of the director in the intermediate states. We have developed a fundamental model based on the effect of spontaneous director fluctuations to explain these unusual NMR observations. [ABSTRACT FROM AUTHOR]