Liquid Crystal Alignment Research Articles

Controlled Diels–Alder “Click” Strategy to Access Mechanically Aligned Main‐Chain Liquid Crystal Networks

AbstractAligned liquid crystal polymers are materials of interest for electronic, optic, biological and soft robotic applications. The manufacturing and processing of these materials have been widely explored with mechanical alignment establishing itself as a preferred method due to its ease of use and widespread applicability. However, the fundamental chemistry behind the required two‐step polymerization for mechanical alignment has limitations in both fabrication and substrate compatibility. In this work we introduce a new protection‐deprotection approach utilizing a two‐stage Diels–Alder cyclopentadiene‐maleimide step‐growth polymerization to enable mild yet efficient, fast, controlled, reproducible and user‐friendly polymerizations, broadening the scope of liquid crystal systems. Thorough characterization of the films by DSC, DMA, POM and WAXD show the successful synthesis of a uniaxially aligned liquid crystal network with thermomechanical actuation abilities.

Electrically tunable dual-layer twisted nematic liquid crystal THz phase shifters with intermediate composite polymer thin film

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

Robotic Pick‐and‐Place Operations in Multifunctional Liquid Crystal Elastomers

Pick‐and‐place operations for transporting objects precisely to a target position are a prominent function of (soft‐) robotic systems. Therefore, there is great interest in industry to improve the characteristic gripping, holding, and releasing methods involved in pick‐and‐place operations. Within living organisms such as octopi, nature demonstrates that multiple types of conjointly working actuators are required for flexible pick‐and‐place operations. Herein, a multifunctional soft robotic arm is developed, capable of transporting an object within 3D space. The soft robotic arm consists of two structural actuators (rotating base and lifting unit) and a suction cup‐based gripper. The structural actuator acts as both the load bearing and actuating components of the robotic system. Yet, the gripper is the crucial innovation within the robotic arm. A cephalopod‐limb‐inspired gripper functioning through the reversible flat‐to‐conical deformation of azimuthally aligned liquid crystal elastomer (LCE) films is proposed. The pressure‐generating actuation mechanism of the gripper means that no external device is needed to operate the gripping function. Akin to natural systems, the in‐tandem operation of the actuators in the soft robotic arm allows for multifactored tasks. Yet, the design achieves this through the use of a single material, which is not innate in natural archetypes.

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

Wide field of view chiral imaging with a liquid crystal planar lens enabled by digitalized nanogratings.

To compensate for the inability for polarization imaging by conventional methods, metasurface optics with compactness and multi-function emerge as an approach to provide images with different linear and circular polarizations. Here, we propose a liquid crystal (LC) geometric phase-based chiral imaging lens (CIL) that simultaneously forms images of objects with opposite helicity. The CIL (Diameter 2.3 cm) was optimized by a spatial multiplexing algorithm and realized using the digital holography technique, where the LC domains were regulated by pixelated nanogratings with varied orientation. We investigated the potential of the patterning technique toward high order LC alignment by balancing the periodicity and depth of the nanogratings. The CIL exhibited a wide field of view of ±20°, which is attributed to the self- assembling effects of LC molecules. The compactness, lightness, and ability to produce chiral images of the LC CIL even at large angles have significant potential for practical polarization imaging.

4D Printing of Seed Capsule‐Inspired Hygro‐Responsive Structures via Liquid Crystal Templating‐Assisted Vat Photopolymerization

AbstractThe creatures in nature exhibit dynamic responses to environmental stimuli through their hierarchical architectures. Benefiting from gradient porous structures, Delosperma nakurense opens its protective valves of the seed capsules when hydrated with liquid water, increasing the likelihood that seeds are dispersed under conditions favorable to germination. Here, a versatile 4D printing technology, namely liquid crystal templating‐assisted vat photopolymerization (LCT‐VPP), which can fabricate bioinspired porous structures with hygro‐responsive capabilities by utilizing photopolymerization induced phase separation (PIPS) and liquid crystals (LCs) electro‐alignment is reported. PIPS within the LCs/nanofiller composites leads to the formation of submicrometer gradient porous structures after extracting nonreactive LCs. The electric field enables the programmable alignment of LCs, which in turn elongates the porous structures and aligns nanofillers. In addition, the programmable arranged nanofillers by the templated LCs enhance the degree of deformation and thus the resulting composites exhibit high shape control accuracy, fast dynamic response, and high reliability. This study opens a perspective for designing bioinspired smart materials with the special spatial distribution of porous structures. The results reported here can give rise to various potential applications in soft robots, smart anticounterfeiting devices, flexible sensors, and ultrafiltration membrane.

Inducement of an Anisotropic One‐dimensional Surface of Solution‐derived Indium Gallium Oxide Film via Imprint Lithography for Liquid Crystal Alignment

AbstractThis study uses imprint lithography to produce an anisotropic unidirectional indium gallium oxide (IGO) film for the uniform liquid crystal (LC) alignment layer. The curing temperature was controlled to 110, 180, and 250 °C during the process, and the 250 °C cured film presented a distinct one‐dimensional pattern transferred from the polydimethylsiloxane (PDMS) mold during scanning electron microscopy (SEM). X‐ray photoelectron spectroscopy (XPS) confirmed an active thermal oxidation effect at a high curing temperature, contributing to the formation of a unidirectional structure. This unidirectional IGO structure functioned as a guide for LCs and induced a uniform alignment state, verified by polarized optical microscopy (POM) and a pre‐tilt angle analysis. The IGO film also revealed improved thermal endurance to the LC alignment state and optical transmittance compared with the conventional rubbed polyimide (PI) layer. Given this performance, the one‐dimensional pattern‐imprinted IGO film can be a promising LC alignment layer for an advanced LC system.

Electrically Driven Torsional Distortions in Twisted Nematic Volumes

The purpose of this review is to describe the physical mechanism responsible for the appearances of both traveling and non-traveling distortions in a twisted microsized nematic volume under the effect of a large electric field. Both experimental and theoretical works devoted to the excitation of structured periodic domains in initially homogeneously aligned liquid crystal systems under the effects of strong crossed electric and magnetic fields were analyzed. Electrically driven distortions in the microfluidic nematic capillary in the presence of a temperature gradient in it, based on the number of numerical results, were analyzed. We also focus on the description and explanation of the novel mechanism of excitation of the kink- and π-like distortion waves of the director field n^ in a cylindrical nematic micro-volume under the effect of voltage U and temperature gradient ∇T, set up between the cooler inner and hotter outer cylinders. Electrically driven torsional distortions in twisted nematic micro-volumes in the form of the kink-like running front based on the classical Ericksen–Leslie approach were considered.

Physicochemically reformed tin oxide film fabricated for self-alignment of liquid crystals

This study compressed the conventional two-step process of brush coating and orientation for liquid crystal (LC) alignment layer manufacturing into a one-step process. Surface modification and LC alignment were performed by brush coating with a tin oxide solution prepared via the solgel process at different curing temperatures. The effect of the curing temperature on atomic bonds was analyzed through x-ray photoelectron spectroscopy, the physical morphology of the film surface was characterized by scanning electron microscopy and atomic force microscopy, and the LC alignment of the brush-coated tin oxide film was verified via polarized optical microscopy and pretilt angle measurements. The shear stress generated during the brush coating process formed a partial groove structure; the resulting anisotropy aligned the LC molecules homogeneously. The brush-coated film showed excellent LC orientation properties, confirming the potential of the proposed and efficient fabrication process as a new orientation technology.

Anisotropic Surface Formation Based on Brush-Coated Nickel-Doped Yttrium Oxide Film for Enhanced Electro-Optical Characteristics in Liquid Crystal Systems

This paper introduces anisotropic nickel yttrium oxide (NYO) film formed by the brush coating technique. X-ray photoelectron spectroscopy confirmed well-formed NYO film after the curing process, and the morphology of the surface was investigated using atomic force microscopy. The shear stress driven from brush hair movements caused the nano/micro-grooved anisotropic surface structure of NYO. This anisotropic surface induced uniform liquid crystal (LC) alignment on the surface, which was confirmed by pre-tilt angle analysis and polarized optical microscopy. The contact angle measurements revealed an increase in hydrophilicity at higher temperature curing, which contributed to homogenous LC alignment. The NYO film achieved good optical transmittance and thermal stability as an LC alignment layer. In addition, the film demonstrated good electro-optical properties, stable switching, and significantly enhanced operating voltage performance in a twisted-nematic LC system. Therefore, we expect that this brush coating method can be applied to various inorganic materials to achieve an advanced LC alignment layer.

Self-Folding Liquid Crystal Network Filaments Patterned with Vertically Aligned Mesogens.

Fibrous soft actuators with high molecular anisotropy are of interest for shape morphing from 1D to 2D and 3D in response to external stimuli with high actuation efficiency. Nevertheless, few have fabricated fibrous actuators with controlled molecular orientations and stiffness. Here, we fabricate filaments from liquid crystal networks (LCNs) with segmental crosslinking density and gradient porosity from a mixture of di-acrylate mesogenic monomers and small-molecule nematic or smectic liquid crystals (LCs) filled in a capillary. During photopolymerization, phase separation between the small-molecule LCs and LCN occurs, making one side of the filament considerably denser than the other side. To direct its folding mode (bending or twisting), we control the alignment of LC molecules within the capillary, either along or perpendicular to the filament long axis. We show that the direction of UV exposure can determine the direction of phase separation, which in turn direct the deformation of the filament after removal of the small-molecule LCs. We find that the vertical alignment of LCs within the filament is essential to efficiently direct bending deformation. By photopatterning the filament with segmental crosslinking density, we can induce a reversible folding/unfolding into 2D and 3D geometries triggered by deswelling/swelling in an organic solvent. Moreover, by taking advantage of the large elastic modulus of LCNs and large contrast of the modulus before and after swelling, we show that the self-folded LCP filament could act as a strong gripper.

Monomolecular vertical alignment layer with room temperature processibility for flexible liquid crystal displays

In this article, we disclose a vertical alignment material using alkylphosphonic acid for liquid crystals that only requires simple room temperature treatments to secure good vertical alignment for liquid crystals. Alkylphosphonic acid forms a monomolecular layer on the substrate providing strong vertical alignment. The proposed alignment layer shows decent electro-optical properties, such as high voltage holding ratio, low residual direct current voltage, low image sticking parameter, etc. Later, we combine the alkylphosphonic acid with photo-rewritable azo-dye to fabricate a hybrid alignment of nematic cells and study its stability and anchoring energy. The material shows great compatibility with flexible substrates. Thus, the proposed alignment layer of alkylphosphonic acid can potentially find application as the alignment layer for flexible liquid crystal displays.

Self-Aligned Liquid Crystals on Anisotropic Nano/Microstructured Lanthanum Yttrium Strontium Oxide Layer.

We propose an efficient alignment method for liquid crystals (LCs). A brush-coating method handles film deposition and LC alignment treatment simultaneously herein, meaning a reduction in the conventional alignment layer treatment process steps. A lanthanum yttrium strontium oxide (LaYSrO) film prepared by the sol–gel process was used for the alignment layer. Topographical details of the brush-coated LaYSrO films (compared with spin-coated films) were investigated by atomic force microscopy. Spin-coated LaYSrO meant that the film formation alone without orientation treatment represented an isotropic surface. On the other hand, the 270 °C-cured brush-coated LaYSrO showed nano/microstructure with directionality. It indicates that brush-hair sweeping induced shearing stress on the sol state of the LaYSrO, which results in surface anisotropy for LC alignment. The uniform LC alignment state was confirmed by polarized optical microscopy and pretilt analysis. The brush-coated LaYSrO shows fine optical transparency compared to plain and indium-tin-oxide coated glasses, and thermal stability up to 150 °C for LC alignment. Competitive electro-optical performances of the brush-coated LaYSrO were verified in a twisted-nematic LC system compared to those of the conventionally used polyimide layer. Consequently, we expect that the brush-coating process can be an innovative technology for LC alignment.

A Simulation of Liquid Crystal Grating with Field‐Induced Twist Deformation for Beam‐Steering Applications

Beam‐steering device using liquid crystal (LC) grating with field‐induced twist deformation, which has symmetric LC director distribution, is demonstrated. The proposed LC cell can convert the polarizations without multidomain LC alignments, and the polarization of the output light can also be changed with different input polarization. Therefore, the polarization of the input light is controlled, and the polarization of the output light is analyzed by additional layers that each consists of a switchable quarter‐wave plate (QWP) and a linear polarizer. The transmitted light of the proposed LC cell can be diffracted in three different directions by changing the bias voltage. The calculated efficiency and angle of steering are 46% and 8°, respectively. Potential applications of such a grating for diffraction optics, optical interconnect, and beam‐steering devices are foreseeable.

Nanostructured hafnium-doped strontium oxide film for homeotropic/homogeneous convertible liquid crystal alignment depending on the curing temperature

ABSTRACT A hafnium strontium oxide (HfSrO) liquid crystal (LC) alignment film was efficiently created through brush coating, and its ability to change the LC alignment direction was confirmed. A brush was applied to HfSrO sol coated on an indium-tin oxide substrate, after which the coating was solidified at various curing temperatures. It was confirmed that a directional micro/nanostructure was formed above 280°C due to the shear stresses caused by the movement of the brush hairs. Surface chemical changes were analyzed by using X-ray photoelectron spectroscopy and contact angle measurements. As the curing temperature increased, the prevalence of oxygen bonds increased and the contact angle decreased, thereby increasing the surface energy. The anisotropic boundary of the microgrooves and the van der Waals forces due to an increase in surface energy changed the alignment direction of LC molecules from vertical to horizontal, as verified through polarized optical microscopy and pretilt angle measurements. Thus, the efficiency of the brush-coating method, which dramatically simplifies the LC alignment film process, was confirmed. The homeotropic/homogeneous LC alignment property of the HfSrO film produced through brush coating depending on the curing temperature provides an innovative approach for LC alignment.

Polymer beads dispersed liquid crystal devices (PBLCD) achieved by predesigned radially constructed polymeric particles

AbstractIn this article, we demonstrate a novel design of liquid crystal (LC) displays via predesigned radially constructed polydopamine (PDA) particles. The synthesized PDA particles were modified with dimethyloctadecyl‐3‐(trimethoxysilyl)propyl ammonium chloride (DMOAP) showing radial constructions. Both PDA particles and surface‐modified PDA particles were used to disturb the order of the LC arrangement, showing an opaque appearance. Based on the results, the radially constructed PDA particles efficiently enhanced the performance of cholesteric liquid crystal (CLC) devices. Adding 0.1 wt% DMOAP‐modified PDA particles into the CLC device shows a higher contrast of 82.2%, a lower threshold voltage of 2.4 V and a fast response time of 7.7 ms. Without adding polymer beads, no any display contrast was observed. The results suggest that the synthesized surface‐modified PDA particles disturb the alignment of LCs dispersed in LC cells, leading to a great improvement in the contrast of the CLC devices. In addition, a prepared traditional polymer stabilized CLC (PSCLC) cell shows only 2.8% contrast. The best performance, with a high contrast of 93.8%, a low threshold voltage of 10.4 V and a fast response time of 10.2 ms, was achieved by adding 0.1 wt% modified PDA particles into the CLC device.

Structural and Optical Characteristics of Flexible Optically Rewritable Electronic Paper

A comprehensive theory of light-reflective characteristics and experimental technique of liquid crystal layer thickness control for flexible optically rewritable electronic paper is presented. Cylindrical pillars were used to control the gap between flexible substrates. The introduced prototype of optically rewritable electronic paper has shown very promising performance. In this regard, we report theoretical results of structural photosensitive alignment of nematic liquid crystals on flexible substrate. The focus of theoretical study is on understanding the self-assembled complex structure, governed by the interplay between surface anchoring and liquid crystal elasticity. Mueller matrix spectroscopic ellipsometry was used to study light-reflecting characteristics and polarization properties of the twisted nematic film.

Temperature-controlled liquid crystals alignment on solution-derived bismuth lanthanum tin oxide films prepared through brush process

ABSTRACT In this study, a bismuth – lanthanum–tin oxide film was fabricated via brush coating to obtain a process-efficient liquid crystal (LC) alignment layer. The brush-coating solution was prepared through a sol – gel process, applied onto an indium tin oxide glass, and cured at various temperatures. The shearing stress caused by the brush coating process formed a micro-/nanogroove structure, as confirmed via atomic force microscopy and line profile analyses. The chemical composition of the surface was analysed through X-ray photoelectron spectroscopy, and the optical transmittance of the film was measured. As regards the LC alignment, polarised optical microscopy observations, thermal stability tests, and pretilt angle measurements were performed, confirming a stable alignment; the grooves generated during the shearing and hardening phases were most clearly formed after curing at 180°C, which led to stable LC orientation.

Long-Ranged Order and Flow Alignment in Sheared p-atic Liquid Crystals.

We formulate a hydrodynamic theory of p-atic liquid crystals, namely, two-dimensional anisotropic fluids endowed with generic p-fold rotational symmetry. Our approach, based on an order parameter tensor that directly embodies the discrete rotational symmetry of p-atic phases, allows us to unveil several unknown aspects of flowing p-atics, that previous theories, characterized by O(2) rotational symmetry, could not account for. This includes the onset of long-ranged orientational order in the presence of a simple shear flow of arbitrary shear rate, as opposed to the standard quasi-long-ranged order of two-dimensional liquid crystals, and the possibility of flow alignment at large shear rates.

Physicochemically Constructed Unidirectional Aluminum Bismuth Gallium Zinc Oxide Film for Enhanced Nematic Liquid Crystal System Using a Brush‐Coating Method

We propose a convenient brush coating method for liquid crystal (LC) alignment. This method enables film deposition and an alignment layer treatment process simultaneously. Aluminum bismuth gallium zinc oxide (AlBiGaZnO) is used as the alignment layer. After the curing process, a unidirectional AlBiGaZnO film is formed; its surface morphology and chemical composition were verified using scanning electron microscopy and X-ray photoelectron spectroscopy. This oriented structure of the surface was produced by shear-stress which originated from brush movement. That structure induced a surface anisotropic characteristic and resulted in a uniform LC alignment. The uniform and homogeneous LC alignment state on the film was confirmed using polarized optical microscopy and pre-tilt angle analysis. The brush coated AlBiGaZnO film exhibited excellent thermal budget for advanced LC system. The film exhibited enhanced electro-optical performance with a low operating voltage. These results demonstrate the potential of LC alignment technology via the brush coating method.