Liquid Crystal Modulator Research Articles

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic, and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry.

High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of providing simultaneous information on the temperature dependence of the specific enthalpy h(T) and on the specific heat capacity c_{p}(T), has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (N_{F}) and a high-temperature paraelectric nematic phase (N). However, in DIO these two phases are separated by an intermediate phase (N_{x}). From the detailed data of h(T) and c_{p}(T), we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (x_{DIO}). The x_{DIO} dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the N_{x}-N transition of DIO was as low as L=0.0075±0.0005J/g and L=0.23±0.03J/g for the N_{F}-N_{x} transition, which is about twice the previously reported value of 0.115 J/g for the N_{F}-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing x_{DIO}. At all the N_{x}-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the N_{F} to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c_{p}(T) resulted in an effective critical exponent α=0.88±0.1 for DIO and this value decreased in the mixtures with decreasing x_{DIO} toward α=0.50±0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.

Hybrid-Type Transparent Organic Light Emitting Diode with High Contrast Using Switchable Windows

Transparent organic light emitting diode (OLED) display is one of the most promising devices among next-generation information displays because of beneficial characteristics, such as self-emissive and optically clear properties. Nevertheless, in conventional transparent OLED display devices, there are serious intrinsic problems in terms of the transmittance in the dark state because of empty windows in the cell, so the contrast ratio of the transparent OLED display would be deteriorated even though it can exhibit excellent bright state. In general, the transparent mode using the OLED device applies an empty area in each pixel because an emitting device could never reveal the background image, so the transparent OLED should contain the empty area in the pixel for transparent images. This may cause the optical degradation in the dark state. To solve this problem, we propose hybrid-type transparent OLED display modes that apply a liquid crystal (LC) to the transparent window part of the empty space. In this paper, we applied two dichroic LC modes- which use an electrically controlled birefringence (ECB) mode (Heilmeier type) for the polarized mode and a cholesteric LC mode (Guest-Host mode) for the non-polarized mode-to the empty area. In each hybrid mode, we have observed optical performance, including the transmittance in the dark/bright state, contrast ratio and response time as a function of cell parameters. As a result, we confirmed that the dark state and the contrast ratio could be improved by applying the proposed modes without serious decay of the transmittance in the bright state.

Defect transition of smectic liquid crystals confined in spherical cavities.

The formation and transformation of defects in confined liquid crystals are fascinating fundamental problems in soft matter. Here, we use molecular dynamics (MD) simulations to study ellipsoidal liquid crystals (LCs) confined in a spherical cavity, which significantly affects the orientation and translation of LC molecules near the surface. The liquid-crystal droplet can present the isotropic to smectic-B phase transition through the smectic-A phase, as the number density of the LC molecules increases. We further find the change of LC structure from bipolar to watermelon-striped during the phase transition from smectic-A (SmA) to smectic-B (SmB) phases. Our results reveal the transition from bipolar defects to the inhomogeneous structures with the coexistence of nematic and smectic phases in smectic liquid-crystal droplets. We also study the influence of the sphere size in the range of 10σ0 ≤ Rsphere ≤ 50σ0 on the structural inhomogeneities. It shows a weak dependence on the sphere size. We further focus on how the structures can be affected by the interaction strength εGB-LJ. Interestingly, we find the watermelon-striped structure can be changed into a configuration with four defects at the vertices of a tetrahedron upon increasing the interaction strength. The liquid crystals at a strong interaction strength of εGB-LJ = 10.0ε0 show the two-dimensional nematic phase at the surface. We further present an explanation for the origin of the striped-pattern formation. Our results highlight the potential for using confinement to control these defects and their associated nanostructural heterogeneity.

Sub-nL thin-film differential scanning calorimetry chip for rapid thermal analysis of liquid samples.

Differential scanning calorimetry (DSC) is a popular thermal analysis technique. The miniaturization of DSC on chip as thin-film DSC (tfDSC) has been pioneered for the analysis of ultrathin polymer films at temperature scan rates and sensitivities far superior to those attainable with DSC instruments. The adoption of tfDSC chips for the analysis of liquid samples is, however, confronted with various issues including sample evaporation due to the lack of sealed enclosures. Although the subsequent integration of enclosures has been demonstrated in various designs, rarely did those designs exceed the scan rates of DSC instruments mainly because of their bulky features and requirement for exterior heating. Here, we present a tfDSC chip featuring sub-nL thin-film enclosures integrated with resistance temperature detectors (RTDs) and heaters. The chip attains an unprecedented sensitivity of 11 V W-1 and a rapid time constant of 600 ms owing to its low-addenda design and residual heat conduction (∼6 μW K-1). We present results on the phase transition of common liquid crystals which we leverage to calibrate the RTDs and characterize the thermal lag with scan rates up to 900 °C min-1. We then present results on the heat denaturation of lysozyme at various pH values, concentrations, and scan rates. The chip can provide excess heat capacity peaks and enthalpy change steps without much alteration induced by the thermal lag at elevated scan rates up to 100 °C min-1, which is an order of magnitude faster than those of many chip counterparts.

Optical approaches for the simple and sensitive detection of aflatoxin B1 via a liquid crystal aptasensor

ABSTRACT We report a direct observation method for the detection of aflatoxin B1 (AFB1), utilising the orientational properties of liquid crystals (LCs). The imaging principle is based on the orientation transition of an LC film, which can be influenced by an aptamer and AFB1. AFB1 is captured by aptamers fixed on a substrate, which causes a conformational change of the aptamers that induces a significant morphological change of the substrate. The AFB1–aptamer interaction induces an orientation transition of LCs from a homeotropic to a planar state at the LC – substrate interface, leading to changes from ‘dark’ to ‘bright’ responses that can be easily seen through crossed polarisers. The developed sensor exhibits high specificity for AFB1 determination and has a low detection limit of 5 fM. The sensor allows AFB1 to be determined in complex samples such as corn and rice powder. The sensing method is highly selective and sensitive for other toxins and common molecules in food. Also, the sensor is stable and easily regenerated. The proposed LC-based aptasensor is a simple, rapid, and convenient platform for label-free detection of AFB1 in real samples, and the design strategy may also provide a novel way for detecting multiple analytes.

Liquid-crystalline behaviors of single-walled carbon nanotube aqueous dispersions with different nanotube aspect ratios and surfactants

We investigate the liquid crystal (LC) phase behaviors of the sodium cholate stabilized single-walled carbon nanotube (SWCNT) aqueous dispersions with different SWCNT aspect ratios. The isotropic-to-nematic phase transition occurs at a lower concentration for the SWCNT dispersion with a larger aspect ratio, which is expected by the Onsager theory. The well-aligned film is successfully fabricated from the dispersion with the higher SWCNT aspect ratio by the simple blade coating. We also prepare the SWCNT dispersions with different surfactants, sodium deoxycholate, and sodium taurodeoxycholate, changing the surface condition of SWCNTs, which may affect the LC transition concentrations.

Design of a three-phase amplitude macro-pixel full-color complex spatial light modulator with an in-cell geometric phase retardation layer.

Complex spatial light modulation, which can simultaneously control the amplitude and phase of light, is an essential optical technology for holographic display. We propose a twisted nematic liquid crystal (TNLC) mode with an in-cell type embedded geometric phase (GP) plate for full-color complex spatial light modulation. The proposed architecture provides an achromatic full-color complex light modulation capability in the far-field plane. The feasibility and working characteristics of the design are validated through numerical simulation.

Spontaneous Interface Healing by a Dynamic Liquid-Crystal Transition for High-Performance Perovskite Solar Cells.

Low-temperature solution processing of thin-film semiconductors is more cost-effective than traditional vacuum processing; however, it leads to more defects during fast bulk crystallization and residual tensile stress. Herein, a new strategy of dynamic liquid-crystal transition (DLCT) is developed to solve these problems in one step. The design principle is used to suggest that the DLCT molecule should firstly interact with the perovskite grains in the bulk and meanwhile go through a dynamic transition to spontaneously heal the interface. A thermotropic LC molecule (CBO6SS6OCB) is then designed to demonstrate the strategy. The LC interacting with perovskite colloid forms an intermediate adduct to retard the crystallization. The annealing processes stimulate the concentrated LC solid, causing it to flow to the electron transport layer to release the residual stress to attain improved electron extraction. Consequently, the device efficiency is increased to 24.38%, where its VOC of 1.184V is among the best for the formamidine-based perovskite solar cells. Furthermore, the ambient stability (93.0% of initial efficiency after 2000h of aging) and light stability (96.3% of initial efficiency after 500h of aging) are much improved. This work conceives a new engineering of additive phase transition for high-performance perovskite solar cells.

Design of automatic processing and testing furniture system based on SCM

In the automatic processing of products, most factories only put forward improvement schemes in processing efficiency, ignoring qualified rates after processing. Therefore, an automatic processing and testing furniture system based on SCM is proposed in this paper. Based on automatic processing, the workpiece detection device is added. In this paper, MCU as the core, using a vision sensor, stepper motor module, system alarm module, wireless transmission module and LIQUID crystal display module, the workpiece processing and detection. It is used to solve the subsequent detection of processed workpieces. In the whole machining process, the first complete processing and testing process, through a transmission belt, can connect processing and testing using the Keil and Proteus simulation. It is realized the size of the visual sensor, step motor positive &negative, buzzer alarm and LCD display, screening unqualified products for secondary processing, further improve the workpiece factory pass rate, has important application value.

Crystallization kinetics of charged PNIPAM microgels dispersions at low volume fractions

This work studies the kinetics of crystallization of charged microgels suspensions of Poly-N-Isopropylacrylamide (PNIPAM) at low ionic strength. The liquid-crystal transition is induced by suddenly decreasing the temperature of the microgel dispersion, and the crystallization process is monitored by measuring the temporal evolution of the static structure factor of the dispersion using light scattering. We find that the crystal growth rate, indicated by the temporal evolution of the crystallinity factor, strongly depends on the quenching temperature.

Ordering Transitions of Liquid Crystals Triggered by Metal Oxide-catalyzed Reactions of Sulfur Oxide Species

Liquid crystals (LCs), when supported on reactive surfaces, undergo changes in ordering that can propagate over distances of micrometers, thus providing a general and facile mechanism to amplify atomic-scale transformations on surfaces into the optical scale. While reactions on organic and metal substrates have been coupled to LC-ordering transitions, metal oxide substrates, which offer unique catalytic activities for reactions involving atmospherically important chemical species such as oxidized sulfur species, have not been explored. Here, we investigate this opportunity by designing LCs that contain 4'-cyanobiphenyl-4-carboxylic acid (CBCA) and respond to surface reactions triggered by parts-per-billion concentrations of SO2 gas on anatase (101) substrates. We used electronic structure calculations to predict that the carboxylic acid group of CBCA binds strongly to anatase (101) in a perpendicular orientation, a prediction that we validated in experiments in which CBCA (0.005 mol %) was doped into an LC (4'-n-pentyl-4-biphenylcarbonitrile). Both experiment and computational modeling further demonstrated that SO3-like species, produced by a surface-catalyzed reaction of SO2 with H2O on anatase (101), displace CBCA from the anatase surface, resulting in an orientational transition of the LC. Experiments also reveal the LC response to be highly selective to SO2 over other atmospheric chemical species (including H2O, NH3, H2S, and NO2), in agreement with our computational predictions for anatase (101) surfaces. Overall, we establish that the catalytic activities of metal oxide surfaces offer the basis of a new class of substrates that trigger LCs to undergo ordering transitions in response to chemical species of relevance to atmospheric chemistry.

Ferroelectric liquid-crystal modulator with large switching rotation angle for polarization-independent binary phase modulation

In this work a ferroelectric liquid crystal (FLC) modulator with a non-standard large switching rotation angle, close to 90∘, is fabricated and characterized. The modulator acts as a switchable wave-plate with an in-plane rotation of the principal axis under the action of a bipolar voltage. In the ideal situation of half-wave retardance, the device is shown to behave as a binary π phase modulator independently of the input state of polarization. We provide physico-chemical properties of the liquid crystalline mixture used to fabricate the FLC modulator with such large switching angle. The characterization method of the device optical properties is presented, which allows the localization of the two LC stable states, the unambiguous determination of the rotation angle, and the evaluation of the spectral retardance function. We demonstrate the polarization-independent π phase shift using an adapted Young's type interferometer for real-time measurements, where we further analyze the operational frequency limits of the device. This FLC operational mode can be exploited to produce binary-phase polarization-independent diffractive optical elements.

Nonequilibrium phase transitions of a nematic liquid crystal under ac field-driven electroconvection.

We report on phase transitions between isotropic and nematic liquid crystal phases in nonequilibrium systems (NESs). ac field-driven electroconvection (EC) provides an NES, which can be well controlled by the voltage V and frequency f; it arises electrohydrodynamically at a threshold voltage V_{c}. In continuous cooling and heating processes with various rates R, the critical temperature T_{c} was determined at a critical time t_{c} for phase transitions. Moreover, the morphological and dynamical features in the phase transitions were examined using an electro-optical image processing method. In comparison with an equilibrium system (V=0), two typical turbulent ECs (i.e., NESs), which are called dynamic scattering mode 1 (DSM1 for V>V_{c}) and DSM2 (for V≫V_{c}), were examined to understand the nonequilibrium phase transitions. In particular, our results show that in high voltage-induced turbulence (i.e., DSM2), T_{c} can be determined effectively without considering R; this provides a possibility for a material technology application in nonequilibrium-based circumstances.

Phase transitions and physical properties by a color texture analysis: Results for liquid crystals

A new approach to investigate subtle liquid crystal transitions and some of their light wavelength dependent physical properties is developed and demonstrated. In this method color texture changes obtained from a polarizing optical microscope are processed, and their color features are exploited. The significance of color features in image processing is discussed with reference to a color extension of the GLCM method which demonstrates smooth phase transitions occurring in two types of chiral liquid crystals (where ferroelectric smectic phases are observed). In the proposed method representation of the physical light wavelength distribution is obtained from suitable image conversion from RGB color space model to the Hue-Saturation-Lightness (HSL) model. This allows the identification of local color changes caused by external factors e.g., electric field or stresses. In this way the analysis of certain physical quantities is possible. The approach was applied to liquid crystal blue phases to show changes in the light wavelength under an electric field, which are related to the electrostriction coefficients. The result from image analysis are compared to results from a conventional spectrophotometer.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals.

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.

22‐2: Field Sequential Color See‐Through Panel Development

We have succeeded in developing a liquid crystal display mode with overwhelming display performance for Field Sequential Color (FSC) driven see‐through panel applications. The FSC‐LCD panel which we have developed a newly liquid crystal display mode 2D‐UV2 A II demonstrates a liquid crystal mode efficiency (transmittance) of over 97%, a color gamut of 93.5% NTSC ratio when driven by FSC, a contrast of 16,000:1, and excellent viewing angle performance.

P‐116: Student Poster: Transparent Displays Using Vertically Aligned Polyimide‐Free Liquid Crystal‐Polymer Composite

In this work, we propose a normally transparent polymer‐networked liquid crystal (PNLC) mode to use as a transparent display with energy saving, high contrast ratio, easy manufacturing process (Polyimide‐free). In this system, we adopt a homeotropic alignment material (HAM), which serves as both vertical alignment inducer and photopolymerizable reactive mesogen (RM). With no polarizers, the ground state shows clear transparency, and applying voltage induces an opaque scattering state successfully. By using double‐sided UV irradiation strategy, we can minimize hysteresis on voltage‐dependent transmittance. we believe the proposed concept will open a way to reach a novel transparent display with high transparency.

Finger front propagation in smectic-A Fréedericksz transition.

Systems with multistability are characterized by exhibiting complex nonlinear waves between equilibria. Experimentally, near the smectic-A to chiral nematic transition in a liquid crystal mixture cell with planar anchoring, we observe finger fronts emerge in the smectic-A phase when applying an electric field, a reorientation transition. Finger fronts propagate in the direction orthogonal to the anchoring. Colorimetry characterization allows us to describe the molecular reorientation transition and front dynamics. We reveal that the reorientation transition is of the first-order type and determine their critical points. The front speed is determined as a function of the applied voltage. Theoretically, based on a prototype model of liquid crystal transitions, we qualitatively describe the experimental observations. We have analytically determined the bifurcation diagram and the propagation speeds of finger fronts, finding a fair agreement with the experimental observations.

Non-equilibrium cooling and heating of an MBBA liquid crystal at the first-order phase transition in a long cell

The first-order phase transition of an MBBA nematic liquid crystal with benzene impurity has been studied by the critical opalescence method at a large cell thickness. Opalescence has been observed at two critical transition points. The spatial transverse parameters of the domains are determined from the Fraunhofer diffraction. The hypothesis about the longitudinal correlation parameter of domains is equal to the cell length has been proposed.

Bistability of Dielectrically Anisotropic Nematic Crystals and the Adaptation of Endothelial Collectives to Stress Fields.

Endothelial monolayers physiologically adapt to flow and flow‐induced wall shear stress, attaining ordered configurations in which elongation, orientation, and polarization are coherently organized over many cells. Here, with the flow direction unchanged, a peculiar bi‐stable (along the flow direction or perpendicular to it) cell alignment is observed, emerging as a function of the flow intensity alone, while cell polarization is purely instructed by flow directionality. Driven by the experimental findings, the parallelism between endothelia is delineated under a flow field and the transition of dual‐frequency nematic liquid crystals under an external oscillatory electric field. The resulting physical model reproduces the two stable configurations and the energy landscape of the corresponding system transitions. In addition, it reveals the existence of a disordered, metastable state emerging upon system perturbation. This intermediate state, experimentally demonstrated in endothelial monolayers, is shown to expose the cellular system to a weakening of cell‐to‐cell junctions to the detriment of the monolayer integrity. The flow‐adaptation of monolayers composed of healthy and senescent endothelia is successfully predicted by the model with adjustable nematic parameters. These results may help to understand the maladaptive response of in vivo endothelial tissues to disturbed hemodynamics and the progressive functional decay of senescent endothelia.