Liquid Crystal Mixture Research Articles

Effects of graphene on the smectic-A to chiral smectic-C* phase transition

ABSTRACT The smectic-A to chiral smectic-C* (SmA–SmC*) phase transition in the mixture of graphene nanoparticles and ferroelectric liquid crystal is studied. Combination of Flory–Huggins free energy of isotropic mixing and Landau theory is applied to explain the effects of graphene nanoparticles on the SmA–SmC* phase transition. The temperature and concentration of graphene nanoparticles dependence of tilt angle, spontaneous polarization and dielectric susceptibility in the smectic-C* phase of the SmA–SmC* phase transition are discussed. A qualitative agreement between theoretical results and experimental results is found.

Eutectic Mixtures of Metallomesogens as Potential Materials for Electro-Optical Applications

In this work, we present a new approach for utilization of eutectic mixtures of metallomesogens (MOM) as potential materials in electro optical devices. We studied the phase diagrams of blends of synthesized rod-like Palladium (Pd) alkyl/alkoxy-azobenzene complexes with their parent ligands and commercial nematic liquid crystal mixtures. The results indicate complete mesgenic miscibility and eutectic points of MOM-ligand mixtures. In addition, the MOMligand eutectic mixture also exhibited a complete nematic miscibility with three commercial liquid crystals TN10427, TNO623 and E43 that qualify them model materials in the guest-host electro-optical devices.

Photothermal Dual Passively Driven Liquid Crystal Smart Window.

Photochromic or thermochromic liquid crystal (LC) smart windows have attracted wide attention due to their spontaneous transmittance modulation under different environments. There remains a challenge for the LC smart windows that can be modulated with light and temperature simultaneously owing to the difficulty in selecting photothermal molecules. Herein, we selected a photothermal molecule, isobutyl-substituted diimmonium borate (IDI), which shows excellent characteristics of a photothermal material used in smart windows, such as transparency in the visible light range with a slight brown color, good compatibility with the LC system, and excellent photothermal effect compared with common photothermal materials. Thus, a photothermal dual-driven smart window is developed by doping IDI into chiral LC mixtures, which can efficiently modulate the transmittance at different temperatures (or light intensities) by varying the phase state from the homeotropically oriented smectic phase (transparent) to the focal conic cholesteric phase (opaque). The transmittance is high (70%) when the ambient temperature is low and the light intensity is weak, allowing more sunlight to enter the room. The transmittance is low (20%) when the ambient temperature is high and the light intensity is strong, which prevents sunlight from entering the room. The proposed smart window will have a promising application in terms of energy saving and personalized privacy protection.

34‐4: Invited Student Paper: UBplus/UB‐FFS‐Premium Performance for Liquid‐Crystal TV and IT Displays

Premium displays that offer outstanding characteristics are becoming more important in liquid crystal display market. Globally, researchers are looking for ways to improve contrast, transmittance, viewing angle dependency, resolution and switching speed while keep the high reliability level of liquid crystal devices. UB (Ultra Bright)‐technology (UBplus and UBFFS) can increase panel transmittance by using negative dielectric anisotropy LC mixtures compared with traditional FFS Mode, supporting higher resolution devices. In this paper we demonstrate the advantage of UB‐LC technology for TV as well as IT applications and summarize the recent developments of such liquid crystal mixtures in terms of contrast, switching speed and reliability.

34‐5: Invited Paper: C‐PS‐VA — Innovative LC Materials Enabling Super‐High Transmittance for High‐Resolution Displays

In recent years, and for the next generation of TVs and monitors, increasing resolution is a key trend. One challenge is keeping the transmittance while pixel density increases. Chiral Polymer Stabilized Vertical Alignment (C‐PS‐VA) technology uses LC mixtures with higher birefringence and a chiral dopant and increases transmittance by more than 10% compared to conventional Polymer Stabilized Vertical Alignment (PS‐VA). This paper summarizes the recent developments of liquid crystal mixtures for C‐PS‐VA.

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.

Hierarchical Organization of Single Crystal Polymorphs of Azobenzene Chromophore in Anisotropic Media Subjected to Local Thermal Gradients upon Cold Spraying.

The present article entails the emergence of diverse crystal polymorphs following thermal quenching into various coexistence regions of binary azobenzene chromophore (ACh)/diacrylate (DA) solution and of azobenzene/nematic liquid crystal (E7) mixture. Development of various crystal topologies encompassing rhomboidal and hexagonal shapes can be witnessed in a manner dependent on thermal quenched depths into the crystal + liquid coexistence region of ACh/DA system. Upon spraying with compressed carbon dioxide (CO2 ) fluid, the local temperature gradient is generated resulting in spherulitic morphology showing discrete lamellae undergoing twisting locally in some regions and branched dendrites or seaweeds in another. When ACh/E7 blend is sprayed using compressed CO2 fluid, hierarchical organization of various discrete faceted single crystals including needle, rectangular, rhombus, and truncated hexagonal crystals radiating from the spherulite core can be discerned in a brighter region (off cross-polarization) polarized optical microscopy (POM) and nematic disclination in a darker cross-polarized region. Of particular interest is that the observed faceted single-crystal polymorphs in ACh/E7 may be contrasted to the lamellar twisting and branching observed in the ACh/DA system and plausible mechanisms of polymer spherulitic growth are discussed.

A Tunable Multi-Mode Dual-Band Terahertz Polarization Converter

The polarization conversion of electromagnetic waves plays a crucial role in practical application. Here, a tunable multi-mode dual-band high-efficiency and reflection-type polarization converter based on liquid crystal (LC) in the terahertz region is proposed. It is composed of periodic unit structures, each unit of which is constituted by the designed structure, LC mixture and fully reflected gold mirror. It numerically reveals that in the operating bandwidth 1.03–1.53 and 1.27–1.86 THz corresponding to the refractive index of LC mixtureñoandñe, the converter can convert linear-polarized and circular-polarized waves to its corresponding cross-polarizations, whose polarization conversion ratio is larger than 90%. Meanwhile, the proposed converter can realize the conversion from linear-polarized wave to circular-polarized one in dual-band, 1.10–1.25 and 1.92–2.2 THz or 0.93–1.04 and 1.57–1.88 THz corresponding toñoandñewith axis ratio lower than 3 dB. The insensibility of incident angle is analyzed as well. In addition, the physical mechanism of the polarization conversion is disclosed. The ultra-thin and compactness characteristic of this designed polarization converter are very attractive for a variety of applications in EM measurement, sensing, terahertz technology and 6G communication.

A second-order time accurate and fully-decoupled numerical scheme of the Darcy-Newtonian-Nematic model for two-phase complex fluids confined in the Hele-Shaw cell

We consider the numerical approximation of the binary immiscible mixture of nematic liquid crystals and viscous Newtonian fluids confined in a Hele-Shaw cell, where the free interface motion is simulated by using the phase-field approach via the energy variational method. The governing system is highly complicated nonlinear and coupled, consisting of the Darcy equations for the flow field, the Cahn-Hilliard equations for the free moving interface, and the constitutive equation for the nematic liquid crystal. The numerical scheme developed in this paper is the first “ideal” scheme, namely, it not only has the following characteristics: linearity, second-order time accuracy, unconditional energy stability, and decoupling structure, but also at each time step, only needs to solve a few elliptic equations with constant coefficients. We strictly prove that the scheme satisfies the unconditional energy stability and give a detailed implementation process. Various numerical experiments are further carried out to prove the effectiveness of the scheme, in which the influence of the initial orientations and anchoring elastic energy of the liquid crystal on the Saffman-Taylor fingering instability are studied.

Mesomorphic, electro-optic and dielectric behavior of self-assembled nanocomposite materials: Nematic mixture doped with carbon coated cobalt nanoparticles

In the present study, different concentrations of carbon coated cobalt nanoparticles (0.25 wt% & 0.5 wt%) into eutectic nematic liquid crystal mixture (E7) have been dispersed and studied their mesomorphic, electro-optic and dielectric properties in planar as well as homeotropic aligned cells. Optical transmission method has been employed in planar aligned cell to study the electro-optic responses while dielectric spectroscopy in the frequency span 20 Hz-10 MHz has been performed in planar and homeotropic cells. Nearly 10°C increase in nematic-isotropic transition temperature is noticed at higher doping concentration. Dispersion of cobalt nanoparticles induces changes in dielectric permittivity, dielectric loss and relaxation frequency. Suppression of free ion concentration (in case of 0.5 wt% Co) suggests large dielectric anisotropy values. Entropy of the system is more or less affected with the nano inclusions but the free energy of the doped samples has been decreased. An improvement in threshold voltage, birefringence and order parameter by ∼ 10%, 15% & 5% respectively have been observed at low doping concentration. However, contrast ratio is reduced with the doping of cobalt nanoparticles. The present study shows that the doped systems have improved response than pure E7 and find potential applications in phase shifters as well as display devices.

Liquid Crystal Templated Chiral Plasmonic Films with Dynamic Tunability and Moldability

AbstractThin films sustaining plasmonic circular dichroism (PCD) have acquired high scientific relevance and a great potential for applications. While most efforts in PCD thin film structures focus on lithographically fabricated static metasurfaces, the bottom‐up fabrication of active chiral plasmonic films constitutes an alternative approach. Herein, the preparation of PCD thin films by melting and freezing a mixture of liquid crystal (LC), a chiral dopant, and gold nanoparticles (Au NPs), serving as helical matrix, symmetry breaking inducer, and plasmonic component, respectively is reported. UV–vis and circular dichroism spectroscopies, as well as theoretical modeling are used to disclose the interactions among thin film components, toward maximizing the PCD dissymmetry factor (g‐factor). Variation of substrate temperature affords reversible off/on switching of the chiroptical response. The soft nature of LC matrix enables patterning of the films via a thermal nanoimprinting method, using a polydimethylsiloxane mold for transfer‐printing onto a flexible substrate, leading to stretchable PCD films. The PCD wavelengths can be readily tuned by varying the geometry of the Au NPs. This work provides an efficient technique to produce PCD thin films with active plasmonic properties and mechanical tunability.

UV-Vis-NIR Light-deformable Shape-memory Polyurethane Doped with Liquid-crystal Mixture and GO towards Biomimetic Applications

UV-Vis-NIR Light-deformable Shape-memory Polyurethane Doped with Liquid-crystal Mixture and GO towards Biomimetic Applications

Mm-Wave Metasurface Unit Cells Achieving Millisecond Response Through Polymer Network Liquid Crystals

The slow response time of planar Liquid Crystal (LC)-based phase-shift metasurface and Reconfigurable Intelligent Surfaces (RIS) cells is addressed in this paper by introducing a polymer network LC (PNLC) mixture suitable at mm-wave bands. Since the conventional effective isotropic model used in optical cells for describing the PNLC is not suitable in RF, an effective anisotropic and uniaxial model for such mixture is provided and experimentally validated at 100 GHz for the first time. In order to compare the temporal performance and tunability of the PNLC, transmissive and reflective cells, containing conventional LC and PNLC, have been manufactured and measured at optical and mm-wave frequencies. The temporal responses of PNLC are also compared for both RF and optical cells, obtaining relevant differences between their improvement factors, which are also discussed. Specifically, a 50 fold response time improvement is attained in cells designed to work at 100 GHz, although at the expense of a 3X tunability reduction. The model, which is robust to varying angle of incidence and cell dimensions, has been experimentally validated by designing and manufacturing a PNLC reflectarray cell of a different geometry. The cell shows reconfigurability times of 210ms, representing a significant improvement with respect to state-of-the-art response time in mm-wave cells, which are in the order of several seconds.

The effect of the structure of a helical nanofilament of the B4 phase of bent-core liquid crystals on the nano-phase separation mixed with a rod-like cholesteric liquid crystal mixture.

We studied the structure of a helical nano-filament of the B4 phase in mixtures of a cholesteric liquid crystal mixture and a bent-core molecule using a resonant soft X-ray scattering (RSoXS) technique. In this system, nanophase separation occurs and it was already found that an unexpected new functional chiral smectic structure in the rod-like molecule rich region is constructed by the strong interaction between bent-core and rod-like molecules. In this paper, we focused on the structure of the helical filament in the bent-core liquid crystalline molecule rich region in this mixing system, and it was found that the pitch of the helical filament decreases and the coherence of the helical structure increases.

Optical, dielectric and elastic studies of high birefringent ambient temperature nematic liquid crystal mixtures having similar molecular cores

We report various physical properties such as optical, dielectric, and elastic studies of two high birefringent climate temperature nematic liquid crystal mixtures having similar molecular cores such as quarter phenyl and phenyethnyltolane cores. In both the mixtures, one compound is common, and, in another compound, the lateral group ethyl group is replaced with a methyl group. These mixtures exhibit high birefringence and high elastic constants in comparison to regular liquid crystals. Both the compounds exhibit high birefringence, positive dielectric anisotropy whereas the parallel component shows anomalous temperature dependence. Our results show that with a variation of lateral group, correlation among the cores varies and that reflects in optical and dielectric anisotropies. These studies will be useful in synthesis and exploring appropriate liquid crystals for various display and smart window applications.

Electro-optical characteristics of cholesteric liquid crystal films based on CuS nanoparticles

The CuS nanoparticles (NPs) coated by poly(vinylpyrrolidone) (PVP) layer were doped into bistable liquid crystal (LC) film to provide an electric, thermal or optical phase-shift controllable films. The CuS/PVP NPs could form a compatible NIR absorbing film, which could generate heat to switch LC film from planar texture to focal conic texture. The appropriate content of CuS/PVP NPs was provided to achieve fast thermal response, satisfactory dispersion and clear display brightness. The novel LC film containing 0.8 wt.% CuS/PVP NPs could save almost 51% time at thermal elimination, comparing to LC mixture without NIR absorbent. The hybrid organic-inorganic functional bistable LC material reported here extend and open up new application of LC writing tablets.

On the Miscibility of Nematic Liquid Crystals with Ionic Liquids and Joint Reaction for High Helical Twisting Power Product(s).

Mixtures of nematic liquid crystals (LCs) with chiral ionic liquids (CILs) may find application as active materials for electrically driven broadband mirrors. Five nematic liquid crystal hosts were mixed with twenty three ionic liquids, including chiral ones, and studied in terms of their miscibility within the nematic phase. Phase diagrams of the mixtures with CILs which exhibited twisted nematic phase were determined. Miscibility, at levels between 2 and 5 wt%, was found in six mixtures with cyanobiphenyl-based liquid crystal host—E7. On the other hand, the highest changes in the isotropization temperature was found in the mixtures with isothiocyanate-based liquid crystal host—1825. Occurrence of chemical reactions was found. A novel chiral binaphtyl-based organic salt [N11116][BNDP] was synthesized and, in reaction to the 1825 host, resulted in high helical twisting power product(s). Selectivity of the reaction with the isothiocyanate-based liquid crystal was found.

Probing the impact of bismuth-titanate based nanocomposite on the dielectric and electro-optical features of a nematic liquid crystal material

We report here the concentration and temperature dependent optical, electro-optical and dielectric studies on bismuth titanate (Bi2Ti2O7/Bi4Ti3O12) nanocomposite (BT2/BT4 NC, ∼42 nm) doped nematic liquid crystal (NLC, 5CB) mixtures using optical polarising microscope and dielectric spectroscopic techniques. The optical textures confirm the uniform dispersion and miscibility of NCs in 5CB for all concentrations (i.e. 0.1, 0.25, 0.5, 1 and 2 wt%) and mixtures appear to be almost agglomeration free. The dielectric studies demonstrate the maximum changes in the dielectric parameters (dielectric permittivity, dielectric loss, loss factor and dielectric anisotropy) of 5CB sample for the 0.1 wt% mixture. Interestingly, the dielectric anisotropy of 5CB in 0.1 wt% mixture is increased by ∼ 11%. However, the dielectric memory effect (bias voltage ON-OFF) is observed maximum in the case of 1 wt% mixture. Similarly, the optical memory examined by bias voltage dependent (ON-OFF) optical textures is also significantly enhanced for 1 wt% mixture. The significant changes observed in dielectric properties of 5CB sample could be due to plausible interaction among NLC molecules and ionic impurities with BT2/BT4 NCs. Moreover, the enhanced volatile memory in BT2/BT4 NC-NLC mixture could be attributed to the dipole–dipole coupling between individual permanent dipole moment of anisotropic NLC molecules with the dipole moments generated by the ionic impurities agglomerated on the surface of high dielectric BT2/BT4 NCs. We strongly believe that such NCs-NLC mixtures would be certainly useful in the advancement of wearable devices (such as smart plenoptic cameras, watches etc.) and smart switchable windows.

Enhancement of the SPR Effect in an Optical Fiber Device Utilizing a Thin Ag Layer and a 3092A Liquid Crystal Mixture.

This paper is a continuation of previous work and shows the enhancement of the surface plasmon resonance effect in a tapered optical fiber device. The study investigated liquid crystal cells containing a tapered optical fiber covered with a silver nanolayer, surrounded by a low refractive index liquid crystal in terms of the properties of light propagation in the taper structure. Silver films with a thickness of d = 10 nm were deposited on the tapered waist area. Measurements were performed at room temperature; liquid crystal steering voltage U from 0 to 200 V, with and without any amplitude modulation with a frequency of f = 5 Hz, and the wavelength λ ranged from 550 to 1200 nm. A significant influence of the initial arrangement of liquid crystals molecules on light propagation was observed. Three types of liquid crystal cells—orthogonal, parallel, and twist—were considered. During the measurements, resonant peaks were obtained—the position of which can also be controlled by the type of liquid crystal cells and the steering voltage. Based on the obtained results, the best parameters, such as highest peak’s width reduction, and the highest SNR value were received for twisted cells. In addition, the present work was compared with the previous work and showed the possibility of improving properties of the manufactured probes, and consequently, the surface plasmon resonance effect. In the presented paper, the novelty is mainly focused on the used materials as well as suitable changes in applied technological parameters. In contrast to gold, silver is characterized by different optic and dielectric properties, e.g., refractive index, extension coefficient, and permittivity, which results in changes in the light propagation and the SPR wavelengths.

Optical and Thermal studies on CNT doped Liquid Crystal Mixtures

In the present study, effect of CNT on optical and thermal properties of mixed thermotropic liquid crystal mixture was studied by various techniques to understand their physical behavior. Some new mesophases have investigated by optical methods viz. Polarizing Optical Microscopy (POM) and Fabry–Perot Scattering Studies (FPSS). It has been observed that these new phase transition occur along with the known phase transition temperatures. These new phase transitions corresponding to new mesophases were also confirmed by thermal study using Differential Thermal Analysis (DTA). The CNT doped thermotropic liquid mixtures can be used in various display application such as LCD monitor with color variation to enhance performance. Copyright © 2018 VBRI Press.