Homeotropic alignment

About: Homeotropic alignment is a research topic. Over the lifetime, 3356 publications have been published within this topic receiving 56569 citations.

Discotic liquid crystals: a new generation of organic semiconductors

Abstract: Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Surface-mediated alignment of nematic liquid crystals with polarized laser light

Abstract: THE control of molecular alignment in liquid-crystal phases at macroscopic scales has been investigated extensively because of its importance in optical or optoelectronic applications, such as liquid-crystal displays1. It is well established that liquid crystals can be aligned by an applied electric field, a magnetic field, a shear-flow field, mechanical grooving of the substrate surface or stretching of liquid-crystal polymer thin films2,3. Here we report a new mechanism for liquid-crystal alignment that uses polarized laser light. We find that nematic liquid crystals in an illuminated region become oriented perpendicular to the direction of the electric-field polarization of the laser and remain aligned in the absence of the laser radiation. The liquid crystals can be reoriented again by subsequent illumination. This technique might have applications for large-area displays, optical memories, binary optics, adaptive optics and molecular micro-assembly.

Electrooptic Effects in Liquid Crystal Materials

Abstract: 1 Liquid Crystalline State.- 1.1 Structure of Liquid Crystal Phases.- 1.1.1 Molecules.- 1.1.2 Thermotropic Mesophases Formed by Achiral Rod-Like Molecules.- 1.1.3 Thermotropic Chiral Mesophases.- 1.1.4 Mesophases of Disc-Like and Lath-Like Molecules.- 1.1.5 Polymer Liquid Crystals.- 1.1.6 Lyotropic Liquid Crystals.- 1.2 Mixtures.- 1.2.1 Nematic Eutectics.- 1.2.2 Reentrant Phases.- 1.2.3 Mixtures of Smectics.- 1.2.4 Nemato-Cholesteric Compositions.- 1.2.5 Ferroelectric Mixtures.- 1.3 Liquid Crystalline Materials.- 1.3.1 Chemical Classes.- 1.3.2 Chemical Structure and Transition Temperatures.- 1.3.3 Material.- 1.4 Direct Influence of an Electric Field on the Structure of Liquid Crystals.- 1.4.1 Field-Induced Shifts of the Phase Transition Temperatures.- 1.4.2 Influence of the Field on the Order Parameters.- 1.4.3 Field-Induced Changes in Symmetry.- References.- 2 Properties of the Materials.- 2.1 Dielectric Permittivity.- 2.1.1 Isotropic Liquids.- 2.1.2 Dielectric Anisotropy of Nematics.- 2.1.3 Nematic Mixtures.- 2.1.4 Other Phases.- 2.2 Electrical Conductivity.- 2.2.1 Dependence on Impurity Concentration.- 2.2.2 Conductivity Anisotropy.- 2.3 Optical Anisotropy and Dichroism.- 2.3.1 Optical Anisotropy.- 2.3.2 Dichroism.- 2.4 Viscoelastic Properties.- 2.4.1 Elasticity.- 2.4.2 Viscosity.- 2.4.3 Diffusion Coefficients.- References.- 3 Surface Phenomena.- 3.1 Structure of Surface Layers.- 3.1.1 Surface-Induced Changes in the Orientational Order Parameter.- 3.1.2 Surface-Induced Smectic Ordering.- 3.1.3 Polar Surface Order and Surface Polarization.- 3.2 Surface Energy.- 3.2.1 Wetting of a Solid Substrate.- 3.2.2 Surface Energy and Anchorage of a Nematic Liquid Crystal.- 3.2.3 Techniques for Measuring Anchoring Energies.- 3.3 Cells and Orientation.- 3.3.1 Electrooptical Cells.- 3.3.2 Liquid Crystal Orientation.- 3.3.3 Anchoring Transitions.- References.- 4 Electrooptical Effects Due to the Uniform Distortion of Nematic Liquid Crystals.- 4.1 Electrically Controlled Birefringence.- 4.1.1 Director Distribution.- 4.1.2 Tilted Directors at the Boundaries.- 4.1.3 Different Geometries. Simultaneous Action of Electric and Magnetic Fields.- 4.1.4 Effect of Electrical Conductivity.- 4.1.5 The Frederiks Transition for a Weak Anchoring at the Boundaries.- 4.1.6 Dynamics of the Frederiks Transition.- 4.1.7 The Frederiks Transition in Ferronematic Liquid Crystals.- 4.1.8 Optical Characteristics of the Electrically Controlled Birefringence Effect.- 4.2 Twist-Effect.- 4.2.1 Preparation of Twist Cells, Optical Properties at Zero Field.- 4.2.2 Transmission-Voltage Curve for Normal Light Incidence.- 4.2.3 Electrooptics of the Twist Cell for Oblique Incidence.- 4.2.4 Matrix Addressed Displays and Multiplexing Capability of Twist-Effect Materials.- 4.2.5 Dynamics of the Twist Effect.- 4.2.6 New Possibilities.- 4.3 Supertwist Effects.- 4.4 "Guest-Host" Effect.- 4.4.1 Change in Intensity of the Coloring.- 4.4.2 Colorimetry of "Guest-Host" Displays.- 4.4.3 Color Switching.- 4.4.4 Change in Fluorescence.- 4.5 The Flexoelectric Effect.- 4.5.1 Physical Reasons.- 4.5.2 Static Flexoelectric Distortion in Different Geometries Determination of Flexoelectric Moduli.- 4.5.3 Dynamics of the Flexoelectric Effect.- 4.5.4 Microscopic Approach to Determination of the Flexoelectric Coefficients.- 4.6 Reflectivity in an Electric Field.- 4.6.1 Optical Properties of Nontwisted Nematic Layers.- 4.6.2 Various Techniques.- 4.7 Field Behavior of the Isotropic Phase.- 4.7.1 The Kerr Effect in the Isotropic Phase.- 4.7.2 Reorientation of Surface Quasi-Nematic Layers.- 4.8 Electric Field Effects in Nematic Polymers.- 4.8.1 Thermotropic Mesophases.- 4.8.2 Lyotropic Polymers 212.- 4.9 Electrooptical Properties of Polymer Dispersed Liquid Crystal Films.- References.- 5 Modulated and Nonuniform Structures in Nematic Liquid Crystals.- 5.1 Orientational Modulated Structures.- 5.1.1 Flexoelectric Domains.- 5.1.2 Dielectric Two-Dimensional Structure in the Frederiks Transition.- 5.1.3 Other Types of Modulated Structures.- 5.2 Electrohydrodynamic Modulated Structures.- 5.2.1 Low-Frequency Limit The Kapustin-Williams Domains.- 5.2.2 Different Types of Low-Frequency Electrohydrodynamics.- 5.2.3 Electrohydrodynamic Instability in Nematics with Oblique Director Orientation at the Boundaries.- 5.2.4 Electrohydrodynamic Instability: "Chevron" Mode.- 5.2.5 Anisotropic Instabilities for Different Field and Cell Configurations.- 5.2.6 Allowance for Flexoelectricity in Anisotropic Domain Structures.- 5.2.7 High-Frequency Inertia Anisotropic Mode.- 5.2.8 Modulated Structures with Large Periods in Homeotropic Nematics.- 5.2.9 "Isotropic" Mechanism of the Excitation of Electrohydrodynamic Domains.- 5.2.10 Instabilities in Homeotropic Nematics with ?? >0.- 5.2.11 Classification of Threshold Conditions for Different Instabilities in Nematics.- 5.2.12 Electrohydrodynamic Instabilities in Polymer Nematics.- 5.2.13 The Instabilities above the Threshold Voltage. Dynamic Scattering of Light.- 5.3 Nematics in Spatially Nonuniform Fields.- 5.3.1 Homeotropic Orientation.- 5.3.2 Homogeneous Alignment.- 5.3.3 Twist Cells.- References.- 6 Electrooptical Properties of Cholesterics and Nonferroelectric Smectics.- 6.1 The Pitch of Helix and the Optical Properties of Cholesterics.- 6.1.1 Textures.- 6.1.2 Methods of Measuring the Pitch.- 6.1.3 Optical Properties of Planar Cholesteric Textures.- 6.1.4 Diffraction on the Focal-Conic Texture.- 6.1.5 Pitch Dependence on Cell Thickness.- 6.2 Field-Induced Dielectric Instabilities of Cholesterics.- 6.2.1 Texture Transitions.- 6.2.2 Instability of the Planar Cholesteric Texture.- 6.2.3 Field Untwisting of a Cholesteric Helix.- 6.2.4 Electrically Switched Bistable Structures.- 6.3 Electrohydrodynamic Instabilities in Cholesterics.- 6.4 Flexoelectric Effects.- 6.4.1 Fast Linear-in-Field Rotation of the Cholesteric Helix.- 6.4.2 Flexoelectric Domains.- 6.5 Electrooptical Effects in Blue Phases.- 6.5.1 Optical Features.- 6.5.2 Field Behavior.- 6.6 Electric Field Behavior of Nonferroelectric Smectics.- 6.6.1 The Frederiks Transition in a Smectic A.- 6.6.2 Dielectrically Induced Texture Transitions.- 6.6.3 The Frederiks Transition in a Smectic C.- 6.6.4 Electrohydrodynamic Instabilities in Smectics A and C.- References.- 7 Ferroelectric Liquid Crystals.- 7.1 The Physical Properties of Ferroelectric Liquid Crystals. Methods of Measurement.- 7.1.1 The Symmetry.- 7.1.2 The Microscopic Approach. Ferroelectric Mixtures.- 7.1.3 Physical Parameters.- 7.1.4 Tilt Angle.- 7.1.5 Spontaneous Polarization.- 7.1.6 Flexoelectric Polarization.- 7.1.7 Rotational Viscosity.- 7.1.8 Helix Pitch.- 7.1.9 Dielectric Properties.- 7.1.10 Optical Properties.- 7.1.11 Total Free Energy with Allowance for Anchoring.- 7.2 Electrooptical Effects in Ferroelectric Liquid Crystals.- 7.2.1 The Clark-Lagerwall Effect.- 7.2.2 Deformed Helix Ferroelectric Effect.- 7.2.3 Electroclinic Effect Near the Smectic A ? C* Phase Transition.- 7.2.4 Other Electrooptical Effects.- 7.2.5 Orientation of Samples.- 7.2.6 Problems of Bistability Realization.- 7.3 Ferroelectric Liquid Crystal Polymers.- 7.3.1 Introductory Remarks.- 7.3.2 Chemical Structures.- 7.3.3 Ferroelectricity.- 7.3.4 Electrooptical Switching.- References.- 8 Applications of Electrooptical Liquid Crystalline Materials.- 8.1 Displays.- 8.1.1 Active Matrix Addressed Displays.- 8.1.2 Supertwist Displays for Personal Computers.- 8.1.3 Projection Displays.- 8.1.4 Guest-Host Large Area Information Boards.- 8.1.5 General Trends in Display Applications.- 8.2 Optical Data Processing Devices.- 8.2.1 Light Valves.- 8.2.2 Modulators, Shutters.- 8.2.3 Deflectors of Light.- 8.2.4 Integrated Optical Devices.- 8.2.5 Matrix Spatial Light Modulators or Controlled Transparencies.- 8.2.6 Liquid Crystal Logic Elements.- 8.2.7 Optical Filtration.- 8.2.8 Application of Polymer Liquid Crystals in Optoelectronics.- 8.3 Other Applications.- 8.3.1 Storage Devices.- 8.3.2 Stereoscopic Liquid Crystal Sytems.- 8.3.3 Nondestructive Testing.- 8.3.4 Large Area Glass Light Shutters on Polymer Dispersed Liquid Crystal Films.- References.

Reflective Liquid Crystal Displays

Abstract: Light modulation mechanisms liquid crystal materials homogeneous cells pi cells homeotropic cells hybrid aligned nematic cells twisted nematic cells super twisted nematic cells guest-house cells cholesteric reflective displays liquid crystal and polymer composites system aspects of reflective displays.

Direct Visualization of Individual Cylindrical and Spherical Supramolecular Dendrimers

Abstract: Electron microscopy methods have been used to visualize individual spherical and cylindrical supramolecular dendrimers, providing definitive confirmation of the structures suggested by previous x-ray diffraction analysis that assumed a microsegregated model. These dendrimers are self-assembled, self-organized, and aligned spontaneously and simultaneously in hexagonal columnar or cubic thermotropic liquid-crystal phases with high uniformity. Homeotropic and planar ordering of the hexagonal columnar liquid crystal was precisely controlled by a variety of surfaces. The stiffness of these cylinders was evaluated by examining their planar texture and its defects.