Backflow-affected relaxation in nematic liquid crystals

A complete numerical study of a two-dimensional nematic backflow problem is presented. Nematodynamic equations are reviewed, and characteristic scales are introduced. The relaxation under the application and suppression of a magnetic field is studied in square- and rectangular-shaped cells. Solutions for the flow fields, director fields, and director time derivative fields are given and these are interpreted to gain a qualitative understanding of the problem. The backflow is found to depend critically on the geometry of the cell. The complete solution is compared with the simplified approach in which the backflow is neglected. The discrepancy depends strongly on the cell geometry.     

Pre-electrohydrodynamic relaxation phenomenon in nematic liquid crystals

Study of electrohydrodynamic instabilities in thicker nematic liquid crystal samples (800–1200 μm) of MBBA is reported. An initial rise in the intensity of transmitted light immediately after the application of an electric field has been found to precede the attenuation of the transmitted beam due to the gradual development of hydrodynamic instability in the sample. The dependence of the relative dominance of the two relaxation phenomenon on the applied electric field intensity has been studied.     

Dielectric relaxation in polymer dispersed nematic liquid crystal films

Dielectric absorption studies for polymer dispersed liquid crystal (PDLC) samples of different nematic (4-hexyl-4'-cyanobiphenyl, 6CB) contents (20, 30, and 40 wt%) have been carried out in the frequency range from 10kHz to 10MHz. A method for evaluation of the fraction of the molecules which are not affected by the polymer surface anchoring forces in the nematic droplets is proposed.     

Preliminary Communication Dielectric relaxation in monotropic liquid crystals

Abstract

A way of investigating dielectric relaxation phenomena occurring in the isotropic and liquid crystalline states by the use of monotropic mesogens is suggested. The application of common methods of dielectric spectroscopy in the radio frequency range (10²-10⁸Hz) to strongly supercooled samples for the investigation of the amorphous liquid and low temperature monotropic liquid crystalline states is described. The possibility of experimental separation and investigation of the different dipole polarization mechanisms responsible for the dielectric properties of the monotropic mesogen 4-cyano-3-chlorophenyl 4-hexyl-3-chlorobiphenyl-4′-carboxylate (CChEC), whose molecules have longitudinal and normal permanent dipole moment components, is established.     

Preliminary Communication Dielectric relaxation in monotropic liquid crystals

A way of investigating dielectric relaxation phenomena occurring in the isotropic and liquid crystalline states by the use of monotropic mesogens is suggested. The application of common methods of dielectric spectroscopy in the radio frequency range (10²-10⁸Hz) to strongly supercooled samples for the investigation of the amorphous liquid and low temperature monotropic liquid crystalline states is described. The possibility of experimental separation and investigation of the different dipole polarization mechanisms responsible for the dielectric properties of the monotropic mesogen 4-cyano-3-chlorophenyl 4-hexyl-3-chlorobiphenyl-4'-carboxylate (CChEC), whose molecules have longitudinal and normal permanent dipole moment components, is established.     

Dielectric spectroscopy analysis in employing liquid crystal phthalonitrile derivative in nematic liquid crystals

Dielectric anizotropy and relaxation properties of 2,3-dicyano-1,4-di[3,4,5-tri(dodecyloxy)phenylcarbonyloxy] benzene (DCDPB)-doped E7 and E7 liquid crystal have been investigated by the dielectric spectroscopy method. Dielectric anisotropy property of the LCs changes from the positive type to negative type. Dielectric relaxation properties suggest that LCs exhibit a monodispersive dielectric property. The relaxation frequency of E7 and E7/DCDPB liquid crystals was calculated by means of Cole-Cole plots. Consequently, DCDPB dopant changes the dielectric anizotropy and relaxation parameters of E7 LC.     

Slow relaxation processes in nematic liquid crystals at weak surface anchoring

We present new results of experimental investigations of azimuthal director reorientation dynamics for a nematic liquid crystal on solid substrates. Two types of substrate with weak anchoring were studied: glass/polystyrene and glass/UV-activated dye. Slow and fast relaxation processes were observed in both cases under the action of a strong 'in-plane' electric field. The slow surface reorientation and memory effects were controlled by two parameters: the electric voltage and the excitation time. It was established that the increase of the excitation time results in a slowing of the relaxation of the system to the initial state after turning off the electric field. A phenomenological model of a gliding of easy axes is proposed to explain the slow relaxation process.     

New method for measuring polar anchoring energy of nematic liquid crystals

A new method for measuring a polar anchoring energy of nematic liquid crystals (LCs) is proposed. A variation of LC tilt angle on the surface with an applied electrical field was determined by a reflective method. The twisted LC cell configuration was selected to compensate a contribution of the induced birefringence in the reflective spectra. The electrical field controlled reflectance was used to analyse the potential form of the polar anchoring energy and to define the anchoring strength. The proposed method is applicable for 2–5 μm thick LC cells.     

Enhancement of the nematic order of polar liquid crystals in thin cells

Experimental observations show that the nematic order parameter (S) is significantly enhanced as the thickness of the cell is reduced. However, calculations based on earlier theories are not in good agreement with the experimental data. We earlier developed a molecular mean field model that assumes that the medium with polar molecules consists of inter-converting anti-parallel and parallel pairs, to explain the molecular origin of the ‘two lengths’ model. The model has been used to explain effects such as double re-entrance and the effect of electric field on the nematic–isotropic transition temperature, T _NI. We have recently extended the Maier–Saupe (M–S) theory using an empirical variation of surface potential, to account for the enhancement in which a molecule near the surface is assumed to feel the mean field potential (M–S type) and also the surface-induced potential. Variation in the density of the medium due to variation of S has been ignored. In the present paper, the change in the relative proportion of the parallel pairs due to changes in the density of the medium is included in the M–S theory with the surface potential. This results in improved agreement with the experimental data.     

Dielectric properties of some nematic liquid crystals for dynamic scattering displays

The dielectric constants of three room-temperature nematic mixtures suited for dynamic scattering displays are reported. The asymmetrically substituted azoxybenzenes involved exist as two isomers. One isomer has a positive dielectric anisotropy (Δ =

- ?_⊥ > 0), the other a negative one.     

Carbazole nematic liquid crystals

A number of calamitic 2,7-diary-N-alkyl-substituted carbazoles with an enantiotropic nematic phase have been prepared. Branching of the aliphatic chain attached to the nitrogen atom in the carbazole ring leads to significantly lower liquid crystal transition temperatures. These new materials show a lower ionisation potential than fluorene analogues and blue photoluminescence in solution and as thin solid films.     

Dielectric analysis of the interaction of nematic liquid crystals with carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) have been shown to self-organise, and when added as a guest to form a nanocomposite, their director couples with an organic liquid crystal (LC) host. Here, effects of MWCNTs on the low-frequency dielectric properties and Fréedericksz transition of the LC 4-cyano-4?-pentylbiphenyl (5CB) are studied. Anti-parallel electro-optic cells were filled with nanocomposites at weight percent concentrations of MWCNT to 5CB of: 0 (neat), 0.01, 0.10, 0.20, and 0.50. Low concentration was chosen to minimise Van der Waals attraction normally responsible for aggregation of MWCNTs. Dielectric relaxation spectroscopy was used to study interactions between MWCNTs and 5CB at frequencies from 20 Hz to 1 MHz. We propose a mechanism based upon measurements of the complex dielectric function which suggests that MWCNTs act as a slow-moving boundary within the sample cell at low frequencies and low applied electric fields, where the MWCNTs reorient along with the 5CB LC molecules. At higher frequencies and larger applied electric fields, the 5CB molecules rotate about their long axes while motion of the MWCNTs is frozen out.     

Magnetic-field induced biaxiality in nematic liquid crystals. Consequences for nuclear spin relaxation

When a uniaxial nematic liquid crystal is subjected to a magnetic field making a non-zero angle with the C_∞ axis, the uniaxial symmetry is broken. The principal effect is a field-induced biaxiality in the long-wavelength region of the director fluctuation spectrum. Whereas the induced biaxiality has little effect on the mean square director fluctuation amplitudes 〈n²_x〉 and 〈n²_y〉, which are dominated by short-wavelength modes, it can profoundly affect the nuclear spin relaxation behaviour, which is sensitive to long-wavelength modes. Motivated by the increasing number of nuclear spin relaxation studies of director fluctuations in thermotropic, amphiphilic, and polymeric nematic liquid crystals, we present here a theoretical analysis of the effects of field-induced biaxiality on nuclear spin relaxation.     

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from epsilon*(omega) (E n) to epsilon*(omega) (E n) allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.     

The polar anchoring energy of nematic liquid crystals with polyvinylidene fluoride alignment layer

It has been proved that the high dielectric polyvinylidene fluoride (PVDF) could be used as an alignment layer in liquid crystal device. In this paper, the transmittance, pretilt angle and the polar anchoring energy of the substrates with PVDF alignment layer were researched. Theoretical results and the experimental results about the reflectivity R_SS (polarisation-conserving signals) recorded by full-leaky guided mode in liquid crystal technique are analysed to evaluate the anchoring energy of PVDF alignment layer. The result shows that the polar anchoring energy between PVDF alignment layer and liquid crystal molecules is 2.80 × 10^?4 J/m².     

Multiple short time power laws in the orientational relaxation of nematic liquid crystals

Relaxation in the nematic liquid crystalline phase is known to be sensitive to its proximity to both isotropic and smectic phases. Recent transient optical Kerr effect (OKE) studies have revealed, rather surprisingly, two temporal power laws at short to intermediate times and also an apparent absence of the expected exponential decay at longer times. In order to understand this unusual dynamics, we have carried out extensive molecular dynamics simulations of transient OKE and related orientational time correlation functions in a system of prolate ellipsoids (with aspect ratio equal to 3). The simulations find two distinct power laws, with a crossover region, in the decay of the orientational time correlation function at short to intermediate times (in the range of a few picoseconds to a few nanoseconds). In addition, the simulation results fail to recover any long time exponential decay component. The system size dependence of the exponents suggests that the first power law may originate from the local orientational density fluctuations (like in a glassy liquid). The origin of the second power law is less clear and may be related to the long range fluctuations (such as smecticlike density fluctuations)--these fluctuations are expected to involve small free energy barriers. In support of the latter, the evidence of pronounced coupling between orientational and spatial densities at intermediate wave numbers is presented. This coupling is usually small in normal isotropic liquids, but it is large in the present case. In addition to slow collective orientational relaxation, the single particle orientational relaxation is also found to exhibit slow dynamics in the nematic phase in the long time.     

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

This study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.     

Defect-induced melting in nematic liquid crystals

In this paper we employ a relatively simple theory to show how a nematic disclination line can act as a nucleation site for the growth of the isotropic phase. With this theory we are able to find analytical expressions for the critical temperature of nucleation and the behaviour of the core radius as a function of temperature. We are then able to compare these results with a previous numerical model of this effect.