Showing papers on "Liquid crystal published in 2004"
TL;DR: Rod-shaped cellulose microcrystals can be obtained from different cellulose sources such as wood, cotton, or animal origin this article, and their properties are discussed in this contribution.
Abstract: Summary: In this article we present some interesting properties of rodlike cellulose microcrystals (so-called “whiskers”). These microcrystals can be obtained from different cellulose sources such as wood, cotton, or animal origin. When submitted to acid hydrolysis, the cellulose fibers yield stable aqueous suspensions because of the presence of negative charges on the surface of the microcrystallites during the hydrolysis process. The obtained microcrystals are rod-shaped particles, the dimensions of which depend on the cellulose origin. For instance, the cotton whiskers have typical dimensions varying from 100 to 300 nm in length, L, and 8 to 10 nm in diameter, d, while those of the tunicate whiskers range from 100 nm to few micrometers in length and 10 to 20 nm in diameter. At very low concentrations, these whiskers are randomly suspended in water and form an isotropic phase. When the concentration reaches a critical value, the whiskers spontaneously display ordered phases showing interesting liquid crystal properties (nematic and chiral nematic). The chiral nematic orders can be retained after evaporation of the solvent (generally water), leaving iridescent films. The reflected color can be controlled by changing either the ionic strength or by applying an electric field. These colloidal particles have been investigated using several techniques including small-angle neutron scattering (SANS), small angle X-ray scattering, rheology, and more recently dynamic and static light scattering techniques (DLS and SLS) to highlight their static and dynamic behavior. Because of their geometry, important axis ratio (L/d), and high crystallinity, these rods have been also extensively used to process nanocomposites based on polymer matrices, to reinforce their mechanical properties. All these properties are discussed in this contribution.
Rodlike nanocrystals in aqueous suspension (left, Tunicate, 1 wt.-%) and film (right), observed between cross-polarizers.
804 citations
Journal Article•
TL;DR: Liquid Crystal Photonic Bandgap Fibers are described, which are PCFs infiltrated with Liquid Crystals (LCs) in order to obtain increased fiber functionality and a thermo-optic fiber switch with an extinction ratio of 60dB and tunable PBGs using thermosensitive tuning of the LC.
517 citations
TL;DR: Mixing the nonmesomorphic components 4-alkoxystilbazole with pentafluoroiodobenzene leads to a 1:1 halogen-bonded complex whose integrity is shown by X-ray single-crystal analysis and which shows thermotropic smectic A and nematic phases.
Abstract: It is shown for the first time that liquid crystal behavior can be induced through halogen bonding. Thus, mixing the nonmesomorphic components 4-alkoxystilbazole with pentafluoroiodobenzene leads to a 1:1 halogen-bonded complex whose integrity is shown by X-ray single-crystal analysis and which shows thermotropic smectic A and nematic phases.
489 citations
TL;DR: The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality.
Abstract: A biaxial nematic phase had been predicted with ${D}_{2h}$ symmetry, wherein the mesogen's long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, $\mathbf{n}$ and $\mathbf{m}$, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.
444 citations
TL;DR: New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials that would be useful for anisotropic transportation of ions at the nanometer level.
Abstract: New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotrop...
427 citations
TL;DR: In this article, single-walled carbon nanotubes (SWNTs) form a highly unusual nematic phase consisting of spaghetti-like self-assembled supermolecular strands of mobile, solvated tubes in equilibrium with a dilute isotropic phase.
Abstract: Single-walled carbon nanotubes (SWNTs) can be dispersed at high concentration in superacids; the protonation of SWNTs sidewalls eliminates wall-wall van der Waals interactions and promotes the dispersion process. At very low concentration, SWNTs in superacids dissolve as individual tubes which behave as Brownian rods. At higher concentration, SWNTs form a highly unusual nematic phase consisting of spaghetti-like self-assembled supermolecular strands of mobile, solvated tubes in equilibrium with a dilute isotropic phase. At even higher concentration, the spaghetti strands self-assemble into a polydomain nematic liquid crystal. Upon the introduction of small amounts of water, the liquid crystal phase separates into needle-shaped strands (20 Im long) of highly aligned SWNTs, termed alewives. Under anhydrous condition, the liquid crystalline phase can be processed into highly aligned fibers of pure SWNT without the aid of any surfactants or polymers.
355 citations
TL;DR: More than 500 triphenylene derivatives have been synthesized to explore the possibilities of one-dimensional charge carrier systems useful in electrical conduction, photoconduction, electroluminescence, photovoltaic solar cells, gas sensing, optical data storage and other devices as mentioned in this paper.
Abstract: Triphenylene-based discotic liquid crystals, which have already been used commercially in phase compensation films to improve the viewing angle of liquid crystal display devices, also have application potential as one-dimensional charge carrier systems useful in electrical conduction, photoconduction, electroluminescence, photovoltaic solar cells, gas sensing, optical data storage and other devices. Over the past 25 years, more than 500 triphenylene derivatives have been synthesized to explore these possibilities. Cammidge and Bushby reviewed the chemistry and physical properties of about 100 triphenylene-based discotic liquid crystals prepared up to 1995. This review summarizes advances in the chemistry of triphenylene-based discotic liquid crystals since late 1995.
330 citations
327 citations
TL;DR: The results of an experimental and theoretical investigation of voltage-controlled ‘walk-off’ and steering of self-localized light in nematic liquid crystals find not only that the propagation direction of individual spatial solitons can be tuned by several degrees, but also that an array of direction-tunable solitONS can be generated by modulation instability.
Abstract: In certain materials, the spontaneous spreading of a laser beam (owing to diffraction) can be compensated for by the interplay of optical intensity and material nonlinearity The resulting non-diffracting beams are called ‘spatial solitons’ (refs 1–3), and they have been observed in various bulk media4,5,6 In nematic liquid crystals7,8,9, solitons can be produced at milliwatt power levels10,11,12 and have been investigated for both practical applications13 and as a means of exploring fundamental aspects of light interactions with soft matter14,15 Spatial solitons effectively operate as waveguides, and so can be considered as a means of channelling optical information along the self-sustaining filament But actual steering of these solitons within the medium has proved more problematic, being limited to tilts of just a fraction of a degree16,17,18,19,20 Here we report the results of an experimental and theoretical investigation of voltage-controlled ‘walk-off’ and steering of self-localized light in nematic liquid crystals We find not only that the propagation direction of individual spatial solitons can be tuned by several degrees, but also that an array of direction-tunable solitons can be generated by modulation instability21,22,23,24,25 Such control capabilities might find application in reconfigurable optical interconnects, optical tweezers and optical surgical techniques
321 citations
TL;DR: In this article, a model describing the temperature effect of liquid crystal refractive indices is derived and confirmed by experiment Two single liquid crystal compounds (5CB, 5PCH) and two mixtures (MLC-6241-000 and UCF-35) with different birefringence values were used to validate the model.
Abstract: A model describing the temperature effect of liquid crystal (LC) refractive indices is derived and confirmed by experiment Two single LC compounds (5CB, 5PCH) and two mixtures (MLC-6241-000 and UCF-35) with different birefringence values were used to validate the model This model fits all the experimental data well For a low-birefringence LC mixture, if the operating temperature is far below its clearing point, the temperature-dependent refractive indices can be approximated as a parabolic form This prediction is also experimentally validated In addition, a linear relationship between 〈n2〉 and temperature is found
300 citations
TL;DR: An all-optical modulator is demonstrated, which utilizes a pulsed 532nm laser to modulate the spectral position of the bandgaps in a photonic crystal fiber infiltrated with a dye-doped nematic liquid crystal.
Abstract: Photonic crystal fibers (PCFs) have attracted significant attention during the last years and much research has been devoted to develop fiber designs for various applications, hereunder tunable fiber devices. Recently, thermally and electrically tunable PCF devices based on liquid crystals (LCs) have been demonstrated. However, optical tuning of the LC PCF has until now not been demonstrated. Here we demonstrate an all-optical modulator, which utilizes a pulsed 532nm laser to modulate the spectral position of the bandgaps in a photonic crystal fiber infiltrated with a dye-doped nematic liquid crystal. We demonstrate a modulation frequency of 2kHz for a moderate pump power of 2–3mW and describe two pump pulse regimes in which there is an order of magnitude difference between the decay times.
TL;DR: In this paper, the electro-optical effect in 90° twisted nematic cells of pristine and doped liquid crystals under an applied dc voltage was investigated. And it was shown that doping with nanotubes can effectively reduce the dc driving voltage and improve the switching behavior.
Abstract: We present results of the electro-optical effect in 90° twisted nematic cells of pristine and doped liquid crystals under an applied dc voltage. The doped cells were fabricated with a minute addition of either buckminsterfullerene C60 or multiwalled carbon nanotubes (CNTs). Investigated were the switching behaviors as well as the hystereses and time evolutions of both the optical transmittance and electrical capacitance of the display samples. It is shown that doping with nanotubes can effectively reduce the dc driving voltage and improve the switching behavior.
TL;DR: The authors summarizes the diverse material requirements of the main Liquid Crystal Display (LCD) technologies used for televisions (TVs) and discusses recent advances in liquid crystal materials for this application.
Abstract: This review summarizes the diverse material requirements of the main Liquid Crystal Display (LCD) technologies used for televisions (TVs). We discuss recent advances in liquid crystal materials for this application. We highlight key materials and discuss synthesis, structure–property relations and LC mixture properties. For all technologies we have achieved fast switching LC mixtures (16 ms) and are en route to 8 ms thus improving the switching times for the next generation of superior performance LCD TVs.
TL;DR: In this article, the Vuks equation was used to describe the wavelength and temperature-dependent refractive indices of liquid crystal compounds and mixtures, which fit experimental data well in the off-resonance spectral region.
Abstract: The extended Cauchy equations are derived based on the Vuks equation for describing the wavelength- and temperature-dependent refractive indices of liquid crystal compounds and mixtures. This model fits experimental data well in the off-resonance spectral region. Correlations between the Cauchy coefficients for the nematic and isotropic phases are derived and validated by experiments.
TL;DR: In this article, a tunable-focus spherical lens using two flat substrates and inhomogeneous electric field over a homogeneous liquid crystal (LC) layer is demonstrated, which can be tuned continuously from infinity to 0.6 m by the applied voltage.
Abstract: A tunable-focus spherical lens using two flat substrates and inhomogeneous electric field over a homogeneous liquid crystal (LC) layer is demonstrated. The top flat substrate has an imbedded spherical indium–tin–oxide (ITO) electrode and the bottom has a planar ITO electrode on its inner surface. The inhomogeneous electric field generates a centrosymmetric gradient refractive index profile within the LC layer which causes the focusing behavior. The focal length of the LC lens can be tuned continuously from infinity to 0.6 m by the applied voltage.
Patent•
26 Jan 2004TL;DR: A phase difference film and a production method which are controlled in the orientation direction of a phase difference layer with a high accuracy and are low in production costs are presented in this article, where a liquid containing a polymer and a liquid crystal compound that react to a polarized ultraviolet ray is applied onto the optically anisotropic layer and dried.
Abstract: A phase difference film and a production method therefore which are controlled in the orientation direction of a phase difference layer with a high accuracy and are low in production costs. Referring to Fig. 1, a base material-carrying anisotropic layer (12) in which an optically anisotropic layer (11) is laid on a transparent base material (10) is provided. Next, a liquid containing a polymer and a liquid crystal compound that react to a polarized ultraviolet ray is applied onto the optically anisotropic layer (11) and dried. Then a polarized ultraviolet ray is applied to orient the liquid crystal compound, and, as required, a non-polarized ultraviolet ray is applied to cross-link the liquid crystal compound, whereby a phase difference film (1) having a phase difference layer (13) directly formed on the optically anisotropic layer (11) is produced.
TL;DR: This work introduces two different types of spatial distributions of forces, detected between the particles accompanied by hyperbolic hedgehog defects, which lead to specific particle arrangements, which are stabilized by the balance of the orientational stress field of nematics.
Abstract: Interparticle forces in a nematic liquid-crystal colloid have been directly observed by the dual beam laser trapping method with pN sensitivity. We introduce two different types of spatial distributions of forces, detected between the particles accompanied by hyperbolic hedgehog defects. These force distributions lead to specific particle arrangements, which are both stabilized by the balance of the orientational stress field of nematics. On the basis of these results, we propose novel artificial construction for multiparticle regular arrangements.
01 Jun 2004
TL;DR: The development of display device technology is described in detail in this paper, where anisotropic, polymeric, and elastomeric liquid crystals are used for display device development.
Abstract: Liquids. The Interwar Period: Anisotropic Fluids or Mesomorphic Phases. The Modern Physical Picture. The Development of Display Device Technology. Lyotropic, Polymeric and Elastomeric Liquid Crystals.
TL;DR: Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapode tetrapodes and unambiguously show the existence of a biaxial nematic phase below a uniaXial nematics phase.
Abstract: Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase.
TL;DR: By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.
Abstract: We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.
TL;DR: Composite materials comprised of nematic liquid crystals (LCs) and SiO(2) inverse opal films were fabricated and it was found that the change in optical properties was derived from the orientation of the LC molecules in the voids in the inverseOpal film.
Abstract: Composite materials comprised of nematic liquid crystals (LCs) and SiO2 inverse opal films were fabricated. Their optical properties were quite different from those of inverse opal films without the LCs. The optical properties could be controlled by changing the refractive indices of the LCs, which vary with orientation, phase, and temperature. In particular, the optical properties were drastically changed by thermal or photoinduced isothermal phase transitions of the LCs. This means that the photonic band structure could be controlled, and tunable photonic crystals have been achieved, based on the inverse opal structure. The mechanism of this change was investigated by the evaluation of the effective refractive indices. As a result, it was found that the change in optical properties was derived from the orientation of the LC molecules in the voids in the inverse opal film. Furthermore, once the mechanism was understood, it was also possible to control the position of the reflection peak by changing the a...
TL;DR: This work reports on random lasing in a disordered system in which the multiple scattering feedback mechanism can be switched from a three-dimensional random walk to a quasi-two-dimensional type of transport.
Abstract: We report on random lasing in a disordered system in which the multiple scattering feedback mechanism can be switched from a three-dimensional random walk to a quasi-two-dimensional type of transport. The emission from this system is anisotropic, extraordinary polarized, and is controlled via an external electric field. The phenomenon is observed in dye-doped polymer dispersed liquid crystals and makes use of the strong scattering anisotropies in these materials.
TL;DR: Using video microscopy coupled with particle tracking routines, the Brownian fluctuations of micrometer-sized particles were analyzed to yield a quantitative measurement of the diffusion coefficients parallel and perpendicular to the nematic director.
Abstract: The Stokes-Einstein relation relates the diffusion coefficient of a spherical Brownian particle in a viscous fluid to its friction coefficient. For a particle suspended in anisotropic liquid, theory predicts that the drag coefficient should also be anisotropic. Using video microscopy coupled with particle tracking routines, the Brownian fluctuations of micrometer-sized particles were analyzed to yield a quantitative measurement of the diffusion coefficients parallel and perpendicular to the nematic director. The experimental values agree quite well with recent numerical calculations that take into account the distortions of the director field in the vicinity of the particles.
Patent•
27 May 2004
TL;DR: In this article, a multi-domain homeotropic alignment mode liquid crystal display device is proposed, where one substrate has a multisdomain pattern thereon to divide a pixel into multiple domain regions, and the other has a plurality of strip patterns thereon.
Abstract: Two substrates constitute a liquid crystal display device, wherein one substrate has a multi-domain pattern thereon to divide a pixel into multi-domain regions, and the other substrate has a plurality of strip patterns thereon. Having injecting liquid crystal into cell between the two substrates, the multi-domain pattern and the strip patterns will divide the pixel to a form multi-domain homeotropic alignment mode liquid crystal display device.
TL;DR: In this article, a review of recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order disorder change) and change of liquid crystal directors (order-order change).
Abstract: Recent progress in alignment modulation of azobenzene-containing liquid crystal systems by photochemical reactions has been reviewed by dividing the modulation methods into two types: phase transitions (order–disorder change) and change of liquid crystal directors (order–order change). First, photochemical phase transitions and alignment changes of liquid crystals in guest/host mixtures and polymers are summarized. Then, alignment control of liquid crystals by linearly polarized light and photoactive surface layers is discussed. Finally, recent applications of alignment change and photochemical phase transitions of liquid crystals in holographic technology and photomechanical effects are introduced. In addition, future possible applications for a variety of practical devices, such as display devices, optical switching and reversible optical image storage, are mentioned.
TL;DR: The small deflection limit of these problems of photoinduced mounds and pits is analyzed and it is shown that beams made of these materials can have two elastically neutral planes, and that plates of these Materials have a typical saddle shape.
Abstract: Photoresponsive solids such as nematic photoelastomers can undergo large deformations induced by light absorbed into rodlike molecules which bend and disrupt liquid crystal order. Significant variation of photoabsorption through the solid leads to nonuniform elastic deformations such as bending of beams and plates and pitting of layers. Such effects are also found in the presence of inhomogeneous thermal or swelling fields in solids or gels. We analyze the small deflection limit of these problems and show that beams made of these materials can have two elastically neutral planes, and that plates of these materials have a typical saddle shape. We also give a scaling analysis of the elasticity of photoinduced mounds and pits and speculate on their applications.