Light-controlled reorientation of nematic liquid crystal driven by an electric field

The joint influence of optical and (quasi-)static electric fields on the orientation of liquid crystal gives rise to peculiar effects. In this article we report on the generation of transient domains in liquid crystals, which are an order of magnitude larger than the size of the optical field profile. The formation of such a domain is due to the fact that the initially present optical field reverses the pre-tilt, and the voltage that is then applied gives rise to an amplification of the tilt angle. The resulting reorientation of the director strongly depends on the starting conditions of the preliminary present optical field. We demonstrate different switching conditions, depending on the relation between the incident angle of the beam and the pre-tilt angle. The resulting refractive index profiles give rise to lensing effects.     

Photoinduced gliding of the surface director in azo-dye doped nematic liquid crystals

We report photoinduced surface director gliding with azo dye-doped nematic liquid crystals. This gliding has the same characteristics as that induced by an applied magnetic field. Fast and slow dynamic regimes are observed, the latter fitting to a stretched exponential. In addition, the gliding demonstrates a 'sensitizing' effect for sequential measurements.     

On the simulation of the director field of a nematic liquid crystal

A numerical method for the analysis of the static and dynamic behaviour of the director field, n, of a nematic liquid crystal is presented, where the equivalence of n and - n is taken into account. The occurrence of defects characteristic of nematics is studied for a hybrid cell, and for a material with a negative anisotropic susceptibility subject to an external field. Typical director configurations with defects are displayed graphically for some two dimensional arrangements and, in addition, the resulting pictures corresponding to an observation between crossed polarizer and analyser have been computed.     

Theoretical model for layer reorientation in smectic A* liquid crystals subject to an oblique magnetic field

Recently, Carlsson and Osipov reported on observations of the rotation of smectic layers under certain experimental conditions, and a dynamic theory describing such rotations was presented [Carlsson, T., and Osipov, M. A., 1999, Phys. Rev. E , 60, 5619]. In the present work, the rotational motion of the smectic layers of a SmA* liquid crystal, over which an oblique magnetic field has been applied, is studied theoretically by the aid of this theory. The role of the interaction between the smectic layers and the substrates is discussed and it is shown that if this interaction is small enough, the smectic layers orient themselves in such a way that the layer normal is almost parallel to the magnetic field. The relevant material parameters governing the dynamical response of the induced tilt and the layer normal are identified, and the time dependence of the response of these two dynamical variables is calculated. From this calculation, expressions for the response times of the system are given and a numerical value of the rotational viscosity of the smectic layers is estimated.     

Theoretical model for layer reorientation in smectic A* liquid crystals subject to an oblique magnetic field

Recently, Carlsson and Osipov reported on observations of the rotation of smectic layers under certain experimental conditions, and a dynamic theory describing such rotations was presented [Carlsson, T., and Osipov, M. A., 1999, Phys. Rev. E, 60, 5619]. In the present work, the rotational motion of the smectic layers of a SmA* liquid crystal, over which an oblique magnetic field has been applied, is studied theoretically by the aid of this theory. The role of the interaction between the smectic layers and the substrates is discussed and it is shown that if this interaction is small enough, the smectic layers orient themselves in such a way that the layer normal is almost parallel to the magnetic field. The relevant material parameters governing the dynamical response of the induced tilt and the layer normal are identified, and the time dependence of the response of these two dynamical variables is calculated. From this calculation, expressions for the response times of the system are given and a numerical value of the rotational viscosity of the smectic layers is estimated.     

Calculation of the birefringences of nematic liquid crystals at optical and infrared wavelengths

In this paper we critically examine computational methods for predicting the birefringences of nematic liquid crystals, with a view to screening molecules for potential use in infrared applications. Using the liquid-crystal 5CB as a test molecule, we calculate molecular electronic polarizabilities using ab initio quantum-mechanical techniques and a wide range of basis sets. We show that the polarizabilities tend to a limiting value as the quality of the basis set is improved. However, the biggest hurdle remains the determination of the refractive index from the polarizability data. We examine several methods for performing this conversion and conclude that the simplest equation, due to Vuks, is adequate for predicting the birefringence, given the uncertainties involved in other parameters. The agreement between calculation and experiment is best described as "semiquantitative." We perform similar calculations for a wide range of nematic liquid crystals at both 589 and 1550 nm, taking into account the likely impact of molecular vibrations at the longer wavelength. We demonstrate that there is a simple scale factor, for conventional nematics, between the birefringence at visible wavelengths and in the infrared. Thus knowledge of the birefringence at optical wavelengths, as widely available in the literature, is a good guide to the usefulness of conventional nematic liquid crystals as active elements for optical switching in the telecommunications industry.     

Temperature dependence of the density of calamitic and discotic nematic lyotropic liquid crystals containing sodium lauryl sulphate/decanol/water

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (±1 ×10^-5 g cm^-3), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.     

Enhancement of the optical absorption in cholesteric liquid crystals due to photonic effects: an experimental study

An anomalous strong optical absorption was measured in a cholesteric liquid crystal (CLC) at both edges of its photonic band gap. The experiment was carried out by studying the luminescence generated by the CLC sample doped with a small amount of fluorescent dye. The material was excited with monochromatic light at different angles of incidence and polarisations. Clear peaks were found in the luminescence response at angles for which the pumping wavelength coincides with the positions of the gap edges. The effect is especially noticeable for excitation under circularly polarised light of the same handedness as that of the CLC helix, and it is the highest at the long-wavelength edge. The modification of the absorption is originated by the helicoidal (photonic) structure of the material, which drastically influences the propagation of electromagnetic waves at certain frequencies and polarisations. The results were analysed numerically using an extension of the Berreman method that incorporates absorption effects. Good agreement with the experiment was found.     

Molecular director and layer response of chevron surface stabilized ferroelectric liquid crystals to low electric field

Recent papers on chevron surface stabilized ferroelectric liquid crystal cells claim that the chevron layer structure can be reversibly uprighted by application of the low to moderate electric fields typically employed to produce director reorientation. In this paper we show, using optical microscopy and X-ray scattering, that there is no significant change in the smectic layer thickness or chevron layer structure of our chevron surface stabilized ferroelectric liquid crystal cells under typical director switching conditions. Furthermore, we present arguments, based on the known elastic properties of smectics, that there is not likely to be a significant elastic layer response to these levels of applied electric field in any surface stabilized ferroelectric liquid crystal cell with anchored layers. Both the switching and observed continuous optical response to applied field can be understood on the basis of electric field induced reorientation of a non-uniform molecular director distribution. We further show that the typically observed broad distribution of layer orientations about the mean chevron structure arises from localized layering defects.     

Electric field transport of biphilic ions and anchoring transitions in nematic liquid crystals

Anchoring interaction of MBBA and MBBA + 5CB nematic layers with monomolecular films of CTAB on ITO glass supports is studied by videomicroscopy in the presence of an electric field. Planar-planar or homeotropichomeotropic electrode substrates are used to make the nematic cells. The substrate symmetry is broken by coating only one electrode with self-assembled CTAB film. In a DC electric field we observe some new effects; (i) polarity-dependent breaking of anchoring and switching to two oblique states in dielectrically stable planar cells; (ii) a polarity-dependent flow-induced metastable anchoring transition in homeotropic cells to a planar or tilted alignment after the field is switched off. These results are discussed in terms of a surface transition assisted by electric transport of biphilic CTAB ions and by a surface memorization of the flow-induced planar alignment.     

Temperature dependence of the second-order susceptibility in calamitic ferroelectric liquid crystals

The dij coefficients of the second-order susceptibility tensor for second harmonic generation were detemined for a calamitic ferroelectric liquid crystal (FLC) specifically designed for applications in non-linear optics. The measurements were performed at different temperatures in the SmC* phase. In accordance with the design methodology of these materials, the coefficient along the polar axis d22 is the greatest, and depicts an expected behaviour in the whole range of the ferroelectric phase. On the other hand, the temperature dependence of d21, d23, and d25, is anomalous to some extent, including a sign inversion of d23 at a certain temperature. These results, which contrast with those reported recently for non-calamitic FLCs, were qualitatively interpreted in the light of different conformations which, by virtue of plausible distortions of the molecular core, could coexist in the sample.     

Theory of anomalous ultrasonic absorption and dispersion of nematic liquid crystals just above the clearing point

The dynamic heat capacity of nematic liquid crystals just above the clearing point is calculated as an extension of our previous theory of static heat capacity. The present theory explains semi-quantitatively the available experimental results of anomalous ultrasonic absorption and dispersion.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4'-butylaniline (MBBA) and 4-n-pentyl-4'-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

The synthesis of nitroaniline monomers and polymers as non-linear optical ferroelectric liquid crystals

Chiral 2-amino-4-alkoxy-5-nitrobenzoate and 5-amino-4-alkoxy-2-nitrobenzoate derivatives as well as the corresponding biphenyl derivatives were synthesized. Some of them were also derivatised to the corresponding acrylates and polyacrylates. Many of the new substances exhibit a large spontaneous polarization and large second order NLO coefficients. In addition some of them show a broad range S*_c phase. All these properties depend strongly on small changes in the molecular structures. Here we present the synthesis of these novel NLO FLC materials and discuss some of their properties.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Abstract

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Anomalous temperature dependence of the dielectric and optical properties in ferroelectric liquid crystals

The anomalous temperature dependence of the sign of the spontaneous polarization in a biphenyl ester series of ferroelectric liquid crystals is reported. The reversal of the sign of the spontaneous polarization in the chiral smectic C phase of 2MBNCBC ((S)-2′-methylbutyl-4′-n- nonylcarbonyloxy-(l,l′-biphenyl)-l-carboxylate) is observed at T _m. This anomaly is studied in detail as functions of the length of the terminal alkyl chain, the applied electric field, the frequency and the applied hydrostatic pressure. T _m has been confirmed to depend on the length of the terminal chain and the applied pressure. However, it is independent of the applied field and the measuring frequency. In the mixture of these compounds, this anomaly is also observed.     

Anomalous temperature dependence of the dielectric and optical properties in ferroelectric liquid crystals

The anomalous temperature dependence of the sign of the spontaneous polarization in a biphenyl ester series of ferroelectric liquid crystals is reported. The reversal of the sign of the spontaneous polarization in the chiral smectic C phase of 2MBNCBC ((S)-2'-methylbutyl-4'-n- nonylcarbonyloxy-(l,l'-biphenyl)-l-carboxylate) is observed at T_m. This anomaly is studied in detail as functions of the length of the terminal alkyl chain, the applied electric field, the frequency and the applied hydrostatic pressure. T_m has been confirmed to depend on the length of the terminal chain and the applied pressure. However, it is independent of the applied field and the measuring frequency. In the mixture of these compounds, this anomaly is also observed.     

Design and synthesis of nematic liquid crystals with negative dielectric anisotropy

Axially fluorinated cyclohexane derivatives are a promising new type of liquid crystals with negative dielectric anisotropy. Up to now, the technical application of these materials in LCDs was impeded by their strong tendency to form smectic B mesophases. We present a convenient synthetic pathway leading to new homologues carrying alkenyl side chains which induce nematic phases.     

Evolution of coercivity and its angular dependence in CoFe nanostructures subjected to field cooling

Evolution of coercivity and its angular dependence in CoFe nanostructures subjected to field cooling has been investigated in this work. Spherical CoFe grains with an average diameter of 30 nm were grown on a silicon substrate using electron beam evaporation. Further, the as‐deposited sample was subjected to field cooling. The morphology and topography of the sample before and after field cooling were characterized by atomic force microscopy and SEM. Magnetic force microscopy indicated that there is a good uniformity of magnetization throughout the sample after field cooling. Vibrating sample magnetometer measurements indicate that the coercivity in CoFe nanostructures is dependent on shape of the nanostructures. Copyright © 2013 John Wiley & Sons, Ltd.     

Intramolecular disorder and its relation to mesophase structure in lyotropic liquid crystals

NMR SPDE measurements are reported for the lamellar (dispersions and multibilayer stacks) and hexagonal phases of sodium octanoate/octanol/D₂O mixtures. In the lamellar L_β and L_γ (gel) phases the octyl chains are rigid and perfectly ordered, while in the lamellar L_α and hexagonal phases they are flexible and disordered. In particular, the measurements show that in the fluid lamellar L_α phase, there is a marked discontinuity in the octyl chain flexibility at the C₅-C₆ segment; this behaviour is identical to that previously reported for the alkyl end-chains in smectic 4,4′-di-n-octyloxyazoxybenzene. In contrast, in the hexagonal phase, there is an effectively continuous flexibility gradient along the whole length of the octyl chain as in nematic 4,4′-di-n-octyloxyazoxybenzene. The behaviour in the lamellar phase is attributed to interference between cooperative conformational modes and localized random thermal fluctuations.