Numerical approach to the physical behavior of fluorinated nematic liquid crystals

Abstract— A numerical approach to the physical behavior of fluorinated liquid crystals has been investigated. The molecular parameters, such as dipole moment and polarizability were calculated using a semiempirical molecular orbital (MO) method. Using these molecular parameters and the Maier-Meier theory, the temperature dependence of the dielectric anisotropy divided by the order parameter (Δ?/S) has been estimated. In addition, taking the temperature dependence of the order parameter S into account, the temperature dependence of the dielectric anisotropy Δ? has been estimated. The calculated results are in good agreement with experimental results.     

Theory of nematic liquid crystals

Theories of nematic liquid crystals are briefly reviewed, and the generalized van der Waals (GVDW) theory is analyzed in some detail. In the latter approach, the short-range repulsion is approximated by hard volume exclusion, and the long-range attraction by dispersion interaction, mean-field averaged. It is shown that if the full London dispersion potential is used the anisotropy in the attractive dispersion potential substantially modifies the mean-field potential anisotropy caused by volume exclusion and affects the nematic-isotropic phase transition temperature and the slope of the coexistence curve, statements can be found in the literature to the contrary. Overall the agreement between theory and experiment, however, is poor.A rigorous formulation is presented of the configurational free energy functions in terms of one-particle distribution functions and correlation functions, applicable to any ordered system at arbitrary density, and practical means for improving the GVDW approach to nematic liquid theory are suggested.     

Studying nematic liquid crystals by spectroscopic ellipsometry

Abstract— The optical characterization of liquid crystals, in a wide spectral range, is becoming a very important technical task because of their expanding applications in displays, optical telecommunications and other advanced areas of science and engineering. One of the most versatile, sensitive, and well‐established technique for the optical characterization of solid and liquid materials is spectroscopic ellipsometry. In this paper, an outline is presented on the use of ellipsometry for nematic liquid‐crystal characterization: anisotropic refractive‐indices measurements and their temperature dependence, anchoring energy, and tilt distribution inside cells will be discussed. The paper is an extended version of a previously published paper.¹     

Electro-optical limits of the electrically controlled birefringence effect in nematic liquid crystals

The main theoretical limitations — of response time, multiplexing rate and contrast — of the electrically controlled birefringence (ECB) effect are outlined. The performances of a pictorial TV screen and a colour graphic and alphanumeric display are reviewed and compared with the theoretical ECB expectations.     

Fast phase modulation using dual‐frequency nematic liquid crystals

Abstract— Experimental study and numerical simulations of optical phase modulators based on nematic liquid crystals characterized by frequency sign inversion of the dielectric anisotropy has been performed. The received results point out an extreme role of back‐flow effects and electro‐hydrodynamic instability, which make restrictions for creating high‐speed phase modulators with large switchable values of phase retardation. It is shown by numerical simulations that the hydrodynamic destabilization can be suppressed by decreasing the ratio of Leslie coefficients |α²/α^4,5|.     

Chemical gas sensors using chiral nematic liquid crystals and its applications

This paper clarifies the relationship between the molecular structures of chiral nematic liquid crystals and the shift in their reflection spectrum resulting from contact with volatile organic compounds (VOCs). We also propose a mechanism to explain the red‐ or blue‐shift in the peak wavelength upon contact with VOC vapor. Furthermore, enhancement of the sensitivity of this method of VOC detection is discussed.     

Temperature-dependent electrical and dielectric properties of nematic liquid crystals doped with ferroelectric particles

We report on the temperature dependence of dielectric properties of nematic liquid crystals impregnated with BaTiO₃ (BTO) ferroelectric nanoparticles. The behavior of ion transport at low frequencies is discussed by means of dielectric spectroscopy, which allows the ionic concentration and the relaxation time of electrode polarization to be deduced. The experimental results imply that the ferroelectric nanoparticles can not only increase the traveling time of ions between two electrodes but also suppress the buildup of the electric double layers. Verified by the voltage holding ratio of cells containing various contents of BTO nanoparticles, it is obvious that doping BTO into liquid crystals is a low-cost and easy way to improve the device performance.     

Superellipsoid-based, real symmetric traceless tensor glyphs motivated by nematic liquid crystal alignment visualization

A glyph-based method for visualizing the nematic liquid crystal alignment tensor is introduced. Unlike previous approaches, the glyph is based upon physically-linked metrics, not offsets of the eigenvalues. These metrics, combined with a set of superellipsoid shapes, communicate both the strength of the crystal's uniaxial alignment and the amount of biaxiality. With small modifications, our approach can visualize any real symmetric traceless tensor.     

Techniques for visualization of carbohydrate molecules

Standard molecular visualizations, such as the classic ball-and-stick model, are not suitable for large, complex molecules because the overall molecular structure is obscured by the atomic detail. For proteins, the more abstract ribbon and cartoon representations are instead used to reveal large scale molecular conformation and connectivity. However, there is currently no accepted convention for simplifying oligo- and polysaccharide structures. We introduce two novel visualization algorithms for carbohydrates, incorporated into a visualization package, CarboHydra. Both algorithms highlight the sugar rings and backbone conformation of the carbohydrate chain, ignoring ring substituents. The first algorithm, termed PaperChain, emphasizes the type and conformation of the carbohydrate rings. The second, Twister, emphasizes the relative orientation of the rings. We further include two rendering enhancements to augment these visualizations: silhouettes edges and a translucent overlay of the ball-and-stick atomic representation. To demonstrate their utility, the algorithms and visualization enhancements are here applied to a variety of carbohydrate molecules. User evaluations indicate that they present a more useful view of carbohydrate structure than the standard ball-and-stick representation. The algorithms were found to be complementary, with PaperChain particularly effective for smaller carbohydrates and Twister useful at larger scales for highlighting the backbone twist of polysaccharides.     

Relationship between rubbing‐induced anisotropy and surface azimuthal anchoring energy of nematic liquid crystals

The relationship between the rubbing‐induced anisotropy of alignment films and the surface azimuthal anchoring energy of nematic liquid crystals was investigated using three kinds of rubbing cloths. Reflection ellipsometry revealed that the optical surface anisotropy of polyimide alignment films increases monotonically with increasing RS. The surface azimuthal anchoring energy measured by the torque balance method depends on the rubbing‐induced anisotropy of alignment films. This indicates that liquid crystal molecular alignment can be controlled by monitoring the rubbing‐induced optical anisotropy of alignment films when a suitable rubbing cloth is used.     

Optimization of liquid crystals for television

Abstract— LCD TVs have dramatically improved in performance during the last 2 years. At the same time, the sale prices decreased by more than 50%. Together with the introduction of digital terrestrial broadcasting, this resulted in increasing sales of LCD TVs. This paper gives an overview of the main liquid‐crystal display (LCD) technologies used for TVs. We discuss key materials, synthesis, structural property relationships, and the optimization of LC mixture properties. For all technologies, we have achieved fast‐switching LC mixtures (16 msec). Novel materials for LC mixtures for the next generation of superior performance LCD TVs with 8‐msec switching times are shown.     

应用于人体关节缺损修复的建模与可视化

目的 关节缺损疾病治疗目前存在的主要问题是缺乏精确的关节模型以及个体化的修复方案,为此提出量化关节骨缺损的精确建模与可视化方法。方法 利用骨骼图像增强、多模态影像融合、关节结构分割、关节病变结构建模与定量分析等核心关键技术,从关节CT或MRI影像中构建和恢复缺损关节的空间立体结构,为关节缺损信息提供量化参数和3维模型,从而帮助医生快速准确地对关节缺损疾病进行诊疗。结果 针对建模与可视化方法中的核心关键技术进行了深入研究,实验结果显示上述方法能够为关节缺损修复提供精确的3维量化模型。结论 基于CT、MRI影像的关节结构建模与可视化技术为评价骨缺损大小提供了精确有效的方法,在关节盂或肱骨结构等疾病的诊疗方面具有重要的临床意义,此项技术的发展对关节缺损疾病的修复发挥着重要作用。     

Multifunctional glassy liquid crystals for photonics

Abstract— As an emerging class of photonic materials, morphologically stable glassy liquid crystals, were developed following a versatile molecular design approach. Glassy cholesteric liquid crystals with elevated phase‐transition temperatures and capability for selective‐wavelength reflection and circular polarization were synthesized via deterministic synthesis strategies. Potential applications of glassy cholesteric liquid crystals include high‐performance polarizers, optical notch filters and reflectors, and circularly polarized photoluminescence. A glassy nematic liquid crystal comprising a dithienylethene core was also synthesized for the demonstration of nondestructive rewritable optical memory and photonic switching in the solid state.     

Nanostructured alignment layer for liquid crystals

A new type of nanostructured alignment surface is proposed and demonstrated. As in previous nanostructured alignment surfaces, a large pretilt angle of near 45° can be produced reliably. Moreover, this new structure is fabricated with a mask, and there is no randomness involved. The uniformity and anchoring properties of this new surface have been measured to be of excellent quality.     

Three-dimensional interactive Molecular Dynamics program for the study of defect dynamics in crystals

The study of crystal defects and the complex processes underlying their formation and time evolution has motivated the development of the program ALINE for interactive molecular dynamics experiments. This program couples a molecular dynamics code to a Graphical User Interface and runs on a UNIX-X11 Window System platform with the MOTIF library, which is contained in many standard Linux releases. ALINE is written in C, thus giving the user the possibility to modify the source code, and, at the same time, provides an effective and user-friendly framework for numerical experiments, in which the main parameters can be interactively varied and the system visualized in various ways. We illustrate the main features of the program through some examples of detection and dynamical tracking of point-defects, linear defects, and planar defects, such as stacking faults in lattice-mismatched heterostructures.

Program summary

Title of program:ALINECatalogue identifier:ADYJ_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADYJ_v1_0Program obtainable from: CPC Program Library, Queen University of Belfast, N. IrelandComputer for which the program is designed and others on which it has been tested: Computers:DEC ALPHA 300, Intel i386 compatible computers, G4 Apple ComputersInstallations:Laboratory of Computational Engineering, Helsinki University of Technology, Helsinki, FinlandOperating systems under which the program has been tested:True64 UNIX, Linux-i386, Mac OS X 10.3 and 10.4Programming language used:Standard C and MOTIF librariesMemory required to execute with typical data:6 Mbytes but may be larger depending on the system sizeNo. of lines in distributed program, including test data, etc.:16 901No. of bytes in distributed program, including test data, etc.:449 559Distribution format:tar.gzNature of physical problem:Some phenomena involving defects take place inside three-dimensional crystals at times which can be hardly predicted. For this reason they are difficult to detect and track even within numerical experiments, especially when one is interested in studying their dynamical properties and time evolution. Furthermore, traditional simulation methods require the storage of a huge amount of data which in turn may imply a long work for their analysis.Method of solution:Simplifications of the simulation work described above strongly depend also on the computer performance. It has now become possible to realize some of such simplifications thanks to the real possibility of using interactive programs. The solution proposed here is based on the development of an interactive graphical simulation program both for avoiding large storage of data and the subsequent elaboration and analysis as well as for visualizing and tracking many phenomena inside three-dimensional samples. However, the full computational power of traditional simulation programs may not be available in general in programs with graphical user interfaces, due to their interactive nature. Nevertheless interactive programs can still be very useful for detecting processes difficult to visualize, restricting the range or making a fine tuning of the parameters, and tailoring the faster programs toward precise targets.Restrictions on the complexity of the problem:The restrictions on the applicability of the program are related to the computer resources available. The graphical interface and interactivity demand computational resources that depend on the particular numerical simulation to be performed. To preserve a balance between speed and resources, the choice of the number of atoms to be simulated is critical. With an average current computer, simulations of systems with more than 10⁵ atoms may not be easily feasible on an interactive scheme. Another restriction is related to the fact that the program was originally designed to simulate systems in the solid phase, so that problems in the simulation may occur if some particular physical quantities are computed beyond the melting point.Typical running time:It depends on the machine architecture, system size, and user needs.Unusual features of the program:In the program, besides the window in which the system is represented in real space, an additional graphical window presenting the real time distribution histogram for different physical variables (such as kinetic or potential energy) is included. Such tool is very interesting for making demonstrative numerical experiments for teaching purposes as well as for research, e.g., for detecting and tracking crystal defects. The program includes: an initial condition builder, an interactive display of the simulation, a set of tools which allow the user to filter through different physical quantities the information—either displayed in real time or printed in the output files—and to perform an efficient search of the interesting regions of parameter space.     

RTR-trees for space robotics behavior simulation and visualization

New types of trees of structures and linked lists with variable order relations (RTR-trees and RTR-lists) are discussed. Using them enables application of the direct kinematic scheme for simulation of 3D-objects with reorderable structure, making behavior simulation more natural.     

Ripple phenomenon of displays with nematic liquid crystal

Liquid crystal displays will show ripple if the display surface or display bracket is subjected to tactile forces. In this paper, the ripple of liquid crystal displays is investigated by dealing with elastic wave propagation in a liquid crystal layer. The model proposed for a visco‐elastic medium like liquid crystals (LCs) is generalized by combining the properties of a crystalline solid and an anisotropic fluid. The governing equation is derived by using visco‐elastic and wave equations. In the experiments, a linear motor is used to touch the display panel for producing ripple. Displays of three different amounts of LCs are compared. Experimental results also show that each display panel has its own wave propagation velocity that is not changed by different motor touch speeds. In addition, both theoretical analysis and experimental results depict that displays with a larger amount of LCs lead to slower ripple speed.     

A novel radial orientation treatment of nematic liquid crystals using magnetic field lines with permanent magnet and applying the assisted electric field

Recently, there has been considerable research on optical devices, such as liquid crystal (LC) lenses and special optical plates, using LCs. In such devices, relatively small LC cells are frequently used, or unique LC orientations are required. As an LC orientation process, we focused on the LC director's orientation induced by the magnetic force line distribution of a small neodymium magnet. We propose a simple method for obtaining radial orientation, which is rather difficult to obtain using the ordinary rubbing method. The initial orientation in the LC cell is a vertical orientation cell with almost zero azimuth anchoring. With the proposed method, the reorientation process is performed with an assisting electric field and a small permanent magnet, unlike the conventional magnetic field orientation process that requires a large electromagnet. Furthermore, a polymer stabilization treatment is used to fix the obtained radial orientation pattern in the LC cell. After the treatment, the applying voltage can control the tilt angle of the director in weak polymer treatment, and a completely fixed orientation pattern can be obtained that in strong.