Spatially periodic reorientation of liquid crystal director at planar Freedericksz transition

Conditions for the formation of a spatially periodic structure in a liquid crystal cell at the threshold planar-planar director reorientation in a dc electric field are considered. The dependences of the threshold and the wave number of the arising structure on (i) the polar and azimuthal anchoring energy at the surface of the cell substrates, (ii) the ratio r of the elastic constants of the liquid crystal, and (iii) the value of the flexoelectric parameter v are calculated. It is shown that the range of the parameters r and v corresponding to the spatially periodic director reorientation depends strongly on the anchoring energy. The range of allowable values of the parameter r narrows with a decrease in the azimuthal anchoring energy and expands with a decrease in the polar anchoring energy, while the range of allowable values of the parameter v widens with decreasing azimuthal anchoring energy and depends nonmonotonically on the polar anchoring energy.     

Threshold spatially periodic structure of the director in a nematic flexoelectric cell with finite anchoring energy

The influence of the finiteness of the anchoring energy of a director and the value of flexoelectric polarization on the threshold of a spatially periodic reorientation of the director and the period of the arising structure is considered for a planar nematic cell. The threshold and the period are calculated numerically and the corresponding analytical expressions are obtained for the case of a strong anchoring of the director. It is shown that for a finite azimuthal anchoring energy the range of admissible values of the flexoelectric parameter ν widens, while for a finite polar anchoring energy this range narrows as compared to the case of an absolutely rigid orientation of the director at the cell surface.     

Effect of surface-Induced Anchoring on Nic Light Scattering Characteristics

The effect of surface anchoring energy on NLC light scattering cross-section and temperature dependence of anchoring parameter are discussed.     

Nanomesh Aluminum Films for LC Alignment. Theoretical and Experimental Modeling

A porous system for LC alignment is reviewed. Fabrication of nanomesh aluminum films and their porous structure are described. Methods of the nanomesh parameters for optimal LC alignment are discussed. A model of the LC alignment in a porous system is proposed. The LC orientation type is determined by the free anchoring energy and the micropore diameter. The difference between planar and homeotropic anchoring energies appears to be lower than the interaction energy by two orders of magnitude.     

The effects of surface morphological mechanics on liquid crystal alignment characteristics using photo-activators

Abstract

Photo-activators, which produce carbon dioxide, are highly reactive radical generators upon UV light illumination. In this study, the generated radicals selectively reacted with the polyimide (PI) main chains and created a liquid crystal alignment layer with a high level of azimuthal anchoring energy. The thickness of the photo-irradiated PI alignment layer was reduced dramatically by photo-induced radical crosslinking, which induced surface wrinkling and roughness. Moreover, the carbon dioxide and methane gases that were generated during photo-irradiation produced many micro-pores, which also attributed to holding the LC molecules tightly on the PI surfaces. The level of azimuthal anchoring energy that was obtained by photo-alignment was better than what was obtained by the rubbing method with the same PI, specifically, the maximum value of the photo-alignment with the photo-activator was 6.92 x 10^?5 J/m² than 1.11 x 10^?5 J/m² of the rubbing. We proposed a mechanism based on a high anchoring energy, a rough surface, a hydrophilic surface, and rapid photo-reactions.     

Liquid Crystal Alignment Behaviors of Polystyrene Derivatives Containing Coumarin Moieties

We synthesized a series of polystyrene derivatives containing coumarin side groups, poly(7-(4-vinylbenzyloxy)coumarin) (P7COU#) and poly(7-(4-vinylbenzyloxy)4-phenylcoumarin) (P7COU4P#), where # is the molar content of 7-hydroxycoumarin and 7-hydroxy-4-phenyl coumarin, respectively, using polymer analogous reaction, in order to study the effect of the 7-hydroxycoumarin and 7-hydroxy-4-phenyl coumarin side groups on the liquid crystal (LC) alignment properties. The LC alignment behavior of these two series was investigated by photoalignment or rubbing alignment. The LC cells made from photoirradiated P7COU# and P7COU4P# films showed homogeneous planar LC alignment. We found that LC aligning ability of the LC cells made from rubbed P7COU# and P7COU4P# films were affected by the structure and molar content of coumarin side groups. For example, anchoring energy of the LC cell fabricated with rubbed P7COU82 (7 × 10^?5 J/m²) and P7COU4P81 (7 × 10^?7 J/m²) film was increased drastically and slightly compared to polystyrene (10^?7?10^?8 J/m²), respectively. Particularly, anchoring energy of the LC cell made from rubbed P7COU# having more than 51 mol % of the 7-hydroxycoumarin as a side groups is comparable to that of the conventional polyimide in the LCD industry.     

Temperature Induced Surface Transitions in Liquid Crystal Planar-Tilted Hybrid Cells

Temperature induced surface transitions in liquid crystal hybrid cells with asymmetrical anchoring, planar to one surface and tilted to the other, are experimentally studied. The results are theoretically explained by an elastic theory and the resulting values for the effective splay-bend elastic constant for a given liquid crystal, as well as the parameters characterising the anchoring on the boundary surface with tilted anchoring are compared with data from previous experiments, with good agreement.     

Surface Order Parameter and Polar Anchoring Energy in Nematic Liquid Crystals

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.     

On orientational fluctuations in nematic liquid crystals

The factors determining the spectral density of intrinsic optical noise of a nematic liquid crystal (NLC), caused by fluctuations of director orientation in a frequency range of 0.01–1 Hz, have been experimentally established for the first time for (10–100)-μm layers, characterized by rigid anchoring at the boundaries and a quasi-homeotropic macrostructure. A model developed for estimating the spectral density of this noise for an NLC layer with a previously deformed macrostructure and finite molecular anchoring energy has been approved.     

The Interfacial Layer of Inorganic Solid Surfaces and Liquid Crystals-Implication on the Anchoring Energy

Liquid crystal anchoring energy varies with the distance to the solid substrate in the interfacial layer. Experimental values correspond to a low anchorage as the liquid crystal slides on its interfacial layer. The question of whether it will be possible to observe low anchorage in a parallel alignment is raised.     

Influence of flexoelectric effect on the correlation of thermal director fluctuations in filled nematic liquid crystals

An electric field generated by the deformation of the liquid crystal director in a filled nematic with flexoelectric properties is shown to reduce correlations of thermal director fluctuations. The contribution to the correlation reduction from the flexoelectric mechanism increases with the energy of director anchoring at the surface of impurity particles and may dominate over the contribution from the director interaction with the particle surface in the absence of flexoelectricity. The reduction of the Rayleigh light scattering by thermal director fluctuations in a filled nematic should be higher if the nematic has flexoelectric properties.     

Influence of magnetic field on the director profile in nematic layer with curved surface

A nematic liquid crystal layer with a curved upper boundary exposed to a magnetic field directed along the layer plane has been studied. Strong nematic anchoring at the lower boundary surface is assumed. A phenomenological expression for the director profile is proposed to solve the problem on the director profile distribution in this system and its stability with respect to the rotational deformations in relation to the degree of the upper-surface roughness and the magnetic field value. The distribution parameters are found by optimizing the expression for the free energy of the liquid crystal system.     

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.     

Energy exchange between electromagnetic waves on the director diffraction grating in planar waveguide with nematic layer

The energy exchange between two coupled TE modes on the diffraction grating of the director in a planar waveguide containing a nematic liquid crystal layer is calculated. The diffraction grating is induced by an external electric field in the nematic layer with periodic anchoring energy at the waveguide surface. The intensity of the signal mode at the output of the nematic layer is calculated as a function of the amplitude and period of the anchoring-energy modulation, the nematic layer sizes, and the electric-field strength. The cases of modes with the same and opposite directions are considered. Analytical expressions for the maximum intensities of the signal mode are derived. In both cases the maximum intensity of the signal mode increases with an increase in the electric-field strength.     

Nematics in Hybrid Cylindrical Cells

A hybrid (weak-strong anchoring) cylindrical cell with antagonistic boundary conditions (planar–homeotropic) at the outer confining surface and at a central coaxial cylindrical core and filled with nematic liquid crystals is investigated by means of the Elastic Theory and Monte Carlo Simulations. The director orientation profile on the surface characterized by weak anchoring is built as a function of the extrapolation length and other relevant parameters. By considering the angle on the surface as an order parameter, a transition in the orientation is found when the extrapolation length becomes smaller than a critical value and the orientation of the surface with strong anchoring is dominant. The order parameter critical exponent is numerically calculated, and the mean field value (β≈1/2) is found in all cases. These transitions are essentially confirmed by Monte Carlo simulations even if some numerical discrepancies are found. The analysis show similarities between planar and cylindrical geometries, but non-usual consequences are found more easily in the latter one.     

On a Periodically Grooved Nematic Cell

The mean permittivity behaviour of a periodically grooved nematic liquid crystal cell has been theoretically investigated as a function of the cell geometry in the strong anchoring hypothesis, with the aim of predicting the degree of homeotropic alignment. The capacitance measurements, performed on two MBBA cells of different thicknesses, are in good agreement with our calculations.     

Electrooptic behaviour of a nematic MBBA layer with a thick interfacial surfactant layer

The electro-optic behaviour of MBBA nematic layers with various surfactants added of anionic, cationic, zwitterionic and non-ionic character and a considerable amount (2wt% to 8 wt%) was studied. The formation of surface-induced flexoelectric domains in all cases clearly showed that the anchoring of the liquid crystal layers has been weak. Such a weak anchoring can be explained with the formation of a perioidc (in the most simple case sinusoidal) MBBA-surfactant interface boundary with a mutual penetration of the liquid crystal and surfactant molecules with some gradient.     

Complex zigzag flexoelectric walls in the Schlieren textures of MBBA: possibility for determination of two basic Schlieren textures with singular or non‐singular surface points

The flexo‐dielectric behaviour of a homeotropic MBBA nematic layer has been experimentally studied. Asymmetric strong‐weak anchoring of the homeotropic nematic layer was achieved by treating the glass plates – one of them with lecithin ensuring the strong anchoring and the other with usual soap ensuring the weak anchoring. The application of a dc voltage with a sufficient amplitude led to the appearance of a complex texture consisting of gradient flexo‐dielectric deformations including Schlieren texture with many singular points and zigzag flexoelectric walls. The application of additional orienting a.c. voltage brought clarification of the Schlieren texture resembling that of the smectic‐C liquid crystal. Inversely, the application of an ac voltage across the homeotropic nematic layer led to formation of a nice Schlieren texture. The additional application of a d.c. voltage created complex zigzag gradient flexoelectric walls which connected the singular points in the Schlieren texture. In this way, one can determine for the first time how many points in the initial Schlieren texture are singular and how many points are non‐singular. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interface Anchoring of p-alkyl Phenols in Disc-micelles of Mesophases

Five para-alkyl substituted phenols have been studied as solutes in two chemically different type II DM mesophases. The degrees of order for the aromatic ring, are not significantly affected by the different short alkyl chains, showing that the anchoring of the -OH group at the aqueous interface is the dominant determinant of local motion. The principal change in the diagonal degrees of order between the chemically different mesophases resides in the change in size of disc micelles with consequent changes in their motional freedom. The anchoring of the phenol -OH at the aqueous interface shows little or no perturbation from the chemical variation of neighboring amphiphiles in the micelle bilayer.     

ALIGNMENT INSTABILITY INDUCED BY IRRADIATION OF VISIBLE LIGHT IN FRUSTOELECTRIC LIQUID CRYSTALLINE CELLS SHOWING THE V-SHAPED SWITCHING

Abstract

Irradiation of the powerful visible light from the laser expedites the deterioration of the frustoelectric liquid crystalline cells (Inui mixture) showing the V-shaped switching. This deterioration is caused by the light absorption in the aligning layer, which strongly influences the molecule-surface polar interaction; this cannot be observed in the cell with the transparent aligning material in the used visible light region. This is observed just in the tilted smectic X* phase, not in the SmA phase. Irradiation during the switching leads to hysteresis in the V-shaped pattern and changes even the surface molecular alignment. These results can be explained by the shielding of the surface charge due to the alignment of the spontaneous polarization and the disturbed polar anchoring.