Comparisons of the vector method and tensor method for simulating liquid crystal devices

In calculating the director configuration in a liquid crystal device, two methods are commonly employed: a vector model and a tensor model. In this paper, we compare and contrast these methods for liquid crystal devices consisting of a layer of liquid crystal sandwiched between two plates. We compare the reliability and accuracy of the results, the speed of computation and the complexity of implementations of each method.     

Polarization holographic techniques: a method to produce diffractive devices in polymer dispersed liquid crystals

We report a study of holographic gratings recorded by exposing a homogeneous mixture of prepolymer and liquid crystal to polarization light patterns. The method is based on the alignment control of the liquid crystal domains during the photo-induced polymerization process. In contrast, the usual methods to write diffraction gratings in polymer dispersed liquid crystals are based on an intensity holographic technique that controls the polymerization process. The polarization patterns here are made using different geometries: in particular we report results from exposure to polarization patterns realized by the superposition of two waves having orthogonal linear polarizations, or two waves having opposite circular polarizations. Analyses of the recorded gratings are reported.     

Optical modeling of liquid crystal biosensors

Optical simulations of a liquid crystal biosensor device are performed using an integrated optical/textural model based on the equations of nematodynamics and two optical methods: the Berreman optical matrix method [J. Opt. Soc. Am. 62, 502 (1972)] and the discretization of the Maxwell equations based on the finite difference time domain (FDTD) method. Testing the two optical methods with liquid crystal films of different degrees of orientational heterogeneities demonstrates that only the FDTD method is suitable to model this device. Basic substrate-induced texturing process due to protein adsorption gives rise to an orientation correlation function that is nearly linear with the transmitted light intensity, providing a basis to calibrate the device. The sensitivity of transmitted light to film thickness, protein surface coverage, and wavelength is established. A crossover incident light wavelength close to lambda(co) approximately 500 nm is found, such that when lambda>lambda(co) thinner films are more sensitive to the amount of protein surface coverage, while for lambda相似文献   

Sound Velocity in Liquid Binary Alloys as Calculated via the Percus-Yevick Liquid Phonon Dispersion Relation

Abstract

In this note, we utilize the recently calculatd long wavelength limit of the “liquid virtual crystal” model of binary alloy liquid structure factor at the melting temperature, ST_M ^AB (0), to estimate the long-volume binary liquid alloys. We compare the calculate sound velocities to the available experimental sound velocities in these liquid binary alloys and find reasonable agreement.     

Computer simulations of molecular motion in liquid crystals by the method of Brownian dynamics

Computer simulations of the molecular motion of polymer chains in the presence of a strong nematic field were carried out by the method of Brownian dynamics. Two models were studied: the first model (linear liquid crystal) is a freely jointed chain with rigid bonds, the second model (comb-like liquid crystal) is a chain with fixed bond angles and rigid side groups. The influence of ordering on chain conformations, orientational and translational mobility and spectra of relaxation times was investigated.     

Optical wave propagation in a cholesteric liquid crystal using the finite element method

Bragg reflections of cholesteric liquid crystals at normal and oblique incidences were investigated using the finite element method (FEM). Detailed FEM derivations together with the consideration of boundary conditions are given. Two methods for achieving broadband Bragg reflection are analysed: one is to use high birefringence liquid crystal in the uniform pitch structure, the other is to use the gradient pitch structure. In each case, the number of cholesteric pitches required for establishing the Bragg reflection was simulated.     

Optical wave propagation in a cholesteric liquid crystal using the finite element method

Bragg reflections of cholesteric liquid crystals at normal and oblique incidences were investigated using the finite element method (FEM). Detailed FEM derivations together with the consideration of boundary conditions are given. Two methods for achieving broadband Bragg reflection are analysed: one is to use high birefringence liquid crystal in the uniform pitch structure, the other is to use the gradient pitch structure. In each case, the number of cholesteric pitches required for establishing the Bragg reflection was simulated.     

Nematic liquid crystal mixtures for 3D active glasses application

Methods of 3D imaging present on the market can be divided into two basic groups: autostereoscopic and stereoscopic. At the stereoscopic methods of 3D imaging it is necessary to use special glasses for separation of images constituting stereo-pair. The most commonly used active glasses consist of two electro-optical transducers based on twisted nematic or other electro-optical effects. Here the evaluation of liquid crystal mixtures is designed intentionally for 3D active glasses in order to eliminate drawbacks of active glasses available on the contemporary market. Material parameters of new designed liquid crystal mixtures such as: elastic constants, viscosity, refractive indices, dielectric and spectral properties as well as response times have been determined. In addition to the symmetrisation of the switching times of 3D active glasses, a dual frequency nematic liquid crystal has been proposed.     

Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

The effect of electric fields on blue phase III with negative dielectric anisotropy

In order to further understand the structure of the blue phase III (BPIII) phase of chiral liquid crystals, we have observed the effect of electric fields on the reflectivity of BPIII phases in which the liquid crystal has negative dielectric anisotropy. The reflectivity increases dramatically with increasing field, in contrast to the behaviour for materials of positive dielectric anisotropy. We compare the data with cubic domain and double-twist models for BPIII structure and we find that the data best agree with the double-twist model and possibly with an O⁵ cubic domain model.     

Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

Liquid crystal dimers and oligomers

A liquid crystal dimer is composed of molecules containing two mesogenic groups linked via a flexible spacer. Initial interest in these materials stemmed from their ability to act as model compounds for semi-flexible main chain liquid crystal polymers but are now of fundamental interest in their own right because their behaviour is significantly different to that of conventional low molar mass liquid crystals. Recently research has begun to focus also on higher monodisperse oligomers such as trimers and tetramers consisting of molecules containing either three or four mesogenic units, respectively, linked via flexible spacers. In this review the most recent developments in our understanding of structure–property relationships in liquid crystal dimers and higher oligomers is discussed.     

液晶二聚体

液晶二聚体作为半柔性主链型液晶聚合物的简化模型, 通过对其液晶性质的研究, 有助于理解更复杂聚合物体系的液晶行为. 另外, 液晶二聚体作为一类特殊的液晶也有其自身的相结构和相行为. 以分子结构与液晶态的相互关系为主线, 系统介绍了目前文献报道的对称及非对称棒状液晶二聚体(线形、H形、U形、T形)、盘状液晶二聚体(对称的盘-盘状液晶二聚体和非对称的盘-棒状液晶二聚体)和香蕉形液晶二聚体(对称的香蕉-香蕉形液晶二聚体和非对称的香蕉-棒状液晶二聚体)等各种不同类型的液晶二聚体的研究进展, 以期为新型液晶二聚体的分子设计提供一些借鉴.     

Computational modelling of light propagation in textured liquid crystals based on the finite-difference time-domain (FDTD) method

Light propagation through uniaxial rod-like nematic liquid crystal films containing singular (thin) and non-singular (thick) line disclinations is computed using the finite-difference time-domain method (FDTD), which is based on accurate numerical solutions to the governing Maxwell equations. The results obtained by the FDTD method are compared with classical matrix-type methods, including the aggregate model and Berreman's method. It is found that the optical signals for singular and non-singular defects predicted by the matrix methods deviate significantly from the FDTD method because director gradient effects on the plane normal to the incident light are not properly taken into account . It is also found that the FDTD optical signal for singular thin lines has a characteristic length scale associated with the wavelength of the incident light, while for non-singular thick lines the scale is associated with the defect escaped core dimensions. The FDTD method offers an accurate quantitative tool for use in new applications including liquid crystal-based biosensors and rheo-optical characterization of liquid crystalline polymers.     

A new anisotropic soft-core model for the simulation of liquid crystal mesophases

A new anisotropic soft-core model is presented, which is suitable for the rapid simulation of liquid crystal mesophases. The potential is based on a soft spherocylinder, which can be easily tuned to favor different liquid crystal mesophases. The soft-core nature of the potential makes it suitable for long-time step molecular dynamics or dissipative particle dynamics simulations, particularly as a reference model for mesogens or as an anisotropic solvent for use in combination with atomistic models. Results are presented for two variants of the new potential, which show different mesophase behaviors. Variants of the potential can also be linked together to produce more complicated molecular structures. Here, as an example, results are provided for a model multipedal liquid crystal, which has eight liquid crystalline groups linked to a central core via semiflexible chains. Here, despite the complexity of molecular structure, the model succeeds in showing the spontaneous formation of a liquid crystal phase. The results also demonstrate that there is a very strong coupling between the internal structure of the multipedal mesogen and the molecular order of the phase, with the mesogen spontaneously undergoing major structural rearrangement at the transition to the liquid crystal phase.     

Calculation of optical properties of liquid crystal devices using the transmission line matrix (TLM) method

The TLM method uses the voltages and currents on a three-dimensional network of transmission lines to model the electric and magnetic field and loss components of Maxwell's equations. Optical fields within anisotropic media may be easily modelled and results are presented for calculation of the transmission spectra of a Freedericksz transition liquid crystal device. The method is well suited to the calculation of transmission spectra; calculations agree well with spectrometer measurements on an actual liquid crystal device.     

A comparison of temperature and mobile phase composition effects for bonded liquid crystal and polymeric stationary phases in HPLC

Summary Using two polycyclyic aromatic hydrocarbons as solutes, a comparison is made between a bonded liquid crystal stationary phase and a conventional polymeric C-18 phase. The bonded nematic liquid crystal phase was the silanized form of 4-[4-(allyloxy)benzoyl-oxy]biphenyl and the polymeric phase was Vydac 201TP. Both phases display shape and planarity selectivity as indicated by the results of the variable temperature and mobile phase composition studies. The slot theory of retention can be used to explain these results. However, the liquid crystal phase is more sensitive to molecular geometry, probably due to its more ordered structure on the surface. Variable temperature experiments which compare retention during both heating and cooling provides additional support for this conclusion. With the polymeric bonded C-18 phase, each solute had identical retention at the same temperature during both the heating and cooling cycles. On the bonded liquid crystal phase, measurable differences in retention were observed at identical temperatures depending on whether the column was heated or cooled. This effect is attributed to a degree of partially reversible disordering which occurs as the column temperature was increased. However, conditioning with the appropriate mobile phase can restore the original retention characteristics of the bonded liquid crystal phase.     

Alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions

Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.     

Computer simulations of the interactions between liquid crystal molecules and polymer surfaces III. Use of pseudopotentials to represent the surface

Results are presented from atomistic computer simulations of single molecules of the liquid crystals 4-n-octyl-4'-cyanobiphenyl and 4-n-heptyl-2-fluorophenyl 4-octyloxybiphenyl-4'-carboxylate in contact with crystalline polymeric surfaces. The simulations were performed as part of a study of the nature of the alignment interactions in liquid crystal displays and other devices. In contrast to previous atomistic simulations of this type, the crystalline polymer surface was represented by a pseudopotential, effectively replacing the parallel array of polymer chains with a periodic corrugation. The use of a pseudopotential has two main advantages. Firstly, it allows an exploration of the general principles behind liquid crystal alignment on crystalline surfaces, free from the obscuring effect of specific chemical interactions. Secondly, it permits a significant saving in computer time compared with using a surface constructed from explicit atom-pair potentials. In the present work, the aligning capabilities of two simple sinusoidal pseudopotential functions were tested. In each case the wavelength and amplitude of the surface corrugations were varied. It was found that the degree of orientational order of liquid crystal molecules in contact with the surfaces increased with increasing amplitude and decreasing wavelength of the corrugations. Aspects of the two potentials were then combined to produce a pseudopotential designed to represent specific polymeric crystal surfaces. In this case, the (1 0 0) and (1 1 0) faces of polyethylene were modelled. Comparisons with earlier simulations employing atomistic surfaces indicate a good agreement between the orientation functions produced by the two methods. However, the pseudopotential approach uses significantly less computer time, allowing a more reliable determination of orientation within a given timescale.     

Calculation of optical properties of liquid crystal devices using the transmission line matrix (TLM) method

The TLM method uses the voltages and currents on a three-dimensional network of transmission lines to model the electric and magnetic field and loss components of Maxwell's equations. Optical fields within anisotropic media may be easily modelled and results are presented for calculation of the transmission spectra of a Freedericksz transition liquid crystal device. The method is well suited to the calculation of transmission spectra; calculations agree well with spectrometer measurements on an actual liquid crystal device.