Homeotropic State Research Articles

Fast switching cholesteric liquid crystal optical beam deflector with polarization independence

Optical beam deflectors based on the combination of cholesteric liquid crystals and polymer micro gratings are reported. Dual frequency cholesteric liquid crystal (DFCh-LC) is adopted to accelerate the switching from the homeotropic state back to the planar state. Polarization independent beam steering components are realized whose transmission versus the polarizing angle only varies 4.4% and 2.6% for the planar state and the homeotropic state, respectively. A response time of 451 ms is achieved for DFCh-LC-grating beam deflectors, which is fast compared to other nematic LC beam steerers with similar LC thickness.

High-contrast electrically switchable light-emitting liquid crystal displays based on α-cyanostilbenic derivative

ABSTRACTLight-emitting liquid crystal displays (LE-LCDs), serving as emissive displays, are considered as promising alternatives to conventional LCDs because of their superior power efficiency. A dichroic fluorescent dye (Z)-2-(4-aminophenyl)-3-(4-(fluorophenyl)acrylonitrile (CN-AFAN) is synthesised, which exhibits strong fluorescence in the solution and solid states. Moreover, an electrically switchable optical switch based on CN-AFAN and a cholesteric liquid crystal is demonstrated, which combines the internal scattering of excitation light and the dichroism of CN-AFAN to improve fluorescence contrast. The photoluminescence and transmittance of the optical switch is modulated by an electric field between the planar state, focal conic state, and homeotropic state. The resultant device is cost-effective and easy to fabricate, thereby demonstrating immense potential for security and display applications.

Light shutter using dye-doped cholesteric liquid crystals with polymer network structure

ABSTRACTProposed herein is a light shutter using dye-doped cholesteric liquid crystals with a polymer network structure for a high-visibility see-through display. The proposed light shutter shows an initially focal-conic state by forming the polymer network structure at the focal-conic state. The proposed light shutter can be easily switched between the focal-conic and homeotropic states without a complicated drive scheme.

Magnetic-field-induced stepwise director reorientation and untwisting of a planar cholesteric structure with finite anchoring energy.

Within the continuum approach we study the equilibrium configurations of a cholesteric liquid crystal confined between two parallel plates, when a magnetic field is applied perpendicularly to the plates. We analyze the role of soft anchoring boundary conditions on magnetic-field-induced cholesteric-nematic transitions in a finite thickness cholesteric cell, treated to induce soft planar alignment. We study the stepwise behavior of cholesteric pitch as a function of the anchoring energy, the thickness of a layer, and the field strength. We analyze some kinds of soft anchoring potentials, including the case of degeneration of the easy axes. We show that the variation of the thickness or intrinsic pitch induces the the stepwise behavior of a pitch of the planar cholesteric structure, and the stepwise variations of the average tensor of diamagnetic susceptibility. The values of these jumps are determined by the anchoring energy. We find the values of critical parameters for the transitions between planar and confocal cholesteric states, and homeotropic nematic state. We show that the variation of the anchoring energy leads to change of the phase transition character; the conditions for hysteresis behavior are obtained. We show that for rather soft anchoring the confocal state is metastable, and the increase of a magnetic field leads to the direct transition between the planar cholesteric and homeotropic nematic phases. We also give a detailed derivation of the threshold and saturation properties of planar cholesteric to homeotropic nematic transition.

Frequency-Driven Self-Organized Helical Superstructures Loaded with Mesogen-Grafted Silica Nanoparticles.

Adding colloidal nanoparticles into liquid-crystal media has become a promising pathway either to enhance or to introduce novel properties for improved device performance. Here we designed and synthesized new colloidal hybrid silica nanoparticles passivated with a mesogenic monolayer on the surface to facilitate their organo-solubility and compatibility in a liquid-crystal host. The resulting nanoparticles were identified by 1 H NMR spectroscopy, TEM, TGA, and UV/Vis techniques, and the hybrid nanoparticles were doped into a dual-frequency cholesteric liquid-crystal host to appraise both their compatibility with the host and the effect of the doping concentration on their electro-optical properties. Interestingly, the silica-nanoparticle-doped liquid-crystalline nanocomposites were found to be able to dynamically self-organize into a helical configuration and exhibit multi-stability, that is, homeotropic (transparent), focal conic (opaque), and planar states (partially transparent), depending on the frequency applied at sustained low voltage. Significantly, a higher contrast ratio between the transparent state and scattering state was accomplished in the nanoparticle-embedded liquid-crystal systems.

Submillisecond Inverse TN Bidirectional Field Switching Mode

We demonstrate a new fast electro-optical switching mode in homeotropically-aligned layer of a nematic liquid crystal. The electric field spatial distribution in the LC layer is switched between twisted-planar and normal states by means of driving the electric potentials over a system of interdigitated electrodes, which have mutually orthogonal orientation at the top and bottom sides of the LC layer. A twisted-planar field characterized by twisted spatial distribution induces a twisted state of the LC, whereas switching back to the homeotropic state is driven by a normal field that is oriented along the LC layer normal. The bidirectional driving eliminates the stage of free relaxation to the homeotropic state, thus allowing for very fast electro-optical switching between the optical states: total switching time is less than 1 ms, which is an order faster than for the conventional twist effect in nematics. The advantages of the suggested driving mode include gradual grayscale and deep-black state that is due to initial homeotropic alignment with a zero pretilt angle.

A new liquid crystal-based method to study disruption of phospholipid membranes by sodium deoxycholate

ABSTRACTIn this study, we have used liquid crystals (LCs) to investigate the mechanism and dynamics of structural change of phospholipid membranes caused by sodium deoxycholate (NaDC). Addition of the NaDC aqueous solution to the phospholipid [1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DOPG)] modified aqueous/LC interface resulted in the interaction-induced change of the orientational arrangement of the LCs from a homeotropic state to a planar state. The importance of contributing parameters was determined by observing the changes in the orientation of LCs. We showed that this interaction was affected by reaction time, reaction pH, concentration of NaDC and the presence of cholesterol. Moreover, the phospholipid membrane, which became defective after being exposed to NaDC, was capable of self-repairing by excess Tris-buffered saline solution, indicating that the reaction of NaDC with the phospholipid membrane is reversible. The obtained results proved the feasibility of the method deploying the DOPG/LC interface to monitor the membrane reaction stemming from the interaction between a bioactive molecule and a phospholipid membrane.

Free Surface Command Layer for Photoswitchable Out-of-Plane Alignment Control in Liquid Crystalline Polymer Films.

To date, reversible alignment controls of liquid crystalline materials have widely been achieved by photoreactive layers on solid substrates. In contrast, this work demonstrates the reversible out-of-plane photocontrols of liquid crystalline polymer films by using a photoresponsive skin layer existing at the free surface. A polymethacrylate containing a cyanobiphenyl side-chain mesogen adopts the planar orientation. Upon blending a small amount of azobenzene-containing side-chain polymer followed by successive annealing, segregation of the azobenzene polymer at the free surface occurs and induces a planar to homeotropic orientation transition of cyanobiphenyl mesogens underneath. By irradiation with UV light, the mesogen orientation turns into the planar orientation. The orientation reverts to the homeotropic state upon visible light irradiation or thermally, and such cyclic processes can be repeated many times. On the basis of this principle, erasable optical patterning is performed by irradiating UV light through a photomask.

Matched elastic constants for a perfect helical planar state and a fast switching time in chiral nematic liquid crystals.

Chiral nematic liquid crystals possess a self-assembled helical structure and exhibit unique selective reflection in visible and infrared light regions. Their optical properties can be electrically tuned. The tuning involves the unwinding and restoring of the helical structure. We carried out an experimental study on the mechanism of the restoration of the helical structure. We constructed chiral nematic liquid crystals with variable elastic constants by doping bent-dimers and studied their impact on the restoration. With matched twist and bend elastic constants, the helical structure can be restored dramatically fast from the field-induced homeotropic state. Furthermore, defects can be eliminated to produce a perfect planar state which exhibits high selective reflection.

The effect of external applied fields on the third order nonlinear susceptibility of ferro-nematics

This experimental work reports measurement of the third order nonlinear susceptibility of the nematic liquid crystals with the impurity of Fe3O4 nanoparticles with the mean diameter of 34nm and weight concentration of 1wt.%. The study was performed without and under the influence of electric and magnetic field consequently by applying different voltages and induction 0.5T. The results show in the case of nematic liquid crystals with magnetic nanoparticles, the parameter changes caused by the external fields depend on the direction of external fields with respect to the director axis n→ and then the type of initial alignment. The direction of applied external fields is perpendicular to the cell surface, so there was no observable shift in the nonlinearity in the case of homeotropic alignment sample. The experimental results show that the change in the third order nonlinear behavior of homogeneous-aligned cells reoriented the ferro-nematic molecules to their homeotropic state.

Spontaneous reorientation from planar to homeotropic alignment in dual-frequency mixture doped with chiral dopant

We investigate a dual-frequency (DF) nematic mixture by means of the dielectric spectroscopy. A chiral dopant has been added to the DF matrix. A pure DF matrix as well as three samples with increasing concentration of chiral dopant (DF*) have been prepared. Investigated samples have shown a selective reflection band, according to a helical pitch induced in proportion to the chiral dopant amount. Such system promises a great application potential as a selective light reflecting shutter or spectral filter due to nearly symmetrical switching times and defectless structures.When we put the non-doped DF mixture into homogenous (HG) oriented cell we found that planar orientation is permanent on both cooling and heating cycles. In the case of DF* chiral mixtures, the behavior changes substantially. One can observe that the oriented virgin planar structure of DF* within the HG cell starts to change its orientation to homeotropic one at different temperature. When heating is continued the homeotropic orientation in the HG cell is preserved up to the clearing temperature. Upon a subsequent cooling from the isotropic state, the homeotropic orientation is preserved within the whole mesogenic state. The effect of induction of the homeotropic state observed within HG cells is analyzed and discussed.

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Electrically assisting crystal growth of blue phase liquid crystals

We proposed an electrically assisting crystal growth of blue phase liquid crystals (BP-LCs) to generate uniform crystal orientation of BP-LCs. With an applied electric field, a phase transition from BP-LCs to focal conic state and homeotropic state occurs. As the electric field is removed, the crystal orientation of [220] quickly dominates the crystal growth process and other crystal orientations grow less at the same time. The BP-LCs with a single crystal orientation can be achieved with multiple cycles of the electrical treatment. We also provide a possible mechanism of crystal growth with electrical treatment which is applicable for both BP-LCs and PSBP-LCs. In addition, we conclude that the hysteresis effect is mainly affected by the domain size of PSBP-LCs rather than uniformity of crystal orientation. This study helps further understanding of the origin of hysteresis effect in PSBP-LCs which is important for development of many BP-LCs based photonic devices, such as micro lenses, displays, and lasers.

DNA Hybridization-Mediated Liposome Fusion at the Aqueous Liquid Crystal Interface.

The prominence of receptor-mediated bilayer fusion in cellular biology motivates development of biomimetic strategies for studying fusogenic mechanisms. An approach is reported here for monitoring receptor-mediated fusion that exploits the unique physical and optical properties of liquid crystals (LC). PEG-functionalized lipids are used to create an interfacial environment capable of inhibiting spontaneous liposome fusion with an aqueous/LC interface. Then, DNA hybridization between oligonucleotides within bulk phase liposomes and a PEG-lipid monolayer at an aqueous/LC interface is exploited to induce receptor-mediated liposome fusion. These hybridization events induce strain within the liposome bilayer, promote lipid mixing with the LC interface, and consequently create an interfacial environment favoring re-orientation of the LC to a homeotropic (perpendicular) state. Furthermore, the bi-functionality of aptamers is exploited to modulate DNA hybridization-mediated liposome fusion by regulating the availability of the appropriate ligand (i.e., thrombin). Here, a LC-based approach for monitoring receptor (i.e., DNA hybridization)-mediated liposome fusion is demonstrated, liposome properties that dictate fusion dynamics are explored, and an example of how this approach may be used in a biosensing scheme is provided.

Discontinuous Thermal Diffusivity Change due to the Anchoring Transition of a Liquid Crystal on a Perfluoropolymer Surface

Thermal diffusivity of a liquid crystal, 4'-butyl-4-heptyl-bicyclohexyl-4-carbonitrile, was measured using a temperature wave method. The liquid crystal was sandwiched by two glass substrates, which were treated with three different surface agents for providing distinct molecular orientations. Here, we demonstrate that: 1) a large thermal diffusivity anisotropy arising from different orientations, that is, planar and homeotropic states, was found in the nematic and smectic A phases; 2) when substrates were coated with a perfluoropolymer, abrupt changes of the thermal diffusivity were observed in the nematic phase both on cooling and heating due to the discontinuous anchoring transition between planar and homeotropic states. The temperature dependence of the thermal diffusivity anisotropy was well described by a power law, with an exponent of 0.27 according to the mean-field theory.

Electro-optic and molecular relaxation behaviour of fluoro substituted achiral unsymmetrical four-ring bent-core mesogen

Electro-optic and molecular relaxation studies in a new achiral unsymmetrical four-ring bent-core mesogen are presented in this study. The compound consists of a laterally fluoro-substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the two phenyl rings as the central unit. The mesomorphic properties were characterised by polarising optical microscopy and differential scanning calorimetry. The compound exhibited enantiotropic B7–Bx phase sequence. The electro-optic studies revealed a bistable switching from planar to a homeotropic state under an electric field in B7 phase. Further memory effect was observed between planar and homeotropic textures after the removal of electric field in B7 phase. Temperature dependent permittivity studies as a function of frequency revealed that the molecular relaxation increases with the increasing temperature in B7 phase than in Bx phase.

Light shutter using dichroic-dye-doped long-pitch cholesteric liquid crystals

We propose a light shutter device using dichroic-dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is set to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). A dye-doped long-pitch ChLC cell is switchable between the dark planar state and the transparent homeotropic state. It has the advantages of high transmittance, low operation voltage, and an easy fabrication process relative to previous LC light shutter devices. The proposed light shutter device is expected to achieve high visibility for transparent organic light-emitting diode displays and emerging smart windows, which can be used in airplanes, cars, and other similar applications.

Electro-optical effects in hybrid aligned flexoelectric nematic layers

Liquid crystal cells with hybrid boundary anchoring, filled with nematic possessing flexoelectric properties, and subjected to external electric field, were studied numerically in order to find the influence of flexoelectricity on their behavior. Such layers may adopt three kinds of director structures: uniform planar, uniform homeotropic, and non-uniform which is intermediate between the former two. Stability of these structures depends on flexoelectric coefficients, anchoring strengths, thickness of the layer, dielectric anisotropy, and elastic constants. Changes of bias voltage cause transitions between them, which lead to electro-optical effects if the layers are placed between crossed polarizers. Three cases of transitions were considered: (i) transition between bright planar and dark non-uniform states, (ii) between dark planar and bright non-uniform states, and (iii) between dark homeotropic and bright non-uniform states. The director distributions in various states corresponding to various grey levels were calculated, and the electro-optic characteristics were obtained. The dynamics of the transitions between dark, bright, and intermediate states was determined taking into account the backflow effect. It was found that the transitions are faster when the nematic is devoid of flexoelectric properties.

Lower operation voltage in dual-frequency cholesteric liquid crystals based on the thermodielectric effect

Dual-frequency cholesteric liquid crystal (DFCLC) devices characteristically require high operation voltage, which hinders their further development in thin-film-transistor driving. Here we report on a lower-voltage switching method based on the thermodielectric effect. This technique entails applying a high-frequency voltage to occasion dielectric oscillation heating so to induce the increase in crossover frequency. The subsequent change in dielectric anisotropy of the DFCLC allows the switching, with a lower operation voltage, from the planar state to the focal conic or homeotropic state. The temperature rise incurred by the dielectric heating is described.

Domain Structures in Nematic Liquid Crystals on a Polycarbonate Surface

Alignment of nematic liquid crystals on polycarbonate films obtained with the use of solvents with different solvations is studied. Domain structures occurring during the growth on the polymer surface against the background of the initial thread-like or schlieren texture are demonstrated. It is established by optical methods that the domains are stable formations visualizing the polymer surface structures. In nematic droplets, the temperature-induced transition from the domain structure with two extinction bands to the structure with four bands is observed. This transition is shown to be caused by reorientation of the nematic director in the liquid crystal volume from the planar alignment to the homeotropic state with the pronounced radial configuration of nematic molecules on the surface. The observed textures are compared with different combinations of the volume LC orientations and the radial distribution of the director field and the disclination lines at the polycarbonate surface.