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Full-Text Articles in Physics

Surface Induced Phase Separation And Pattern Formation At The Isotropic Interface In Chiral Nematic Liquid Crystals, Rafael S. Zola, L. R. Evangelista, Y.-C Yang, Deng-Ke Yang Jan 2013

Surface Induced Phase Separation And Pattern Formation At The Isotropic Interface In Chiral Nematic Liquid Crystals, Rafael S. Zola, L. R. Evangelista, Y.-C Yang, Deng-Ke Yang

Chemical Physics Publications

We study the pattern formation of a chiral nematic liquid crystal under a wetting transition. In the isotropic-liquid crystal transition, a surface-enhanced effect happens and a thin liquid crystal layer forms at the substrates of the cell. In this confined system, chirality, elastic anisotropy, surface anchoring, and wetting strength interplay. A striped pattern is formed due to the chiral nature of the material and the tilted anchoring at the isotropic boundary. As the wetting layer grows from cooling the sample, first the stripes rotate through a process where dislocation defects are formed. As the wetting layer grows further, the periodicity ...


A Cellulose Liquid Crystal Motor: A Steam Engine Of The Second Kind, Yong Geng, Pedro Lucio Almeida, Cheng Cheng, Peter Palffy-Muhoray, Maria Helena Godinho, Susete Nogueira Fernandes Jan 2013

A Cellulose Liquid Crystal Motor: A Steam Engine Of The Second Kind, Yong Geng, Pedro Lucio Almeida, Cheng Cheng, Peter Palffy-Muhoray, Maria Helena Godinho, Susete Nogueira Fernandes

Chemical Physics Publications

The salient feature of liquid crystal elastomers and networks is strong coupling between orientational order and mechanical strain. Orientational order can be changed by a wide variety of stimuli, including the presence of moisture. Changes in the orientation of constituents give rise to stresses and strains, which result in changes in sample shape. We have utilized this effect to build soft cellulose-based motor driven by humidity. The motor consists of a circular loop of cellulose film, which passes over two wheels. When humid air is present near one of the wheels on one side of the film, with drier air ...


The Effect Of Siox Alignment Layer Thickness On The Switching Of Smc* Bistable Liquid Crystal Devices, Mitya Reznikov, Philip J. Bos, Michael J. O'Callaghan Jan 2010

The Effect Of Siox Alignment Layer Thickness On The Switching Of Smc* Bistable Liquid Crystal Devices, Mitya Reznikov, Philip J. Bos, Michael J. O'Callaghan

Chemical Physics Publications

SiOx alignment layers have been shown to allow defect-free SmC* devices with near optimum bistable orientation of the director. In this paper we investigate the effect of the thickness of this type of alignment layer on the required amplitude of an applied voltage pulse to cause bistable switching. The results of an experimental investigation and simple model are presented. We find that for thicker layers, the amplitude is controlled by the voltage drop across the alignment layers and by the effect of polar interactions between the liquid crystal (LC) and the alignment layers. For thin alignment layers the amplitude is ...


Spatiotemporal Patterns In A Langmuir Monolayer Due To Driven Molecular Precession, Satyendra Kumar, R. K. Gupta, K. A. Suresh, L. M. Lopatina, R. L. B. Selinger, Jonathan V. Selinger Oct 2008

Spatiotemporal Patterns In A Langmuir Monolayer Due To Driven Molecular Precession, Satyendra Kumar, R. K. Gupta, K. A. Suresh, L. M. Lopatina, R. L. B. Selinger, Jonathan V. Selinger

Physics Publications

Langmuir monolayers of chiral liquid crystals on the surface of water exhibit orientational waves with complex spatiotemporal patterns. These patterns arise from a collective precession of the mesogenic molecules, driven by the evaporation of water through the monolayer. We investigate the behavior of these orientational waves around topological defects in the molecular orientation. Through Brewster angle microscopy, we find that the waves form a reversing spiral pattern, which rotates about the central vortex. With increasing relative humidity, the rotation slows and then stops. We model the system theoretically, and show that predicted patterns are in good agreement with the experiments.


What Aligns Liquid Crystals On Solid Substrates? The Role Of Surface Roughness Anisotropy, Satyendra Kumar, Jae-Hoon Kim, Yushan Shi Feb 2005

What Aligns Liquid Crystals On Solid Substrates? The Role Of Surface Roughness Anisotropy, Satyendra Kumar, Jae-Hoon Kim, Yushan Shi

Physics Publications

The mechanism responsible for liquid crystal (LC) alignment on mechanically buffed or UV exposed polymer films is poorly understood. A comprehensive study of LC alignment on variously prepared substrates unequivocally shows that the anisotropy in the surface roughness of the substrate completely determines the direction of LC alignment. In all the cases studied, including those where an anchoring transition occurs with temperature, the LC director (re)aligns in the directions of low roughness.


Light-Induced Changes Of Optical And Electrical Properties In Bent-Core Azo Compounds, Antal Jakli, V. Prasad, D.S.S. Rao, Guangxun Liao, I. Janossy Feb 2005

Light-Induced Changes Of Optical And Electrical Properties In Bent-Core Azo Compounds, Antal Jakli, V. Prasad, D.S.S. Rao, Guangxun Liao, I. Janossy

Chemical Physics Publications

We have studied the optical and electrical properties of two bent-core substances with an azo linkage in their cores. Pump-probe laser studies, direct textural observations, and spectrophotometric recordings show an initial decrease of light transmission, which at larger light intensities (similar to 1 mW/mm(2)) is followed by a bleaching. Simultaneously the electrical properties (electric conductivity, antiferroelectric polarization, switching threshold, and switching time) decreased monotonically with increasing light intensities. The monotonic decrease of electrical properties indicates that the darkening and bleaching have the same origin, namely, the photochemical isomerization of the azo linkage from the trans to the cis ...


Photoinduced Three-Dimensional Orientational Order In Side Chain Liquid Crystalline Azopolymers, O. V. Yaroshchuk, K. D. Kiselev, Yu. Zakrevskyy, T. Bidna, J. Kelly, Liang-Chy Chien, J. Lindau Jul 2003

Photoinduced Three-Dimensional Orientational Order In Side Chain Liquid Crystalline Azopolymers, O. V. Yaroshchuk, K. D. Kiselev, Yu. Zakrevskyy, T. Bidna, J. Kelly, Liang-Chy Chien, J. Lindau

Chemical Physics Publications

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by ...


Electrically Controllable Microlens Array Fabricated By Anisotropic Phase Separation From Liquid-Crystal And Polymer Composite Materials, Hee-Su Ji, Jae-Hoon Kim, Satyendra Kumar Jul 2003

Electrically Controllable Microlens Array Fabricated By Anisotropic Phase Separation From Liquid-Crystal And Polymer Composite Materials, Hee-Su Ji, Jae-Hoon Kim, Satyendra Kumar

Physics Publications

Anisotropic phase separation has been used to fabricate an electrically switchable microlens array from nematic liquid crystals. Nematic liquid-crystal-based microlens arrays have been built with diameters of similar to400 mum and natural focal lengths as small as 1.6 mm. The focal length of each microlens in the array can be changed in milliseconds by an applied electric field. These devices, which have no internal substructures to scatter light, offer higher efficiency and greater light throughput than polymer dispersed devices.


Anchoring-Mediated Interaction Of Edge Dislocations With Bounding Surfaces In Confined Cholesteric Liquid Crystals, Ivan I. Smalyukh, Oleg Lavrentovich Feb 2003

Anchoring-Mediated Interaction Of Edge Dislocations With Bounding Surfaces In Confined Cholesteric Liquid Crystals, Ivan I. Smalyukh, Oleg Lavrentovich

Chemical Physics Publications

We employ the fluorescent confocal polarizing microscopy to image edge dislocations in cholesteric liquid crystals. Surface anchoring at the bounding plates determines the structure and behavior of defects. Two types of plates set in-plane director orientation but differ in the type of associated anchoring potentials. Plates with strong polar anchoring and nonzero azimuthal anchoring repel the dislocations, while plates with weak polar anchoring and no azimuthal anchoring allow the dislocations to escape through the boundary. To explain the results, we propose a coarse-grained model of cholesteric anchoring.


Effect Of Director Distortions On Morphologies Of Phase Separation In Liquid Crystals, D. Voloschenko, Oleg P. Pishnyak, Sergij V. Shiyanovskii, Oleg Lavrentovich Jun 2002

Effect Of Director Distortions On Morphologies Of Phase Separation In Liquid Crystals, D. Voloschenko, Oleg P. Pishnyak, Sergij V. Shiyanovskii, Oleg Lavrentovich

Chemical Physics Publications

We study phase separation from a nematic liquid crystal with spatially nonuniform director gradients. Particles of a phase-separated component, which is either an isotropic fluid (silicone oil) or a nonmesogenic photopolymer, accumulate in the regions with the strongest director distortions, thus reducing the overall energy of the system.


Optical Studies Of The Liquid Crystal B2 Phase Formed By Banana-Shaped Molecules, D. A. Olson, M. Veum, A. Cady, M. V. D'Agostino, P. M. Johnson, H. T. Nguyen, Liang-Chy Chien, C. C. Huang Apr 2001

Optical Studies Of The Liquid Crystal B2 Phase Formed By Banana-Shaped Molecules, D. A. Olson, M. Veum, A. Cady, M. V. D'Agostino, P. M. Johnson, H. T. Nguyen, Liang-Chy Chien, C. C. Huang

Chemical Physics Publications

Null-transmission ellipsometry and depolarized light microscopy have been performed on free-standing films of three achiral banana-shaped compounds in the B2 phase. Our results support a two-layer unit cell previously proposed to explain the observed antiferroelectricity in thin films and bulk samples. We have studied thicker films than previously reported and have found no deviations in the film structure from the earlier findings. Moreover, we can determine the layer spacing, the molecular tilt from the layer normal, and the three principal indices of refraction in the molecular reference frame.


Light Induced Structures In Liquid Crystalline Side-Chain Polymers With Azobenzene Functional Groups, O. Yaroschuk, Tatiana Sergan, J. Lindau, S. N. Lee, J. Kelly, Liang-Chy Chien Mar 2001

Light Induced Structures In Liquid Crystalline Side-Chain Polymers With Azobenzene Functional Groups, O. Yaroschuk, Tatiana Sergan, J. Lindau, S. N. Lee, J. Kelly, Liang-Chy Chien

Chemical Physics Publications

We applied ellipsometry to study the distribution of azobenzene fragments in the films of two types of comblike polymers with azobenzene moieties in the side chains before and after ultraviolet ~UV! light irradiation. The polymer with an alkyl chain at the end of the azobenzene fragment forms structures with preferred homeotropic alignment of the fragments. Irradiation of this polymer with nonpolarized UV light at normal incidence induces a homeotropic alignment of azobenzene fragments. Oblique irradiation induces tilted structures. Polarized UV light irradiation at normal beam incidence induces biaxial structures with a fanlike distribution of azobenzene fragments and preferably out-of-plane alignment ...


Photoalignment Of Liquid Crystals By Liquid Crystals, Yuriy Reznikov, Oksana Ostroverkhova, Kenneth D. Singer, Jae-Hoon Kim, Satyendra Kumar, Oleg Lavrentovich, Bin Wang, John L. West Feb 2000

Photoalignment Of Liquid Crystals By Liquid Crystals, Yuriy Reznikov, Oksana Ostroverkhova, Kenneth D. Singer, Jae-Hoon Kim, Satyendra Kumar, Oleg Lavrentovich, Bin Wang, John L. West

Physics Publications

We observed light-induced alignment of the nematic liquid crystal, 4, 4'-n-pentylcyanobiphenyl (5CB), on a fused quartz. Irradiation of the adsorbed layer with polarized ultraviolet light produced homogeneous alignment in a 5CB-filled liquid crystal cell with the easy orientation axis perpendicular to thr polarization direction. The measured anchoring energy increases with illumination up to 10(-4) erg/cm(2). Phase retardation and pretilt measurements confirmed near homogeneous alignment of the LC in a cell, while the polarization dependence of second harmonic generation suggested a near normal alignment of the adsorbed layer. We believe that light-induced rearrangement or phototransformation of the ...


Material-Independent Determination Of Anchoring Properties On Rubbed Polyimide Surfaces, Bharat R. Acharya, Jae-Hoon Kim, Satyendra Kumar Dec 1999

Material-Independent Determination Of Anchoring Properties On Rubbed Polyimide Surfaces, Bharat R. Acharya, Jae-Hoon Kim, Satyendra Kumar

Physics Publications

A material-independent method for determining liquid-crystal (LC) anchoring energy on rubbed polyimide (PI) surfaces has been devised. This method exploits the changes in the easy axis of rubbed PI film induced by exposure to linearly polarized UV (LPUV) Light. The distribution of PI chains in a rubbed film is approximated by a Gaussian function and its width determined from the measured rotation of the LC easy axis as a function of exposure time. A quasimicroscopic free energy of the LC-substrate interface is used to model LC anchoring properties. The experimental and calculated values of the azimuthal anchoring energy are in ...


Dislocation Profile In Cholesteric Finger Texture, T. Ishikawa, Oleg Lavrentovich Nov 1999

Dislocation Profile In Cholesteric Finger Texture, T. Ishikawa, Oleg Lavrentovich

Chemical Physics Publications

The displacement field around an isolated elementary dislocation in a medium with one-dimensional periodicity is established experimentally. The system studied is a cholesteric fingerprint texture with a macroscopic (∼10 μm) periodicity. The characteristic elastic length is smaller than the “interlayer” distance. As a result, the experimental dislocation profile deviates from the classic pattern predicted by the linear elastic theory but fits well with the recently suggested nonlinear theory of dislocation [Phys. Rev. E 59, R4752 (1999)].


Observation Of Pattern Evolution During Homeotropic-Focal Conic Transition In Cholesteric Liquid Crystals, Yang-Ming Zhu, Deng-Ke Yang Jun 1999

Observation Of Pattern Evolution During Homeotropic-Focal Conic Transition In Cholesteric Liquid Crystals, Yang-Ming Zhu, Deng-Ke Yang

Chemical Physics Publications

We have observed an interesting pattern evolution of cholesteric liquid crystals during the homeotropic-focal conic transition in a dispersed polymer network. From the electrical field induced homeotropic state, if the field is:reduced to an appropriate bias level focal conics grow at a constant speed, being an open or compact structure depending on the field strength. When the field is increased, these patterns change to ramified structures. We phenomenologically explained the observation.


Orientation And Surface Anchoring Of Nematic Liquid Crystals On Linearly Polymerized Photopolymers, H. G. Galabova, David W. Allender, J. Chen Feb 1997

Orientation And Surface Anchoring Of Nematic Liquid Crystals On Linearly Polymerized Photopolymers, H. G. Galabova, David W. Allender, J. Chen

Physics Publications

The liquid crystal orientation on a poly(vinyl cinnamate) or poly(vinyl 4-methoxy-cinnamate) coated solid substrate exposed to linearly polarized UV light has been studied. The angular distribution of the polymer side chains and the photoreaction products was derived as a function of exposure time using a simple assumption about the probability for crosslinking of two side chains. The liquid crystal was assumed to be weakly anchored to the surface but have a degree of order and orientation determined from those of the polymer side chains. The polar and the azimuthal surface coefficients were estimated as a function of exposure ...


X-Ray Studies Of The Hexatic Phase In Liquid-Crystals With A Crystal-B-Hexatic-B-Smectic-A Phase Sequence, Ewa Gorecka, Li Chen, Wieslaw Pyzuk, Adam Krowczynski, Satyendra Kumar Oct 1994

X-Ray Studies Of The Hexatic Phase In Liquid-Crystals With A Crystal-B-Hexatic-B-Smectic-A Phase Sequence, Ewa Gorecka, Li Chen, Wieslaw Pyzuk, Adam Krowczynski, Satyendra Kumar

Physics Publications

Free-standing monodomain thick films of three liquid crystalline compounds were examined by x-ray scattering. These compounds exhibit a unique enantiotropic crystal-B-hexatic-B-smectic-A phase sequence with no evidence of herringbone order. The hexatic order parameter Cg changes in the hexatic phase with critical exponent beta=0.15+/-0.03. Temperature dependence of the in-plane positional order correlation length suggests a strongly first-order hexatic-B to crystal-B phase transition with indiscernible enthalpy change.


Surface-Layer Phase-Transitions In Nematic Liquid-Crystals, Natasha Kothekar, David W. Allender, R. M. Hornreich Apr 1994

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

Physics Publications

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 ...


Surface-Induced Discrete Smectic Order In The Isotropic-Phase Of 12cb In Cylindrical Pores, Germano S. Iannacchione, Joseph T. Mang, Satyendra Kumar, Daniele Finotello Mar 1994

Surface-Induced Discrete Smectic Order In The Isotropic-Phase Of 12cb In Cylindrical Pores, Germano S. Iannacchione, Joseph T. Mang, Satyendra Kumar, Daniele Finotello

Physics Publications

No abstract provided.


Theory Of A Symmetry-Breaking Phase-Transition In A Surface-Layer Of Nematic Liquid-Crystal, Y. L'Vov, R. M. Hornreich, David W. Allender Aug 1993

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

Physics Publications

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 ...