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Articles 1 - 14 of 14

Full-Text Articles in Physics

Nematic Order In Polymer-Stabilized Liquid Crystals, R. E. Kraig, P. L. Taylor, Ruiqing Ma, Deng-Ke Yang Oct 1998

Nematic Order In Polymer-Stabilized Liquid Crystals, R. E. Kraig, P. L. Taylor, Ruiqing Ma, Deng-Ke Yang

Chemical Physics Publications

We have studied both experimentally and theoretically how the presence of polymer fibrils can influence the ordering in liquid crystal systems. When such fibrils are formed by polymerization in a nematic solution, the polymer structure can reflect the director orientation at the time of polymerization. This anisotropic structure can be retained even when the liquid crystal is heated into its isotropic phase. We report measurements of this effect and also calculations based on a simple model of the mechanism by which the polymer fibrils can be expected to influence the nematic order of a liquid crystal matrix. A numerical solution ...


Capillary Filling Of Nematic Liquid Crystals, Xiang-Dong Mi, Deng-Ke Yang Aug 1998

Capillary Filling Of Nematic Liquid Crystals, Xiang-Dong Mi, Deng-Ke Yang

Chemical Physics Publications

We studied the orientation of nematic liquid crystals when they were being filled into cells by capillary action. We found that, in the flow front, the liquid crystal was always aligned along the flow direction. Far behind the flow front, the orientation of the liquid crystal depended on the anchoring direction of the alignment layers and the direction and speed of the flow. In cells with the anchoring direction parallel to the flow direction, the liquid crystal always oriented along the flow direction. In cells with the anchoring direction perpendicular to the flow direction, at low flow speed gradients, the ...


Cholesteric Cellular Patterns With Electric-Field-Controlled Line Tension, Oleg Lavrentovich, Deng-Ke Yang Jun 1998

Cholesteric Cellular Patterns With Electric-Field-Controlled Line Tension, Oleg Lavrentovich, Deng-Ke Yang

Chemical Physics Publications

A field-induced instability of wall defects, commonly called oily streaks, is observed in cholesteric liquid crystals. The electric field favors reorientation of cholesteric layers and drives the line tension of the oily streaks negative. Defects with negative line tension elongate, preserving their width. The electrically controlled line tension is a nonmonotonic function of the width; streaks that are too narrow or too wide remain straight. The phenomenon is explained within a coarse-grain model that accounts for the surface anchoring.


Effect Of Surface Alignment Layers On The Configurational Transitions In Cholesteric Liquid Crystals, J. V. Gandhi, X. D. Mi, Deng-Ke Yang Jun 1998

Effect Of Surface Alignment Layers On The Configurational Transitions In Cholesteric Liquid Crystals, J. V. Gandhi, X. D. Mi, Deng-Ke Yang

Chemical Physics Publications

To understand the mechanism behind the pitch change during the transient planar to the planar state transition of a cholesteric liquid crystal (while it is relaxing from the homeotropic state to the planar state), we studied a more controlled situation of pitch change in a thermochromic liquid crystal. We studied the pitch change in the liquid crystal, as a function of temperature, under various surface alignment conditions such as homogeneous and homeotropic surface alignments. In a cell with homogeneous surface treatment, the liquid crystal showed a discontinuous pitch change, while the pitch change was rather continuous in a cell with ...


Mechanism Of Unidirectional Liquid-Crystal Alignment On Polyimides With Linearly Polarized Ultraviolet Light Exposure, M. Nishikawa, Bahman Taheri, John L. West May 1998

Mechanism Of Unidirectional Liquid-Crystal Alignment On Polyimides With Linearly Polarized Ultraviolet Light Exposure, M. Nishikawa, Bahman Taheri, John L. West

Chemical Physics Publications

Unidirectional liquid-crystal(LC) alignment by a linearly polarized ultraviolet light(UV) exposure was examined using two types of polyimide (PI). PI with a fluorene unit incorporated in a side chain showed LC alignment perpendicular to those of conventional PIs, i.e., perpendicular to the rubbing direction and parallel to the exposure polarization of UV. The results of the dichroic ratio measurement of LCcells,UVabsorption spectra,birefringence, infrared absorption spectra, and PI conformation calculations using molecular mechanics suggest that the photodecomposition of PI by UV exposure produces anisotropic van der Waals forces, which align LC along its optical axis.


A Method For The Formation Of Polymer Walls In Liquid Crystal Polymer Mixtures, Yoan Kim, Jim Francl, Bahman Taheri, John L. West May 1998

A Method For The Formation Of Polymer Walls In Liquid Crystal Polymer Mixtures, Yoan Kim, Jim Francl, Bahman Taheri, John L. West

Chemical Physics Publications

We have investigated the formation of polymer walls for high polymer content Liquid crystal (LC) formulations, using a patterned electric field to induce phase separation. The effect of this field on the phase separation temperature of a LC/monomer mixture is studied as a function of the photopolymerizable monomer concentration. The phase separation temperature increases with the patterned field strength. The application of a patterned field results in segregation of the LC molecules in the high electric field regions, i.e., pixels, whereas the monomers segregate in the low-field regions, i.e., interpixels. Subsequent photopolymerization results in the formation of ...


Polymer-Stabilized Diffraction Gratings From Cholesteric Liquid Crystals, S. N. Lee, Liang-Chy Chien, Samuel N. Sprunt Feb 1998

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

Chemical Physics Publications

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.


Non-Newtonian Hele-Shaw Flow And The Saffman-Taylor Instability, Ljubinko Kondic, Michael J. Shelley, Peter Palffy-Muhoray Feb 1998

Non-Newtonian Hele-Shaw Flow And The Saffman-Taylor Instability, Ljubinko Kondic, Michael J. Shelley, Peter Palffy-Muhoray

Chemical Physics Publications

We explore the Saffman-Taylor instability of a gas bubble expanding into a shear thinning liquid in a radial Hele-Shaw cell. Using Darcy's law generalized for non-Newtonian fluids, we perform simulations of the full dynamical problem. The simulations show that shear thinning significantly influences the developing interfacial patterns. Shear thinning can suppress tip splitting, and produce fingers which oscillate during growth and shed side branches. Emergent length scales show reasonable agreement with a general linear stability analysis.


Effects Of Cell Structure On The Reflection Of Cholesteric Liquid Crystal Displays, Ming Xu, Fudong Xu, Deng-Ke Yang Feb 1998

Effects Of Cell Structure On The Reflection Of Cholesteric Liquid Crystal Displays, Ming Xu, Fudong Xu, Deng-Ke Yang

Chemical Physics Publications

Cholesteric liquid crystal displays employ cells consisting of parallel glass plates which have indium-tin-oxide coating as the electrode and polyimide as the alignment layer. We have performed a systematic study of the reflections from the interfaces and the liquid crystal. We measured the reflection spectra using various incident light, various polarizations, and detection polarizations. Using the Berreman 4×4 method, we simulated the reflection spectrawithout fitting parameters; the results agreed well with the experimental results.


Domain Size In The Presence Of Random Fields, E. Weinan, Peter Palffy-Muhoray Jan 1998

Domain Size In The Presence Of Random Fields, E. Weinan, Peter Palffy-Muhoray

Chemical Physics Publications

We consider the size of domains formed in ordered systems in the presence of quenched random fields. We argue that below the critical dimension, the, domain size shows a nonmonotonic dependence on the correlation length of the random field. If the random field is slowly varying in space, the order parameter follows the field, and the domain size is comparable to the correlation length. If the field is rapidly varying, the domain size becomes larger than the correlation length, and diverges as the correlation length of the random field goes to zero.


Nonlinear Schrodinger Equation In Nematic Liquid Crystals, Peter Palffy-Muhoray, J. A. Reyes Jan 1998

Nonlinear Schrodinger Equation In Nematic Liquid Crystals, Peter Palffy-Muhoray, J. A. Reyes

Chemical Physics Publications

We derive the amplitude equation, in the weakly nonlinear regime, for an optical wave packet that propagates in an initially undistorted nematic liquid crystal. By using the dyad representation Q(ij), we find the retarded and nonlocal equation for the nematic configuration and solve it in Fourier space. This allows us to calculate the amplitude dependent dispersion relation for a nematic liquid crystal in a given initial undistorted stationary state. We consider a linearly polarized wave packet that travels along the principal axis of the nematic dielectric tensor. We find a nonlinear Schrodinger equation for the amplitude, which includes an ...


Dynamical Studies Of Gratings Formed In Polymer-Dispersed Liquid Crystal Films, Andy Y. G. Fuh, T. C. Ko, M. S. Tsai, C. Y. Huang, Liang-Chy Chien Jan 1998

Dynamical Studies Of Gratings Formed In Polymer-Dispersed Liquid Crystal Films, Andy Y. G. Fuh, T. C. Ko, M. S. Tsai, C. Y. Huang, Liang-Chy Chien

Chemical Physics Publications

The use of polymer-dispersed liquid crystal films to fabricate gratings was demonstrated. The written gratings are permanent, but are electrically switchable. In this article, we report the results obtained from the dynamical studies of gratings formed in polymer-dispersed liquid crystal films having various liquid crystal-polymer mixing ratios. The results showed that during the initial phases of formation, a thermal grating effect dominated. Later, it was offset or quenched by the photopolymerization effect which eventually dominated and determined the characteristics of the final grating.


Tilted Photoalignment Of A Nematic Liquid Crystal Induced By A Magnetic Field, Denis Andrienko, Yuri Kurioz, Yuri Reznikov, Charles Rosenblatt, Rolfe Petschek, Oleg Lavrentovich, Darius Subacius Jan 1998

Tilted Photoalignment Of A Nematic Liquid Crystal Induced By A Magnetic Field, Denis Andrienko, Yuri Kurioz, Yuri Reznikov, Charles Rosenblatt, Rolfe Petschek, Oleg Lavrentovich, Darius Subacius

Chemical Physics Publications

Nematic liquid crystal cells with polyvinyl cinnamate coated substrates were subjected to ultraviolet light. When this was done in the presence of an oblique magnetic field the photoalignment was found to be temporally and thermally robust, with a large pretilt angle and weak polar anchoring. Moreover, two easy axes with equal and opposite pretilt angle were obtained, such that a magnetic field could switch the director from one easy axis to the other. (C) 1998 American Institute of Physics.


Unipolar Electro-Optic Effect In A Nematic Cell, V. G. Nazarenko, R. Klouda, Oleg Lavrentovich Jan 1998

Unipolar Electro-Optic Effect In A Nematic Cell, V. G. Nazarenko, R. Klouda, Oleg Lavrentovich

Chemical Physics Publications

We report on a unipolar electro-optic effect in a nematic liquid crystal cell. The cell placed between two polarizers behaves as an "optical diode:" only one polarity of the applied voltage makes the cell transparent. The phenomenon is caused by a double symmetry breaking at the two plates that bound the nematic slab.