smectic-C Liquid Crystals Research Articles

Theoretical model of the transition between C1 and C2 chevron structures in smectic liquid crystals

We present a study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. The coexistence of C1 and C2 chevron structures within a single cell causes zigzag defects to occur and may affect the optical characteristics of the cell. By standard Euler-Lagrange minimisation of the total energy of the system, we obtain analytical expressions for the equilibrium director cone angle in the two chevron states. These in turn allow us to compare the total energies of the states and determine the globally stable chevron profile. We show that analytical predictions for the critical transition temperature, which depends on anchoring strength and pretilt angle, are in good agreement with those obtained numerically.

Formation of tilted smectic-C liquid crystal phase in polar Gay–Berne molecules

We perform molecular dynamics simulation for a system of Gay–Berne molecules having two terminal dipole moments to generate tilted smectic-C liquid crystal phase. We investigate the effect of dipolar orientation with respect to the long molecular axis on phase behaviour. The study indicates that larger dipolar angle can give rise to greater tilt in molecular organization within a layer.

Traveling waves in ferroelectric smectic-C liquid crystals.

This article investigates, by means of Lie point symmetries, traveling wave solutions to a dynamic equation that frequently arises in the theory of ferroelectric smectic- C liquid crystals under the influence of an electric field. The equation considered has three sinusoidal nonlinearities and possible time-dependent solutions are discussed in the context of minima and maxima of the electric energy density for these liquid crystals: solutions travel between such constant equilibrium states. Implicit solutions to an approximation of the governing dynamic equation are determined. Nondimensional control parameters that characterize changes in the availability of equilibrium states as the magnitude of the field increases are also identified.

Surface Anchoring and Chevron Structure in Ferroelectric Liquid Crystals

Applying a sufficiently high electric field to a ferroelectric smectic-C liquid crystal deforms the so-called chevron structure into one in which the layers are flat. This transition happens in three different ways depending on the type of liquid crystal phase. In all three cases, the flattening has been believed to begin from the middle of the smectic layers. We show that a previously proposed model can predict the possibility of forming a new type of partially flattened layer in which the flattening begins at the edges of the layer. This interesting behavior takes place when the surface anchoring is strong.

Piezoelectricity of a ferroelectric liquid crystal with a glass transition.

Pressure-electric (hydrostatic piezoelectric) measurements are reported on bookshelf textures of a ferroelectric smectic-C (Sm C*) liquid crystal with a glass transition. The continuous variation of a partially fluid state to the solid glass enables one to trace how the piezoelectric effect depends on the consistency of the material. It was observed that in the Sm C* samples with poled glass the piezoelectric constants are comparable to conventional piezoelectric crystals and poled piezoelectric polymers. This implies their application possibilities. The magnitude of the piezoelectric constant in the glassy state depends very much on the poling conditions. The studies indicate that there are two counteracting effects, which cancel each other out in the Sm C* phase near the glass transition. Our analysis indicates that the pressure-induced director tilt change has a dominating effect both in the fluid and the glassy Sm C* states.

Kinetics of the photoferroelectric effect in chiral smectic-C liquid crystals studied by time-resolved measurements of spontaneous electric polarization and director tilt angle.

The origin of the photoferroelectric effect in liquid crystals, where the spontaneous polarization of a chiral ferroelectric smectic-C* (SmC*) host phase is changed by the E,Z-photoisomerization of azobenzene dopant molecules, was investigated by kinetic studies on the molecular isomerization and the subsequent changes in the SmC* order parameters, the director tilt angle, and the spontaneous electric polarization. The photoresponse of a liquid-crystal mixture consisting of 5 mol% 4,4'-bis-[(2-methyl)butyloxy]azobenzene dissolved in the SmC* host phase FLC 6430 was studied at low UV-light intensities (lambda = 366 nm, 15 microW cm-2) using an electrooptical technique that measured the desired parameters with a time resolution of about 1 s. The time-resolved measurements of optical absorption, tilt angle, and spontaneous polarization during the thermal reisomerization after ending the sample irradiation showed that the photoinduced changes in the spontaneous polarization simultaneously followed the molecular isomerization with the same rate constant and activation energy, while the director tilt angle remained basically unchanged. Minor changes in the tilt are explained by the local heating of the sample due to the optical absorption. Since the photoinduced change in polarization was observed at constant tilt, we conclude that in the limit of low UV intensity the photoferroelectric effect originates from a photo-induced change of the bilinear coupling coefficient between the polarization and the tilt. In the molecular theory of chiral SmC* liquid crystals, the coupling coefficient is related to the bias of molecular rotations. This bias may be considerably disturbed by the formation of the bent Z-isomers during the photoisomerization.

Electrorheological Properties of Chiral Smectic-C Liquid Crystals Observed by Using Shear Horizontal Wave Propagation

An electrorheology (ER) effect of a chiral smectic- C (Sm- C *) liquid cyrstal is observed using a shear horizontal (SH) wave propagating in a liquid-crystal cell. Propagation loss and acoustic phase-delay in the liquid-crystal cell are measured and the propagation characteristics of the SH wave in the cell structure is numerically analyzed in the form of considering the stress tensor of Sm- C * phase. The experimental results are explained qualitatively in comparison with the numerical analysis results. A relative viscosity change under electric field is evaluated, which indicates that the ER effect in the Sm- C * liquid crystal is predominated by the orientations of the liquid-crystal directors and the Sm- C * layers. The present technique has the advantage of evaluating the ER properties without disturbing the Sm- C * layers.

Electrorheological Properties of Chiral Smectic-C Liquid Crystals Observed by Using Shear Horizontal Wave Propagation

An electrorheology (ER) effect of a chiral smectic- C (Sm- C *) liquid cyrstal is observed using a shear horizontal (SH) wave propagating in a liquid-crystal cell. Propagation loss and acoustic phase-delay in the liquid-crystal cell are measured and the propagation characteristics of the SH wave in the cell structure is numerically analyzed in the form of considering the stress tensor of Sm- C * phase. The experimental results are explained qualitatively in comparison with the numerical analysis results. A relative viscosity change under electric field is evaluated, which indicates that the ER effect in the Sm- C * liquid crystal is predominated by the orientations of the liquid-crystal directors and the Sm- C * layers. The present technique has the advantage of evaluating the ER properties without disturbing the Sm- C * layers.

DISTORTIONS INDUCED BY A MAGNETIC FIELD IN PLANAR ALIGNED SAMPLES OF SMECTIC-C LIQUID CRYSTALS

This article presents some preliminary theoretical results for the onset of the Helfrich-Hurault transition in smectic-C liquid crystals induced by a magnetic field applied parallel to the smectic layers. An energy in terms of the smectic layer displacement u is minimized via averaging to enable the calculation of a critical field strength H c for the onset of layer distortions.

Optically induced periodic structures in smectic-Cliquid crystals

We explore periodic structures of smectic-C (SmC) liquid crystals, induced optically by a polarization grating. The studied cells contain a passive surface of rubbed polyimide and an active photosensitive substrate of azo-dye doped polyimide. In a nematic phase the director field can be periodic independent of the angle between the grating vector and the rubbing direction. In the SmA phase periodic structure can be induced only by layer undulations. The SmC behaves similarly to the nematic phase, but the director can rotate only on a cone, which results in a more complex geometry. The periodic pattern is superimposed with four different director and layer structures. In spite of the coexistence of the nonuniform structures the diffraction efficiency is better in the SmC, than in the nematic phase.

Comparison between the switching dynamics of homeotropic and planar cells of chiral smectic-C liquid crystals

The molecular dynamics during the switching of homeotropic and planar cells of chiral smectic-C liquid crystals has been investigated using electrooptic measurements and second-harmonic-generation interferometry. Two ferroelectric liquid crystals with great differences in their molecular structures have been studied. It has been found that the molecular motion is rather different depending on the type of cell used. For planar cells, the azimuthal rotation of the molecules is limited within half the smectic cone and the molecules rotate in opposite directions in the two halves of the chevron structure. In contrast, for homeotropic cells there is no chevron structure and a uniform macroscopic optical indicatrix can be defined all over the sample during the whole switching process. However, the molecular reorientation takes place within numerous microdomains of size smaller than the optical wavelength. There are two types of microdomains that occur with different probabilities depending on the material. Within each domain the molecules rotate in opposite directions, and the molecules describe a complete cone during a whole switching period.

Berreman-matrix formulation of light propagation in stratified anisotropic chiral media

We critically analyze the problem of light propagation in stratified birefringent gyrotropic media by introducing a \(\) matrix formulation. As a test model we consider the case of short-pitch chiral smectic-C liquid crystals, for which simple descriptions in terms of an effective gyrotropic homogeneous model and of a microscopic periodic structure are available. By comparing reflectances and transmittances from finite and semi-infinite slabs, we show that the homogeneous description is unable to account for some of the properties of the underlying periodic structure, whatever small is the pitch with respect to the light wavelength.

Anomalous heat capacity of smectic-C and chiral smectic-C liquid crystals above the transition point

The anomalous parts of the specific heat capacity of smectic-C and chiral smectic-C liquid crystals above the transition point is calculated on the basis of the fluctuation theory of Landau. The present theory explains the available experimental results of heat capacity. The analysis shows that only the Gaussian model is sufficient to explain the anomalous heat capacity of smectic-C liquid crystals above the transition point.

Stability of traveling waves in smectic-Cliquid crystals

Stability of traveling waves in smectic-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi></mml:math>liquid crystals

Optical unwinding and reentrance phenomena in chiral smectic-C liquid crystals

We optically measure the molecular reorientation induced by an argon laser in homeotropically aligned chiral smectic-C films. Optical unwinding of the helix is observed and the critical unwinding intensity is determined. We have also developed a theory which not only explains the observed effect but also predicts a novel reentrance phenomenon of the unwinding state.

Flow Alignment in Smectic Liquid Crystal

The experimental results of Cladis et al. ill about the behaviour of smectic-C liquid crystal free-standing films subjected to a two-dimensional shear flow are reproduced via a Ginzburg-Landau model which is simulated numerically. The model takes into account effects that were first neglected in the theoretical analysis made by Cladis et al. particular emphasis is put on the influence of the elastic anisotropy and the nonrotational part of the flow.

Subharmonics arising from the application of an electric field across a smectic-C liquid crystal.

Subharmonics arising from the application of an electric field across a smectic-C liquid crystal.

Modulations in the diffracted intensity in chiral smectic-C liquid crystals.

Recently [K. A. Suresh, Yuvaraj Sah, P. B. Sunil Kumar, and G. S. Ranganath, Phys. Rev. Lett. 72, 2863 (1994)] the intensity and polarization features associated with the optical diffraction in chiral smectic-C liquid crystal were studied in the phase grating mode. In the computations it was found that the diffracted intensity, as a function of sample thickness, has modulations of different length scales. In this paper, using a perturbative approach, we show that these modulations are a consequence of a coupling between different orders of scattering. \textcopyright{} 1996 The American Physical Society.

Twisting transition in a fiber composed of chiral smectic-C liquid crystal polymer.

Twisting transition in a fiber composed of chiral smectic-C liquid crystal polymer.

Covariant elasticity and dislocations in smectic-C liquid crystals.

We derive a rotationally invariant form for the elastic energy of a smectic-C liquid crystal. The magnitude of the c director, but not its direction, can screen layer strain just as two independent components of the director screen strain in smectic-A liquid crystals. Type I and type II smectic-C liquid crystals can be classified according to whether the c-director penetration depth ${\ensuremath{\lambda}}_{\mathit{c}}$ is less than or greater than the smectic coherence length \ensuremath{\xi}. We calculate the energies of isolated dislocations and the interaction energies of parallel dislocations as a function of direction. We find an attractive interaction at large distances between like-sign screw dislocations and a repulsive interaction at short distances, leading to a minimum energy separation. (c) 1995 The American Physical Society