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Full-Text Articles in Physics

Periodic Stripe Domains And Hybrid-Alignment Regime In Nematic Liquid-Crystals - Threshold Analysis, A. Sparavigna, Oleg Lavrentovich, A. Strigazzi Feb 1994

Periodic Stripe Domains And Hybrid-Alignment Regime In Nematic Liquid-Crystals - Threshold Analysis, A. Sparavigna, Oleg Lavrentovich, A. Strigazzi

Chemical Physics Publications

Recently we investigated the occurrence of static periodic stripes in a hybrid aligned nematic cell. Assuming that the tilt anchoring was stronger at the planar wall than at the homeotropic wall, we have found the critical thickness of the cell for the transition from planar to periodic alignment as a function of the surface energy in the presence of a magnetic field. Here we study, for the same kind of cell, the critical thickness between the periodic and the aperiodic deformed structure by means of an appropriate numerical technique. As expected, such a threshold was found to be greater than ...


Field-Driven 1st-Order Structural Transition In The Restricted Geometry Of A Smectic-A Cell, Oleg Lavrentovich, M. Kléman Jul 1993

Field-Driven 1st-Order Structural Transition In The Restricted Geometry Of A Smectic-A Cell, Oleg Lavrentovich, M. Kléman

Chemical Physics Publications

An external field gives rise to a first-order transition in a smectic-A cell; the transition occurs by nucleation of focal conic domains. The physical mechanisms that govern the behavior of the new domain phase are different for small (ρ≪1) and large (ρ≫1) values of the order parameters ρ [ρ=(domain radius)/(cell thickness)]. The nucleation (ρ≪1) is governed by the elastic and field energies and is facilitated by bulk or surface irregularities, while the expansion (ρ≫1) is determined by the balance of the field and anisotropic surface energy.


Cholesteric Liquid-Crystal Polymer Dispersion For Haze-Free Light Shutters, Deng-Ke Yang, Liang-Chy Chien, J. W. Donae Jun 1992

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

Chemical Physics Publications

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.


Method To Obtain Uniform Bookshelf Textures In Smectic C-Asterisk Liquid-Crystals, Antal Jakli, A. Saupe May 1992

Method To Obtain Uniform Bookshelf Textures In Smectic C-Asterisk Liquid-Crystals, Antal Jakli, A. Saupe

Chemical Physics Publications

A new method is described to obtain homogeneous bookshelf textures in thin films of ferroelectric smectic C* material with short pitch and high P(s). The surfaces were coated with silane to obtain a homeotropic texture. The sample was realigned to a homogeneous bookshelf texture under simultaneous application of an electric field and shearing. The texture is stable and gives a very good contrast (approximately 80) under permanent drive condition.


Spontaneous Transition From Chevron To Striped Texture Of A Planar Smectic-C-Asterisk Liquid-Crystal, Antal Jakli, A. Saupe Apr 1992

Spontaneous Transition From Chevron To Striped Texture Of A Planar Smectic-C-Asterisk Liquid-Crystal, Antal Jakli, A. Saupe

Chemical Physics Publications

We observed a spontaneous transition from a vertical chevron texture to a striped texture (striped "bookshelf" texture) in a surface-stabilized film of ferroelectric liquid crystal mixture (ZLI 4237-000, from E. Merck). The transition takes place gradually, at temperatures more than 20-degrees-C below the smectic-A-smectic-C* transition. The layers in the stripe texture are perpendicular to the plate, but in successive stripes, the orientation of the layer normal alternates by +/- 23-degrees. The polarization is perpendicular to the film and alternates between up and down, while the director orientation is in plane and alternates by +/- 5-degrees. The width of the stripes was typically ...


Effect Of A Polymer Network On The Alignment And The Rotational Viscosity Of A Nematic Liquid-Crystal, Antal Jákli, D. R. Kim, Liang-Chy Chien, A. Saupe Jan 1992

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

Chemical Physics Publications

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 ...


Nematic Director Orientation In A Liquid-Crystal-Dispersed Polymer - A Deuterium Nmr Approach, Ralf Stannarius, G. P. Crawford, Liang-Chy Chien, J. William Doane Jun 1991

Nematic Director Orientation In A Liquid-Crystal-Dispersed Polymer - A Deuterium Nmr Approach, Ralf Stannarius, G. P. Crawford, Liang-Chy Chien, J. William Doane

Chemical Physics Publications

The nuclear-magnetic-resonance (NMR) technique is employed to liquid-crystal-dispersed-polymer (LCDP) systems. Deuterium NMR is used to determine director distributions in LCDP cells in the presence of a magnetic field. The samples consist of a nematic liquid crystal mixed with small percentages of a monomer that is polymerized under different conditions after dissolving it in the liquid crystal. NMR spectra of the deuterated liquid-crystal molecules give information on orientation and order in such systems. The orientation of the polymer skeleton formed during the polymerization is found to be stable. It determines the orientation of the nematic director even in the presence of ...


Ordering And Self-Diffusion In The 1st Molecular Layer At A Liquid-Crystal Polymer Interface, G. P. Crawford, Deng-Ke Yang, S. Zumer, D. Finotello, J. William Doane Aug 1990

Ordering And Self-Diffusion In The 1st Molecular Layer At A Liquid-Crystal Polymer Interface, G. P. Crawford, Deng-Ke Yang, S. Zumer, D. Finotello, J. William Doane

Chemical Physics Publications

H-2 NMR of a liquid crystal confined to submicron cylindrical cavities, in a 17-K interval above the nematic-isotropic transition, reveals a weakly orientationally ordered molecular layer at the cavity wall governed by local interactions. The order (S0 = 0.02) is temperature independent so that a partial wetting is realized with no prewetting transition expected. A molecular exchange rate between the surface layer and the bulk is measured to be approximately 10(3) s-1. The coherence length measuring the order penetrating further into the cavity is described by alpha = 0.4 and xi-0 = 0.6 nm.


Molecular And Relative Segmental Order In The Nematic And Smectic C-Phases - A Nuclear Magnetic-Resonance Study, Philip J. Bos, A. Shetty, J. William Doane, Mary E. Neubert Jun 1980

Molecular And Relative Segmental Order In The Nematic And Smectic C-Phases - A Nuclear Magnetic-Resonance Study, Philip J. Bos, A. Shetty, J. William Doane, Mary E. Neubert

Chemical Physics Publications

No abstract provided.


Nmr Measurement Of Biaxial Molecular Order In The Smectic C-Phase, Demetri J. Photinos, Philip J. Bos, J. William Doane Nov 1979

Nmr Measurement Of Biaxial Molecular Order In The Smectic C-Phase, Demetri J. Photinos, Philip J. Bos, J. William Doane

Chemical Physics Publications

No abstract provided.


Nmr In Spinning Samples Of Biaxial Liquid-Crystals, Demetri J. Photinos, Philip J. Bos, Paul Ukleja, J. William Doane Jan 1979

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

Chemical Physics Publications

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.


Nmr Measurements Of Orientational Order In Nematic And Smectic C Phases, Philip J. Bos Jun 1978

Nmr Measurements Of Orientational Order In Nematic And Smectic C Phases, Philip J. Bos

Chemical Physics Publications

No abstract provided.


Nmr Measurement Of Biaxial Order, P. J. Collings, Philip J. Bos, Paul Ukleja, J. William Doane Jun 1978

Nmr Measurement Of Biaxial Order, P. J. Collings, Philip J. Bos, Paul Ukleja, J. William Doane

Chemical Physics Publications

No abstract provided.


Molecular Order Versus Conformation Changes In Liquid-Crystal Phases, Philip J. Bos, J. William Doane Jan 1978

Molecular Order Versus Conformation Changes In Liquid-Crystal Phases, Philip J. Bos, J. William Doane

Chemical Physics Publications

Deuteron magnetic resonance splittings and their temperature dependence from deuterated liquid-crystal compounds are analyzed in terms of molecular conformation and orientational order. Three order parameters are found to be necessary to explain the smectic-C (SC) spectra whereas only two are required for the nematic (N), SA and SB phases. The data are consistent with the concept that the preferred conformation of the molecule remains unchanged at the N-SC, N-SA, and SA-SB phase transitions as well as throughout the temperature range of these phases.