Bend-stripes in hybrid aligned nematic layers

Abstract

The occurrence of bend-stripes also involving twist distortion is investigated in hybrid aligned nematic layers, with stronger tilt-anchoring at the homeotropic substrate than at the planar one. The modulated structure is found to exist above the thickness threshold d _a for the aperiodic bend-splay deformation. The periodicities of the director azimuth and polar angle are incommensurate along the normal to the cell plates. In principle, two independent periodic modes are possible, the first one stable, with threshold d _p = d _a, the second one metastable, with threshold d _p > d _a, according to the values of the torsional extrapolation lengths. Both transitions are continuous with respect to the in-plane wavenumber. The second mode can appear only if the difference between the torsional and the tilt extrapolation lengths is positive at the planar substrate. Moreover, both thresholds are independent of the saddle-splay elastic constant.     

A hybrid aligned nematic reflective liquid crystal display driven by a fringe-field

We have designed a novel reflective nematic liquid crystal (LC) cell driven by a fringe electric field, in which the LCs are hybrid aligned in the initial state. Due to the hybrid alignment of the LC, the effective retardation value of the cell is greatly reduced when viewed in the normal direction and such a cell retardation value of 0.28 µm equals a quarter wave plate viewed in the normal direction. This means that the new reflective device can have a large cell gap of greater than 3 µm, which is advantageous when manufacturing the cell and, in addition, the device shows excellent electro-optic characteristics.     

Light-induced hydrodynamic reorientation of hybrid aligned nematic liquid crystals caused by direct volume expansion

The dynamics of the direct volume expansion mechanism of light-induced hydrodynamic motions in hybrid-oriented nematic liquid crystal (NLC) have been studied experimentally. It was shown that heating caused by the absorption of a laser beam results in expansion of a NLC volume and induces hydrodynamic flow in a planar capillary. The hydrodynamic flow reorients the NLC molecules and thus the optical properties of the NLC layer are changing. A change of the tilt direction of the NLC director is demonstrated experimentally, and it is shown that hybrid-oriented NLC could be considered as ‘flexible ribbon’. Velocity gradient brings about an increase of curvature when hydrodynamic flow velocity is directed out of the ‘flexible ribbon’s’ curvature. The ‘flexible ribbon’ reverses its curvature when velocity is directed into ‘flexible ribbon’s’ curvature.     

Oscillations in thin nematic layers

In the present paper a thin nematic liquid crystal layer between two identical boundary surfaces (solid walls or free surfaces in the case of a freely suspended film) is considered. In a mean field approximation it is shown that the interference between the boundary surface-induced smectic density waves results in oscillations of the free energy of the nematic layer and disjoining pressure acting on the boundary surfaces. Theoretical dependence of disjoining pressure on the nematic layer thickness is in qualitative agreement with experiment. Also we have considered a thin film of polar nematic in which in addition to an ordinary monolayer smectic A phase (S_A1) with the layer thickness d equal to the molecular length l the partial bilayer smectic A phase (S_Ad) occurs. It is shown that the variation of the distance between the boundary surfaces can result in the oscillatory S_A1-A_Ad phase transitions in this nematic film     

The in-plane switching of homogeneously aligned nematic liquid crystals

We have investigated the electro-optical effects and physical switching principle of homogeneously aligned nematic liquid crystals when applying an in-plane electric field with interdigital electrodes. By using the in-plane switching (IPS) of the liquid crystals which is achieved by the in-plane electric field, the viewing angle characteristics of the electro-optical effects were confirmed to be far superior to those of the conventional twisted nematic mode in which the electric field is applied along the direction perpendicular to the substrates. The non-reversal region of grey scales was extremely wide in which a high contrast ratio was kept, even along quite an oblique direction in the IPS mode. In order to clarify the switching principle of the liquid crystals in the IPS mode, a simplified expression describing the threshold behaviour of the device was derived with the assumption that a uniform in-plane electric field was applied along a direction perpendicular to the director and parallel to the homogeneously aligned nematic slab, and found to be sufficiently able to explain the experimental results. First, a critical field at which the liquid crystals just began to twist, was found to be proportional to the reciprocal of the cell gap. Second, it was the electric field and not the voltage that drives the liquid crystals. This relationship was due to the independence of the electric field regarding the liquid crystal layer normal direction. So the threshold voltage in the IPS mode was strongly dependent on the variation of the cell gap. For the dynamical response mechanism of the liquid crystals to the in-plane electric field, the switching on and off processes of the liquid crystals were analysed quantitatively. The relaxation time of the liquid crystals when removing the electric field could be described as proportional to the square of the cell gap. A thinner cell gap also proved to be effective in obtaining a fast response time in the IPS mode. In contrast, the switching on time when applying the in-plane electric field was found to be inversely proportional to the difference between the square of the electric field strength and the square of the critical electric field strength at which the liquid crystals began to deform.     

Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.     

Polarizing-alignment layers for twisted nematic cells

We describe polarizing films formed from the lyotropic liquid crystalline phases of dyes dissolved in water. The dye polarizers possess a high dichroic ratio close to that of conventional polarizers. In addition, the dried dye films provide a uniform homogeneous orientation for thermotropic nematic materials. Therefore, thin dye films can simultaneously serve as internal polarizers and as alignment agents. These properties make them especially suitable for twisted nematic devices based on low cost birefringent plastic substrates.     

Polarizing-alignment layers for twisted nematic cells

We describe polarizing films formed from the lyotropic liquid crystalline phases of dyes dissolved in water. The dye polarizers possess a high dichroic ratio close to that of conventional polarizers. In addition, the dried dye films provide a uniform homogeneous orientation for thermotropic nematic materials. Therefore, thin dye films can simultaneously serve as internal polarizers and as alignment agents. These properties make them especially suitable for twisted nematic devices based on low cost birefringent plastic substrates.     

Polarized absorption spectroscopy of molecules aligned in stretched polymers bl

Molecules absorbed in stretched polymers are partially aligned. We review procedures suitable for the preparation of such samples, ways in which polarized light can be used to obtain their spectra, and methods that permit an interpretation of the results. This yields information on the orientation distribution and on molecular optical anisotropy, such as electronic and vibrational transition moments directions. These are valuable for the determination of molecular and electronic structure, for state symmetry assignments, and for the detection of ‘hidden’ transitions     

Threshold flexoelectric deformations in homeotropic nematic layers

Elastic deformations of nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. Homeotropic alignment with finite surface anchoring strength was assumed. The director orientation and the electric potential distribution were calculated; the space charge density was also determined. It was found that the threshold voltage strongly depended on the parameters of the system. In particular, a threshold as low as a few tenths of a volt occurred under suitable circumstances. In the case of a negative dielectric anisotropy, Δ ε, such low values of the threshold voltage existed when the ion concentration was sufficiently high, and given sufficiently large magnitudes of the flexoelectric coefficients and a sufficiently small anchoring energy. If the ion concentration was low or if the flexoelectric coefficients were small or if the surface anchoring was strong, the threshold was equal to several volts. In the case of positive dielectric anisotropy, the threshold amounted to several tenths of a volt for a weakly anisotropic and highly conductive material. If the dielectric anisotropy was sufficiently high or if the ion concentration was sufficiently low, the threshold voltage increased with Δ ε and reached tens of volts. These results can be explained as the effect of the inhomogeneous electric field arising in the vicinity of the surfaces, due to the ionic space charge redistributed by the external voltage. They are qualitatively consistent with earlier experiments which show the effect of the ion concentration on the elastic deformations in flexoelectric nematics. They correspond also with theoretical results concerning the effect of the electric field produced by the surface polarization or by the adsorption of ions.     

Experimental study of hybrid nematic wetting films

Liquid crystal layers, with thickness less than 1 μm, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as "hybrid nematics". The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.     

Inversion walls in homeotropic nematic and cholesteric layers

We describe the experimental properties of metastable domains associated with the presence of an Inversion Wall (IW ) and observed with homeotropically anchored nematic and cholesteric liquid crystals sandwiched between parallel glass plates. Such a distorted situation, stabilized by the application of an electric field parallel to the plates as described in reference [1], can also be obtained transiently either when filling a sample cell by capillarity or in some studies of directional solidification at the N-I interface [2]. The application of an electric field perpendicular to the plates with Delta epsilon 0 materials allows control of the reversal region thickness in the bulk of the sample and its associated birefringence. This IW can be stabilized in the particular case of low lateral extension globules in which the line tension of the looped disclination separating the pi wall regions from the homeotropic regions counterbalances the unfavourable bulk free energy. Particular attention is devoted to the defects of these walls, whose appearance using polarizing microscopy is similar to the umbilics of the Freedericksz transition. The structure of these 'four brush' defects is nevertheless here singular, corresponding to point defects of a 3D uniaxial nematic medium. In the case of a chiral nematic, these IW undergo a particular undulation instability which is also observed in 2D simulations.     

Ultrafast to slow orientational dynamics of a homeotropically aligned nematic liquid crystal

The orientational dynamics of a homeotropically aligned nematic liquid crystal, 4'-pentyl-4-biphenylcarbonitrile (5-CB), is studied over more than six decades of time (500 fs to 2 mus) using optical heterodyne detected optical Kerr effect experiments. In contrast to the dynamics of nematogens in the isotropic phase, the data do not decay as a highly temperature-dependent exponential on the longest time scale, but rather, a temperature-independent power law spanning more than two decades of time, the final power law, is observed. On short time scales (approximately 3 ps to approximately 1 ns) another power law, the intermediate power law, is observed that is temperature dependent. The power law exponent of the correlation function associated with the intermediate power law displays a linear dependence on the change in the nematic order parameter with temperature. Between the intermediate power law and the final power law, there is a crossover region that displays an inflection point. The temperature-dependent orientational dynamics in the nematic phase are shown to be very different than those observed in the isotropic phase.     

Stability of the periodic deformations in planar nematic layers

The periodic deformations induced by external fields have been analysed by means of the Taylor expansion method based on the theorems of catastrophe theory. The analysis is restricted to the planar nematic slab influenced by a magnetic field. Two different configurations of the field which lead to periodic deformations with prevailing splay or twist we considered. The ranges of material constants at which the periodic state is stable and the threshold magnetic field strength have been found. The problem of transitions between the undeformed, uniformly deformed and periodic states is discussed.     

Anisotropic diffusion of elongated and aligned polymer chains in a nematic solvent

The translational diffusion constant, D, of a polymer solute in a single-domain, nematic liquid crystal solvent (5CB) is measured in directions parallel and perpendicular to the nematic director using a fluorescence two-beam, cross-correlation technique. The solute under investigation is the stiff, conjugated polymer, MEH-PPV. The ratio D parallel/D perpendicular) of diffusion constants (parallel and perpendicular to the director) is observed to be 1.9 +/- 0.3. This is surprisingly small considering that MEH-PPV is known to be both elongated and highly aligned along the liquid crystal director of 5CB. We therefore argue that the structural order parameter of the solvent governs the anisotropy of the diffusion of the solute.     

Polarized infrared spectra of 1,4-dihydroxy-9,10-anthraquinone aligned in stretched polyethylene

The i.r. absorption spectrum of 1,4-dihydroxy-9,10-anthraquinone (quinizarin) has been studied in a stretched polyethylene matrix using linearly polarized radiation. The resulting two linearly independent spectra show that only three different transition moment directions are present in the 30 observed transitions, and this leads to a classification of the 30 peaks according to the point group symmetry of the excited state involved. This classification makes a complete assignment possible, and settles several questions on the assignment which have been discussed in the literature for several years.     

Polarized surface enhanced Raman and absorbance spectra of aligned silver nanorod arrays

Polarized surface-enhanced Raman scattering (SERS) and UV-vis absorbance spectra were measured for a nonplanar Ag nanorod array substrate prepared by oblique angle vapor deposition. The anisotropy of the SERS polarization was shown to differ from that of the polarized UV-vis absorbance. The maximum SERS intensity was observed in the polarization direction perpendicular to the long axis of the Ag nanorods, while the UV-vis absorbance was strongly polarized along the direction of the long axis of the nanorod array. Analysis of the polarization data showed that molecular orientation was not the cause of the anisotropic SERS scattering. Rather, the SERS anisotropy was primarily attributed to the lateral arrangement of the three-dimensional tilted nanorod lattice in which highly localized plasmon modes are created by strong electromagnetic coupling between adjacent metallic nanorods.     

Polymer-dispersed liquid crystals From the nematic curvilinear aligned phase to ferroelectric films

Polymer-dispersed liquid crystals (PDLC) are widely used for electro-optic applications such as flexible displays, privacy windows or projection displays. Besides these applications, the confinement of a liquid crystal to small cavities is of fundamental interest. The present paper contains a review of the work on nematic and cholesteric PDLCs. Moreover, some very recent developments are summarized such as the use of ferroelectric liquid crystals for PDLC applications.