Polarity-dependent dielectric torque in nematic liquid crystals

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this "dielectric memory" leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the "switch-off" phase of director dynamics.     

Dielectric relaxation behaviour of liquid crystals with opposite orientation of -COO ester group in molecular core

Dielectric measurements were performed for two smectogens, being structurally analogous with the opposite space orientation of the-COO ester group in the molecular core, using a HP4192A impedance analyser. Gold coated electrode cells of different thicknesses were used. Temperature dependences of the relaxation frequency fR and the inverse of the dielectric strength Δε′_┴ ⁻¹ obtained by fitting experimental values of perpendicular components, i.e., the real ε′_┴ and the imaginary ε″_┴ parts of the complex dielectric permittivity to the Cole-Cole equation as well as the investigation of modification of relaxation processes under bias were determined for the SmC_*, SmA_*, and N_* phases of studied compounds. One can conclude, on the basis of the above results, that dielectric relaxation processes observed by us in the studied compounds are similar to those of the soft and Goldstone mode typically observed by others in the SmC_*, SmA_*, and N_* phases. It is concluded from acomparison of their properties with other related compounds that the link between the biphenyl moiety and-COO ester group is closely related to the stability of smectic phases.     

Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

Indium tin oxide (ITO) layers are widely used to make electrodes in measuring cells, because these layers are transparent and electrooptical investigations can be performed using such prepared cells. It was found during the dielectric spectroscopy measurements, performed for smectic liquid crystalline mixture, that it is not possible to detect some important relaxation modes in paraelectric SmA*, ferroelectric SmC*, and antiferroelectric SmC_A* phases for the frequencies higher than 300 kHz. The measuring cell does not allow to measure relaxations, because its own dielectric behaviour covers the dielectric response of a liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to the finite resistance of ITO layers. The theoretical model was introduced, which shows how to calculate relaxations related to liquid crystals from dielectric response of the empty and filled measuring cell. The proof of strong influence of cell properties on effective (measuring) values of dielectric permittivities was shown.     

Dielectric spectroscopy of liquid crystals. Electrodes resistivity and connecting wires inductance influence on dielectric measurements

Dielectric spectroscopy is a very useful experimental method for liquid crystal investigation. Electrodes made from indium tin oxide (ITO) or gold are widely used in measuring cells. During the dielectric spectroscopy measurements performed for smectic liquid crystalline mixture it was found that detection of some important relaxation modes in paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC_A* phases for frequencies higher than 0.2–0.5 MHz is not possible. The measuring setup does not allow us to measure such relaxations due to its own dielectric response covering the dielectric response of liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to non-zero resistivity of electrode material or non-zero inductivity of connecting wires. In this paper, the new model was introduced. Its final equations show how to calculate parameters of relaxations observed in liquid crystals, from dielectric response of the empty and filled measuring cell. The experimental proof of strong influence of measuring setup properties on effective (measured) values of dielectric permittivities was shown.     

Nematic liquid crystals with positive dielectric and negative diamagnetic anisotropy

p-Alkyl-p′-cyanocyclohexylcyclohexanes CCH represent a new class of thermotropic nematic liquid crystals which no longer exhibit UV absorption abobe 200 nm, are distinguised by an optical anisotropy of less than 0.1, and in contrast to all previously known “dielectric positive”, thermotropic nematic liquid crystals possess a negative diamagnetic anisotropy.     

Dielectric measurements of new antiferroelectric liquid crystals

Three new pure antiferroelectric materials, synthesized at the Military University of Technology (MUT), were investigated by dielectric means. Clear relaxation modes were detected, i.e., Soft, Goldstone, low frequency in-phase (P_L) as well high frequency anti-phase (P_H) modes for paraelectric Sm A*phase, ferroelectric Sm C* phase and antiferroelectric Sm C_A* phase, respectively. Additionally, a high relaxation mode in Sm C_A* was found. The relaxation frequencies f_R of the observed modes were calculated from Cole-Cole diagrams.     

The problem of linear dielectric dispersion in crystals and polymers

Some general laws of the dispersion behavior of electric characteristics are considered for various dielectric materials. The mechanisms of relaxation processes and linear dielectric response in the ?″(?′) diagram are discussed.     

Dielectric dispersion and its temperature variation in calcite single crystals

The variation of dielectric constant and loss of calcite single crystals with frequency and temperature over wide ranges (10² to 2.4×10¹⁰c/s and ?193 to 450 °C) in two orientations of the crystal, namely, the electric field (i) parallel to the optic axis and (ii) perpendicular to the optic axis, has been studied and the results reported. The dielectric constant at 25 °C in both orientations (E‖, 7.8 andE⊥, 8.2) is frequency-independent. The dielectric loss shows a minimum in the 10⁵ to 10⁸ c/s region. Dielectric constant as a function of temperature exhibits two distinct regions: frequency-independent (?193 to 50 °C) and frequency-dependent (50 to 450 °C). The temperature variation of specific conductivity can be represented by a sum of two exponentials, e.g., \(\sigma = Ae^{ - E_1 /kT} + Be^{ - E_2 /kT}\) with the usual notation; the constants have different values in the two orientations. The intrinsic conductivity with larger activation energy (E‖ 0.91 eV andE⊥ 1.05 eV) is attributed to the movement of lattice vacancies in the crystal. The 15% larger value for the intrinsic activation energy alongE⊥ optic axis over the other orientation may be due to a probably large electrostatic interaction between the charge carriers and the lattice ions in this orientation.     

Planar photonic crystals infiltrated with liquid crystals: optical characterization of molecule orientation

Nematic liquid crystals are infiltrated into InP-based planar photonic crystals. Optical measurements as a function of temperature and polarization are used to study the average director field configuration in the nanometer-size holes: a planar equilibrium state is found.     

Interface dynamics in liquid crystals

We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out. Received 5 August 1999 and Received in final form 13 September 1999     

Fluctuational spatial dispersion in achiral liquid crystals

An investigation is made of the spatial dispersion of the permittivity of achiral liquid crystals with large-scale fluctuations. It is observed that for large correlation lengths the terms with spatial dispersion are anomalously large. The specific form of these terms is obtained for orientational fluctuations in a nematic and fluctuations of the deformations of the smectic layers in a smectic-A in an orienting magnetic field. It is shown that these effects may be observed optically by means of accurate measurements of the angular dependence of the refractive indices of electromagnetic waves. Zh. éksp. Teor. Fiz. 114, 182–190 (July 1998)     

Dielectric properties of liquid crystals in polycapillary matrices

This paper reports on the results of investigations into the dielectric properties of liquid crystals embedded in polycapillary matrices and describes a technique for their measurement. It has been revealed that the chemical structure of the rigid core and the length of mobile alkyl groups of liquid-crystal molecules of the alkylcyanobiphenyl group substantially affect the equilibrium configuration of the liquid-crystal director in capillaries. The reorientation of liquid-crystal molecules embedded in capillaries in the nematic phase under the influence of an external magnetic field has been investigated.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

Dielectric heating and relaxations in smectic a liquid crystals

The dependence of the temperature changes ΔT on frequency occurring in the smectic A, nematic and isotropic phase of a positive dielectric liquid crystal material initiated by dielectric heating is investigated. The same relaxation mechanism governing the strong increase of ΔT in nematics is shown to be responsible for inducing comparably strong dielectric heating effects in smectic A phases.     

Orientation polarization and dielectric dispersion characteristics of nonpolar liquid solutions

Taking account of dipole-multipole interactions of molecules, the Debye dispersion _p — _p is computed in nonpolar liquid solutions. The influence of the multipole interactions between molecules of different components of the solution on the Debye dispersion is demonstrated. It is shown that the theory is in good agreement with experiment for the solutions CCl₄-C₆H₆, CCl₄-CS₂, C₆H₆-CS₂, and C₄H₈O₂-C₆H₆.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 13–17, November, 1986.     

Static dielectric pretransitional effects in thermotropic liquid crystals

The paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.     

Orientational dynamics in nematic liquid crystals

We examine the behaviour of single-particle orientational time correlation functions in nematic liquid crystals. As well as the expected dynamics involving oscillation in a mean-field potential, and occasional jumps between orientations parallel and antiparallel to the director, we provide the first simulation evidence of long-time tails characteristic of coupling to director fluctuations.