Anisotropic Surface Order in the Isotropic Phase of Nematic Liquid Crystals

A spinning-drop technique was used to study the thermal variation of surface tension of two nematic liquid crystals at the interface with an isotropic fluid. A sharp increase in interfacial tension was observed in the vicinity of the nematic-isotropic transition. The interfacial tension-temperature characteristics, unlike monotonically decreasing dependence found in most isotropic fluids, showed regions of positive slope, both in the isotropic and anisotropic phases. A positive slope in the isotropic region of the liquid crystal is interesting since it implies that excess order has developed at the surface while the buk interior remains isotropic.     

Surface Order Parameter and Polar Anchoring Energy in Nematic Liquid Crystals

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.     

Viscometry of Nematic Liquid Crystals

Viscometric experiments have played a significant part in establishing continuum theory for nematic liquid crystals. Such theory supports the hypothesis that observed non-Newtonian behaviour stems from competition between the aligning influences of flow and solid surfaces, and as a consequence predicts rather unusual scaling for the apparent viscosity. This paper first describes such scaling and its subsequent experimental confirmation emphasising the full implications of the latter. The relevance of the theory thus established, there follows a simple analysis of alignment in shear flow which leads to conditions on material coefficients necessary to ensure consistency with observations. Our discussion turns next to an account of solutions exhibiting non-Newtonian behaviour, and also mention of a recent stability analysis which attempts to discriminate between the different solutions that are possible. The final section considers oscillatory shear flow and presents an analysis of an instability which can occur in certain nematics.     

Dipole-Dipole Correlation in Nematic Liquid Crystals

The theory of dipole-dipole correlation in nematic liquid crystals is presented and it is shown that different models for the internal field lead to different values for dipole correlation factors. Results of calculations are presented for three liquid crystals and commented upon. Using the calculated correlation factors and a simplified dielectric theory, predictions are made of the temperature derivatives of the mean permittivity and permittivity anisotropy and compared with experimental values.     

Field-Induced Order in Nematic Liquid Crystals

The effect of an external magnetic field on the orientational order of a nematic liquid crystal has been examined using both Landau-de Gennes and Maier-Saupe theories. In the Maier-Saupe approach a rotationally invariant form of the pseudo-potential is introduced, which in the absence of an external field leads to three degenerate isomorphic solutions for the order parameter, corresponding to alignment along three principal axes; a similar result is obtainable from the Landau-de Gennes theory. Application of a magnetic field lifts the degeneracy of these solutions, and for materials having a positive diamagnetic susceptibility anisotropy, the uniaxial solution with alignment along the field direction is always energetically favorable. For materials with a negative susceptibility anisotropy, a biaxial solution minimizes the free energy at low temperatures, but on increasing the temperature there is a transition from a biaxial phase to an uniaxial phase. The field dependence of the transition temperatures is evaluated, and for positive materials there is a critical field, corresponding to a second order transition above which the nematic and isotropic phases are indistinguishable. A contrasting behavior is predicted for negative materials, and above a certain critical field the biaxial/uniaxial transition changes from first order to second order. For weakly ordered systems it is shown that the Landau-de Gennes expression for the free energy is identical to that obtained from the Maier-Saupe theory. However, for more ordered systems, the results of the two approaches differ, and in particular the Maier-Saupe theory predicts a susceptibility divergence temperature T* which increases with applied field, in agreement with recent experiments.     

Influence of Additives and Surface Topography on the Alignment of Nematic Liquid Crystals

Observations of the influence of a simultaneous action of different surface topographies and polar or nonpolar additives in small concentration on the alignment properties of liquid crystal molecules are reported. The additive dependence of the threshold behaviour of planar aligned liquid crystal cells is also studied.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.     

Local Field Anisotropy in Nematic Liquid Crystals

The anisotropy of the local electric field in the interior of ordered fluids provides a measure of the anisotropy of the molecular distribution. In nematics, both the orientational order parameter and the anisotropy of the local field can be calculated from refractive index and density measurements; results are presented for the liquid crystal butyl p-(p-ethoxyphenoxycarbonyl) phenyl carbonate. It is found that the anisotropy of the local field is very nearly proportional to the orientational order parameter. A simple model incorporating steric intermolecular repulsion predicts this behaviour. The ratio of the local field anisotropy to the uniaxial and biaxial order parameters is simply related to the length-to-breadth ratio of the effective molecular hard core.     

Molecular Dynamics Study of the Nematic Liquid Crystals

Since nematic liquid crystals have found a wide use for solving a number of practical problems, the theoretical prediction of thermodynamic and transport properties of liquid crystal systems, as well as an explanation of the structure and dynamic characteristics on the basis of reasonable molecular models have become a point of great importance. The modern molecular- statistical theories, e.g. Maier-Saupe theory and its modifications, use rough approximations (mean field approximations for example) and therefore may lead to wrong conclusions.     

TSD of Liquid Crystals in the Supercooled Nematic Phase

MBBA and EBBA are used to apply the techniques of thermally stimulated depolarization (TSD) to liquid crystals in their glassy state. The orientational mobilities of the butyl and methoxy resp, ethoxy group show significant differences compared to the crystalline phases. A small TSD band in MBBA at 69.5 K and in EBBA at 64.5 K probably comes from relaxations of the butyl group. A ten times stronger TSD band at 117.5 K resp. at 124 K is due to relaxations of the methoxy respect to the ethoxy group. Using the partial heating technique and the thermal sampling technique a detailed analysis of this band allows more insight into the distribution of reorientation enthalpies and relaxation times.     

Diamagnetic Anisotropy Measurements of Nematic Liquid Crystals

Electro-optic effect of liquid crystals in the presence of a magnetic field was investigated. Two independent techniques: magnetic field dependent threshold voltage and magnetic field-assisted relaxation time, were used to evaluate the diamagnetic anisotropy for three nematic liquid crystals: BDH-E7, RO-TN-103 and ZLI-1132. Correlation between the diamagnetic anisotropy and the molecular structures was obtained. This information is particularly important for synthesizing liquid crystal components with desired diamagnetic anisotropy.     

Anisotropy of Ultrasonic Attenuation in Nematic Liquid Crystals

Anisotropic ultrasonic attenuation in nematic liquid crystals is accounted for by an intramolecular mechanism. Anisotropy calculated for n-(p-methoxybenzylidene)-p-butylaniline (MBBA), on the basis of the proposed model, is in good agreement with experimental results.     

Modulated Microdomain Switching of Nematic Liquid Crystals

Abstract

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.     

Laser Induced Reorientation of Nematic Liquid Crystals

A significant increase of the divergence and appearance of a fringe system has been observed by illuminating of MBBA (p-n-methoxybenzilidene-p-butylaniline) and OCB (octyl-cyano-biphenyl) nematic liquid crystalline samples with a collimated beam of an argon ion laser. The dependence of this effect on laser power, beam polarization, and angle of incidence has been studied in homeopolar and planar sandwich-like cells of 50-150 μm thickness, in the nematic phase. At a homeopolar cell of MBBA (150 μm thickness), at normal incidence a threshold laser power of 45 m W was found. The phenomena can be explained as deformation of the orientation by the Fredericks effect due to light fields. The observations are in good agreement with the theoretical predictions including an estimation of the laser power threshold.     

Landau Theory of Biaxial Nematic Liquid Crystals

The orientational order of molecules in a liquid crystal is traditionally described by spatial or temporal averages of functions that are quadratic in the direction cosines of molecular axes with respect to laboratory fixed axes. The free energy describing the nematic phase must be independent of the choice of coordinate systems and therefore rotationally invariant with respect to both laboratory and molecular frames. A Landau theory for biaxial and uniaxial nematics based on six fundamental invariants is developed which is properly rotationally invariant. Four possible nematic phases, two uniaxial and two biaxial, result. NMR experiments should be able to identify which of these actually occur in specific materials.     

Molecular Orientational Order in Reentrant Nematic Liquid Crystals

The predictions of orientational order parameter in reentrant nematic liquid crystals using a Landau-type theory are compared with those from the McMillan-type molecular theory. Recent experimental results from birefringence, NMR and ESR studies are discussed.     

Nematic Liquid Crystals with a Trifluoromethyl Group

Abstract

We have synthesised a new class of compounds incorporating a trifluoromethyl group in the terminal alkoxy or alkenyloxy chain. Compounds containing several aromatic rings were synthesised with a view to producing compounds of high birefringence. This also included the synthesis of compounds containing a carbon-carbon triple bond. Compounds with three 1,4-disubstituted rings were synthesised in order to produce materials with a high nematic clearing point and as low a melting point as possible. Laterally fluoro-substituted compounds were prepared to generate a high positive value of the dielectric anisotropy, a low melting point and no smectic mesophases. Substances containing a cyclohexyl ring, which tend to induce a lower viscosity and compounds incorporating a bicyclo[2.2.2]octane ring were also synthesised in order to produce a high nematic clearing point and to influence the elastic constants. The (E)-trifluorobut-2-enyloxy-compounds were prepared in attempts to produce compounds with appropriate elastic constants and a high birefringence.     

Magnetic Field Induced Deformations in Nematic Liquid Crystals

Abstract

Unlike most organic materials, liquid crystals respond readily to magnetic fields. This response originates in the anisotropic magnetic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large optical birefringence of liquid crystals, these orientational deformations are easy to observe. An interesting demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers which is illuminated from below. As the magnetic field reorients the liquid crystal, beautiful interference colours appear, indicating director deformations.