Electric field‐driven director oscillations in nematic liquid crystals

Abstract

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.