Physical Sciences and Mathematics Commons^™

Model For The Director And Electric Field In Liquid Crystal Cells Having Twist Walls Or Disclination Lines, G. Panasyuk, David W. Allender

David W Allender

Two examples of the director structure and electric field in patterned electrode liquid crystal cells are studied using a recently developed calculational model. First, a display cell that exhibits a homeotropic to multidomainlike transition with twist wall structures has been considered for a liquid crystal with positive dielectric anisotropy. The model elucidates the behavior of the electric field. Calculations show good agreement between the model and direct computer solution of the Euler-Lagrange equations, but the model is at least 30 times faster. Second, the possibility that a cell has +/-1/2 disclination lines instead of a wall defect is probed. A …

The Effect Of Surface Alignment On Analog Control Of Director Rotation In Polarization Stiffened Smc* Devices, Mitya Reznikov, Lena M. Lopatina, Michael J. O'Callaghan, Philip J. Bos

Philip J. Bos

The effect of surface alignment on the achievement of analog ("V"-shaped) electric field control of director rotation in SmC* liquid crystal devices is investigated experimentally and through numerical modeling. Ferroelectric SmC* liquid crystals are intrinsically analog and thresholdless, i.e. the director can be rotated freely around the tilt cone. Whether or not a SmC* liquid crystal cell exhibits thresholdless switching depends strongly on the influence of the cell's alignment layers, on the magnitude of the liquid crystal's spontaneous polarization, and on whether smectic layers adopt a bookshelf or chevron configuration. To study the effect of the surface alignment layers, we …

Finite Molecular Anchoring In The Escaped-Radial Nematic Configuration: A 2-H-Nmr Study, G. P. Crawford, David W. Allender, J. William Doane, M. Vilfan, I. Vilfan

David W Allender

The director-field configuration of a nematic liquid crystal confined to cylindrical cavities of polycarbonate Nuclepore membranes ranging from 0.3 to 0.05-mu-m in radius is determined using deuterium nuclear magnetic resonance (H-2 NMR). Spectral patterns from cavities of radius 0.3-mu-m reveal the escaped-radial configuration with singular point defects, but as the cylinder size is decreased, the elastic energy imposed by the curvature of the confining walls competes with the anchoring energy to tilt the directors away from their preferred perpendicular anchoring direction, preventing the expected transition to the planar-radial configuration. A surface fitting parameter is directly determined by simulating H-2-NMR line …

Surface States In Nearly Modulated Systems, A. E. Jacobs, D. Mukamel, David W. Allender

David W Allender

A Landau model is used to study the phase behavior of the surface layer for magnetic and cholesteric liquid-crystal systems that are at or near a Lifshitz point marking the boundary between modulated and homogeneous bulk phases. The model incorporates surface and bulk fields and includes a term in the free energy proportional to the square of the second derivative of the order parameter in addition to the usual term involving the square of the first derivative. In the limit of vanishing bulk field, three distinct types of surface ordering are possible: a wetting layer, a nonwet layer having a …

Surface-Layer Phase-Transitions In Nematic Liquid-Crystals, Natasha Kothekar, David W. Allender, R. M. Hornreich

David W Allender

Surface-layer transitions in nematogenic materials characterized by a preferential planar surface interaction linear in the order parameter have been studied theoretically at temperatures above the bulk transition (T(NI)). The coupled Euler-Lagrange nonlinear differential equations obtained from the Landau-de Gennes free energy were solved exactly by numerical integration. This problem had been studied previously employing various limits and approximations with several differences in the phase diagram reported. The exact results allow one to determine which of these differences are artifacts of the approximations used and which are dependent upon the ratio of elastic constants. It is found, for physically relevant elastic …

Nmr In Spinning Samples Of Biaxial Liquid-Crystals, Demetri J. Photinos, Philip J. Bos, Paul Ukleja, J. William Doane

Philip J. Bos

Magnetic resonance measurements on a spinning sample of deuterated n-heptyloxyazoxybenzene (HOAB dsp) are presented. The resulting spectral patterns for the deuterated terminal methyl groups display the biaxial character of the smectic-C phase. Values for those biaxial order parameters which survive under free molecular rotation are directly determined from the shape of the spectral patterns. Parameters expressing biased rotation are also present and contribute significantly to the biaxiality.

Piezoelectric Effects In Cholesteric Elastomer Gels, C. C. Chang, Liang-Chy Chien, R. Meyer

Liang-Chy Chien

We report a piezolectric effect in cholesteric liquid crystalline elastomer gels. When we apply a shear stress to a cholesteric elastomer gel in the direction perpendicular to its helical axis, a polarization can be induced in the direction perpendicular to both the stress and the helix. This experimental observation agrees with the theory proposed by Pelcovits and Meyer [J. Phys. II France 5, 877 (1995)].

Formation And Localization Of Internal Polymer Columnar Structures In Liquid Crystal Cells, Qingbing Wang, Liang-Chy Chien, Satyendra Kumar

Liang-Chy Chien

Internal polymer columnar structures in liquid crystal (LC) cells have been produced using the method of thermally-induced phase separation of LC/prepolymer solution and solidified by photopolymerization. These polymer columns adhere to both substrates and enhance control of the LC cell spacing and mechanical stability. The existence and profile of the polymer columns were investigated by optical and scanning electron microscopy techniques. Supertwisted nematic LC cells with polymer columnar structures have been constructed using this method and their electro-optical characteristics measured.

Electro-Optic Characteristics Of A Liquid Crystal Cell In A Homogeneous To Twisted Planar Mode, S. H. Lee, H. Y. Kim, H. G. Galabova, David W. Allender

David W Allender

The electro-optic properties of a novel liquid crystal device associated with a homogeneous to twisted-planar (HTP) transition have been investigated. The cell plates are prepared in such a way that in the absence of an electric field the liquid crystal orientation is uniform throughout the cell, and thus between crossed polarizers the cell appears black (off state). When an electric field of a particular configuration is applied, a 90° twist is imposed, and the cell transmits light (on state). It was found that the HTP device exhibits wide viewing angle and excellent color characteristics. The dependence of the electro-opticeffect on …

Optimizing The Nematic Liquid Crystal Relaxation Speed By Magnetic Field, Bin Wang, Xinghua Wang, Philip J. Bos

Philip J. Bos

Nematic liquid crystalline materials have been widely used in electro-optical control of visible light. However, when IR light is considered, the thickness of the liquid crystal layer must be increased which causes the relaxation time of the material to be slower than required for many applications. In this paper we use a magnetic field to increase the speed of thick nematic devices. We show that above a particular magnetic field strength, thicker cells relax more quickly than thinner ones. Also, we find that there exists an optimal voltage range for devices of a particular thickness and with a particular applied …

Electro-Optical Study Of Nematic Elastomer Gels, C.C. Chang, Liang-Chy Chien, R. B. Meyer

Liang-Chy Chien

We report on an electro-optical study of a liquid-crystal-polymer composite system. This system is made by dissolving a small amount of acrylate monomers (8%) plus crosslinkers in a nematic liquid crystal, and then photopolymerizing the homogeneous mixture. These nematic gels show properties different from those of ordinary nematics or polymer stabilized liquid crystals. By studying the electric field induced Frederiks transition, some basic propel-ties of the nematic gels can be deduced. We compare the experimental results to a simple phenomenological model of the nematic gel, involving a characteristic length scale of order 1 mu m, which is also visible as …

Switchable Diffraction Gratings Based On Inversion Of The Dielectric Anisotropy In Nematic Liquid Crystals, S. W. Kang, Samuel N. Sprunt, Liang-Chy Chien

Liang-Chy Chien

A switchable liquid crystal diffraction grating, responsive to both the frequency and magnitude of an applied voltage, is demonstrated. The grating is based on polymer stabilization of modulated states of the liquid crystal optic axis induced at frequencies near a sign inversion of the dielectric anisotropy. In a particular case of frequency-based switching, 70% of the transmitted intensity of a 633 nm laser beam can be shifted into or out of a +/-12 degrees forward are by changing the frequency of an applied voltage between 10 and 16 kHz at a fixed amplitude of 27 V. (C) 2001 American Institute …

Charge Instabilities And Phase-Separation In 3-Band Extended Hubbard Models: Effect Of O-O Hopping, N. Kothekar, K. F. Quader, David W. Allender

David W Allender

We explore the interplay between various interaction and hopping terms, in particular the effect of O-O hopping, tppo on the charge instabilities and phase separation in the three-band extended Hubbard models. Within a weak-coupling mean-field approach, we find that in two dimensions (2D), tppo moves the phase separation boundary appreciably to higher dopings. This would increase the window of doping over which charge-transfer-mediated superconductivity can occur. Considering a larger phase space, we also obtain a more complete picture of the coexistence lines and spinodal decompositions, and determine the charge-transfer-instability critical point. We also consider the 1D case: tppo is found …

Macroscopic Chirality Of A Liquid Crystal From Nonchiral Molecules, Antal Jákli, Geetha G. Nair, C. K. Lee, R. Sun, Liang-Chy Chien

Liang-Chy Chien

The transfer of chirality from nonchiral polymer networks to the racemic B2 phase of nonchiral banana-shaped molecules is demonstrated. This corresponds to the transfer of chirality from an achiral material to another achiral material. There are two levels of chirality transfers. (a) On a microscopic level the presence of a polymer network (chiral or nonchiral) favors a chiral state over a thermodynamically stable racemic state due to the inversion symmetry breaking at the polymer-liquid crystal interfaces. (b) A macroscopically chiral (enantimerically enriched) sample can be produced if the polymer network has a helical structure, and/or contains chemically chiral groups. The …

Double Twist In Helical Polymer "Soft" Crystals, Christopher Li, Stephen Cheng, Jason Ge, Feng Bai, John Zhang, Ian Mann, Frank Harris, Liang-Chy Chien, Donghang Yan, Tianbai He, Bernard Lotz

Liang-Chy Chien

In natural and synthetic materials having non-racemic chiral centers, chirality and structural ordering each play a distinct role in the formation of ordered states. Configurational chirality can be extended to morphological chirality when the phase, structures possess low liquid crystalline order. In the crystalline states the crystallization process suppresses the chiral helical morphology due to strong ordering interactions, In this Letter, we report the first observation of helical single lamellar crystals of synthetic non-racemic chiral polymers. Experimental evidence shows that the molecular chains twist along both the long and short axes of the helical lamellar crystals, which is the first …

Liquid-Crystal Photoalignment Using Low-Molecular-Weight Photo-Cross-Linkable Composites, O. Yaroshchuk, L Cada, M. Sonpatki, Liang-Chy Chien

Liang-Chy Chien

A liquid-crystal (LC) photoalignment method, based on the use of low-molecular-weight photo-cross-linkable (LMWPC) composites is proposed. The basic idea of this method is that cross linking or both cross linking and polymerization of LMWPC composites are realized immediately on the substrate simultaneously with the photoalignment or as a stage of the backing process. Providing the advantages of a conventional photoalignment method, the use of LMWPC composites simplifies the synthesis and alignment procedure, makes possible wide range control of pretilt angles, and enhances the thermal stability of the LC alignment. The abilities of this method are demonstrated using a series of …

Landau Theory Of Wall-Induced Phase Nucleation And Pretransitional Birefringence At The Isotropic-Nematic Transition, David W. Allender, G. L. Henderson, D. L. Johnson

David W Allender

The Landau-deGennes theory is used to study the isotropic-nematic phase transition of a semi-infinite system having a wall which induces local nematic order. Analytic solutions are obtained and it is found that (1) for sufficient surface order, the briefringence-induced optical phase shift diverges at the transition rather than at the supercooling limit, and (2) the regions of superheating and supercooling are severely restricted.

Cholesteric Liquid-Crystal Polymer Dispersion For Haze-Free Light Shutters, Deng-Ke Yang, Liang-Chy Chien, J. Donae

Liang-Chy Chien

A new dispersion involving a polymer in low concentration with a cholesteric liquid crystal is reported. Two types of light modulators from these materials are described as a normal mode shutter which is opaque (light scattering) in the field-OFF state and transparent in the field-ON state and a reverse-mode shutter with the opposite field conditions. The transparent state of both cells is haze-free for all viewing angles making the material attractive for window applications and direct view displays.

Effect Of A Polymer Network On The Alignment And The Rotational Viscosity Of A Nematic Liquid-Crystal, Antal Jákli, D. Kim, Liang-Chy Chien, A. Saupe

Liang-Chy Chien

Polymer dispersions were made by photoinduced polymerization of 4,4'-bisacryloyl-biphenyl in liquid crystal 4-4'-cyano pentyl biphenyl as nematic solvent. Samples were prepared with 0.5, 1, and 1.5 wt % monomer. The polymerization was done in the isotropic phase. The rotational viscosities of the two lower concentration samples increased only moderately after UV curing (3 mW/cm2, 25 min). The 1.5 wt % sample gave under the same condition a nematic with a fairly rigid anisotropic network that did not realign in magnetic field up to 20 kG. We estimate that the mesh size of the network was in the order of 1 …

Theory Of A Symmetry-Breaking Phase-Transition In A Surface-Layer Of Nematic Liquid-Crystal, Y. L'Vov, R. M. Hornreich, David W. Allender

David W Allender

For a surface interaction linear in the order parameter and favoring an orientation in which liquid-crystalline molecules lie parallel to the surface (but which is independent of whether the orientation is uniaxial or isotropic in the bounding plane), a symmetry-breaking phase transition in a surface layer is possible at temperatures above that of the bulk isotropic to nematic transition. At the surface transition, the high-temperature in-plane isotropic state becomes unstable with respect to a biaxial phase, the transition being continuous and therefore of-the Berezinskii-Kosterlitz-Thouless (BKT) type, It is expected to occur for intermediate surface couplings but not at very weak …

Polymer-Stabilized Diffraction Gratings From Cholesteric Liquid Crystals, S. N. Lee, Liang-Chy Chien, Samuel N. Sprunt

Liang-Chy Chien

We report the use of a low concentration polymer network to stabilize electric field-induced diffraction gratings in cholesteric liquid crystals. The stabilized gratings can be electrically switched between a zero-field "on" state, which exhibits approximately 75% diffraction efficiency, and a moderate field (typically 3 V/mu m) "off" state. Grating spacings between 1.7 and 30 mu m are obtained in a 10 mu m thick cell by variation of the concentration of chiral dopant used to form the cholesteric liquid crystal. (C) 1998 American Institute of Physics.

Biaxial Order Parameters In Liquid-Crystals - Their Meaning And Determination With Nuclear-Quadrupole Resonance, David W. Allender, J. William Doane

David W Allender

A simple formulation i presented for the analysis of nuclear quadrupole spectra in biaxial liquid crystals based on orientational order parameters. The analysis contains no tilt angle or other model-dependent parameters not directly measured in NMR experiments. The observability and effect of various order parameters on the asymmetry of the electric field gradient η is discussed. It is shown that η is insensitive to rotational freeze-out of the variety expected to be observed in some biaxial phases. Nuclear quadrupole resonance experiments are described which are expected to yield rotational freeze-out parameters.

City Notifications As A Data Source For Traffic Management, Pramod Anantharam, Biplav Srivastava

Kno.e.sis Publications

A common problem for cities of developing countries like India in managing traffic is the lack of basic automated instrumentation to track road conditions or vehicle locations. Still, to help their citizens make informed travel decisions based on changing city dynamics; many cities have an authorized, city-initiated, notification service in place to alert subscribing commuters about road conditions. Here, alternative means may be used to create informal textual notifications e.g., inputs from field personnel, citizen updates, and pre-authorized events from city calendar. In this paper, we show that collections of such notifications, when processed with information extraction techniques, can turn …

Magnetic Properties And Magnetocaloric Effect Of Ndmn2−Xtixsi2 Compounds, M F. Md Din, Jianli Wang, S J. Campbell, R Zeng, W D. Hutchison, M Avdeev, S J. Kennedy, S X. Dou

Australian Institute for Innovative Materials - Papers

The structural and magnetic properties of the intermetallic compounds NdMn_2−xTi_xSi₂(x = 0, 0.1, 0.2, and 0.3) have been studied by x-ray and high resolution neutron powder diffraction, specific heat, dc magnetization, and differential scanning calorimetry measurements over the temperature range 3–450 K. The Curie temperature and Néel temperature of NdMn₂Si₂ decrease from T_C = 36 K and T_N = 380 K to T_C = 14 K and T_N = 360 K, respectively, on substitution of Ti (x = 0.3) for Mn. The magnetocaloric effect …

Potassium Metasomatism At The Polymetallic Nico Deposit, Northwest Territories, Canada, Gregory B. Robinson

Electronic Thesis and Dissertation Repository

High temperature K-metasomatism pervasively replaced Paleoproterozoic volcanic and metasedimentary rocks at Lou Lake NWT. This replacement developed primarily at the structural interface juxtaposing non-metamorphosed Lou Lake volcanic rocks over Treasure Lake Group metasedimentary rocks. This crustal-scale fault developed coeval with the emplacement of anorogenic magmatism within a post-collision extensional setting. Emplacement of porphyry dykes into the fault breccia coincided with K-metasomatism at peak upper-greenschist to amphibolite-facies thermal conditions.

The K-metasomatism resulted from co-mingling of descending crustal fluids and ascending potassic magmatic fluids along the decollement surface. Pervasive K-metasomatism resulted in complete replacement of precursor rocks by potassium feldspar. The pronounced …

Study On The Surface Chemistry Behavior Of Pyrochlore During Froth Flotation, Saeed Chehreh Chelgani

Electronic Thesis and Dissertation Repository

Separation of pyrochlore from its associated minerals is typically accomplished by froth flotation. The surface chemistry of pyrochlore is similar to many of its associated non-value minerals. Therefore, understanding the surface chemical properties of the pyrochlore for selective pyrochlore flotation will potentially aid in the design of flotation strategies for optimized recovery. In this study, pyrochlore samples were collected from various points in the flotation scheme at the Niobec plant, Quebec, Canada. The SEM-EDX analysis revealed that pyrochlore from the Niobec deposit occurs as high and low iron varieties, and that recovery favours varieties with a lower Fe content. To …

Active Presecription Drug Safety Surveillance: Exploring Omop 2011-2012 Experiments, Susan Gruber, James M. Robins

Susan Gruber

The Observational Medical Outcomes Partnership (OMOP), a consortium of pharmaceutical, FDA, and academic researchers focuses on developing and evaluating electronic records-based methods for enhancing post-market drug safety surveillance. The OMOP 2011-2012 experiment consists of applying variants of seven analysis methods to five different EMR or claims databases to estimate the increase (decrease) in risk associated with drug-outcome pairs whose causal association has been previously established, and serves as a gold standard for comparison. Variants of each method can produce very different effect estimates, sometimes at odds with the gold standard. We explore the reasons behind this heterogeneity, and in doing …

Limits On Tensor Coupling From Neutron Β Decay, Robert W. Pattie, Kevin P. Hickerson, Andrew R. Young

Robert W. Pattie Jr.

Ontology-Guided Extraction Of Structured Information From Unstructured Text: Identifying And Capturing Complex Relationships, Sushain Pandit

Sushain Pandit

Many applications call for methods to enable automatic extraction of structured information from unstructured natural language text. Due to the inherent challenges of natural language processing, most of the existing methods for information extraction from text tend to be domain specific. This thesis explores a modular ontology-based approach to information extraction that decouples domain-specific knowledge from the rules used for information extraction. Specifically, the thesis describes: 1. A framework for ontology-driven extraction of a subset of nested complex relationships (e.g., Joe reports that Jim is a reliable employee) from free text. The extracted relationships are semantically represented in the form …

Cholesteric Polymer-Dispersed Light Shutters, John L. West, R. B. Akins, J. Francl, J. W. Doane

John L West

We report cholesteric/polymer dispersed (CPD) materials that switch from a reflecting, planar state to a relatively transparent, focal conic state. The reflective wavelength of the CPD materials can be continuously adjusted by varying the cholesteric pitch length. The electro‐optic properties of these materials are similar to the recently reported polymer stabilized cholesteric materials; however, the polymer concentration is sufficient to provide a self‐sustaining and self‐adhering film, offering the potential of large area devices fabricated on flexible substrates.