Flexoelectric polarization in nematic liquid crystals measured by a field on-off pyroelectric technique

A novel technique based on the pyroelectric effect is developed for making direct measurements of the flexoelectric polarization P _f in hybrid aligned nematic cells. The pyroelectric response is measured first in the field-off regime and then with the bias field applied. The latter allows a direct comparison of P _f with the field-induced polarization over the whole range of the nematic phase of the standard compound 5CB. The sum of the flexoelectric coefficients (e ₁+e ₃) is shown to be negative, and its dependence on the nematic order parameter does not follow a simple law (linear or quadratic), as has been predicted theoretically. The dynamics of P _f is discussed in terms of the order parameter fluctuations. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 220–225 (10 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Short range order in nematic liquid crystals

The effect of short range order on the nematic isotropic transition is investigated by means of a generalization of the socalled Bethe approximation. The results are compared with the Maier-Saupe theory, Monte Carlo calculations, Kimura's results and experimental data for PAA.     

Short range orientational order in nematic liquid crystals

The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.     

Influence of second order elasticity on the piezoelectrical bending of homeotropic nematic layers

It is theoretically shown that a more exact formula for determining the piezoelectric coefficient of bending e_3x is obtained by taking into account the elastic energy with the second derivatives of the director with respect to the coordinates included.     

Paramagnetic nematic liquid crystals

The paper reports new nematic liquid crystals that are paramagnetic.     

Correlation functions for nematic liquid crystals

We show that the correlation functions of a system of hard ellipsoids whose axes are constrained to be parallel are simply related to the corresponding functions of a hard sphere fluid with the same packing fraction. The existence of simple, accurate approximations for the latter system make the oriented ellipsoid fluid a suitable ‘reference system’ for the nematically ordered phase of liquid crystals and other orientationally ordered molecular fluids such as polar fluids in an external field. The properties of such fluids, with a given angular distribution f(ω), may then be obtained via ‘standard’ perturbation theory. Methods for obtaining f(ω) near the perfectly ordered state are discussed.     

A comparison of orientational order measurements in cholesteric and nematic liquid crystals

We present a quantitative comparison of orientation order parameter measurements at constant pressure and constant density in cholesteric and nematic liquid crystals. The two phases show great similarity. In addition, the volume dependence of the mean-field potential is estimated for this cholesteric, and found to differ from measurements in nematics.     

On long-range order in low-dimensional lattice-gas models of nematic liquid crystals

The problem of the orientational ordering transition for lattice-gas models of liquid crystals is discussed in the low-dimensional case d = 1, 2. For isotropic short-range interactions, orientational long-range order at finite temperature is excluded for any packing of molecules on the lattice Z^d; on the other hand, for reflection-positive long-range isotropic interactions, we prove the existence of an orientational ordering transition for high packing (μ > μ₀) and low temperatures (β > β_c(μ)).     

Phase and structural order in mixture of nematic liquid crystals and anisotropic nanoparticles

We studied the structural order in randomly perturbed nematic liquid crystals using a random anisotropic nematic-type lattice model. Simulations were carried out at zero temperature and at various other temperatures, for which we varied the concentration of impurities p, anchoring strength W and imposed external strength fields B at two different histories: temperature-quenched history (TQH) and field-quenched history. The Haller approximation method utilized the temperature dependence of the order parameter to determine the pseudo-critical exponent β at different values of p and W. Consequently, the values of β increased when p and W were increased. Additionally, applying an external field B for investigating the parameters revealed that the TQH established long-range order for high magnitudes of B at zero temperature (T* = 0) and T* = 0.5.     

Relation between orientational order parameters of matrix and probe in nematic liquid crystals

Summary The phenomenological relation between the orientational order parameters of matrix (S) and probe (S _p) has been obtained and confirmed for a large number of impure nematic liquid cyrstals. The temperature-dependent parts of the parametersS andS _p are proportional to one another and have the same critical behaviour over the whole mesophase range, although the absolute values ofS andS _p as well as their temperature dependencesS(T) andS _p(T) may be sufficiently different. The relation observed is not the consequence of the known molecular-statistical theories of impure nematic liquid crystals. Work presented at the second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

The effect of spontaneous polarization on two-dimensional elasticity of smectic liquid crystals

The influence of polarity on orientational elasticity and on structures formed in the director field is studied in free-standing smectic films. Periodic stripe patterns and 2π-walls in a magnetic field are investigated. Measurements are performed on a nonpolar racemic mixture, on an optically pure ferroelectric compound, and in mixtures with different concentrations of the chiral isomers of opposite signs. The structure of periodic stripes changes drastically with the polarity of the film. The ratio of the bend K _B and splay K _S elastic constants for smectic films is determined as a function of polarization from the structure of periodic stripe patterns and 2π-walls. We find that the elastic anisotropy K _B /K _S increases essentially with increasing the polarity of the film. Changes of the elasticity and the structure of periodic stripes are explained by polarization charge effects.     

One- and two-dimensional deformations in nematic liquid crystals: Influence of the flexoelectric polarization

Summary The elastic deformations in nematic liquid crystals are analysed in one and two dimensions. The influence of the flexoelectric polarization on the elastic properties of the medium is studied. It is shown that for one-dimensional deformations, the presence of the flexoelectric polarization introduces a renormalization of the elastic constants. On the contrary in the bidimensional case, the effect of the flexoelectricity is not a simple renormalization of the elastic constants. The profiles of the electric potential and of the tilt angle are evaluated for the cases in which the substrate is an insulating or conducting material. The validity of the calculations is discussed by considering the value of Debye's screening length of the nematic liquid crystal.     

Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.     

Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to ) and thick (7 to cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to ) of ( , where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far ( C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.Received: 22 February 2004, Published online: 24 May 2004PACS: 61.30.-v Liquid crystals - 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitionsSurajit Dhara: Present address: Department of Physics, Birla Institute of Technology and Science, Pilani 333 031, India.     

An equation of state for nematic liquid crystals

A derivation of an equation of state for nematic liquid crystals is given based upon slightly non-spherical molecules interacting via anisotropic attractive forces. The results are in reasonable agreement with the experimental data for p-azoxyanisole.     

Density functional theory for chiral nematic liquid crystals

ABSTRACT

In this note, the density functional theory for chiral nematic liquid crystals is expressed in terms of rotational invariant expansion coefficients of the pair-excluded area at fixed separation. This modification from the standard approach, without making any additional assumptions, yields a more efficient procedure for minimising the free energy, as well as providing insight into the origins of the effect away from the long-pitch limit.     

Electroconvection in homeotropic nematic liquid crystals

Electroconvection is a classical example of pattern-forming phenomena in liquid crystals, typically observed in nematics with negative dielectric and positive conductivity anisotropies. This article focuses on how electroconvection in the homeotropic geometry differs from that in planar alignment. The influence of an additional magnetic field on the pattern characteristics and on secondary instabilities (the normal roll–abnormal roll transition) is discussed. The homeotropic alignment offers unique possibilities also for studying defect motion. Basic characteristics of some patterns of large wavelength are presented and compared with those of the classical Carr–Helfrich structures. Finally, electroconvection in substances with negative conductivity anisotropy is addressed.     

Bistability in dual-frequency nematic liquid crystals

Different modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.