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Experimental investigations of the effects of limiting of a beam traversing a different light-scattering medium based on liquid crystals are presented. It is shown that the result of limiting of the beam at the output of the medium depends on the magnitudes of the phase delays of the singly forward scattered partial signals. Investigations of the effects of limiting has been carried out using nematic liquid crystals MBBA with carbon nanotubes (CNT) in comparison wihs a solution of the same liquid crystals doped by carbon nanodots. Liquid crystals dopped by carbon nanotubes or nanodots, located between crossed polarizers, delays external optical radiation as its power increases to a certain critical level. This can be explained by the phase changes in the LC dopped by the nanoparticles under the action of heating aimed by radiation or an applying external electromagnetic field. Observed phenomena strongly influenced by shape, size and concentration of CNT, as well as the sensitivity of the liquid crystal to the illuminating beam power rate has increased.

Photonic crystal fiber (PCF) is employed as a refractive index sensor (RIS) for solving a lot of problems in biological, physicochemical, medical, engineering fields and many environmental challenges, where it is used in many industries of food, medicines, chemical materials and material diagnosis. The kind of the PCFs was HC-1550 with three wavelengths of laser source were used; 638 nm, 850 nm, and 1550 nm that are useful for recording the intended transmitted signal intensity. Therefore, the measured RI was in the range (1.469 - 1.455 RIU) at the temperature range (36 - 70)°C for the EBBA and it was (1.621 - 1.612 RIU) at the temperature range (22 - 42)°C for the MBBA. The results showed that the highest RI sensitivity was 96.335 dBm/RIU for the HC-1550 infiltrated with MBBA using the laser of wavelength 638 nm, Also, the highest temperature sensitivity was 0.2505 dBm/°C for empty HC-1550 using the laser of wavelength 1550 nm.

We study the influence of confinement on the rheology and structure of nematic liquid crystals (NLCs). NLCs get confined in networks of titania (TiO2, primary particle size = 21 nm) nanoparticles in suspensions of TiO2 and NLC, N-(4-methoxybenzylidene)-4-butylaniline (MBBA). Suspensions with TiO2 nanoparticle volume fraction (ϕ) of 0.006–0.017, form viscoelastic solids with low elastic modulus (G′) of 101 Pa–102 Pa and short relaxation times. Increase in TiO2 nanoparticle ϕ leads to a rise in G′ with TiO2 nanoparticles forming a percolating network at a critical volume fraction (ϕ c) = 0.023, and G′ of ~103 Pa. TiO2/MBBA NLC suspensions at and above ϕ c = 0.023 show G′ ~ ω x−1 scaling, where ω is the angular frequency and the minimum in loss modulus (G′′) with ω. The effective noise temperature, x decreases and approaches 1 with the increase in the TiO2 nanoparticle ϕ from 0.023–0.035, is indicative of an increase in the glassy dynamics. Through the polarized light microscopy and differential scanning calorimetry experiments, we propose that the progressive addition of TiO2 nanoparticles introduces a quenched random disorder (QRD) in the NLC medium which disturbs the nematic order. This results in metastable TiO2/MBBA NLC suspensions in which NLC domains get confined in the network of flocs of TiO2 nanoparticles. We also show that the salient rheological signatures of soft glassy rheology develop only in the presence of NLC MBBA and are absent in the isotropic phase of MBBA.

This study presents a new method to develop a thin controllable retarder/modulator with spatial axial symmetry by rotating a freely suspended of 4-Cyano-4- n -pentylbiphenyl (5CB) and N -(4-methoxybenzylidene)-4-butylaniline (MBBA) liquid crystal films using mechanical method or as a liquid film motor.

A novel modified binary bat algorithm (MBBA) with chaos and Doppler effect in echoes is presented in this paper to deal with the fault diagnosis problem of analog circuits with tolerance. The proposed method focuses on further mimicking the bats' behaviors and improving the binary bat algorithm (BBA) in view of biology. The proposed algorithm utilizes the bats' self-adaptive compensation for Doppler effect in echoes and the chaotic optimization method to improve the BBA. The elitist strategy is also used to store all the possible global best solutions, which will provide us with more possible choices in practice. Since single feature extraction method has limitations during fault diagnosis of analog circuits with tolerance, several kinds of feature extraction methods are used together to build a big feature set firstly in our method, and then, the new proposed MBBA is used to select the optimal feature subset simultaneously optimize the classifier parameters for the analog fault diagnosis. Three analog circuits' examples are given to demonstrate the effectiveness and generalization ability of our method, and other methods are also used to do comparisons. The results indicate that combining different kinds of feature extraction method together to optimally select the feature subset simultaneously optimize the classifier parameters by MBBA can obtain better performance in dealing with analog fault diagnosis problems, and our proposed method provides a better tradeoff between fault diagnosis accuracy and total cost, which is superior to other methods. Therefore, it is an effective method for analog fault diagnosis and has broad application prospects in practice.

The prediction of a theory developed with allowance for the processes of orientational order parameter relaxation in liquid crystals about two independent acoustic mechanisms of orientational instability in the planar layer of a cholesteric liquid crystal (CLC) in a wave field, manifesting itself in the occurrence of a system of 2D domains, is experimentally substantiated for the first time. The distortions of the layer macrostructure for the mesophase of this type was observed in the field of longitudinal waves in a wide frequency range, including frequencies above the relaxation frequency of the orientational order parameter in CLC. The values of the spatial domain period at the effect threshold and the threshold compression amplitudes are determined for CLC layers 10–100 μm thick with a helix pitch of 2–30 μm in the frequency range of 0.3–45 MHz. It is shown that a complete theoretical description of the phenomenon, consistent with the experimental data, can be obtained only with allowance for the action of the convection and the nonlinear relaxation mechanisms of the mesophase layer structure destabilization, which have different natures.

Keywords: nematic liquid crystal, grid pattern, image proces sing, dimensional analysis. Grid pattern (GP) is observed on planarly aligned nematic liquid crystal 4-methoxy-benziledene-4-n-buthyl-aniline (MBBA) when it is applied by a certain frequency and a certain voltage that exceeds a threshold voltage, VGP. The GP appears due to convective movements of MBBA molecules. Dimensions on GP are distinguished into the length and width of GP rectangular shape. Width-area of GP is a product of the length and the width. Dimensional analysis is carried on image processing using ImageJ software. Observation on the pattern shows that the frequency affects GP dimensions, which are proportional to the GP length and inversely proportional to the GP width. Variation on frequency also shows that the frequency is inversely proportional to GP width-area. Dimensional analysis also describes a quadratic relation between frequency and width-area of GP.

In continuation of our earlier work (Pande et al. in Appl Phys A 122:217–226, 2016), we report the results of dielectric and electro-optical properties of pure MBBA and PSLC (polymer PiBMA dispersed in MBBA) systems. The polymer networks domains formed are found to be anisotropic and are oriented in the direction of electric field for both the planar and homeotropic alignment cells. The dielectric anisotropy, optical anisotropy, response time, threshold voltage, splay elastic constant and rotational viscosity were observed for both the LC and PSLC systems with electric field. The liquid crystal properties are affected significantly with increasing concentration of polymer in pure LC material. It has been observed that the polymer networks interaction plays a major role in changing the properties of PSLC system. The effect of dielectric loss and dielectric permittivity on both pure LC and PSLC systems is also discussed.

Optical properties of two nematic liquid crystals, i.e., 4-methoxybenzylidene-4-butylaniline (MBBA) and 4-cyano-4’-pentylbiphenyl (5 CB) which are parallel oriented by nanofibers has been successfully performed. Planar samples of liquid crystals were made using polyvinyl alcohol (PVA) nanofiber from electrospinning process. Electrospinning method was modified using copper (Cu) as gap collector. These planar samples area are 15 mm x 25 mm. Optical characteristic of these samples were studied by using optical polarizing microscope. The optical intensity changes by a rotationof crossed polarizers is observed. The sinusoidal intensity change was observedin these samples as such as in the planar sample prepared by the rubbing method.

The electrohydrodynamic (EHD) vortices produced by an electric current in freely suspended liquid crystal (LC) films of N-(4-methoxybenzylidene)-4-butylaniline (MBBA), convert to a pure rotation in the presence of external electric field ( $$\it{E}_{{\rm ext}}$$ ) perpendicular to the current direction. Here, the direction and strength of the rotation are precisely under control by our self-made device called “liquid-film motor”. In this paper, we present experimental observations of the EHD fluid flow when external electric field varies from zero to a value in which pure rotation on the liquid crystal (LC) film is observed. We also show experimentally that the presence of external electric field causes a great decrease in the current produced by the voltage V J required for observing EHD vortices in freely suspended films of MBBA. The LC films begin to rotate when E ext V J reaches a threshold value. This threshold is investigated experimentally as a function of voltage V J and the external electrical field $$\it{E}_{{\rm ext}}$$ .

The optical transmission of small aluminum oxide particles in liquid crystal 4-methoxybenzylidene-4′-butylaniline (MBBA) and dual-frequency liquid crystal mixture consisting of 4-n-pentyl-4′-cyanobiphenyl (5CB), 4-hexyloxyphenyl ester 4′-hexyloxy-3-nitrobenzoic acid (C2), 4-n-pentanoyloxy-benzoic acid-4′-hexyloxyphenyl ester (H 22) was studied. It was established that there was a transmission band which was shifted on application of a voltage and depended on temperature. At using the dual-frequency liquid crystal as a matrix, a transmission band maximum switched at changing of the frequency. The experimental results are explained by the optical homogeneity of the system in a narrow wavenumber interval when the refractive indices of the particle material and the matrix are close, and also by re-orientation of the liquid crystal molecules when a voltage is applied. The obtained results can be used in constructing of tunable filters and compact monochromators.

The coexistence of the flexo-dielectric walls and the flexoelectric domains of Vistin'-Pikin-Bobylev has been again observed in the nematic methoxybenzylidene-parabutylaniline (MBBA). On the basis of the first observation of such coexistence by Vistin' et al. and the new theory developed by us, the modulus of the difference between the flexoelectric coefficients of splay e1z and bend e3x, |e1z − e3x|, has been estimated to be around 2 × 10−4 statC/cm. It was experimentally found that the increase of the conductivity σ of the nematic MBBA from 10−9 to 10−8 (Ω · cm)−1 replaced the flexoelectric domains by electrohydrodynamic movement of the fluid. The details of this movement are illustrated for the first time.

We report that the in-plane alignment direction changes by liquid crystal materials on a rubbed polyvinyl-cinnamate (PVCi) film surface. The alignment direction of almost LCs investigated in this study was perpendicular to the rubbing direction. However, a few LCs, for example, MBBA (N-p-methoxybenzylidene p-n-butylaniline) aligned parallel to the rubbing direction. On the other hand, all LCs investigated aligned perpendicular to the polarization direction of the UV light on the photo-alignment PVCi surface. Moreover, the alignment direction of MBBA changed perpendicular to parallel to the rubbing direction with the passage of time after MBBA was put on the irradiated with UV light and subsequently rubbed PVCi surface. These results suggest that the alignment surface is changed by wetting with the LC.

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)

We report electrohydrodynamic instability in a cholesteric liquid crystal—a mixture of p-methoxy benzylidene p-n-butylaniline (MBBA) and a cholesteric dopant cholesteryl nonanoate (CN), in the presence of a superimposed magnetic field H. We have found a certain intensity H* with respect to the instability in this mixture with negative dielectric anisotropy, beyond which a stripe pattern appears. With an increasing electric field, the stripe pattern evolves into a grid pattern (GP). Investigations of the H-dependences of thresholds and wavelengths for these patterns in comparison with those of nematic liquid crystals clearly show that the typical GPs found in cholesteric liquid crystals are a superposition of two kinds of stripe patterns induced by the electrohydrodynamic instability.

A simple microscopic mean-field model is proposed for a homeotropically aligned planar nematic liquid crystal (NLC) in contact with a solid-substrate surface. The intermolecular interaction in the NLC is simulated with the anisotropic McMillan potential, and the orienting effect of the substrate surface on the molecules in the NLC is described as that of an external field acting only on the first surface molecular layer of the sample. This model is used to describe the deformation of the director field of the sample caused by the external field and to determine the anchoring strength coefficient W, which is employed to macroscopically describe the orienting effect of the solid substrate on the NLC. The dependence of this coefficient on the strength of the short-range orienting field of the substrate surface used in the proposed microscopic model is found, and a unique correspondence between W and the profile of the orientational order parameter near the substrate surface is established. The temperature dependence of the anchoring strength coefficient is derived and found to agree well with experimental data for the MBBA NLC.

As a fundamental research for developing new liquid crystalline devices, numerical simulations on flexoelectric polarization in liquid crystals under shear flow has been performed using the Leslie-Ericksen continuum theory. We have used two types of liquid crystals for computation; one is N-(p-methoxybenzylidene)-p'-butylaniline (MBBA), which is an aligning liquid crystal, and another is 4-n-Octyl-4'-cyanobiphenyl (8CB), which is a tumbling liquid crystal. These liquid crystals are placed between two parallel plates, and one plate is moved to its plane direction in order to impose shear flow on the liquid crystals. For simplification, the director is assumed to lie in the shear plane. As the boundary condition of the director, the director orientation on one plate is anchored strongly, and the director on the other plate is weakly anchored so that the director orientation depends on shear rate. The flexoelectric polarization for the aligning liquid crystal increases monotonically to reach a steady state value, while it shows peaks for the tumbling liquid crystal; the number of peaks depends on the Ericksen number (ratio of viscous to elastic stress), and the peak value is considerably larger than the steady state value for the aligning liquid crystal. Since the flexoelectric polarization is estimated from the difference of the director angle between both plates, it is found that the flexoelectric polarization can be controlled by the anchoring strength as well as the Ericksen number.

We report electroconvection in a cholesteric liquid crystal with negative dielectric anisotropy, that is, a mixture of p -methoxy benzilidene p - n -butylaniline (MBBA) with a small amount of a cho...

In this paper, a phase diagram is developed for the molar mixtures of nematic liquid crystals of 5CB and MBBA. In order to understand the interaction of the two systems, dielectric permittivities ˜‖ and ˜⊥ were measured for mixtures of various concentrations. The usual assumption is that in the absence of chemical reactions the bulk physical properties add up as a weighted sum of the individual properties. Our dielectric permittivity data clearly show a correlation to the phase diagram and the existence of the induced phase. In order to understand the interactions from a fundamental level, we modeled the 5CB and MBBA molecules using a Silicon Graphics O2 workstation running the software Spartan 5.1. Different electrical surfaces were calculated for a geometrically optimized molecule. Our investigations support the idea of strong charge interactions between the nematic systems.

This work is the continuation of a previous study [Van Horn BL, Winter HH (2000) Dynamics of shear aligning nematic liquid crystal monodomains. Rheol Acta 39:294–300] on the shear dynamics of monodomains of shear aligning nematic liquid crystals [NLC]. The strain dependence of director orientation has been experimentally investigated for monodomains of a NLC with various initial orientations. Comparison of experimental results to predictions using Ericksen's transversly isotropic fluid [TIF] model supports the validity of the TIF model for systems of low molecular weight NLCs. The TIF model has been used to examine the effect of pre-tilt on the dynamics of flow-aligning NLC monodomains. It is shown that small deviations from planar alignment (no pre-tilt) have a large effect on orientation dynamics.

A continuum model is employed to study systematically the optical response of hybrid-aligned nematic (HAN) liquid crystal cells under the application of an external electric field. The influence of the flexoelectric effect is discussed for a large range of anchoring strengths at the homeotropic alignment layer. It is shown that the optical response of HAN cells is governed by a complicated interplay between the flexoelectric coefficient and homeotropic anchoring strength. In particular, the calculations reveal that, for weak homeotropic anchoring, the flexoelectric effect leads to a non-linear voltage shift of the optical transmittance as a function of flexoelectric coefficient, and gives rise to an asymmetry in the transmittance–voltage curve. Finally, a comparison of the continuum-model simulations with recent experimental observations indicates that both the flexoelectric coefficient and the anchoring strength of the nematic liquid crystal MBBA on a homeotropic polyimide alignment layer are significant...

Surface irregularities (for example, rubbing-induced domains, and bumps) have been observed in MBBA nematic cells after special treatment of the electrodes using a conventional soap. Various techniques such as polarizing microscopy, the shadowgraph technique and computer processing of the images have been used. The behaviour of the rubbing-induced domains under a.c. or d.c. electric fields has been studied. A new flexoelectric effect has been observed leading to the dividision of the rubbing-induced domains. A simple model has been proposed (a modification of the Saupe model), which explains the structure and dividision of the rubbing-induced domains.

The coefficient of electronic polarizability (φ i ) is determined by means of the density and extraordinary/ordinary refractive indexes measurements near the reentrant isotropic (I RE ) - calamitic nematic (N C ) and N C - isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and water. These parameters were related to the extraordinary/ordinary refractive indexes via the temperature derivative of the Vuks's equation. The results showed that near the I RE - N C - I phase transitions the measured value of φ i was found to be about two order of magnitude smaller than that obtained for nematic MBBA system. Sign inversion of this coefficient near these phase transitions is also discussed.

Nematic 8CB and MBBA slabs, with a homeotropic zone between two parallel -1/2 disclinations (and associated splay bend zones), are subject to an electric or magnetic field acting transversely to the disclinations. Optical microscopic study of the static and dynamic characteristics of the boundary zones is carried out for the first time. The growth rate of the zones is linear in field-strength. As the field is progressively elevated, the zones widen nonlinearly, and merge to form a splay-bend wall in the mid-region. At a high enough field, the homeotropic region is squeezed out of the wall and a pair of wedge disclinations forms in its place. One of these combines with a -1/2 line originally present in the field off state, forming a loop that progressively collapses.

A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Two types of chevrons have been found in the conduction regime, as well as the conventional chevrons in the dielectric regime in homeotropically-aligned nematics. These two are named the defect-mediated chevron (DMC) and the defect-free chevron (DFC) by their structure changes. Two different bifurcation sequences are found with increasing the applied voltage, such as Freedericksz transition → normal rolls → abnormal rolls → defect chaos → DMC for low frequencies and Freedericksz transition → prewavy pattern → DFC for high frequencies. Moreover, the double periodicity of the chevrons (i.e., the short wavelength λ1 of the striated rolls and the long wavelength λ2 of the chevron bands) shows quite specific frequency dependence.

An interaction between an elastic wave and a liquid crystal is investigated from the view point of evaluation of the elastic properties of the liquid crystal. A periodical stripe domain pattern is induced by an interaction between the elastic wave and the nematic liquid crystal, the pattern is observed by a polarizing microscope. In the numerical analysis of elastic wave propagation in a nematic liquid crystal cell, the 0th mode depends on the liquid crystal properties, on the other hand the 1st mode is influenced predominantly by the properties of two sheets of glasses composing the cell. The 0th and 1st mode phase velocities are estimated from the periodic length of the stripe domain pattern. The 0th mode phase velocity in the nematic liquid crystal cell strongly depends on the cell layer thickness. The phase velocity obtained from the observed result is in good agreement with the numerical analysis result.

In this work we report preliminary results on the properties of a non-polar bicyclohexane nematic liquid crystal. Moreover, its binary mixtures both with a low viscosity phenylcyclohexane and with a normal polar nematic liquid have been investigated. The elastic, viscous and electro-optical properties of these compounds are presented. The non-polar compound, and its mixture with a low percentage of the well known liquid crystal MBBA, exhibit an electrohydrodynamic behaviour in which the conducting regime is absent, while the dielectric regime spreads to low frequencies. Additionally, at higher frequencies of the applied electric field, a regime whose thresholds are linear in frequency is observed. On the other hand, the mixture formed by 50 wt % of the non-polar compound with MBBA exhibits at low frequencies the usual behaviour, followed at higher frequencies by the linear regime.

Electroconvection in twisted nematics has been studied experimentally and theoretically. The frequency dependence of the threshold voltage and rolls period for the different twist angles as well as the Lifshitz point versus the twist angle have been measured. The experimental results agree well with the predictions of linear stability analysis. Various types of secondary instabilities were found above the roll instability depending on the twist angle. The transition from 1D structure to the 2D domain pattern was investigated.

The temperature behaviour of the anchoring energy of nematic liquid crystal MBBA at the substrates with oblique SiO evaporation and director orientation at the interface have been studied by means of magnetic Freedericksz transition technique. The temperature dependence of the coefficients in the phenomenological model of surface anchoring energy has been determined.

The dependence of the cut-off frequency of the conduction mode electrohydrodynamic instabilities, exhibited by homogeneously aligned negative dielectric anisotropy nematic liquid crystals, on the conductivity of the nematic material, the thickness of the nematic layer and the intensity of an orienting magnetic field, is examined. It is found that for sufficiently high conductivity and/or large layer thickness, the cut-off frequency is proportional to the conductivity of the material, and independent of the intensity of the orienting field. Lowering the conductivity and/or the thickness of the nematic layer results in smaller values of the ratio of the cut-off frequency to conductivity, until a critical limit is reached beyond of which no conduction mode of instability has to be observed. It is found also that for given values of conductivity and thickness, the effect of the orienting magnetic field is to lower the cut-off frequency of the instability and, also, to increase the critical conductivi...

We have investigated the pattern fluctuations of electrohydrodynamic convection in a homeotropically aligned nematic layer by means of the dynamic image analysis. The time correlation function C k ...

The equations of linear and angular momentum for nematic liquid crystals have been described with Ericksen's transversely isotropic fluid [TIF] model and solved for start-up of shear flow at constant rate and varying initial alignment conditions. An analytical solution for the rotation provides predictions of the nematic director which closely agree with experimental results of Boudreau et al. (1999), supporting the validity of Ericksen's TIF model. The solution is limited to flows where the effects of director gradients are negligible.

Spin coated films of a chiral comb-like liquid crystalline copolymer containing azobenzene chromophores in its side chains are optically isotropic in their twisted smectic-like glassy state. In contact with a nematic liquid crystal (5CB, E7, MBBA) they provide a degenerate planar orientation. When irradiated by unpolarized UV light, they orient the same nematics homeotropically. Treated with linearly polarized UV light they orient nematics homogeneously with the director along the electric vector of the exciting light. After a combined irradiation first with unpolarized UV light and then with linearly polarized visible light, the films again provide a homogeneous liquid crystal orientation, this time with the director perpendicular to the visible light electric vector. The phenomena observed are related to the light induced optical anisotropy. Two main processes are responsible for the anisotropy (1) a UV light depletion of trans-isomers of the azobenzene chromophores from the chosen direction and (2) a reorientation of the chromophores by polarized visible light.

The theoretical model proposed by Pieranski et al.3 to describe the phenomena occurring in nematic liquid crystals when the magnetic field is suddenly varied was extended by considering higher powers of the distortion angle for the nematic director. Equations giving the time evolution of the number of extinctions in the light transmission were obtained. These equations depend on some relaxation parameters (the time constants τA and τB intervening when the magnetic field was switched "on" or "off", respectively) and on the ratio of elastic constants. The equations were solved numerically and good agreement with the experimental data was obtained for the nematic MBBA.

Orientational changes in monodomains of flow-aligning liquid crystals, 4-n-pentyl-4′-cyanobiphenyl and N-(4-methoxybenzylidene)-4-butylaniline, were studied during shear and recovery in a linear shearing device fitted to an optical microscope. Planar alignment (director in the shear plane) allows the study of twist effects and was generated by strong planar anchoring at the walls with orientations in a range of 0–90° with the shear direction. While being held back by the anchoring walls, shear caused the bulk director to rotate towards a steady-state alignment angle in the shear direction (Leslie angle θL). The transient director rotation was observed with conoscopy. It was found that increasing the initial alignment towards the vorticity direction increased the measured θL. Upon stopping the flow, the bulk director returned to its initial state. With initial alignment orientation changing from parallel to perpendicular to the flow direction, the rate of the twist-driven recovery process increases. This rate increase is not seen in the splay-driven recovery which is constant and consistently faster than twist-driven recovery at all orientations studied.

We report on the interaction between CO 2 laser radiation and the nematic liquid crystal MBBA. The experimental results can be explained by a reorientation of the director due to the electric field of the laser beam.

We consider planarly oriented nematic liquid crystal subjected to plane, rectilinear, oscillatory Couette or Poiseuille flow in the case, when the director is initially perpendicular to the flow plane. Using the Galerkin method we found the critical amplitude of the flow at which the homogeneous instability develops. The analytical approximations for the frequency dependence of the critical amplitude for different types of flow are obtained. The results are compared with experimental data.

Nonstationary electrohydrodynamic convection of nematic liquid crystals in dc electric field has been investigated experimentally. Two modes of domain oscillations: longitudinal - along the initial orientation of director and azimuth - along the crystallographic axes of hexagonal grid pattern have been found in oscillating domain lattice. It has been established that the transition from stationary grid pattern to oscillating one is similar to the first order phase transition.

The optical response of the nematic liquid crystal confined between plates with strong homeotropic and weak planar anchoring (hybrid geometry) under the oscillatory shear has been investigated. Critical shear amplitude above that the director deviates from the planar orientation at the substrate with weak anchoring was found and the dependence of the tilt angle on the displacement amplitude was obtained. The anchoring strength and surface viscosity for the SiO-evaporated substrate was estimated.

Broadband dielectric spectroscopy was applied to investigate the influence of confinement on the dynamics of dielectric modes in MBBA, dispersed in porous matrix with parallel cylindrical pores, 200 A in diameter. Both molecular modes due to the molecular rotation around its short and long axis were observed in the pores. The confinement results in the modification of the bulk-like modes and of the temperature dependencies of the relaxation times. Two low frequency modes due to relaxation of interfacial polarization (collective mode) and reori-entational dynamics of molecules at liquid crystal-pore wall interface (surface mode) were observed. The relaxation time of the surface mode has strong glasslike temperature dependence.

We have investigated the control of the pretilt angle and its direction appearing in the tilted homeotropic alignment of nematic liquid crystals oriented on polyimide films that are irradiated with an unpolarized obliquely incident UV light. As a result, it is shown that the pretilt angle is well controlled and its value is shown to be determined by the irradiation energy where the direction of pretilt is governed by the temperature of polyimide films during the course of UV irradiation.

The polarized and depolarized Rayleigh light scattering spectra for the isotropic phase of p-methoxybenzylidene-n-butylaniline (MBBA) and 4-cyano-4-penthylbiphenyl (5CB) were measured in order to investigate the coupling between translational and orientational modes of molecular motion. The Rayleigh dip was observed in depolarized light scattering spectra of isotropic phase of both liquid crystals in the wide temperature ranges. The Rayleigh dip spectrum was attributed to the coupling between orientational and translational mode of molecular motions in anisotropic molecules. Data analysis based on the Alms’ theory (J. Chem. Phys. 67(12), 5779 (1977)) leads to the evaluation of the coupling parameter R. The temperature dependence of the coupling parameter of both liquid crystals near the phase transition point was discussed.

The photoflexoelectric effect in homeotropic MBBA/D1 azo-dye nematic layers in a horizontal electric field subjected to UV illumination was investigated. About 10 per cent, reversible increase of the flexoresponse (i.e. the increment of transmitted light intensity between crossed polarisers due to an in-plane electric field) was found under epi-UV-illumination, with relaxation times of the order of 15 s. The d.c. voltage dependences of both dark and light flexoresponses followed the predictions of the theory. An UV-dependent amplification of the amplitude of flexo-oscillations (excited by an a.c. voltage) was also registered. The relative importance of bulk and surface contributions to the photoflexoresponse was discussed.

The behaviour of methoxy benzylidene butyl aniline (MBBA) nematic liquid crystal on the cleavage surface of triglycine sulphate under a strong magnetic field has been investigated. With the saturation regime obtained for the liquid crystal cell in fields up to 100 kOe, for the first time the possibility appears to determine not only the ordinary anchoring coefficient W2, but the effective anchoring energy W ∗ without any fitting procedure. Considerable difference in these parameters (two orders of magnitude) disagrees with “electrical” corrections of the Rapini-Papoular approximation. This discrepancy is connected with the surface polarization caused by spontaneous ordering of longitudinal components of MBBA molecular dipoles on interface. The form of the anisotropic interaction potential for such interfaces has to take into account polar effects in near-surface layers of the liquid crystal.

Abstruct Pattern dynamics in a weakly nonlinear regime near the onset of the electrohydrodynamic instability (EHDI) is experimentally investigated in the homeotropic alignment of nematic liquid crystals, of which morphology is introduced. The wavy, chain and bamboo-chevron patterns are observed in homeotropic samples as new patterns. The Busse balloon is also obtained which is quite different from the conventional balloons in EHDI in planar liquid crystals.

The quantitative experimental study of the Eckhaus instability in one-dimensional systems is carried out, employing electrohydrodynamic convection in a nematic layer. The obtained Busse balloon is in good agreement with that predicted by the Eckhaus theory. However, the observed changes of the wavenumber for unstable roll patterns, caused by the Eckhaus instability, as well as the growth rates for the corresponding spatial modes differ dramatically from those the theory yields for the most unstable Eckhaus modes. Possible reasons for the disagreement are discussed.

Liquid crystal display panels subjected to tactile force will show ripple propagation on screens. Tactile forces change tilt angles of liquid crystal molecules and alter optical transmission so as to generate ripple on screens. Based on the Ericksen-Leslie theory, this study investigates ripple propagation by dealing with tilt angles of liquid crystal molecules. Tactile force effects are taken into account to derive the molecule equation of motion for liquid crystals. Analytical results show that viscosity, tactile force, the thickness of cell gap, and Leslie viscosity coefficient lead to tilt angle variation. Tilt angle variations of PAA liquid crystal molecules are sensitive to tactile force magnitudes, while those of 5CB and MBBA with larger viscosity are not. Analytical derivation is validated by numerical results.

Our measurements by a plasmon technique show that the structure of grazing incidence (85°) evaporated SiO x thin films is highly porous. A large fraction of the voids open and can be penetrated by a nematic liquid crystal put into contact with the SiO x layer. The nematic material in the voids remains disordered (isotropic) in the whole nematic rang. This indicates that the director orientation in the volume of the liquid crystal must, at least in this case, be determined by other mechanisms than the ordering of molecules in the grooves of the rough surface. Further, we have found a transition region where the order parameter gradually decreases from the bulk value to zero (disorder) at the surface with a coherence length of approximately 8 nm.

The nonlinear dielectric effect (NDE) is described with reference to the concentrational, isothermal approach to the weakly discontinuous phase transition point from the isotropic to the nematic phase in MBBA (n-4-methoxybenzylidene-4-butylaniline)-benzene solution (nematogen-nonmesogenic component). It was ascertained that the dependence of the pretransitional effect as a function of benzene concentration (‘density’ variable) may be described, as with temperature or pressure (‘field’ variables) studies in pure MBBA, by the mean-field theory.