Ágnes Buka

DIELECTRIC RELAXATION IN THE NEMATIC AND ISOTROPIC PHASES OF N-HEPTYL- AND N-HEPTOXY-CYANOBIPHENYL.

Abstract The dielectric spectrum of n-heptyl-cyanobiphenyl(7CB) and n-heptoxy-cyanobiphenyl(7OCB) are reported over a frequency range up to 18 GHz and over a temperature range well into the isotropic phase. Two relaxation processes are observed in the isotropic phase of both compounds, as well as in the nematic phase (when measurements are made parallel to the nematic director). Possible mechanisms for the two relaxation processes are discussed.

Flexoelectricity and pattern formation in nematic liquid crystals.

We present in this paper a detailed analysis of the flexoelectric instability of a planar nematic layer in the presence of an alternating electric field (frequency ω), which leads to stripe patterns (flexodomains) in the plane of the layer. This equilibrium transition is governed by the free energy of the nematic, which describes the elasticity with respect to the orientational degrees of freedom supplemented by an electric part. Surprisingly the limit ω→0 is highly singular. In distinct contrast to the dc case, where the patterns are stationary and time independent, they appear at finite, small ω periodically in time as sudden bursts. Flexodomains are in competition with the intensively studied electrohydrodynamic instability in nematics, which presents a nonequilibrium dissipative transition. It will be demonstrated that ω is a very convenient control parameter to tune between flexodomains and convection patterns, which are clearly distinguished by the orientation of their stripes.

Flexoelectricity in liquid crystals : theory, experiments and applications

Preface - the Concept of Flexoelectricity (R Meyer) Molecular Theory of Flexoelectricity in Nematics (M Osipov) Flexoelectro-optics and Measurements of Flexocoefficients (N V Madhusudana) Flexoelectricity of Bent Core Molecules (A Jakli & N Aeber) The Role of Flexoelectricity in Pattern Formation (A Buka et al.) Flexoelectro-optic Chiral Nematic Based Devices and Displays (H Coles & S Morris) Flexoelectricity in Chiral Polar Smectics - The Origin of Interactions to Distant Layers (M Cepic) Flexoelectricity in Biological and Lyotropic Systems (A Petrov) Applications of the Flexoelectric Effect (S T Lagerwall & P Rudquist).

Prewavy Pattern: A Director-Modulation Structure in Nematic Liquid Crystals

Abstract A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Regular dendritic patterns induced by nonlocal time-periodic forcing

The dynamic response of dendritic solidification to spatially homogeneous time-periodic forcing has been studied. Phase-field calculations performed in two dimensions (2D) and experiments on thin (quasi-2D) liquid-crystal layers show that the frequency of dendritic side branching can be tuned by oscillatory pressure or heating. The sensitivity of this phenomenon to the relevant parameters, the frequency and amplitude of the modulation, the initial undercooling and the anisotropies of the interfacial free energy, and molecule attachment kinetics, has been explored. It has been demonstrated that in addition the side-branching mode synchronous with external forcing as emerging from the linear Wentzel-Kramers-Brillouin analysis, modes that oscillate with higher harmonic frequencies are also present with perceptible amplitudes.

Direct transition to electroconvection in a homeotropic nematic liquid crystal

We present an experimental and theoretical investigation of a variant of electroconvection using an unusual nematic liquid crystal in an isotropic configuration (homeotropic alignment). The significance of the system is a direct transition to the convecting state due to the negative conductivity anisotropy and positive dielectric anisotropy. We observe at onset rolls or squares depending on the frequency and amplitude of the applied ac voltage with a strong signature of the zigzag instability. Good agreement with calculations based on the underlying hydrodynamic theory is found. We also construct an extended Swift-Hohenberg model which allows us to capture complex patterns like squares with a quasiperiodic modulation.

Molecular Relaxation in hi Ghly Ordered Orthogonal Smectic SA, SB and SE Phases

Abstract Dielectric relaxation and calorimetric investigations were perforemed on different smectic phases. Discountinuities of the critical frequency were found at both SA-SB and SB-SE phase transitions. SA and SE structures can be regarded as two-dimensional liquid and two-dimensional crystal, respectively. The low frequency dielectric dispersion presented here demonstrates the difference btween the SE and a three-dimensional cyrstal where this phenomena is absent.

Physical properties of a bent-core nematic liquid crystal and its mixtures with calamitic molecules

This article summarizes the results obtained by various experimental methods on the physical properties of a bent-core nematic liquid crystal 4-chloro-1,3-phenylene bis-4-[4′-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB). The material exhibits unusual properties in all aspects tested. Its bend flexoelectric coefficient is 1000 times larger than in calamitics; it is viscoelastic with a large, shear-rate-dependent viscosity. Its bend and twist elastic constants are abnormally low; thus the nematic phase can be rendered to be a blue fog phase with a small amount of chiral dopant. It shows very high flow birefringence and unusually small leading Landau coefficient. It has two types of isotropic phases; at lower temperature it is probably tetrahedratic that can be transferred into the nematic phase with magnetic field. ClPbis10BB has a frequency-dependent conductivity anisotropy which is characterized by a double sign inversion. It exhibits various electroconvection (EC) patterns which are currently not understood in the frame of the standard theory of EC.

Properties of a Homologous Series of Ferroelectric Liquid Crystals

Abstract A homologous series of MBOPEnOBA chiral compounds has heen synthesized and some chemical and physical properties of the components as well as that of a binary mixture has heen studied. Measurements of the spontaneous polarization and the pitch are presented and results of eutectic, dielectric, electrooptic and electromechanical investigations are given.

Phase-field simulations and experiments of faceted growth in liquid crystals

We present numerical simulations directed at the description of smectic-B germs growing into the supercooled nematic phase for two different liquid crystalline substances. The simulations are done by means of a phase-field model appropriate to study strong anisotropy and also faceted interfaces. The most important ingredient is the angle-dependent surface energy, but kinetic effects are also relevant. The simulations reproduce qualitatively a rich variety of morphologies observed in the experiments for different value of undercooling, extending from the faceted equilibrium shape to fully developed dendrites.