Liquid Crystal Materials Research Articles

Molecules structure and viscosity relationship of nematic liquid crystal and BPNN-QSAR model

Nematic liquid crystal materials designed for optics, microwave communication tuning, etc. need high response speed, which is related to the rotational viscosity and the birefringent index of the liquid crystal. In order to achieve a wide tuning range of phase modulation, the nematic liquid crystals often employ large π-electron conjugated systems and large polar groups to enhance the birefringence and dielectric anisotropy of the liquid crystal molecule, which, however, increases the viscosity of the liquid crystal material, deteriorating the response speed of the microwave device. Herein, we explore the viscosity of the nematic liquid crystal from the perspective of liquid crystal compound structure by testing the viscosity of our designed and synthesized forty-two different nematic liquid crystals by using a rotating rheometer at 25 ℃. To the best of our knowledge, the BPNN-QSAR quantitative structure-activity model between nematic liquid crystal molecular structure and viscosity is established for the first time. The correlation coefficient between the predicted value and the experimental value is <i>q</i><sup>2</sup> = 0.607 > 0.5, indicating that the model can be used to predict the viscosity performances of liquid crystal compounds. Besides, the molecular structure descriptors affecting the viscosity properties are explored. Based on the practical application and this model, seven liquid crystal molecules of two series with large birefringent index are designed and tested. The viscosity predicted by the BPNN model is smaller than that of the molecules of the same type and matches with the measured viscosity.

Machine learning methods for liquid crystal research: phases, textures, defects and physical properties.

Liquid crystal materials, with their unique properties and diverse applications, have long captured the attention of researchers and industries alike. From liquid crystal displays and electro-optical devices to advanced sensors and emerging technologies, the study and application of liquid crystals continue to be of paramount importance in the fields of materials science, chemistry and physics. With the ever-increasing complexity and diversity of liquid crystal materials, researchers face new challenges in understanding their behaviors, properties, and potential applications. On the other hand, machine learning, a rapidly evolving interdisciplinary field at the intersection of computer science and data analysis, has already become a powerful tool for unraveling implicit correlations and predicting new properties of a wide variety of physical and chemical systems and structures. Here we aim to consider how machine learning methods are suitable for solving fundamental problems in the field of liquid crystals and what are the advantages of this artificial intelligence based approach.

High transmittance and low degradation liquid crystal materials applied in the 325 nm wavelength range

High transmittance and low degradation liquid crystal materials applied in the 325 nm wavelength range

Tunability Dependence of All-Optical Liquid Crystal Delay Line on Laser Polarization and Incidence Angle

This work experimentally investigates the optical manipulation of trans-cis swaps and resultantly their optical/electromagnetic imprints as a driver for realizing the tunable delay lines in the microwave X band (8-12 GHz) regime utilizing the polarization and incident angle dependencies of azo-admixed liquid crystals (LCs). Measurement results confirm that the tuning efficacy of the azo-LCs-based delay line is highly dependent on the polarization of the laser beam concerning the rubbing and filling directions of LCs. When the laser light beam is polarized parallel to the rubbed direction as well as the LCs filling direction, the achievable differential phase shift is maximized. Full Text: PDF References T. Sakamoto, Optoelectronic Circuits for Control of Lightwaves and Microwaves (London, IntechOpen 2011). CrossRef J.F. Li, "All-optically controlled microwave analog phase shifter with insertion losses balancing", Eng. Lett. 28, 3 (2020). DirectLink J.P.F. Lagerwall, G. Scalia, "A new era for liquid crystal research: Applications of liquid crystals in soft matter nano-, bio- and microtechnology", Curr. Appl. Phys. 12, 6 (2012). CrossRef S. Kim, K. Seiji, "Photochemical On–Off Switching of One-Dimensional Photonic Crystals Consisting of Azo-Functionalized Liquid Crystal Polymer and Polyvinyl Alcohol", Crystals 9, 12 (2019). CrossRef J.F. Li, "Micro-LED Mass Transfer Technologies", ICEPT (Guangzhou, IEEE 2020). CrossRef U. Chodorow, J. Parka, O. Chojnowska, "Liquid Crystal Materials in THz Technologies", Phot. Lett. Poland 4, 3 (2012). CrossRef

Influence of the polymer network on the stability of the heliconical structure in the ferro- and antiferroelectric liquid crystalline phases

The use of polymer networks to stabilise chiral liquid crystalline materials (ChLC), which canfind applications in electro-optical and photonic devices, is becoming increasingly common. Stabilisingthe heliconical structure of a ChLC using a polymer network involves forming a spatial polymer matrix ina liquid crystal medium. The polymer network acts as a kind of scaffold for the liquid crystal molecules,resulting in the helix being stiffened in a particular desired state. This method of helix stabilisation hasbeen mainly used in chiral nematic liquid crystals so far. Effective stabilising the helix with a polymerin chiral smectic liquid crystals could eliminate the main drawback of these materials, i.e., the changein their performance, in electro-optical effects based on the deformation of the helix, with a changein temperature. The main parameter characterising the heliconical structure is its pitch, whose valuechanges rapidly with temperature change, in most smectic liquid crystals, which in turn translates intothe above-mentioned drawbacks. Stabilisation with the polymer should translate into temperaturestability of the helical pitch and the rest of the material parameters. This paper presents the results ofthe temperature dependence of the helical pitch before and after polymerisation of a liquid crystalmaterial with an antiferroelectric phase. A tetrafunctional monomer structurally compatible to thecomponents of the antiferroelectric mixture and, for comparison, a commercially available monomerwith a different structure were used in the study. The results relevant that stabilisation of the heliconicalstructure is easier after the use of a monomer more structurally compatible to the components ofparent liquid crystal material, while maintaining homogeneity of the heliconical structure orderingand a wide temperature range of the antiferroelectric phase occurrence.Keywords: liquid crystals, antiferroelectric phase, helical pitch, polymerization

Detection of glyphosate residues in agricultural products using liquid-crystal-based sensor exploiting competitive binding of glyphosate and Cu2+ at the aqueous/LC interface and capillary tube test strip

Glyphosate is a widely used herbicide that poses both health and environmental risks. In this study, we propose a liquid crystal (LC)-based assay for glyphosate detection that exploits the unique properties of LC materials. The nematic LC 4-cyano-4′-pentylbiphenyl (5CB) was employed as the sensing material and a self-assembled monolayer of octadecyltrichlorosilane (OTS) was used to modify glass substrates. The assay involved strong competition for coordination with Cu2+ for glyphosate, resulting in changes in the LC texture. By monitoring and analyzing the optical images of the LC film using polarizing microscopy, we detected and quantified the glyphosate concentrations. The proposed assay demonstrated high sensitivity and selectivity toward glyphosate in the detection range of 1–300 nM with a limit of detection of 0.26 nM. Moreover, the assay successfully applied to analyze glyphosate in spiked samples, including tap water, soil, and cabbage, and satisfactory recovery rates were achieved. Based on this detection principle, capillary tube test strips were developed for on-site applications. The detection thresholds of the test strips were controlled by varying the Cu2+ concentration. The developed LC-based assay is a rapid and reliable glyphosate detection method with potential applications in environmental monitoring and food safety.

Refractive Fresnel liquid crystal lenses driven by two voltages.

We propose and demonstrate a high-performance refractive Fresnel liquid crystal (LC) lens with a simple electrode design. The interconnected circular electrodes enable the creation of a parabolic voltage distribution within each Fresnel zone using only two driving voltages. By controlling these voltages within the linear response region of LC material, the desired parabolic phase profile can be achieved. We provide a detailed discussion on the electrode structure design methodology and operating principles of the lens. In our experiments, we constructed a four-zone Fresnel LC lens with a total aperture of 8 mm. The results show that the optical power of the lens can be continuously adjusted from -1.30 D to +1.33 D. Throughout the process of electrically tuning the optical power, the phase distribution within each Fresnel zone maintains a parabolic profile. These results demonstrate the high-performance of the proposed Fresnel LC lens.

The Birefringence and Extinction Coefficient of Ferroelectric Liquid Crystals in the Terahertz Range

In this paper, the refractive index and extinction coefficient of ferroelectric liquid crystals have been examined by the terahertz time-domain spectroscopy system. Two modes of ferroelectric liquid crystal materials, deformed helix ferroelectric liquid crystal (DHFLC), and electric suppressed helix ferroelectric liquid crystal (ESHFLC) are tested as experimental samples. Nematic liquid crystal (NLC) was also investigated for comparison. The birefringence of DHFLC 587 slowly increases with the growth of frequency, and it averages at 0.115. Its extinction coefficients gradually incline to their stable states at 0.06 for o-wave and 0.04 for e-wave. The birefringence of ESHFLC FD4004N remains between around 0.165 and 0.175, and both of its e-wave and o-wave extinction coefficients are under 0.1, ranging from 0.05 to 0.09. These results of FLC will facilitate the examination and improve the response performance of THz devices using fast liquid crystal materials.

Polarization Z-Scan Studies Revealing Plasmon Coupling Enhancement Due to Dimer Formation of Gold Nanoparticles in Nematic Liquid Crystals.

We investigate the plasmon coupling of gold nanoparticle (AuNP) dimers dispersed in a nematic liquid crystal matrix using the polarization z-scan technique. Our experimental setup includes the precise control of incident light polarization through polarization angles of 0°, 45°, and 90°. Two distinct cell orientations are examined: parallel and twisted nematic cells. In parallel-oriented cells, where liquid crystal molecules and AuNPs align with the rubbing direction, we observe a remarkable 2-3-fold increase in the nonlinear absorption coefficient when the polarization of the incident light is parallel to the rubbing direction. Additionally, a linear decrease in the third-order nonlinear absorption coefficient is noted as the polarization angle varies from 0° to 90°. In the case of twisted nematic cells, the NPs do not have any preferred orientation, and the enhancement remains consistent across all polarization angles. These findings conclusively establish that the observed enhancement in the nonlinear absorption coefficient is a direct consequence of plasmon coupling, shedding light on the intricate interplay between plasmonic nanostructures and liquid crystal matrices.

Smectic layers reorientation with stripes and heliconical structure in surface stabilised ferroelectric liquid crystal

ABSTRACT High-tilt angle ferroelectric liquid crystal material in surface stabilised geometry (SSFLC) is investigated for the smectic layer reorientation and its molecular dynamics by electro-optical and dielectric spectroscopy techniques. Smectic layer reorientation (5 o to 45 o ) is obtained by controlled cooling rate and applying electric field in the vicinity of transition temperature from chiral nematic ( N ∗ ) to chiral smectic C ( Sm C ∗ ) phase. After slow cooling down to room temperature, the stripe domain textures along the rubbing are observed with quasi-bistable states. Dielectric spectroscopy has revealed the unusual increment in the dielectric permittivity. A dielectric relaxation process at lower frequency than Goldstone mode with the increase in DC bias up to 2 V is observed in the sample cooled without any bias. This increment is found to be originated by the heliconical structure within the stripes which is seen in the optical textures recorded at the time of dielectric scan. However, this increment is not observed in the sample cooled under DC bias. Thus, there exist different phenomena in the high tilt angle SSFLC sample cooled in different conditions of DC bias; stripe domains, smectic layers reorientation, heliconical stricture and twisted structure within stripes which are significant to explore deeply in high-tilt angle ferroelectric liquid crystals.

Low-cost, high-speed multispectral imager via spatiotemporal modulation based on a color camera

Spectral imaging is a powerful tool in industrial processes, medical imaging, and fundamental scientific research. However, for the commonly used spatial/spectral-scanning spectral imager, the slow response time has posed a big challenge for its employment in dynamic scenes. In this paper, we propose a spatiotemporal modulation concept and build a simple, low-cost spectral imager by combining a liquid crystal (LC) cell with a commercial color camera. By the synergic effect of temporal modulation of the LC materials and spatial modulation of the Bayer filter in a color camera, high-quality multispectral imaging is successfully demonstrated with a high rate of 8 Hz, far beyond the counterparts. Experimental results show that even with three tuning states of the LC material, optical signals with a 10-nm band can be resolved in the range between 410 and 700 nm by this method, overcoming the tradeoff between spectral resolution and time resolution. As a proof of demonstration, we present its potential usage for metamerism recognition, showing superiority over traditional color cameras with more spectral details. Considering its low cost, miniaturization and monolithic-integration ability on color sensors, this simple approach may bring the spectral imaging technology closer to the consumer market and even to ubiquitous smartphones for health care, food inspection and other applications.

Extended centroid position method in liquid-crystal-based interrogation system for electric-field-modulated Fabry-Perot sensing

A polarized low-coherence interferometer (PLCI) based on a liquid crystal (LC) wedge is designed, and an associated demodulation method encompassing the tunability feature is proposed for tunable, standalone optical sensing. The application of an electric field to the LC material effectively decreases the birefringence value and the related dispersion relation, which in turn enhances the resolution of detection. The effect of the electric field on the demodulation of the cavity length is addressed by the successive determinations of the centroid positions of the PLCI interferograms. Through a comprehensive study of numerical simulations, the effectiveness of the proposed approach is explored relative to the conventional envelope detection methodology. In order to verify this method, an experiment with a Fabry-Perot-based fiber optic displacement sensor is carried out using a 5CB LC wedge-based PLCI setup in the presence of an electric field. The measurement accuracy of the cavity length is found to be 0.74% of full scale, rendering it more precise and robust than the conventional envelope detection method.

Enhanced thermo-electro-optical and dielectric properties of carbon nanoparticle-doped polymer dispersed liquid crystal based switchable windows

The effect of temperature as well as voltage on switchable smart windows was studied. These smart windows are composites of a polymer and a liquid crystal (LC) and collectively known as polymer dispersed liquid crystal (PDLC) films. In order to enhance the efficiency of the smart windows/PDLC films, the LC material was doped with carbon nano particles (CNP) in various concentrations ranging from 0 to 0.1%. To examine the efficacy and the temperature dependent performance of the CNP doped PDLC (CPDLC) film, firstly morphological and thermo-morphological studies were performed using polarizing optical microscopy (POM). POM in combination with temperature changes allows to record texture changes and the nematic-isotropic transition of the composite films for applied voltage. The complete electro-optical (EO) study comprising of voltage and temperature dependent transmittance, threshold voltage (Vth), saturation voltage (Vsat), contrast ratio (CR) and response time measurement was performed through an assembled EO set-up. To infer intermolecular cooperative processes between polymer and the dipoles of the LC, the relaxation mechanism of the composite films for various temperatures was studied by employing Debye and Cole-Cole models in dielectric spectroscopy. Parameters like dielectric strength and dissipation factor were determined to allow optimization of the CPDLC films.

Liquid crystal-embedded fiber optic fabry perot temperature sensor based on Vernier effect

An optical temperature sensing method with ultra-high sensitivity and a wide measurement range is proposed. The performance enhancements come from the high thermos-optical coefficient of liquid crystal (LC) material, and the Vernier effect produced by LC birefringence characteristics. This paper proposes and demonstrates a method to enhance the measurement range of the LC temperature sensor. A miniature LC temperature sensor was made by using an optical fiber Fabry-Perot interferometer (FPI). To compensate for and attenuation of LC and improve the contrast of the FPI, silver films are coated on one end of the optical fiber. Due to the high birefringence of the LC in the interferometer, a Vernier spectrum can be observed in the reflection. The temperature measurement range of 20–32 °C and the temperature sensitivity of the Vernier peaks of a sensor sample is about 42.18 nm/°C, which is much higher than the sensitivity of the cascaded FBG in structure. The limited measurement range is due to the free spectral range (FSR) limitation. In this context, the FBG peak can be used as a temperature reference while switching the adjacent Vernier peaks. Hence, this LC temperature sensor exhibits high sensitivity as well as a wide measuring range, which would broaden the application scope of the LC temperature sensors.

Recent progress in liquid crystal devices and materials of TFT-LCDs

Thin-film-transistor-liquid crystal displays (TFT-LCDs) still dominate most of the display markets due to a big leap in image quality while keeping low cost and superior reliability, and its evolution is continuing to compete with emerging emissive displays. In this paper, recent progress in liquid crystal technologies mainly focusing on fringe-field switching (FFS) and polymer-stabilized vertical alignment (PS-VA) modes and materials for improving electro-optic performances is reviewed.

Method for increasing the utilization rate of birefringence in liquid crystal lenses.

We propose a method to increase the utilization rate of birefringence in liquid crystal (LC) lenses. The method involves designing electrode patterns based on the phase response curve of the LC material, enabling the lenses to operate across a wide range of driving voltages while generating the desired phase profile. The underlying principle of this technique is discussed in detail. Electrode patterns have been successfully designed for positive and negative cylindrical LC lenses. The experimental results demonstrate that the designed lenses generate a parabolic phase profile even when the driving voltage exceeds the linear response region. The utilization rate of LC birefringence for the positive lens has increased from 41.3% to 69.7%, indicating a 68.8% increase from the original. For the negative lens, the utilization rate has risen from 41.8% to 68.7%, representing a 64.4% increase from the original.

The effect of the number of cyanostilbene building blocks on the self-assembly and photophysical property of pyrene-based liquid crystals

The synthesis, self-assembly and physical properties of two series of di/tetra-substituted pyrene-based liquid crystals were reported. These compounds were synthesized by using Suzuki coupling and Knoevenagel reactions as key steps. The effects of the number of cyanostilbene arms connecting to central pyrene on the mesomorphic and emission behaviors with structurally related compounds were investigated. The lower pyrene-based homologue containing two cyanostilbene arms could self-assemble into rectangular columnar phase in which two molecules aggregated into a disc, whereas the higher pyrene-based homologues could self-assemble into hexagonal columnar phase in which four molecules aggregated into a disc. The pyrene-based liquid crystals containing four cyanostilbene arms could also self-assemble into hexagonal columnar phase in which two molecules aggregated into a disc. The self-assembly process and the models for arrangements within these columnar phases were proposed and discussed in detail. These compounds also exhibited distinct slovatochromism and strong fluorescence in both solution and aggregated states. Therefore, controlling molecular structures of functional liquid crystal materials was an efficient way to tune the self-assemblies and emission properties.

Transition Temperature in Thermochromic Liquid Crystals using Second-Order Features Extraction

<p>Liquid crystals are a type of substance that has solid and liquid properties. One of the types of it is cholesteric. Cholesteric liquid crystals have a characteristic which is called pitch. Pitch is very sensitive to changes in temperature. The pitch will reflect different colors depending on the wavelength at a particular temperature. Thermochromic Liquid Crystals (TLC) is the cholesteric liquid crystals material sold commercially. At the transitional temperature, the texture of TLC changes, so the reflected color will also change. Second-order feature extraction was chosen to determine the change in texture with the transition temperature. The TLC layer was made by thickness of 100 µm. This layer was heated and observed using a polarizing microscope with an angle between the polarizer and analyzer of 90<sup>o</sup>. The obtained result are cross patterns emerged at anisotropic transition temperature and higher temperature on TLC will lead to an isotropic phase.<em></em></p>

Formation of polymerizable layer on a photo-alignment layer of liquid crystal cell and its image sticking characteristics

The effect for polymerizable layer on a vertical-type photo-alignment layer was investigated from the point of image sticking characteristics. Fabrication process of the polymerizable layer is followings; photo-alignment treatment is firstly carried out by exposing polarized ultraviolet (UV) from the oblique direction. After construction of the liquid crystal (LC) cell, the LC material including monomers is injected, and unpolarized UV is exposed for formation of the polymerizable layer. For avoiding pretilt angle shift during the exposure, fast rate constant of polymerization is necessary, and mixed monomers 4,4’-dimethacryloyl-oxy-biphenyl (4,4’-DMABiph) and 4,4’-dimethacryloyl-oxy-benzilketal (4,4’-DMABzk) are appropriate for obtaining small level of image sticking parameters. As the image sticking parameters, residual direct current voltage (V rDC) and difference in pretilt angle (Δtilt) before and after application of alternate current voltage were evaluated, and the results of V rDC and Δtilt indicated that 6% 4,4’-DMABzk in the mixed monomers 4,4’-DMABiph and 4,4’-DMABzk would be useful for obtaining the small level of image sticking.

THz dual-core liquid photonic crystal fiber with high negative dispersion

A novel photonic crystal fiber (PCF) design is proposed and analyzed with highly negative dispersion for THz applications. The reported PCF has TOPAS background material due to its low material loss in THz regime. Further, dual porous cores are constructed and selectively infiltrated with liquid crystal (LC) material to control the dispersion characteristics of the reported PCF. The basic operation of the suggested dual core LCPCF (DC-LCPCF) depends on the optical coupling between the supported modes of the two porous core regions in the THz regime to achieve high negative dispersion for the two fundamental polarizations: transverse electric (TE) and transverse magnetic (TM). The coupling can be switched between the TE and TM modes by applying an external electric field on the LC material via two metallic electrodes. The full vectorial finite element method (FVFEM) is utilized to study the dispersion characteristics of the DC-LCPCF structure. The obtained results reveal that the TE and TM modes have large negative chromatic dispersions of − 44.57 ps/THz/cm and − 30.59 ps/THz/cm at frequencies of 0.386 THz and 0.4027 THz, respectively. So, it will be a solution for further innovation of fiber devices in the THz regime.