Maximizing Orientational Order in Polymer-Stabilized Liquid Crystals Using High Magnetic Fields

A reduction in the temperature of the transition to the biphasic and clearing points by the application of a magnetic field in excess of the critical unwinding field has been observed in a cholesteric liquid crystal. This observation qualitatively is in accordance with the phenomenological expansion of the excess free energy in the order parameter S. Inclusion of a twist term in the gradients of S leads to a rescaling of T* which is inversely proportional to the equilibrium pitch P0. When the pitch of the cholesteric liquid crystal is increased or removed by a magnetic field, a reduction in the transition temperature is expected. The observed effect is apparently insignificant in thermotropic liquid crystals but may be more pronounced in the lyotropic liquid crystal of this study due to its very small latent heat.

Low-voltage electrochromic behaviour of polymer-stabilised liquid crystal with considerable broadband reflection

Recently, we have demonstrated a new approach to fabricate a variable focal lens using polymer-stabilized liquid crystals [1, 2]. The approach is based on curing of a polymer/liquid crystals mixture with a circularly symmetric (e.g. Gaussian) shaped laser beam to induce spatially inhomogeneous polymer network. Applying a uniform voltage to the non-pixilated cell leads to circular-symmetric (lens-like) distribution of refractive index in the cell with plane parallel substrates. In this paper we study and optimize the electro-optical characteristics of such lens by varying the wavelength of the polymerizing laser, temperature regime of the process of polymerization as well as frequency of the lens driving voltage. Obtained results are applied to develop lenses that have no moving components and allow the electro-optical zooming. REFERNCES [1] V.V.Presnyakov, T.V.Galstian, K.E.Asatryan, A.Tork. "Polymer-Stabilized Liquid Crystal for Tunable Microlens Applications", Optics Express, 10, 17, 865-870, 2002. [2] V.V.Presnyakov, T.V.Galstian. "Variable Focal Length Lens Based on Polymer-Stabilized Nematic Liquid Crystals", 19th International Liquid Crystal Conference, Edinburgh, UK, June 30 – July 5, 2002, Book of Abstracts, P754.

This feature article describes recent advances in several areas of research involving the interfacial ordering of liquid crystals (LCs). The first advance revolves around the ordering of LCs at bio/chemically functionalized surfaces. Whereas the majority of past studies of surface-induced ordering of LCs have involved surfaces of solids that present a limited diversity of chemical functional groups (surfaces at which van der Waals forces dominate surface-induced ordering), recent studies have moved to investigate the ordering of LCs on chemically complex surfaces. For example, surfaces decorated with biomolecules (e.g., oligopeptides and proteins) and transition-metal ions have been investigated, leading to an understanding of the roles that metal-ligand coordination interactions, electrical double layers, acid-base interactions, and hydrogen bonding can play in the interfacial ordering of LCs. The opportunity to create chemically responsive LCs capable of undergoing ordering transitions in the presence of targeted molecular events (e.g., ligand exchange around a metal center) has emerged from these fundamental studies. A second advance has focused on investigations of the ordering of LCs at interfaces with immiscible isotropic fluids, particularly water. In contrast to prior studies of surface-induced ordering of LCs on solid surfaces, LC-aqueous interfaces are deformable and molecules at these interfaces exhibit high levels of mobility and thus can reorganize in response to changes in the interfacial environment. A range of fundamental investigations involving these LC-aqueous interfaces have revealed that (i) the spatial and temporal characteristics of assemblies formed from biomolecular interactions can be reported by surface-driven ordering transitions in the LCs, (ii) the interfacial phase behavior of molecules and colloids can be coupled to (and manipulated via) the ordering (and nematic elasticity) of LCs, and (iii) the confinement of LCs leads to unanticipated size-dependent ordering (particularly in the context of LC emulsion droplets). The third and final advance addressed in this article involves interactions between colloids mediated by LCs. Recent experiments involving microparticles deposited at the LC-aqueous interface have revealed that LC-mediated interactions can drive interfacial assemblies of particles through reversible ordering transitions (e.g., from 1D chains to 2D arrays with local hexagonal symmetry). In addition, recent single-nanoparticle measurements suggest that the ordering of LCs about nanoparticles differs substantially from micrometer-sized particles and that the interactions between nanoparticles mediated by the LCs are far weaker than predicted by theory (sufficiently weak that the interactions are reversible and thus enable self-assembly). Finally, LC-mediated interactions between colloidal particles have also been shown to lead to the formation of colloid-in-LC gels that possess mechanical properties relevant to the design of materials that interface with living biological systems. Overall, these three topics serve to illustrate the broad opportunities that exist to do fundamental interfacial science and discovery-oriented research involving LCs.

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices.

Experimental results of studying the p-i-n structures based on n-Si with the resistivity ρ = 120 Ω cm are reported. The dynamic S-shaped current-voltage characteristics were studied at various values of the applied magnetic field. Initiation of the current and voltage large-amplitude oscillations was observed at a magnetic field of 2 kOe. A further increase in the magnetic field leads to an increase in the amplitude of oscillations and to the appearance of a high-frequency oscillation mode with the frequency of ∼ 3 MHz; this mode modulates the low-frequency mode. A magnetic field in excess of 5 kOe reduces the amplitude of both modes of current oscillations and completely suppresses oscillations in the circuit so that the S-shaped form of the current-volt-age characteristic disappears. In accordance with the theory, experimental current-voltage characteristics were observed for the first time to feature portions with negative differential N-type conductivity in the presence of a high magnetic field and with the chosen sample configuration and relevant values of electric field.

Calculation of electric and magnetic induced fields in humans subjected to electric power lines

Electrically induced coloration of polymer-stabilized cholesteric liquid crystal films with broadband reflection capability for smart windows

Epoxy resin-based polymer-wall stabilized liquid crystal films with low driving voltages and improved mechanical performance for smart window applications

We cause a low-molecular-weight liquid crystal in the nematic phase to flow between parallel plate electrodes at a constant rate. Under electric fields (in the range 0–1.0 kV/mm), magnetic fields (in the range 0.0–2.0 T, where 1 T=10000 G), and both magnetic and electric fields, we measured changes in liquid crystal flow resistance and orientation. We also estimated the crystal orientation by measuring the brightness of light through the crystal to investigate the relationship between pressure drop and orientation. As results, the pressure drop increases as electric and magnetic field intensities increase, and remains constant at E > 0.6 kV/mm. Under application of an electric field, the brightness of light through the liquid crystal increases with increasing rates of pressure in the range 0–0.4 kV/mm.

This paper describes a newly developed controllable damper using liquid crystal (LC) as working fluid. LC is homogeneous organic liquid characterized by the long-range order of its molecular orientation. The sample LC used in the present experiment is a thermotropic LC with low molecular-weight which appears in nematic phase, originally developed for display devices. The nematic phase is characterized by the molecular orientation of slender ellipsoidal shape molecules, the main axis of which can be controlled by the applied electric or magnetic field. When an electric field is applied to LC, it is known that the orientation order of the molecule becomes parallel to the applied electric field, which causes the viscosity to increase. This phenomenon is defined as the electroviscous effect. As an application study of electroviscous effect of LC to a controllable mechanical damping device, a prototype controllable damper was constructed and its performance was examined. In the present LC damper, the piston, equipped with several concentric cylindrical electrodes attached to the piston rod, moves in the LC filled damper. During the reciprocal movement of electrodes, LC flow through electrodes is controlled by applying electric voltage to the electrodes. The performance was investigated under various applied DC electric field strength, the piston alternation amplitude and frequency. The results show that the controllable damping force was three times larger for the case with electric field than that without electric field, and that the variation range was kept at the same level regardless of the frequency and amplitude of the piston reciprocation.

In the order parameter of hole-doped cuprate superconductors in the pseudogap phase, two holes enter the order parameter from opposite sides and pass through various [Formula: see text] cells jumping from one [Formula: see text] to the other under the influence of magnetic field offered by the [Formula: see text] ions in that [Formula: see text] cell and thus forming hole pairs. In the pseudogap phase of electron-doped cuprates, two electrons enter the order parameter at [Formula: see text] sites from opposite ends and pass from one [Formula: see text] site to the diagonally opposite [Formula: see text] site. Following this type of path, they are subjected to high magnetic fields from various [Formula: see text] ions in that cell. They do not travel from one [Formula: see text] site to the other along straight path but by helical path. As they pass through the diagonal, they face high to low to very high magnetic field. Therefore, frequency of helical motion and pitch goes on changing with the magnetic field. Just before reaching the [Formula: see text] ions at the exit points of all the cells, the pitch of the helical motion is enormously decreased and thus charge density at these sites is increased. So the velocity of electrons along the diagonal path is decreased. Consequently, transition temperature of electron-doped cuprates becomes less than that of hole-doped cuprates. Symmetry of the order parameter of the electron-doped cuprates has been found to be of [Formula: see text] type. It has been inferred that internal magnetic field inside the order parameter reconstructs the Fermi surface, which is requisite for superconductivity to take place. Electron pairs formed in the pseudogap phase are the precursors of superconducting order parameter when cooled below [Formula: see text].

Recent laboratory and epidemiological results have stimulated interest in the hypothesis that human beings may exhibit biological responses to magnetic and/or electric field transients with frequencies in the range between 100 Hz and 100 kHz. Much can be learned about the response of a system to a transient stimulation by understanding its response to sinusoidal disturbances over the entire frequency range of interest. Thus, the main effort of this paper was to compare the strengths of the electric fields induced in homogeneous ellipsoidal models by uniform 100 Hz through 100 kHz electric and magnetic fields. Over this frequency range, external electric fields of about 25-2000 V/m (depending primarily on the orientation of the body relative to the field) are required to induce electric fields inside models of adults and children that are similar in strength to those induced by an external 1 microT magnetic field. Additional analysis indicates that electric fields induced by uniform external electric and magnetic fields and by the nonuniform electric and magnetic fields produced by idealized point sources will not differ by more than a factor of two until the sources are brought close to the body. Published data on electric and magnetic field transients in residential environments indicate that, for most field orientations, the magnetic component will induce stronger electric fields inside adults and children than the electric component. This conclusion is also true for the currents induced in humans by typical levels of 60 Hz electric and magnetic fields in U.S. residences.

High-field miniband transport in semiconductor superlattices under the influence of a transverse magnetic field

Low-voltage polymer-stabilised liquid crystal windows with highly polar molecules for privacy and thermal comfort