Orientation Of Liquid Crystal Molecules Research Articles

Light-matter coupling and spin-orbit interaction of polariton modes in liquid crystal optical microcavities

In this theoretical study, we explore the dispersion and basic properties of optical microcavities filled with liquid crystal (LC) media that contain embedded quantum wells. As a result of the strong coupling between cavity photons and excitons, exciton polariton quasiparticles arise in these structures. LC-filled microcavities have an advantage of the ability to manipulate the spin (polarization) of the photonic component of the polariton states by controlling the orientation of LC molecules using an external electric field. This enables the engineering of controllable synthetic Hamiltonians for the polariton eigenmodes in microcavity structures. The introduction of synthetic spin-orbit interaction via placing of the quantum wells at particular positions in the LC-filled cavity enables control over the propagation of exciton polaritons, leading to various spatial effects. Through numerical calculations, we successfully reproduce the birefringence and phenomena exhibited by exciton polaritons propagating within the microcavity plane. We also examine the conditions required for strong coupling when utilizing perovskite layers as hosts for excitons. While the strong coupling regime can be also achieved in this material system, the manifestations of the synthetic spin-orbit interaction are suppressed owing to stronger disorder and nonradiative processes. Published by the American Physical Society 2024

Reverse mode LCPG films with self-assembly support of gel networks

ABSTRACT Liquid crystal physical gels (LCPG) with a 3D network structure show considerable promise as novel functional materials with a great impact on scientific and practical applications. LCPGs are not characterised by high hardness and high strength, but this mild nature is capable of forming dynamic supramolecular structures with the self-assembly ability of various non-covalent bonds such as hydrogen bonding, π-π stacking, and van der Waals forces. In addition to anisotropic changes under optical and electrical stimulation, it enables to be one of the candidates for display devices. When the isotropic-anisotropic transition temperature of LCPG solution (TIA) is higher than the sol-gel transition temperature of gel molecules (TSG), the gelation can anchor the orientation of liquid crystal molecules to achieve the alteration of transparency and scattering of the film. In this paper, reverse mode LCPG films with high contrast, low driving voltage, and excellent cycle life were prepared using oriented liquid crystal cassettes and LCPG solutions, it also focused on the effects of chiral compounds (S)-octan-2-yl 4-((4-(hexyloxy)benzoyl)oxy)benzoate (S811) and gel 1,3:2,4-bis-0-(3,4-dimethylbenzylidene)sorbitol (DMDBS) content on electro-optical properties of LCPG films as well as the different functions of TIA and TSG on film orientation.

Facile fabrication of MOF-based composite membranes with liquid crystal ordered microstructure for effective dyes separation

High-performance membranes for effective dye/water separation are urgently desired in chemical industrial manufacture, and Metal-organic frameworks (MOFs) with abundant microporous structures are considered suitable candidates for fabricating separation membranes. However, for MOF-based membranes, maintaining superior separation efficiency while ensuring high water flux remains a challenge. In this study, a series of novel MOF-based composite materials (TFNs) and membranes oriented by electric fields (fio-TFNs) were fabricated using NH2-MIL-53(Fe), a thermotropic liquid crystal, poly (styrene-4-sulfonic acid), and polytetrafluoroethylene. The fio-TFN membranes exhibited characteristics of orientation and uniform distribution, and showed higher water flux and dye rejection compared with ordinary TFN membranes. This highlights the significant role played by the ordered orientation of liquid crystal molecules induced by the electric field in effective dye/water separation. High porosity is the basis of dye absorption, and the electrostatic interaction among structural charges and dyes is the dominant driving forces in these dye/water separation systems. The prepared fio-TFN membranes exhibited excellent performance in effective dye separation. For instance, the fio-TFN-1 membrane demonstrated high water flux (up to 255.9 L·m−2 h−1 bar−1) and excellent dye rejection rates (≥97 %, 92 %, and 82 % for methyl blue, congo red, and rhodamine B, respectively). Moreover, these prepared membranes exhibited excellent long-term stability, reusability, and flexibility, indicating their potential as high-performance separation membranes for wastewater treatment.

Chiral monomer template for designing Low-Driving-Field blue phase liquid crystals

Blue phase liquid crystals (BPLC) are known to be useful for various non-trivial applications due to their nanoscale self-assembled cubic crystalline structures. Even though the BPLC exhibits an efficient optically isotropic phase due to the double helical confinement of liquid crystal (LC) molecules within a few hundred-nanometer range, this hinders the orientation of LC molecules along the applied field direction. Consequently, the BPLC demands larger driving fields to operate, degrading the device’s performance in real-world applications. In this context, we provide a chiral monomer template for BPLC that decreases the driving field by 52 % while the electro-optical responses remain unaffected. Furthermore, our approach provides higher thermal stability, with a broader BPLC phase range (42 °C), including near ambient temperature. As this approach potentially overcomes the fundamental challenges of BPLC, the proposed system could be useful for next-generation display devices.

Miscibility of porphyrin/liquid crystal floating monolayers at the air–water interface

ABSTRACT We found that a new stable state of porphyrin monolayer can be induced by liquid crystal molecules. We measured the surface pressure per area of the monolayer at various molar fractions of liquid crystal. The polarity and orientation of liquid crystal molecules were investigated by recording the surface potential value of the monolayer. We found that the dispersion of porphyrin in a liquid crystal solution led to changes in the surface structure and the surface pressure of the monolayer at the air–water interface. Therefore, the surface potential records can be used to control the orientation of liquid crystal and the formation of mono- or multilayers of Langmuir thin film. The plots obtained for solutions with a mole fraction of liquid crystal greater than 0.333 showed repulsive interactions between porphyrin and liquid crystal molecules.

Tuning Molecular Orientation Responses of Microfluidic Liquid Crystal Dispersions to Colloid and Polymer Flows.

An important approach to molecular diagnostics is integrating organized substances that provide complex molecular level responses to introduced chemical and biological agents with conditions that optimize and distinguish such responses. In this respect, liquid crystal dispersions are attractive components of molecular diagnostic tools. This paper analyzes a colloid system, containing a nematic liquid crystal as a dispersed phase, and aqueous surfactant and polymer solutions as the continuous phases. We applied a microfluidic approach for tuning orientation of liquid crystal molecules in picoliter droplets immobilized on microchannel walls. Introduction of surfactant to the aqueous phase was found to proportionally increase the order parameter of liquid crystal molecules in microdroplets. Infusion of polymer solutions into surfactant-mediated microfluidic liquid crystal dispersions increased the order parameter at much lower surfactant concentrations, while further infusion of surfactant solutions randomized the orientation of liquid crystal molecules. These effects were correlated with the adsorption of surfactant molecules on surfaces of microdroplets, stabilizing the effect of a polymer matrix on bound surfactant ions and the formation of insoluble polymer-colloid aggregates, respectively. The revealed molecular behavior of liquid crystal dispersions may contribute to optimized synthesis of responsive liquid crystal dispersions for in-flow molecular diagnostics of polymers and colloids, and the development of functional laboratory-on-chip prototypes.

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

AbstractPhotodimerization of coumarin derivatives in photoactive polymers under linearly polarized UV (LPUV) light irradiation is a photochemical reaction often used to prepare the surface alignment layer that can orient liquid crystals (LCs) due to the anisotropic interfacial interactions. Herein, the synthesis of a series of novel coumarin‐containing copolymers and their use for the photoalignment surface layer of LCs is reported. These copolymers have the particular feature of bearing two coumarin side groups in each monomer unit, which is designed to have a high content of the chromophores. The alignment of LCs induced by the photoalignment layers of the coumarin‐containing copolymers is investigated. It is found that the photoalignment layers can effectively induce orientation of LC molecules along the direction of the UV light polarization used to prepare the surface layers, with an order parameter of around 0.5 achieved by adjusting the preparation conditions. Moreover, the electro‐optical behaviors of LC cells made with the photoalignment layers are investigated, confirming the alignment of LCs without the use of rubbed surfaces. Optically inscribing the photoalignment layer for nonuniform and organized LC alignment is also demonstrated.

Morphological, dielectric, electro-optic and photoluminescence properties of titanium oxide nanoparticles enriched polymer stabilized cholesteric liquid crystal composites

Herein, TiO2 nanoparticles (NPs) dispersed polymer stabilized cholesteric liquid crystal (PSCLC) composites are prepared and studied. Effect of TiO2 NPs doping in varying concentration into PSCLC, on the phases, molecular states, dielectric, electro-optic and photoluminescence properties are investigated in studied samples. The morphological studies reveal that the TiO2 NPs are responsible to switch the orientation of liquid crystal molecules into homeotropic state at a lower voltage. The permittivity reduced from 9.62 to 7.75 at an optimum doping amount (0.5 wt%) of TiO2 NPs. At this concentration, an improvement in dielectric anisotropy was found. A decrease in threshold voltage from 3.2 V to 1.4 V was observed. Further, the saturation voltage also decreases from 10.5 V to 7.5 V at 0.5 wt% doped TiO2 NPs. The photoluminescence intensity increase with TiO2 NPs concentration however emission wavelength does not change. This work may provide a facile approach to fabricate the low energy consumption based electro-optical devices with the affordable and eco-friendly nature of TiO2 NPs.

Orientation Behavior of Nematic Liquid Crystals at Flow-Wall Interfaces in Microfluidic Channels

This work characterizes the orientation behavior of nematic liquid crystals in pressure-driven flows of microfluidic channels at interfaces between the flow and microchannel walls. The impact of flow velocity and microchannel geometry on the orientation of liquid crystals in single-phase and two-phase flows is discussed. Polarizing optical microscopy images revealed the homeotropic orientation of liquid crystal molecules at microchannel walls at zero flow velocities, which gradually transitioned into planar alignment along the microchannel axis when the flow velocity increased in the 50 μm/s to 5 mm/s range. Liquid crystal droplets demonstrated homeotropic or planar alignment depending on the sizes of droplets and flow velocities. The polarized light pattern from homeotropically aligned droplets deposited on microchannel walls was found to be logarithmically proportional to the flow velocity in the 2 to 40 mm/s range. The revealed behavior of nematic liquid crystals at microchannel wall surfaces in dynamic flow conditions offers new tools for on-demand control of the optical properties of microfluidic devices and can contribute to the development of analytical lab-on-chip tools with internal continuous or discrete liquid crystal layers for flow characterization in microchannel confinement.

High-resolution light field imaging based on liquid crytal microlens arrays with ZnO microstructure orientation

We proposed a liquid crystal (LC) microlens array (MLA) based on zinc oxide (ZnO) microstructure orientation and used it in a light field imaging system. The system uses convex optimization theory to jointly optimize the multi-voltage light field information and obtain a high-resolution imaging method. The core element in this system is an LCMLA, which induces the horizontal orientation of LC molecules by ZnO microstructure to effectively overcome the physical defects caused by orientation, such as mechanical friction. Combined with the proposed joint optimization high-resolution algorithm based on convex optimization theory, high-resolution imaging can be achieved using the redundant characteristics of light field images. The experiments show that the proposed method's PSNR of the reconstructed image is about 32 dB compared with the conventional method. The designed imaging system is flexible, simple, and suitable for high-resolution reconstruction.

Liquid Crystal Biosensors: Principles, Structure and Applications.

Liquid crystals (LCs) have been widely used as sensitive elements to construct LC biosensors based on the principle that specific bonding events between biomolecules can affect the orientation of LC molecules. On the basis of the sensing interface of LC molecules, LC biosensors can be classified into three types: LC–solid interface sensing platforms, LC–aqueous interface sensing platforms, and LC–droplet interface sensing platforms. In addition, as a signal amplification method, the combination of LCs and whispering gallery mode (WGM) optical microcavities can provide higher detection sensitivity due to the extremely high quality factor and the small mode volume of the WGM optical microcavity, which enhances the interaction between the light field and biotargets. In this review, we present an overview of the basic principles, the structure, and the applications of LC biosensors. We discuss the important properties of LC and the principle of LC biosensors. The different geometries of LCs in the biosensing systems as well as their applications in the biological detection are then described. The fabrication and the application of the LC-based WGM microcavity optofluidic sensor in the biological detection are also introduced. Finally, challenges and potential research opportunities in the development of LC-based biosensors are discussed.

Photo-patterned memory state based on nematic-silica-dye suspension

The influence of photoalignment on the memory state of nematic liquid crystals (LCs) doped with silica nanoparticles and azo dyes was studied. The memory state of the LC sample with hybrid alignment was realized with electrophoretically controlled silica nanoparticles, which accumulated on the homogeneously aligned side of the LC sample to stabilize the vertical orientation of LC molecules through a pulse voltage with appropriate polarity. When the photoalignment was executed on the LCs in the hybrid orientation, the adsorbed azo dyes screened the effect of vertical orientation supported by silica networks. The hybrid LC orientation was stabilized because the adsorbents of azo dyes on the substrate provided a stronger surface homogeneous alignment. When the LCs in the memory state of vertical alignment were illuminated, their vertical orientation was maintained by the adsorbed azo dyes that suppressed the electrophoretic movement of the silica nanoparticles. The study also demonstrates a simple display with hidden images by utilizing these results.

Is there only one equilibrium configuration for spontaneous bending of liquid crystal elastomer circular plates with free edges?

In this paper, the spontaneous bending of a liquid crystal elastomer (LCE) circular plate under light illumination is investigated. The large bending deformation configurations for the LCE circular plate with free edges are obtained by minimizing the potential energy. It is found that for an LCE circular plate with random distribution of liquid crystal molecules, if the light intensity is small, there is only one equilibrium configuration for bending of the LCE circular plate. However, when the light intensity reaches a critical value, the circular plate will buckle and there will be three possible equilibrium configurations. On the other hand, for an LCE circular plate with uniform orientation of liquid crystal molecules, the strain induced by light is anisotropic, and there is only one equilibrium configuration. In addition, bending shapes of the LCE circular plate depend on its thickness. These results may be useful for designing light-driven LCE devices.

Rational design of surface-confined nanostructured self-assemblies based on functional comb-shaped copolymers for tunable molecular orientation

Functional ultrathin nanoarchitectures are commonly based on hierarchical molecular self-assemblies that are structurally tunable and facilely fabricable, and have found a broad range of potential applications from nanoelectronics to biosensors. Here, we demonstrate a simple and elegant approach for the rational design of ultrathin molecular nanoarchitectures for tunable interfacial orientation of liquid crystals (LCs) using the surface-confinement of functional amphiphilic copolymers. The surface-confined nanostructured self-assemblies are simply prepared by introducing common functional polymers into amphiphilic systems. The surface-confined copolymeric nanoalignment layer, which is constructed from the simple doping and molecular self-assembly of comb-shaped amphiphilic copolymers in a confined LC medium, comprises a hydrophobic polyacrylate segment with a long side chain moiety capable of uniformly controlling the LC alignment and a hydrophilic polyacrylic acid segment with a carboxylic acid group confined to the hydrophilic surface of indium tin oxide substrate. The combination of structural variations on amphiphilic copolymer system and tunability of molecular orientation makes the surface-confined copolymeric nano-assemblies highly attractive ultrathin nanoarchitectures. The resulting nanostructured self-assemblies exhibit a spontaneous and uniform vertical orientation of LC molecules, simultaneously providing the LC device with a superfast electro-optical switching time and a strong surface anchoring control.

Control of Liquid Crystal Microarray Optical Signals Using a Microspectral Mode Based on Photonic Crystal Structures.

Herein, a novel liquid crystal microarray (LCM) film with optical regulation ability is first constructed by combining liquid crystals (LC) and the highly ordered microporous structure of inverse opal photonic crystals (IOPhCs). The LCM films are fabricated by infiltrating LC molecules into the LC polymer with the structure of IOPhCs, and their properties are very different from those without the LC. Interestingly, the optical property of LCM films can be controlled by changing the orientation of LC molecules, which varies with the interfacial force. In combination with polarization images, spectral reflection peak, circular dichroism spectra, potential difference, and fluorescence images of LCM films, the mechanism of this change is investigated. It is found that the exposed basic group of single-stranded DNA is the key to the change of the optical property of LC microarrays. Meanwhile, the optical signals of LC microarrays based on the PhCs provide a novel LC signal mode for an LC sensing system (microspectral signal mode), and it can be recorded by a fiber-optic spectrometer, which is a great improvement on LC sensing signals. Therefore, the LC microarray sensing signal can be used for accurate analysis of targets by the change of the reflection peak intensity of PhCs. When the LC molecules are induced by different aptamers, the LC microarray sensing interface can be further used for the determination of different targets, such as cocaine and Hg2+. The research on LCM films is of significant value for the development of LC sensing technology and also shows great application prospects in biochemical sensing fields.

Vertical Orientation of Liquid Crystal on Polystyrene Substituted with n-alkylbenzoate-p-oxymethyl Pendant Group as a Liquid Crystal Precursor.

We synthesized a series of polystyrene derivatives modified with precursors of liquid crystal (LC) molecules via polymer modification reactions. Thereafter, the orientation of the LC molecules on the polymer films, which possess part of the corresponding LC molecular structure, was investigated systematically. The precursors and the corresponding derivatives used in this study include ethyl-p-hydroxybenzoate (homopolymer P2BO and copolymer P2BO#, where # indicates the molar fraction of ethylbenzoate-p-oxymethyl in the side chain (# = 20, 40, 60, and 80)), n-butyl-p-hydroxybenzoate (P4BO), n-hexyl-p-hydroxybenzoate (P6BO), and n-octyl-p-hydroxybenzoate (P8BO). A stable and uniform vertical orientation of LC molecules was observed in LC cells fabricated with P2BO#, with 40 mol% or more ethylbenzoate-p-oxymethyl side groups. In addition, the LC molecules were oriented vertically in LC cells fabricated with homopolymers of P2BO, P4BO, P6BO, and P8BO. The water contact angle on the polymer films can be associated with the vertical orientation of the LC molecules in the LC cells fabricated with the polymer films. For example, vertical LC orientation was observed when the water contact angle of the polymer films was greater than ~86°. Good orientation stability was observed at 150 °C and with 20 J/cm2 of UV irradiation for LC cells fabricated with the P2BO film.

Development of a liquid crystal-based α-glucosidase assay to detect anti-diabetic drugs

The detection of α-glucosidase (AGLU) inhibitors is critical for the screening of anti-diabetic drugs. In this study, we first demonstrate a liquid crystal (LC)-based assay to detect anti-diabetic drugs. When the solution of non-ionic surfactant dodecyl α-D-glucopyranoside (DDG) is introduced onto the LCs, self-assembled monolayers are formed at the aqueous/LC interface, which induces the perpendicular orientation of LC molecules at the interface. Accordingly, the LCs show a dark image. However, when a mixture of AGLU and DDG is introduced onto the LCs, a bright image is observed due to enzymatic hydrolysis of DDG by AGLU, which prevents formation of the surfactant monolayers and results in the planar or tilted orientation of LC molecules at the interface. Using the LC-based AGLU assay, the detection of three AGLU inhibitors that are popular anti-diabetic drugs including acarbose, migliol, and voglibose is demonstrated. In addition, the linear detection ranges and the detection limits of these drugs are also determined. This method provides a simple and powerful strategy to rapidly and accurately detect AGLU inhibitors, which is very promising in the applications of screening anti-diabetic drugs.

Vertical Orientation of Liquid Crystal on Comb-Like 4-(trans-4-alkylcyclohexyl)phenoxymethyl-substituted Polystyrene Containing Liquid Crystal Precursor.

We synthesized a series of polystyrene derivatives modified with precursors of liquid crystal (LC) molecules, including 4-(trans-4-ethylcyclohexyl)phenol (homopolymer PECH and copolymer PECH#; # = 5, 10, 15, 20, 40, 60, and 80, where # indicates the molar fraction of 4-(trans-4-ethylcyclohexyl)phenoxymethyl in the side chain), 4-(trans-4-propylcyclohexyl)phenol (PPCH), 4-(trans-4-butylcyclohexyl)phenol (PBCH), and 4-(trans-4-amylcyclohexyl)phenol (PAmCH) via polymer modification reactions in order to investigate the orientation of LC molecules on polymer films exhibiting part of the LC molecular structure. A stable and uniform vertical orientation of LC molecules was observed in LC cells fabricated with PECH#, having 15 mol% or more of 4-(trans-4-ethylcyclohexyl)phenoxymethyl side groups. In addition, the vertical orientation of LC molecules was observed in LC cells fabricated with homopolymers of PECH, PPCH, PBCH, and PAmCH. The water contact angle on the polymer films could be associated with the vertical orientation of the LC molecules in the LC cells fabricated with polymer films. For example, a vertical LC orientation was observed when the water contact angle of the polymer films was higher than ~81°. Good orientation stability was observed at 200 °C and 15 mW/cm2 of UV irradiation for LC cells fabricated with PECH films.

High-Efficiency Responsive Smart Windows Fabricated by Carbon Nanotubes Modified by Liquid Crystalline Polymers

Smart windows can dynamically and adaptively adjust the light transmittance in non-energy or low-energy ways to maintain a comfortable ambient temperature, which are conducive to efficient use of energy. This work proposes a liquid crystal (LC) smart window with highly efficient near-infrared (NIR) response using carbon nanotubes grafted by biphenyl LC polymer brush (CNT-PDB) as the orientation layer. The resultant CNT-PDB polymer brush can provide the vertical orientation of LC molecules to maintain the initial transparency. At the same time, the smart window shows a rapid response to NIR light, which can quickly adjust the light transmittance to prevent sunlight from entering the room. Different from common doping systems, this method avoids the problem of poor compatibility between the LC host and photothermal conversion materials, which is beneficial for improving the durability of the device.

Vertical Orientation of Liquid Crystal on 4-n-Alkyloxyphenoxymethyl-Substituted Polystyrene Containing Liquid Crystal Precursor

We synthesized a series of polystyrene derivatives that were modified with precursors of liquid crystal (LC) molecules, such as 4-ethyloxyphenol (homopolymer PEOP and copolymer PEOP#; # = 20, 40, 60, and 80, where # indicates the molar fraction of 4-ethyloxyphenoxymethyl in the side chain), 4-n-butyloxyphenol (PBOP), 4-n-hexyloxyphenol (PHOP), and 4-n-octyloxyphenol (POOP), via polymer modification reaction to investigate the orientation of LC molecules on polymer films, exhibiting part of the LC molecular structure. LC molecules showed a stable and uniform vertical orientation in LC cells fabricated with polymers that have 4-ethyloxyphenoxymethyl in the range of 40–100 mol%. In addition, similar results were obtained in LC cells fabricated with homopolymers of PEOP, PBOP, PHOP, and POOP. The vertical orientation of LC molecules in LC cells fabricated with polymer films correlated to the surface energy of polymer films. For example, vertical LC orientation was observed when the total surface energies of the polymer films were lower than approximately 43.2 mJ/m2. Good alignment stabilities were observed at 150 °C and 20 J/cm2 of ultraviolet irradiation for LC cells fabricated with PEOP film.