Liquid Crystal Samples Research Articles

Dynamics of the periodic distortions in confined nematic liquid crystals imposed by a strong orthogonal electric field

The dynamics of the periodic distortions in confined nematic liquid crystals (LCs) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in confined LC film, only in response to the suddenly applied strong electric field. It has been shown that there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the non uniform rotation mode rather than the uniform one, whereas the lower values of the amplitude dominate the uniform mode. During the turn-off process, the reorientation of the director to the direction preferred by the surfaces is characterized by the complex destruction of the initially periodic structure to a monodomain state. Analysis of evolution of the director distribution in the LC film under the influence of the pretransitional fluctuations when the temperature goes to the bulk nematic-smectic A transition temperature TNA shows that there is two scenarios of the evolution. First, in the reduced temperature range -5⩽logTTNA-1⩽-2, dominate the non uniform rotation mode and the growing pretransitional fluctuations does not destructed the initially periodic structure, whereas at the lower temperature towards TNA, the reorientation of the director field is characterized by the complex destruction of the initially periodic structure to a monodomain state with the director field aligned parallel to the direction preferred by the surfaces.

Boundary Layers in Pressure-driven Flow in Smectic A Liquid Crystals

This article examines the steady flow of a smectic A liquid crystal sample that is initially aligned in a classical bookshelf geometry confined between parallel plates and is then subjected to a lateral pressure gradient which is perpendicular to the initial local smectic layer arrangement. The nonlinear dynamic equations are derived. These equations can be linearized and solved exactly to reveal two characteristic length scales that can be identified in terms of the material parameters and reflect the boundary layer behavior of the velocity and the director and smectic layer normal orientations. The asymptotic properties of the nonlinear equations are then investigated to find that these length scales apparently manifest themselves in various aspects of the solutions to the nonlinear steady state equations, especially in the separation between the orientations of the director and smectic layer normal. Non-Newtonian plug-like flow occurs and the solutions for the director profile and smectic layer normal share features identified elsewhere in static liquid crystal configurations. Comparisons with numerical solutions of the nonlinear equations are also made.

基于应变液晶技术制备反式电控调光玻璃的研究

Based on the principle of stressed liquid crystal,re-exposal method is used during the phase separation by UV curing,and vertical stretched force is applied in the second exposure process.The reverse mode electronic controlled switchable glass samples of polymer dispersed negative liquid crystal by stress inducing vertical directional polymerization are prepared.The sample has the properties of reverse mode piezo-optical effect and reverse mode electronic controlled switchable glass.The sample is translucent and the transmittance is nearly 30%.When compress force or electronic field is applied,the sample turns to scattering opaque state and the haze is above 90%.The research on reverse mode electronic controlled switchable glass is a difficult problem in the switchable glass field.Preparing the sample by stressed liquid crystal technique has a certain significance in research and application of liquid crystal electro-optic device.

A cloud point extraction procedure for gallium, indium and thallium determination in liquid crystal display and sediment samples

A simple, sensitive and rapid cloud point extraction (CPE) methodology has been developed for the selective separation and preconcentration of gallium, indium and thallium, after complexation with gallic acid in the presence of Triton X-114 as a non-ionic surfactant.

Optically binary liquid crystalline blue phases induced by one-armed cholesterol-linked azobenzene molecules.

A series of one-armed cholesterol-linked azobenzene molecules named CholXAzo with different spacers were synthesized, in which Chol6Azo was found to have induced blue phases (BPs) with a concentration of 4.0 wt%. Under irradiation of 385 nm UV light with a density of 15.0 mW cm(-2), photo-responsive behaviour of the 4.0 wt% Chol6Azo doped sample named B3 shows a sensitive temperature dependence, which means that at 38.0 °C a phase transition from BPs to the isotropic phase is induced; however, at 33.0 °C, this phase transition does not take place. Results from the research show that the optically binary phase transition behaviour of B3 is sensitive to the isomerization degree of Chol6Azo, which is closely related to the stability of the BP structure and there is a critical isomerization degree of 13.7% for the phase transition of the B3 liquid crystals. Further POM observation shows that the liquid crystal samples doped with different concentrations of Chol6Azo have an increasing transition temperature for photo-induced phase transition from the BP to the isotropic phase along with the increasing concentration of Chol6Azo, which are found to have the same changing tendency with phase transition temperature from the isotropic phase to BPs and a phase diagram is made to map the optically binary behaviour of Chol6Azo doped blue phase liquid crystals. At last, a simple pattern with the BP and the isotropic phase arranged at an interval was made in this optically binary liquid crystalline blue phase under a suitable photomask.

Properties of Ferroelectric Liquid Crystal/Multiwall Carbon Nanotube Doped Composite

Carbon nanotubes-ferroelectric liquid crystal composites have been prepared and studied. Doping of carbon nanotubes into ferroelectric liquid crystal in varying concentration has been investigated in thin cells in the frequency range of 50 Hz–1 MHz. Decrease in the dielectric permittivity and dielectric strength is found with the doping of carbon nanotubes, however ∼32% increase in the relaxation frequency has been observed in higher concentration carbon nanotubes doped ferroelectric liquid crystal composite than pure ferroelectric liquid crystal sample. A decrease in conductivity has also been noticed after doping.

English

In this paper an attempt is made to design and develop a high-gain low-noise pre-amplifier and a narrow band-pass filter for photoacoustic spectrometer (PAS). A two-stage pre-amplifier with the gain of 1000 and a second order state variable narrow band-pass filter are designed and fabricated. The pre-amplifier and narrow band-pass filter circuits are designed with a low-noise op-amp (LM308). This arrangement improves the sensitivity and signal-to-noise ratio of the photoacoustic (PA) signal. The frequency response of the PA cell is studied with carbon-black and liquid crystal samples. The resonant frequency of the PA cell was found at 345Hz. The performance of the system is tested in both the presence and absence of modulated laser beam applied on the sample, and it is found that the signal quality has been improved with the pre-amplifier and narrow band-pass filter designed for the present study.

A small-angle X-ray scattering study of nanoparticle assembly in an aligned nematic liquid crystal

The assembly of colloidal particles in a nematic liquid crystal has been investigated using small-angle X-ray scattering. The structure and orientation of nanoparticle assemblies in bulk samples of aligned nematic liquid crystal have been determined. The method offers some advantages over optical microscopy, which is usually restricted to investigations of thin cells and micron-sized particles. The scattering from chains of particles has been calculated, and comparison with experimental results has shown that suspensions of 48 and 105 nm diameter silica nanoparticles formed highly ordered structures perpendicular to the liquid crystal director, consistent with quadrupolar defect-induced assembly.

Dielectric, electro-optical, and photoluminescence characteristics of ferroelectric liquid crystals on a graphene-coated indium tin oxide substrate.

Multilayer graphene was deposited on indium tin oxide (ITO) -coated glass plates and characterized by suitable techniques. A liquid crystal sample cell was designed using graphene deposited ITO glass plates without any additional treatment for alignment. Ferroelectric liquid crystal (FLC) material was filled in the sample cell. The effect of multilayer graphene on the characteristics of FLC material was investigated. The extremely high relative permittivity of pristine graphene and charge transfer between graphene and FLC material were consequences of the enormous increase in relative permittivity for the graphene-FLC (GFLC) system as compared to pure FLC. The presence of multilayer graphene suppresses the ionic impurities, comprised in the FLC material at lower frequencies. The ionic charge annihilation mechanism might be responsible for the reduction of ionic impurities. The presence of graphene reduces the net ferroelectricity and results in a change in the spontaneous polarization of pure FLC. Rotational viscosity of the GFLC system also decreases due to the strong π-π interaction between the FLC molecule and multilayer graphene. The photoluminescence of the GFLC system is blueshifted as compared to pure FLC, which is due to the coupling of energy released in the process of charge annihilation and photon emission.

Electro-optical properties of a nematic liquid crystal aligned with a mixture of nanofibres and polyimide

We examined the electro-optical properties of a nematic liquid crystal sample whose substrates were coated with a mixture of nanofibres (NFs) and polyimide (PI). The samples coated with a NF–PI mixture showed a faster turn-off time than the sample coated with pure PI. We found that the elastic constant and the order parameter of the NF–PI samples were greater than those of the pure PI sample, thus inducing the reduction in turn-off time. The NF–PI mixture-coated surface showed a more homogeneous striped surface topology along the rubbing direction compared to the pure PI surface. The uniform topological anisotropy of the surface seems to be related to the increase in the elastic constant and the order parameter of the NF–PI mixture sample.

Impedance spectroscopy investigation of liquid crystals doped with ionic surfactants

The effect of a direct-current (dc) electric field on the electrophysical characteristics of nematic liquid crystals of the alkyl cyanobiphenyl series nCB (n = 6–8) and the multicomponent liquid-crystal mixture MB-1, which are doped with ionic surfactants, has been investigated using impedance spectroscopy. It has been found that the impedance spectra depend substantially on the dc bias voltage applied to the sample. The approximation of the measured spectra with specially developed equivalent circuits of the measuring cell with a sample has made it possible to determine the capacitance of the electric double layer and the electrical conductivity of the samples, as well as the mobility, concentration, and diffusion coefficient of the ions. It has been shown that a region of the space charge is formed in the frequency range f < 100 Hz near the electrodes of the measuring cell with a liquid-crystal sample, which leads to an increase in the active (resistive) and reactive (capacitive) components of the impedance.

P‐111: Polymer Effect on Polymer‐Stabilized Blue Phase Liquid Crystals

AbstractWe have investigated the polymer effect on the thermal stability and electro‐optical properties of polymer‐stabilized blue phase (PSBP) liquid crystal. The temperature range of polymer stabilized blue phase liquid crystal samples expanded over 54°C including room temperature. The experimental results showed that polymer plays an important role in the thermodynamic stability of blue phase liquid crystals and reducing switching voltage.

Effects of fullerene addition on the thermo-optical properties of smectic liquid crystals at the vicinity of the nematic-smectic-A phase transition.

The present work is devoted to the study of the thermo-optical properties of liquid crystals doped with traces of fullerene C(60) at the vicinity of the nematic-smectic-A phase transition. By using the time-resolved Z-scan technique, we measure the temperature dependence of the thermo-optical coefficient and the thermal diffusivity. Our results reveal that the critical behavior of the thermal diffusivity is strongly affected by the fullerene addition. We provide a detailed discussion under the light of the distinct mechanisms behind the thermal transport in liquid-crystal samples.

Improvement of the relaxation time and the order parameter of nematic liquid crystal using a hybrid alignment mixture of carbon nanotube and polyimide

We examined the electrooptical properties of a nematic liquid crystal (LC) sample whose substrates were coated with a mixture of carbon nanotube (CNT) and polyimide (PI). The relaxation time of the sample coated with 1.5 wt. % CNT mixture was about 35% reduced compared to the pure polyimide sample. The elastic constant and the order parameter of the CNT-mixture sample were increased and the fast relaxation of LC could be approximated to the mean-field theory. We found the CNT-mixed polyimide formed more smooth surface than the pure PI from atomic force microscopy images, indicating the increased order parameter is related to the smooth surface topology of the CNT-polyimide mixture.

Light-Induced Instabilities in Photo-Oriented Liquid Crystal Cells

In a planar liquid crystal sample sandwiched between a photosensitive and a reference plate instabilities occurred, when the cell was illuminated from the reference side. The instabilities were induced both by polarized white light source and monochromatic laser beams. Static and dynamic regimes were found; for laser irradiation dynamic instability was found only in a range of polarization directions. A model, developed for monochromatic excitation, predicts that at certain thicknesses dynamic instability is forbidden. Experiments on a wedge-like cell confirmed this conclusion.

Retardance properties of Twisted Ferro-electric Liquid Crystal

The present work presents, for the first time, the dynamic evolution of the hysteresis loops R(V), specifically based onto the total retardance polarimetric parameter R(t) extracted from the Mueller Matrix (MM) of Twisted Ferro-electric Liquid Crystal (TwFLC) samples. This evolution is given for increased exposure time Texp to a rectangular periodic electric field Eapp preliminary applied to the sample, before performing the polarimetric measurements. The Mueller Matrix formalism used is found as a relevant non-destructive and non invasive full-optical method to characterize and to monitor the Twisted Ferro-electric Liquid Crystal cell dynamic behaviour under external applied electric field, before reaching the so-called stripes regime.

Голографічний запис в системі “ліотропний рідкий кристал – віологен”

Наведено результати нелінійно-оптичного експерименту, проведеного для зразків ліотропного рідкого кристалу (ЛРК) з домішками віологенів. Експериментально здійснено запис динамічних дифракційних ґраток на двошарових зразках ЛРК-віологен. Установлено, що запис відбувається в тонкому прикатодному забарвленому шарі віологенів. Визначено основні характеристики записаних ґраток. Проведено аналіз кінетики затухання залишкових теплових ґраток. Розглянуто вклад теплової нелінійності в дифракційну ефективність зразків.

Twisted ferroelectric liquid crystals dynamic behaviour modification under electric field: A Mueller matrix polarimetry approach using birefringence

A low frequency and high amplitude rectangular voltage V has been applied during different increased duration to Twisted Surface Stabilized Ferroelectric Liquid crystal (TwFLC) samples in which the alignment layers of the two substrates were rubbed along two different directions between 0° and 90°. The optical bistability properties have been evaluated using the specific Mueller Matrix formalism that allows a simultaneous access, through a single-shot measurement, to different polarimetric coefficients. In this new approach, the ellipticity ϵR and the azimuthal αR polarimetric parameters, extracted from the birefringence Mueller Matrix MR will be considered in priority. Several significant parameters, such as the horizontal offset ΔV, the degree of asymmetry DA, the characteristic area S of the hysteresis loop, are used to characterize the degradation observed into the hysteretic behaviour of the samples, for different values of ψ, at different duration T of exposure to V, before reaching the so-called stripes regime, giving a new experimental point of view concerning the evolution of the dynamic properties of the samples studied. The αR(V) and the ϵR(V) hysteresis loops are specifically examined. Static mapping related to ϵR(T) is given too. Among the different possible physical origins of the observed degradation, the in-plane anchoring energy contribution will be particularly examined, and a theoretical model is proposed that also gives access to different physical parameters, through a new approach.

Study of electro-optical properties of liquid crystal/reactive mesogen-coated liquid crystal display fabricated by slit coater

We have recently proposed a slit coater method for printable liquid crystal (LC) devices. In this method, the LC/reactive mesogen (RM) mixture was directly coated uniaxially onto the substrate surface by shear flow between the slit nozzle and the substrate, and the liquid crystalline molecules were anchored by in situ photopolymerisation of RM monomers. In this study, the dependence of electro-optical properties and device parameters of LC sample cells fabricated by the slit coater on ultraviolet (UV) exposure dosage was investigated. Moreover, effects of UV exposure dosage and thermal annealing on the azimuthal anchoring energy of LC sample cells fabricated by the slit coater are presented.

用偏光显微术研究聚合物分散胆甾相液晶微滴形貌

Polymer dispersed cholesteric liquid crystal samples with large droplets were made,and the droplets had strange"gem"texture by polarizing microscope.The new vertical helix and parallel helix configuration in the polymer disperse cholesteric liquid crystal droplets were proposed.Considering the bipolar and radiation configuration of polymer disperse nematic liquid crystal,the"gem"texture may correspond with degeneracy coexistence of multi domain texture of vertical helix and parallel helix configuration.This experiment had basic research significance for liquid crystal optics and reference value to design the new liquid crystal device.