smectic-C Liquid Crystals Research Articles

Optical and Structural Properties of Anisotropic ZnS Nanoparticle Suspensions.

We studied the optical and structural properties of highly ordered arrays of surfactant-capped ZnS nanowires (NWs) and nanorods (NRs) in organic suspensions. The photoluminescence (PL) emission measured under different concentrations and postsynthesis washing cycles interestingly showed increasing emission upon decreasing nanoparticle (NP) concentration. Synchrotron small angle X-ray scattering measurements elucidated the liquid-crystal-like structure of the NPs in suspension under different concentrations and temperatures. The NWs are stacked in a simple structure with a hexagonal cross-section, whereas the structure of the NRs is more complex, resembling a smectic-c liquid crystal, and shows unusual thermal expansion versus temperature. The results point out that a certain amount of bound surfactant must be present on the NP surface to maximize the PL intensity.

Optical diffraction in chiral smectic-C liquid crystals

Optical diffraction in chiral smectic-C liquid crystals

Photorefractive flexoelectric liquid crystal mixtures and their application to laser ultrasonics

A photorefractive effect of mixtures of flexoelectric liquid-crystal (flex LC) was investigated and applied to laser ultrasonics. Mixtures of flex LC, composed of smectic-C liquid crystals, photoconductive chiral compounds, and a sensitizer, are demonstrated to exhibit a large photorefractive effect. The experiments of a two-beam mixing with a photorefractive flexoelectric liquid crystal (PR-flex LC) show that a gain coefficient was measured as 1400 cm–1 and a response time was 960 microseconds, both with an applied electric field of 2.0 V/µm. The large gain and fast response are advantageous for remote ultrasound detection by using two-beam mixing with PR-flex LC. This remote sensing method can be used to probe the internal structure of an object or to measure the thickness of a plate object. The experimental results of acoustic time-of-flight in an aluminum (Al) plate are presented by using an adaptive two-beam interferometer with a PR-flex LC. A 3-D surface topology is shown by using laser ultrasonics with 2-D scanning of a test Al plate. With a fast response time in PR-flex LC, the system is not affected by vibrations in an industrial environment.

Effect of Smectic Layers on Thermal Diffusivity of Side-Chain Polymer Liquid Crystals

The thermal conductivity (TC) of polymers is known to be effectively enhanced by liquid crystallinity. Liquid crystals (LCs) exhibit anisotropic TC with a greater magnitude in the orientation direction. However, the effects of other LC structures than the orientational order, such as smectic layer structures, on TC have not been well studied. This study investigated the correlation between LC structures and thermal diffusivity using a series of LC PMnMA polymethacrylates having azobenzene mesogens in the side chains. PMnMA formed nematic (N), smectic A (SmA), and smectic C (SmC) phases. These LCs were aligned by a magnetic field or large-amplitude oscillatory shear, yielding glassy LC films with mesogen long axes in the in-plane or out-of-plane direction. These glassy LCs increased thermal diffusivities in the LC orientation direction with the degree of LC orientation, whereas N and SmC LCs had smaller values than SmA LCs. These results demonstrated that smectic layers can enhance thermal diffusivity in their normal direction.

Effect of the concentration of chiral compound on the photorefractive effect of flexoelectric smectic liquid crystal blends

The photorefractive effect of flexoelectric smectic liquid crystal blends was investigated. Smectic liquid crystal blends, composed of smectic-C liquid crystals, photoconductive chiral compounds and a sensitizer, are known to exhibit a large photorefractive effect. The chiral compound plays a critical role in the exhibition of large photorefractivity in smectic liquid crystals. The concentration of the chiral compound was varied and the photorefractive properties of the liquid crystal blend were examined. It was found that smectic-C liquid crystals containing small amounts of a chiral compound show a large photorefractivity even though they do not possess spontaneous polarizations.

Dynamics of smectic-C liquid crystals in a stochastic magnetic field

ABSTRACT The dynamical evolution of smectic-C liquid crystals in the presence of a magnetic field is studied. The theoretical analysis is based on dynamic renormalisation group techniques. The starting point of the analysis is the nonlinear elastic model of smectic-C liquid crystals followed by Langevin formulation. The dynamical model predicts the onset of Kosterlitz-Thouless transition at low frequency limit. The nonlinearity drives the system to a Kardar-Parisi-Zhang (KPZ) fixed point. The dynamic renormalisation group calculation predicts that smectic-C liquid crystals belong to the KPZ universality class.

Curvature elasticity of smectic-C liquid crystals and formation of stripe domains along thickness gradients in menisci of free-standing films.

Smectic liquid crystals with a layering order of rodlike molecules can be drawn in the form of free standing films across holes. Extensive experimental studies have shown that smectic-C (SmC) liquid crystals (LCs) with tilted molecules form periodic stripes in the thinner parts of the meniscus, which persist over a range of temperatures above the transition of the bulk medium to the SmA phase in which the tilt angle is zero. The prevailing theoretical models cannot account for all the experimental observations. We propose a model in which we argue that the negative curvature of the surface of the meniscus results in an energy cost when the molecules tilt at the surface. The energy can be reduced by exploiting the allowed (∇·k)(∇·c) deformation which couples the divergence of k, the unit vector along the layer normal, with that of c, the projection of the tilted molecular director on the layer plane. We propose a structure with periodic bending of layers with opposite curvatures, in which the c-vector field itself has a continuous deformation. Calculations based on the theoretical model can qualitatively account for all the experimental observations. It is suggested that detailed measurements on the stripes may be useful for getting good estimates of a few curvature elastic constants of SmC LCs.

Surface and finite-size effects on N-Sm-A-Sm-C phase transitions in free-standing films.

The present study is devoted to the investigation of surface anchoring and finite-size effects on nematic-smectic-A-smectic-C (N-Sm-A-Sm-C) phase transitions in free-standing films. Using an extended version of the molecular theory for smectic-C liquid crystals, we analyze how surface anchoring and film thickness affect the thermal behavior of the order parameters in free-standing smectic films. In particular, we determine how the transition temperature depends on the surface ordering and film thickness. We show that the additional orientational order imposed by the surface anchoring may lead to a stabilization of order parameters in central layers, thus modifying the nature of the phase transitions. We compare our results with experimental findings for typical thermotropic compounds presenting a N-Sm-A-Sm-C phase sequence.

Dispersion relation for electromagnetic propagation in stochastic dielectric and magnetic helical photonic crystals.

We theoretically study the dispersion relation for axially propagating electromagnetic waves throughout a one-dimensional helical structure whose pitch and dielectric and magnetic properties are spatial random functions with specific statistical characteristics. In the system of coordinates rotating with the helix, by using a matrix formalism, we write the set of differential equations that governs the expected value of the electromagnetic field amplitudes and we obtain the corresponding dispersion relation. We show that the dispersion relation depends strongly on the noise intensity introduced in the system and the autocorrelation length. When the autocorrelation length increases at fixed fluctuation and when the fluctuation augments at fixed autocorrelation length, the band gap widens and the attenuation coefficient of electromagnetic waves propagating in the random medium gets larger. By virtue of the degeneracy in the imaginary part of the eigenvalues associated with the propagating modes, the random medium acts as a filter for circularly polarized electromagnetic waves, in which only the propagating backward circularly polarized wave can propagate with no attenuation. Our results are valid for any kind of dielectric and magnetic structures which possess a helical-like symmetry such as cholesteric and chiral smectic-C liquid crystals, structurally chiral materials, and stressed cholesteric elastomers.

UV spectral characterization of a smectic-C liquid crystal: Theoretical support to the experiment

ABSTRACTIn the present article, UV spectral characterization of a smectic-C liquid crystal 4,4′-bis(n-alkoxy)azoxybenzene (n = 14) (C40H66N2O3) has been carried out. Structure of the molecule has been optimized using the Density functional B3LYP with 6-31+G (d) basis set using crystallographic geometry as input. The absorption spectra have been estimated in the UV region by employing the DFT method, semiempirical CNDO/S and INDO/S parameterizations. The oscillator strength (f) and vertical transition energy (EV) have been reported corresponding to absorption wavelength (λmax). These values have been compared with the experimental value reported in the literature to offer theoretical support to the experimental value. Further, some electrochemical properties have been reported for the molecule.

Dual relaxation and structural changes under uniaxial strain in main-chain smectic-C liquid crystal elastomer.

The relationship between strain-dependent macroscopic elastic behavior and the changes in microscopic structure of the smectic-C liquid crystal elastomer (LCE), C11MeHQSi8 were investigated using synchrotron X-ray studies. At very low strains ε ≤ 0.2, the smectic layers are randomly oriented. As the strain increases beyond 0.2, the smectic layers reorient and become parallel to the direction of the applied strain. The polydomain to monodomain (P-M) transition accompanied by the formation of chevron structure ensues for ε > 0.2 and is nearly complete for ε = 0.7. The chevron structure relaxes after the applied strain changes, with a time constant τα ∼ 45 min while the orientation order parameters of the mesogenic and elastomeric components gradually increase and saturate at 0.83 and 0.4, respectively at ε = 1.7 which is near the end of the plateau region. Relaxation rates τα for the tilt angle and τd corresponding to the smectic layer spacing both become about 10 times faster when the strain exceeds 0.7. The LCE remains "locked" into the monodomain state and retains 90% and 80% values of α and S, respectively for 24 hours after the applied strain is removed. The viscoelastic properties of the liquid crystal appear to dominate the equilibration process at low strains while the elastomeric properties control the system's response at high strains.

Smectic C chevrons in nanocylinders

The structure of an achiral smectic-C liquid crystal confined in nanocylinders with a planar surface anchoring is studied by small angle neutron scattering. We observe an invariant alignment of the nematic director with the pore axis, that promotes an original chevron structure with revolution symmetry.

Comment on "Origin of tilted-phase generation in systems of ellipsoidal molecules with dipolar interactions''.

In this Comment, I point out that the physical origin of molecular tilt in the smectic phase, found in the Monte Carlo simulations of systems of rodlike molecules with two terminal antiparallel transverse dipole moments by Bose and Saha [Phys. Rev. E 86, 050701(R) (2012)], is similar to the one proposed by McMillan. In particular, unlike in smectic-C liquid crystals, in which the molecules are known to have practically free rotations about their long axes, the molecular rotations are found to be partially frozen in the simulations. Further, I suggest that the attractive interaction between correlated splay fluctuations of the antiparallel polarized sublayers which lie close to each other in adjacent molecular layers give rise to the tilting, rather than a reduced attractive interaction between dipoles belonging to the two dipolar sublayers within one molecular layer, as proposed by the authors.

Modulated phase of liquid crystals: Covariant elasticity in the context of soft, achiral smectic-Cmaterials

Ginzburg-Landau-de Gennes-type covariant theories are extensively used in connection with twist grain boundary phases of chiral smectogens. We analyze the stability conditions for the linear, covariant elasticity theory of smectic-C liquid crystals in the context of achiral materials, and predict an equilibrium modulated structure with an oblique wave vector. We suggest that a previous experimental observation of stripes in smectic-C is consistent with the predicted structure.

Orientational correlations in two-dimensional liquid crystals studied by molecular dynamics simulation

Orientational correlations in Langmuir monolayers of nematic and smectic-C liquid crystal (LC) phases are investigated by molecular dynamics simulation. In both phases, the orientational correlation functions decay algebraically yet with the different exponents of 1.9 and 0.2 for the nematic and the smectic-C monolayers, respectively. The power law decay, i.e., the absence of long-range orientational order, means the both monolayers should be the ideal 2D system with a continuous symmetry, whereas the large difference in the exponents of power law gives rise to the crucial difference in their optical properties; the nematic monolayer is optically isotropic while the smectic-C monolayer exhibits an anisotropy on the length scale of visible light. Since the exponent is inversely proportional to the molecular exchange energy, the averaged molecular interaction in the nematic monolayer should be an order of magnitude smaller than that in the smectic-C monolayer, which is ascribed to the low molecular density and the weak molecular dipole due to the water molecule. The relation between the molecular interaction and the orientational correlation calculated for the 2D LC system offers much information not only about the 2D LCs but also on the bulk system.

On the application of finite differences, method of lines and pseudo-spectral methods to smectic-C liquid crystals

On the application of finite differences, method of lines and pseudo-spectral methods to smectic-C liquid crystals

A Catastrophe-Theoretic Approach to Tricritical Points with Application to Liquid Crystals

A criterion to locate tricritical points in phase diagrams is proposed. The criterion is formulated in the framework of elementary catastrophe theory and encompasses all the existing criteria in that it applies to systems described by a generally nonsymmetric free energy which can depend on one or more order parameters. We show that tricritical points are necessarily given by free energies which are not 4-determined. An application to smectic-C liquid crystals is briefly discussed.

Understanding self-assembly of rod-coil copolymer in nanoslits

Rod-coil diblock copolymers are a special kind of molecule containing a rigid rod and a flexible part. We present a systematic study on self-assembly of the rod-coil copolymers in nanoslits using a hybrid density functional theory. The self-assembly of the rod-coil molecule is driven by the bulk concentration, and there exists a critical bulk concentration beyond which the rod-coil molecule self-assembled into ordered lamellar structures in the slit, otherwise it is in a disordered state. By monitoring the effect of the interaction (epsilon(TT)(*)) of molecular tail on the self-assembly, we found that in the nanoslit of H=13sigma, it is at epsilon(TT)(*)=8 rather than epsilon(TT)(*)=10 or epsilon(TT)(*)=12 that the minimal critical bulk concentration occurs. It may be because the strong tail-tail interaction leads to aggregation of the copolymer molecules in bulk phase, and the resulting supramolecular structures are fairly difficult to enter the slit due to the depletion effect. At a fixed slit, the structural evolution of the self-assembled film with the bulk concentration is observed, including trilayer and five-layer lamellar structures, smectic-A, smectic-C, and a mixture of smectic-A and smectic-C liquid crystal phases and so on. We found that the critical bulk concentration, corresponding to the disordered-ordered phase transition, greatly depends on the separation between two walls, and it changes periodically with the increase of the slit width. In addition, it is also found that the molecular flexibility is one of key factors determining the self-assembled structure in the slit, and the critical bulk density increases with the molecular flexibility.

Effect of decoupled azimuthal and zenithal anchoring on smectic-Cchevron structures

We present a study of the effect of decoupled azimuthal and zenithal weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. We consider temperatures below the SmA-SmC transition and assume that the value of the smectic cone angle can be regarded as constant through the cell. By standard Euler-Lagrange minimization of the total energy we obtain a simple analytical expression for the equilibrium director twist angle in the C1 and C2 chevron states. Using this analytical form, we are able to compare the total energies of the C1 and C2 chevrons, and determine the globally stable chevron profile. We show that the C2 state is preferred when the azimuthal anchoring strength is relatively large, while C1 chevrons will dominate for strong zenithal anchoring.

Configuration of a chiral smectic-C film with a circular inclusion

It was shown experimentally (P.V. Dolganov et al., Europhys. Lett. 76, 250 (2006)) and by numerical calculations (C. Bohley, R. Stannarius, Eur. Phys. J. E 23, 25 (2007)) that the c -director profile of a two-dimensional chiral smectic-C (SmC) film around a circular inclusion adopts dipolar rather than quadrupolar configuration observed in achiral SmC films. We give an analytical argument on how spontaneous bend inherent in chiral SmC liquid crystals influences the configuration of a SmC liquid crystal film around a circular inclusion imposing tangential anchoring. We find how the angle alpha between two surface defects seen from the center of the inclusion depends on the radius of the inclusion R and the strength of the spontaneous bend q . We show, however, that the contribution of the spontaneous bend to the free energy suffers from mathematical ambiguity; it depends on the mathematical treatment of the outer boundary even when it is at infinity. This might indicate that the shape as well as the treatment of the outer boundary of the film can significantly influence the equilibrium configuration of the c -director and the position of the surface defects.