Showing papers on "Liquid crystal published in 1994"

Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal

Abstract: THE search for organic materials suitable for electronic applica-tions dates back to the early 1950s. But the only organic systems known so far to show electronic charge-carrier mobilities comparable to the amorphous inorganic semiconductors that are the main-stay of the microelectronics industry are zone-refined organic single crystals1–4. Single crystals are difficult and costly to process, however, and are not suitable for device applications. Here we show that a highly ordered columnar (stacked) phase of disk-like organic molecules can exhibit high mobilities for photoinduced charge carriers, of the order of 0.1 cm2 V-1 s-1—higher than for any organic material other than single-crystal phases. Specifically, we study the helical columnar phase of 2,3,6,7,10,11-hexahexylthiotriphenylene, which can be prepared simply by cooling the isotropic liquid melt via the discotic liquid-crystal phase, in which the molecules are already stacked with a high degree of order.

Variational theories for liquid crystals

Abstract: Physical background. Liquid crystals. Early theories. Order parameters. Optical properties. Mathematical preliminaries. Points, vectors and tensors. Algebra. Analysis. Isotropic tensors. Transversely isotropic tensors. Isotropic functions. The classical theory. The bulk free energy. Frank's formula. Frank's constants. Eriksen's inequalities. Instabilities. Electric and magnetic energies. Freederick's transition. Periodic Freedericks' transition. De Gennes' transition. Twisted nematics. Inversion walls. Drops. Surface free energy. Equilibrium configurations. Small and large drops. Wulff's construction. Floating drops. Weak anchoring.

Liquid Crystals: Physical Properties and Nonlinear Optical Phenomena

Abstract: Order Parameter, Phase Transition, and Free Energies. Nematic Liquid Crystals. Cholesteric, Smectic, and Ferroelectric Liquid Crystals. Light Scatterings. Laser--Induced Nonelectronic Optical Nonlinearities in Liquid Crystals. Thermal, Density, and Other Nonelectronic Nonlinear Mechanisms. Electronic Optical Nonlinearities. Introduction to Nonlinear Optics. Nonlinear Optical Phenomena Observed in Liquid Crystals. Index.

Electrically switchable volume gratings in polymer‐dispersed liquid crystals

Abstract: We report electrical switching of the diffraction efficiency in volume Bragg gratings written holographically in polymer‐dispersed liquid crystals (PDLCs). Scanning electron microscopy confirms the volume nature of the gratings and shows that they consist of periodic PDLC planes. The diffraction efficiency can be switched from a high value (∼50%) to a value near zero at fields ∼11 V/μm.

Nuclear Magnetic Resonance of Liquid Crystals

Abstract: Liquid crystals have become ubiquitous in the displays for electronic devices, ranging from wrist watches to laptop computers. Nuclear magnetic resonance is one of the important mechanisms for determining their structures tures and properties. This book covers NMR techniques used in studying liquid crystals and present up to date results from such studies. Ronald Dong has worked on NMR in liquid crystals for much of his professional career. Topics covered include: Nuclear spin dynamics, orientational order, molecular field theories of liquid-crystal molecules, nuclear spin relaxation, spin relaxation, rotational and translational dynamics, internal dynamics of liquid-crystal molecules, NMR in liquid crystals; an appendix covers rotations, Euler angles and Wigner rotation matrices.

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Abstract: Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

Control of reflectivity and bistability in displays using cholesteric liquid crystals

Abstract: It is demonstrated that the reflective properties and bistability of cholesteric liquid crystals can be controlled by proper surface treatment and dispersed polymers. Dispersing a polymer in the liquid crystal or using a cell with an inhomogeneous surface anchoring creates permanent defects which result in long‐term bistability, high contrast at large viewing angles, and gray scale. The wide‐angle, reflective feature makes cholesteric materials suitable for displays without backlights and bistability provides flicker‐free operation.

Observation of orientational photorefractive effects in nematic liquid crystals

Abstract: We report the observation of nonlinear photorefraction in pure aligned nematic liquid crystals in the presence of an applied dc electric field The effect is attributed to nematic axis reorientation created by the photoinduced space-charge field in combination with the applied dc field Strong optical nonlinearity, self-diffraction, and beam-coupling effects are observed

The Cosmological Kibble Mechanism in the Laboratory: String Formation in Liquid Crystals

Abstract: The production of strings (disclination lines and loops) has been observed by means of the Kibble mechanism of domain (bubble) formation in the isotropic-nematic phase transition of the uniaxial nematic liquid crystal 4-cyano-49- n -pentylbiphenyl. The number of strings formed per bubble is about 0.6. This value is in reasonable agreement with a numerical simulation of the experiment in which the Kibble mechanism is used for the order parameter space of a uniaxial nematic liquid crystal.

The Molecular Dynamics of Liquid Crystals

Abstract: 1. A Comparative Survey of the Physical Techniques Used in Studies of Molecular Dynamics.- 1. Introduction.- 2. Molecular Motion in Liquid Crystals.- 3. Spectroscopy in Studies of Molecular Motion.- 4. Applications of Spectroscopy to the Study of Rotational Motion.- 5. Applications to Studies of Translational Diffusion.- 2. On the Description of Ordering in Liquid Crystals.- 1. Introduction.- 2. General Approach.- 3. Purely Positional Order.- 4. Orientational Order.- 5. Positional-Orientational Order in Uniaxial Phases.- 6. Rotameric Molecules.- 3. Diffusion Models for Molecular Motion in Uniaxial Mesophases.- 1. Diffusion Equations.- 2. Solution of the Diffusion Equation.- 3. Diffusion Across Potential Barriers.- 4. Dynamics of Chain Molecules.- 5. Diffusive Coupling with the Solvent.- 4. ESR and Liquid Crystals: Statistical Mechanics and Generalised Smoluchowski Equations.- 1. Introduction.- 2. Rotational and Translational Motion in Ordered Fluids.- 3. Symmetries of the Liquid-Crystalline Potential.- 4. Relative Translational Diffusion: The Pair Correlation Function.- 5. Fluctuating Torques and Slowly Relaxing Local Structures.- 5. Techniques and Applications of Langevin Dynamics Simulations.- 1. Introduction.- 2. Hydrodynamics.- 3. Algorithms and Errors.- 4. Barrier Crossing.- 5. Rotation.- 6. Application to Lipid Bilayers.- 7. Limitations and Extensions of Langevin Dynamics.- 8. Appendix.- 6. An Introduction to the Molecular Dynamics Method and to Orientational Dynamics in Liquid Crystals.- 1. Introduction.- 2. Equations of Motion.- 3. Integration of the Equations of Motion.- 4. Calculation of Static and Dynamic Properties.- 5. General Properties of Orientational Correlation Functions.- 6. Evaluation of Correlation Functions by Molecular Dynamics.- 7. Appendix.- 7. Nuclear Spin Relaxation Formalism for Liquid Crystals.- 1. Introduction.- 2. Spin Dynamics: Density Matrix Description of Relaxation.- 3. Molecular Dynamics.- 4. Cooperative Motion.- 5. Illustrative Experiments.- 6. Summarising Remarks.- 8. Nuclear Spin Relaxation and Molecular Motion in Liquid Crystals.- 1. Introduction.- 2. Experiments and Methods.- 3. Density Operator Theory.- 4. Conclusions.- 9. The Effects of Director Fluctuations on Nuclear Spin Relaxation.- 1. Introduction.- 2. Historical Background.- 3. Theory.- 4. Experiments.- 5. Conclusions.- 10. Nuclear Spin Relaxation Mechanisms in Liquid Crystals Studied By Field Cycling NMR.- 1. Introduction.- 2. Principles and Techniques of Field Cycling NMR.- 3.T1 Relaxation Dispersion in Nematic Mesophases.- 4.T1 Relaxation Dispersion in Smectic Mesophases.- 5. DeuteronT1 Relaxation Dispersion in Methyl Deuteriated MBBA.- 11. Probe Studies of Liquid Crystals.- 1. Introduction.- 2. Orientational Order.- 3. Tools for Molecular Ordering.- 4. Solute-Solvent Interactions.- 5. Interesting Complications.- 6. Conclusions.- 12. ESR and Molecular Motions in Liquid Crystals: Motional Narrowing.- 1. The ESR Spin Hamiltonian: g and A Tensors.- 2. Effective Spin Hamiltonian and Order Parameters.- 3. Spectral Densities and Linewidths.- 4. Rotational Dynamics in Liquid-Crystalline Phases.- 5. Translational Motion in Liquid Crystals.- 13. Thermodynamics of Liquid Crystals and the Relation to Molecular Dynamics: ESR Studies.- 1. Introduction.- 2. Smectic A-Nematic Tricritical Point and Crossover Behaviour.- 3. Universality in Nematic Ordering.- 4. Lipid-Cholesterol Mixtures.- 5. Dynamics: Thermotropics.- 6. Dynamics: Lyotropics.- 14. ESR Studies of Molecular Dynamics at Phase Transitions in Liquid Crystals.- 1. Introduction.- 2. Models of Collective Dynamics: Director Fluctuations.- 3. The Nematic-Isotropic Phase Transition.- 4. The Smectic A-Nematic Phase Transition.- 5. The Dynamic Cluster Model.- 6. Fast versus Slow Collective Motions.- 7. Treatment of Data.- 15. ESR and Slow Motions in Liquid Crystals.- 1. Introduction.- 2. ESR Lineshapes: The Stochastic Liouville Equation.- 3. Methods of Solution: Lanczos and Conjugate Gradient Methods.- 4. Relation to Mori's Method in Statistical Mechanics.- 5. Ordering and Thermodynamics: Behaviour of Large versus Small Probes.- 6. Dynamics in I, N, SA and NR Phases.- 7. Rotational Dynamics in Lyotropics: Lipid Multilayers.- 8. Experimental Techniques: Lineshapes in One and Two Dimensions.- 9. On Fitting the Data.- 16. Raman and IR Fluctuation Spectroscopy of Liquid Crystals.- 1. Introduction.- 2. Determination of Correlation Functions from IR and Raman Lineshapes.- 3. Fluctuation Raman and IR Spectroscopy in Liquid Crystals.- 4. Experimental Results.- 5. Conclusions.- 17. Dielectric Relaxation Behaviour of Liquid Crystals.- 1. Introduction.- 2. Phenomenological Aspects of Dielectric Relaxation.- 3. Measurement of Dielectric Permittivity.- 4. Molecular Aspects of the Dielectric Permittivity.- 5. Experimental Results.- 6. Conclusions.- 18. Neutron Scattering From Liquid Crystals.- 1. Introduction to the Neutron.- 2. Types of Neutron Scattering Experiments.- 3. Coherent and Incoherent Scattering.- 4. Examples of Neutron Diffraction from Liquid Crystals.- 5. Inelastic and Quasi-Elastic Scattering.- 6. Model Incoherent Scattering Laws.- 7. Experiments and Examples of Results.- 19. Molecular Order and Motion in Liquid Crystal Polymers Studied By Pulsed Dynamic NMR.- 1. Introduction.- 2. Experiments and Methods.- 3. Results and Discussion.- 4. Conclusions.- 20. Aggregates of Amphiphiles in Lyotropic Liquid Crystals.- 1. Aggregation of Amphiphiles.- 2. Structure and Aggregates.- 3. Within the Aggregates.- 21. Orientation and Frequency Dependent NMR Relaxation Studies of Bilayer Membranes: Characterisation of the Lipid Motions.- 1. Introduction.- 2. Experiments and Methods.- 3. Results and Discussion.- 4. Conclusions.- 22. Molecular Dynamics in Liquid-Crystalline Systems Studied By Fluorescence Depolarisation Techniques.- 1. Introduction.- 2. Principles of Fluorescence Spectroscopy.- 3. Instrumentation for Fluorescence Spectroscopy.- 4. Principles of Fluorescence Polarisation.- 5. Data Analysis.- 6. Order and Dynamics of DPH and TMA-DPH Molecules in Lipid Bilayer Configurations.- 23. Spectroscopic Studies on Structure and Dynamics of Lyotropic Liquid Crystals: Cubic and Reversed Hexagonal Phases and Lipid Vesicles.- 1. Introduction.- 2. Phase Equilibria and Structural Polymorphism.- 3. Theoretical Aspects on Lipid Self-Assembly.- 4. Nuclear Magnetic Resonance.- 5. Electron Spin Resonance.- 6. Time-Resolved Fluorescence Spectroscopy.- 7. Fluorescence Anisotropy.

Liquid crystal elastomers: Influence of the orientational distribution of the crosslinks on the phase behaviour and reorientation processes

Abstract: The influence of the crosslinking conditions on the phase behaviour and reorientation processes of nematic elastomers is investigated. Two series of elastomers with various network anisotropies crosslinked either in the nematic or in the isotropic state were investigated by means of IR-dichroism and stress-strain measurements. The experimental results clearly demonstrate that for nematic elastomers not only the coupling between the network anisotropy and the state of order has to be considered. It is shown that the influence of the crosslinks and their orientational distribution on the phase behaviour, the state of order as well as on director reorientation processes cannot be neglected.

Polymer-dispersed liquid crystals From the nematic curvilinear aligned phase to ferroelectric films

Abstract: Polymer-dispersed liquid crystals (PDLC) are widely used for electro-optic applications such as flexible displays, privacy windows or projection displays. Besides these applications, the confinement of a liquid crystal to small cavities is of fundamental interest. The present paper contains a review of the work on nematic and cholesteric PDLCs. Moreover, some very recent developments are summarized such as the use of ferroelectric liquid crystals for PDLC applications.

Patterned optical properties in photopolymerized surface-aligned liquid-crystal films

Abstract: MOST practical applications of liquid crystals require control of molecular alignment at macroscopic scales1. This is achieved most simply by confining the liquid-crystalline phase between mechanically rubbed surfaces1. Recent developments2–11 have shown that liquid-crystal alignment can also be controlled by optical means: for example, if azo dyes either on the alignment surface2,3,5,8–11 or dispersed within the liquid crystal itself4,6,7 are oriented by illumination with polarized light, alignment can be induced in the liquid crystal. We show here that the high-resolution alignment patterns obtainable by such techniques may be 'frozen in' by subsequent photopolymerization of the optically patterned liquid-crystalline phase. The resulting polymer films might prove valuable in the development of high-density optical storage media, three-dimensional stereo displays and other optical devices.

Molecular interpretation of the absorption-induced optical reorientation of nematic liquid crystals.

Abstract: A molecular mechanism is proposed to explain the anomalous optical reorientation observed in nematic guest-host systems. According to the model, light-induced reorientation in these systems is due to the interaction between the excited dye molecules and the nematic host. The model justifies a phenomenological description given earlier. Numerical results, based on the present model, are in agreement with the observed order of magnitude of the effect.

Three-terminal adaptive nematic liquid-crystal lens device

Abstract: A 1 mm × 1 mm nematic liquid-crystal three-terminal device for optical beam forming (focusing/spoiling) is fabricated. A thin-film-resistor network on the device substrate layer is used to control the voltages on the 98 internal lens electrodes by use of only one variable external driver. By using a high-resistance thin-film layer of amorphous silicon under the 98-element parallel electrode structure layer, we generate a near-continuous index perturbation to form a cylindrical lens. The focal length of this lens is continuously variable from inifinity to 12 cm by use of a variable 1–4-V-peak 1-kHz square-wave external terminal control signal.

Method and dispenser for filling liquid crystal into LCD cell

Abstract: Methods and dispensers which are used for a new way of filling liquid crystal into a LCD cell. The invented methods use the invented liquid crystal dispenser able to release liquid crystal with sufficient controllability at an evacuated atmosphere. Liquid crystal is filled through an inlet of an assembled LCD cell, or filled by laminating an upper substrate with a lower substrate on which liquid crystal has been dropped. The invented liquid crystal dispensers have an opening, in addition to the opening for a capillary, through which interior air bubbles can be released out so as not to be released from the capillary.

Chemistry and crystal growth

Abstract: Single-crystal materials, along with other forms of condensed matter (ceramics, polymers, liquid crystals, etc.) are fundamental to modern technology. The basic research and production of new materials with “tailored” solid-state physical properties therefore necessitate not only chemical synthesis but also the production of single crystals of a particular morphology (either bulk or thin layer crystals) and well-defined crystal defects (doping). In this review, an attempt is made to broaden the traditional synthetic concept of chemistry to the process of single-crystal synthesis. The methods of the resulting approach, which takes into account the specific properties of solid materials, are discussed and illustrated by experimental set-ups for the solution of a range of problems in chemical crystallization. Also included is recent work on the growing of single crystals of high-temperature superconductors, organic non-linear optical compounds, and proteins.

Partitionable display system

Abstract: An apparatus to display images with partitionable regions allowing visible, invisible, and narrow field of view of displayed information includes a polarizing means to form a plane polarized light background from incident light, a liquid crystal panel capable of forming cross-polarized images against a plane polarized background, and a proximal movable polarizing means to distinguish images on liquid crystal panel from its plane polarized background. Linear translation of the movable polarizer partitions the liquid crystal panel into regions that are visible where there is optical alignment of the liquid crystal panel, background polarizer, and movable cross-polarizer. An optional remote polarizer complements the movable polarizer and provides a narrow viewing angle of images not visible on the liquid crystal panel. By creating a user defined visible, invisible, and narrow field of view of displayed information on an LCD assembly, electronic devices incorporating such a display can be made to provide computing privacy, accessibility to the device, strategic information separation, and information separation to multiple users.

Orientation of a solid particle embedded in a monodomain nematic liquid crystal

Abstract: A problem of the equilibrium orientation of an elongated solid particle put inside a uniformly aligned nematic liquid crystal is studied. The particle is assumed to be sufficiently large so that the orientational distortions it causes may be treated in the framework of the continuum theory. Three kinds of anchoring of nematic material on the particle surface are considered: two of the planar, and one of the homeotropic type. We prove that, depending on the anchoring strength, the stable orientation of the particle major axis may be either perpendicular (weak anchoring) or parallel (strong anchoring) to the unperturbed director of the liquid-crystalline domain. The dimensionless parameter controlling the situation is the ratio \ensuremath{\omega} of the transverse size of the particle to the extrapolation length of the nematic material. The transition from the perpendicular to the parallel orientation of the particle takes place at \ensuremath{\omega}\ensuremath{\sim}1.

Shear-Induced Isotropic-to-Nematic Phase Transition in Equilibrium Polymers

Abstract: We report on a first-order isotropic-to-nematic phase transition induced by shear in concentrated solutions of elongated flexible wormlike micelles. As shear is applied to a disordered micellar solution, the transition shows up in steady-shear experiments as a kink in the shear stress behaviour. Using small-angle neutron scattering under shear, we show that beyond the transition rate, a nematic phase manifests itself by the growth of crescentlike scattering patterns in the direction perpendicular to the flow. A dynamical phase diagram is finally proposed for this system of equilibrium polymers.

Determination of the physical properties of an arbitrary twisted-nematic liquid crystal cell

Abstract: We present an experimental procedure for the determination of the physical properties of an arbitrary twisted-nematic liquid crystal cell. No assumptions are made about the physical properties of the cell, because the commercially available devices are generally produced under proprietary conditions. The techniques developed were then used to evaluate the Jones matrix of an Epson liquid crystal television, and reasonable agreement is observed between theory and experiment. Knowledge of the Jones matrix for these devices helps to evaluate their performance as spatial light modulators in optical processing systems, and such applications are discussed. It also provides an effective means for determining the spatial quality of the liquid crystal layer. We find that there is significant variation in the birefringence across this cell, and the influence that this has on the use of the cell in optical processing systems is discussed.

Diffraction efficiency analysis of a parallel-aligned nematic-liquid-crystal spatial light modulator.

Abstract: An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator is developed for applications in optical information processing and interferometry. Its performance, including diffraction efficiency, is measured, and a theoretical analysis of diffraction efficiency is performed. A comparison between the experimental results and the theoretical analysis shows good agreement. The diffraction efficiency of near-sinusoidal gratings written on the device is of the order of 30%, which is close to theoretical maximum.

Process for producing liquid crystal device whereby curing the sealant takes place after pre-baking the substrates

Abstract: A liquid crystal device is produced through the steps of: providing a pair of glass substrates each having thereon transparent electrodes, applying the substrates to each other with a thermosetting sealant so that the electrodes thereon oppose each other with a prescribed spacing therebetween, and disposing a liquid crystal at the spacing. In the process, prior to the step of applying the substrates, the thermosetting sealant is applied onto a prescribed part of one of the substrates, and the pair of substrates are both pre-baked under an identical condition for pre-baking the thermosetting sealant. As a result, the two substrates constituting the liquid crystal device are caused to have an identical thermal history so that the resultant liquid crystal device is provided with a uniform alignment characteristic free from difference in alignment power between the substrates and a good application accuracy between the substrates.

Molecular and Atomic Arrays in Nano- and Mesoporous Materials Synthesis

Abstract: A model is presented to explain the formation and morphologies of 3-d periodic surfactant-silicate mesostructures. The structures of lamellar, hexagonal tubular and a minimal surface cubic liquid crystal/silicate phase are described.

Soft elasticity : deformation without resistance in liquid crystal elastomers

Abstract: Polymeric nematic liquid crystals crosslinked into elastomers (solid liquid crystals) are shown to display novel and complex elasticity. The internal (nematic) direction can experience a barrier to its rotation which couples to standard elasticity. We predict a new phenomena unique to anisotropic rubber - a «soft elastic response»; uniaxial strain is developed without resistance below a critical deformation λ * due to the relaxation of related shear strains and reorientation of the nematic director. We discuss possible experiments to verify this prediction and interpret the existing experimental observation in terms of the concept of «soft elasticity»

Spiral waves in liquid crystal.

Abstract: We report experimental investigation and theoretical interpretation of the formation of sprial waves in a nematic liquid crystal subjected to a rotating magnetic field and a high frequency electrical field.