In the experimental investigations it is usual to employ a high frequency electric field, typically several kHz, to ensure that disruption of the director orientation by conductive motion of ions is ...eliminated. However, in the torque-balance equation, the electric field is taken to be constant. Clearly the periodic variation in the square of the electric field will influence the director dynamics, thus as the electric field increases so the relaxation time will decrease and as the field decreases so the relaxation time will increase. In addition, the direction of the director alignment will be reversed when the electric field starts to decrease ; however the precise moment when this occurs will depend on the frequency of the electric field in comparison with the composite relaxation rate. The importance of these variations to the validity of the analysis, based on the assumption that the electric field is constant, will also depend on the frequency of the electric field and the composite relaxation rate for the director. Here, we consider this question both theoretically and experimentally.
We report studies using deuterium NMR spectroscopy of perdeuteriated 4-octyl-4'-cyanobiphenyl-d25 (8CB-d_ ) where the director is aligned by a magnetic or an electric field. This particular technique ...was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. For all of the experiments in the nematic and smectic phases of 8CB-d_ we find that the relaxation times are independent of the group used to determine the director orientation during the alignment process.
It is important both for basic science and device applications to investigate the static and dynamic director orientation in a thin nematic liquid crystal (NLC) slab. Optical and electrical ...measurement methods are generally used to determine the director orientation or distribution. It is to be expected that deuterium NMR spectroscopy combined with simultaneous insitu observation of the optical transmittance should give us a good understanding of the director orientation for a thin NLC slab because the former is controlled by the sum of the spectrum over each position in the slab and the latter by the optical anisotropy averaged over the slab. This combined method is used to investigate the director distribution in a thin NLC slab.
Deuterium nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the field-induced director dynamics in a nematic liquid crystal, 4-pentyl-d_2-4'-cyanobiphenyl (5CB-d_2) ...deuteriated in the α-position of the pentyl chain, confined between two glass plates. The NMR spectra have been measured as a function of time after turning an electric field on and off. It is demonstrated that the field-induced director dynamics in the nematic liquid crystal cells can be successfully time-resolved. In addition, it is found that the doublet NMR spectra are changed into powder-like spectra during the turn-on and turn-off processes. It is shown that values for the rotational viscosity and the diamagnetic anisotropy of 5CB-d_2 can be determined from the time-resolved NMR spectra by assuming uniform alignment of the director.
We have used deuterium NMR to investigate the director distribution in a 5CB confined between two glass plates and subject to both magnetic and electric fields. The quadrupolar splitting from a ...sample with a weak anchoring condition decreases with increasing electric field, through zero and then increases again to a value which is essentially half of that in zero electric field. It would seem that the director orientation changes more or less continuously from being parallel to the magnetic field to being orthogonal to it, as the electric field grows. The voltage dependence of the quadrupolar splitting for a strong anchoring condition is nearly the same as that for a weak anchoring case. However two quadrupolar splittings are observed below a critical value of the electric field.
Deuterium NMR is a powerful technique to investigate the director distribution of a nernatic stab, because the NMR spectrum can contain a simple quadrupolar doublet whose separation us affected by ...the angle made by the director with the magnetic field vector. Using a contnnuum theory approach we calculated the director distribution in order to understand the experimental results. In this approach the torque balanced equations, involving the magnetic, electric and elastic energy, and a unified surface anchoring energy, was included. The equations with different boundary conditions were numerically solved to obtain the director distributions, which were employed to simulate the NMR spectra. The observed spectra for the strong anchoring condition showed a bistable director distribution below a critical value of the electric field. It was confirmed from the spectral simulations that two director distributions exist in a nematic slab with strong anchoring conditions. One is compatible with the weak anchoring condition, and the other vanishes at the critical point. At higher electrnc fields, for both strong and weak anchoring conditions, the director alignes parallel to the electric field.
