Nematic Liquid Crystal Film Research Articles

Self‐Assembled Biconvex Microlens Array Using Chiral Ferroelectric Nematic Liquid Crystals

AbstractRecently, it is shown (Popov et al, Sci. Rep, 2017, 7, 1603) that chiral nematic liquid crystal films adopt biconvex lens shapes underwater, which may explain the formation of insect eyes, but restrict their practical application. Here it is demonstrated that chiral ferroelectric nematic liquid crystals, where the ferroelectric polarization aligns parallel to the air interface, can spontaneously form biconvex lens arrays in air when suspended in submillimeter‐size grids. Using Digital Holographic Microscopy, it is shown that the lens has a paraboloid shape and the curvature radius at the center decreases with increasing chiral dopant concentration, i.e., with decreasing helical pitch. Simultaneous measurements of the imaging properties of the lenses show the focal length depends on the pitch, thus offering tunability. The physical mechanism of formation of the self‐assembled ferroelectric nematic microlenses is also discussed.

Dynamic flexoelectric instabilities in nematic liquid crystals.

Electrohydrodynamic phenomena in liquid crystals constitute an old but still very active research area. The reason is that these phenomena play the key role in various applications of liquid crystals and due to the general interest of the physical community in out-of-equilibrium systems. Nematic liquid crystals (NLCs) are ideally representative for such investigations. Our article aims to study theoretically the linear NLCs dynamics. We include into consideration orientation elastic energy, hydrodynamic motion, external alternating electric field, electric conductivity, and flexoelectric polarization. We analyze the linear stability of the NLC film, determining dynamics of perturbations with respect to the homogeneous initial state of the NLC. For the purpose we compute eigenvalues of the evolution matrix for a period of the external alternating electric field. These eigenvalues determine the amplification factors for the modes during the period. The instability occurs when the principal eigenvalue of the evolution matrix becomes unity by its absolute value. The condition determines the threshold (critical field) for the instability of the uniform state. It turns out that one might expect various types of the instability, only partially known and investigated in the literature. Particularly, we find that the flexoelectric instability may lead to two-dimensionally space-modulated patterns exhibiting time oscillations. This type of the structures was somehow overlooked in the previous works. We formulate conditions needed for the scenario to be realized. We hope that the results of our work will open the door to a broad range of further studies. Of especial importance would be a comprehensive understanding of the role of various material parameters and nonlinear effects which is a key step for the rational design of NLCs exhibiting the predicted in this publication multidimensional oscillating in time patterns.

Point and line defects in checkerboard patterned hybrid nematic films: A computer simulation investigation.

We consider a nematic liquid crystal film confined to a flat cell with homeotropic and planar patterned hybrid anchoring and show, using Monte Carlo simulations, the possibility of the system to stabilize line and point defects. The planar anchoring surface is patterned with a chessboardlike grid of squares with alternating random or parallel homogeneous planar anchoring. The simulations show only line defects when the individual domains are small enough, but also point defects when the domain size is significantly larger than the sample thickness. In the latter case, defect lines are not observed in domains with random surface anchoring, although lines and points are connected by a thick line which separates two regions with different director tilts. Increasing the anchoring strength, the defect lines appear a few layers above the surface, with the two ends just above the randomly oriented domains.

Dynamic Transition from Branched Flow of Light to Beam Steering in Disordered Nematic Liquid Crystal

AbstractOrientational ordered soft matter possessing diverse microstructures has become a focal point of scientific research and technological exploration, thanks to its advancements in serving as an indispensable optical platform enabling propagation of light with multidimensional and manipulatable states. Herein, a facile way is developed to manipulate the in‐plane light beam transition dynamics by harnessing a time‐variable nematic liquid crystal (NLC) film through the electrical‐field‐induced topological defects. The results show that the dynamic change of optical branched flow is associated with the growth in the correlation length of optical potential and the reduction in the density of topological defects. The optical branching can continuously transform to deterministic and tunable beam steering at the low‐defect‐density regime through annihilation kinetics of defects. The explored soft matter system provides an excellent planar platform for the fundamental physics of light interacting with topological defects and may offer new perspectives for novel optics elements toward the applications of soft matter photonics.

Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management.

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9°C are demonstrated at noon with solar intensity of 717Wm-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1°C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Ultra-sensitive optical fiber ethanol gas sensor based on Vernier effect in cascaded Mach-Zehnder interferometer and Sagnac interferometer

Optical fiber-based sensors, which show the merits of high sensitivity, real-time, and rapid detection, have important research value in the detection of volatile organic compounds (VOCs). In this paper, a novel ethanol gas sensor was experimentally investigated based on the cascaded optical fiber-based Sagnac interferometer and Mach-Zehnder interferometer. The Sagnac interferometer was constructed with an anisotropic nematic liquid crystal (NLC) film which acted as the reactor with ethanol gas. A tapered single-mode fiber (SMF) was used to build the Mach-Zehnder interferometer (MZI). The two interferometers were parallelly and tandemly cascaded respectively to achieve the magnification of measurement sensitivity. Experimental results showed that the sensitivities were 8.561 pm/ppm with a magnification factor of 3.76 for the parallel cascaded structure and 7.924 pm/ppm with a magnification factor of 3.48 for the tandemly cascaded structure. The designed optical fiber-based ethanol gas sensor, showing the advantages of compact structure, high sensitivity, small size, and low cost, could be a competitive candidate for the detection of ethanol gas leakage.

Electrical tuning of branched flow of light

Branched flows occur ubiquitously in various wave systems, when the propagating waves encounter weak correlated scattering potentials. Here we report the experimental realization of electrical tuning of the branched flow of light using a nematic liquid crystal (NLC) system. We create the physical realization of the weakly correlated disordered potentials of light via the inhomogeneous orientations of the NLC. We demonstrate that the branched flow of light can be switched on and off as well as tuned continuously through the electro-optical properties of NLC film. We further show that the branched flow can be manipulated by the polarization of the incident light due to the optical anisotropy of the NLC film. The nature of the branched flow of light is revealed via the unconventional intensity statistics and the rapid fidelity decay along the light propagation. Our study unveils an excellent platform for the tuning of the branched flow of light which creates a testbed for fundamental physics and offers a new way for steering light.

A Hydrodynamical Model of Nematic Liquid Crystal Films with a General State of Orientational Order

A Hydrodynamical Model of Nematic Liquid Crystal Films with a General State of Orientational Order

Control of Coherent Light through Microperiodic Director Modulation in Nematic Films under Low-Voltage DC Electric Field.

This work addresses the achievement of efficient control of laser light transmission through stationary microperiodic parallel stripe textures formed in films of nematic liquid crystals (NLCs) in planar-oriented cells upon a direct-current (DC) electric field. By varying the field intensity and, thereby, the field-induced periodic modulation of the nematic director and hence the complex transmittance function corresponding to the longitudinal domain texture induced in NLC films with initial planar alignment, the intensity of a linearly polarized laser beam passed through the films can be well controlled. In 25 µm-thick films of room-temperature NLCs pentylcyanobiphenyl (5CB), this results in a low-voltage (~4 V) sharp and deep V-shaped behavior of their electro-optically controlled transmittance. Such a reversible electro-optical effect is interesting for active control of laser beam intensity and other applications. The relevant physical mechanism is analyzed and explained.

A Compact Optical Fiber Sagnac Loop Inserting With A Nematic Liquid Crystal Film for Highly Sensitive Temperature Sensing

Since the high temperature response characteristics of nematic liquid crystal (NLC) molecules, a compact optical fiber Sagnac loop inserted a segment of NLC film for temperature sensing was proposed and experimentally demonstrated. The NLC film was formed by filling into the micron-scale gap between the two single-mode fibers inserted into a ferrule matching sleeve. After two single-mode patch cables were rotated, the NLC film molecules were oriented due to exogenous effects. Experimental results showed that the temperature sensing performance of the designed sensor was improved by decreasing the thickness of NLC film in Sagnac loop. The thickness of NLC film in the ferrule matching sleeve was measured accurately by Fabry-Perot interference. The wide temperature measurement range of 2.2 ∼ 50.2°C and a high temperature sensitivity of -9.34 nm/°C were achieved at the NLC film thickness of 31.30 μm. Owing to the compact structure, large measurement range, and high sensitivity, the designed sensor shows a broad application prospect in high spatial resolution and high sensitivity temperature sensing.

Extensional flow of a free film of nematic liquid crystal with moderate elasticity

The human tear film is a multilayer structure in which the dynamics are often strongly affected by a floating lipid layer. That layer has liquid crystalline characteristics and plays important roles in the health of the tear film. Previous models have treated the lipid layer as a Newtonian fluid in extensional flow. Motivated to develop a more realistic treatment, we present a model for the extensional flow of thin sheets of nematic liquid crystal. The rod-like molecules of these substances impart an elastic contribution to the rheology. We rescale a weakly elastic model due to Cummings et al. [“Extensional flow of nematic liquid crystal with an applied electric field,” Eur. J. Appl. Math. 25, 397–423 (2014).] to describe a lipid layer of moderate elasticity. The resulting system of two nonlinear partial differential equations for sheet thickness and axial velocity is fourth order in space, but still represents a significant reduction of the full system. We analyze solutions arising from several different boundary conditions, motivated by the underlying application, with particular focus on dynamics and underlying mechanisms under stretching. We solve the system numerically, via collocation with either finite difference or Chebyshev spectral discretization in space, together with implicit time stepping. At early times, depending on the initial film shape, pressure either aids or opposes extensional flow, which changes the free surface dynamics of the sheet and can lead to patterns reminiscent of those observed in tear films. We contrast this finding with the cases of weak elasticity and Newtonian flow, where the sheet retains the same qualitative shape throughout time.

High sensitivity ethanol vapor sensor based on a nematic liquid crystal film embedded optical fiber Sagnac interferometer

Interferometric optical fiber sensors have become the preferred choice for ethanol vapor detection because of their high sensitivity and figure of merit. However, the response time of interferometric fiber optic ethanol vapor sensors is very long. To address this issue, we experimentally investigated an ethanol vapor sensor based on a nematic liquid crystal (NLC) film embedded optical fiber Sagnac interferometer. The high birefringent NLC film, which worked as the sensing media of ethanol vapor for its absorption of ethanol vapor, was penetrated into the Sagnac ring to generate the spectral interference. The results showed that the measurement sensitivity of ethanol gas concentration reached 2.22 pm ppm−1. The detection range was about 1210 ppm–10 000 ppm. Most importantly, the response time of the proposed sensor is only 15 s. The designed sensor, which showed the advantages of fast response, high sensitivity, and stability, could be a competitive candidate for ethanol vapor sensing.

Active nematic fluid films

Coupling surface deformations with active stresses in two-dimensional nematic liquid crystal films leads to a rich area of investigation, particularly in biological fluid mechanics across multiple scales from tissue mechanics to cell membrane mechanics. In Al-Izzi & Morris (J. Fluid Mech., vol. 957, 2023, A4), the authors derive the complete set of governing equations for such systems. Their results provide an extended theoretical framework with which active nematic fluid films with in-plane flow and out-of-plane deformation can be analysed. To illustrate the potential applications of this framework, a few specific biologically inspired examples are discussed.

Ultrasensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers with a nematic liquid crystal film

We demonstrated an ultrasensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers (CSIs). One of the Sagnac interferometers (SIs) consisted of a panda polarization-maintaining fiber (PMF) as the reference arm, while the other one contained two single-mode patch cables in a ferrule matching sleeve as the sensing arm, where a nematic liquid crystal (NLC) film was infiltrated into the micron-scale gap of the sleeve. In the sensing SI, the orientation of NLC film was induced by just rotating the two single-mode patch cables without any orientation agent, electric, or magnetic devices. Vernier effect was acquired by adjusting the thickness of NLC film. Experimental results revealed that the temperature sensitivity of the CSIs was up to 43.13 nm/°C, which was enhanced about 5 times compared to the single NLC SI. The designed optical fiber temperature sensor possessed the merits of compact structure, high stability, ultrahigh sensitivity, low hysteresis effect and high resolution, showing a promising application prospect in some scientific filed requiring accurate temperature measurement, such as biomolecules, medicine and other filed.

Tunable Circularly Polarized Luminescence of Excited‐State‐Proton‐Transfer‐Based Chiral Guanidine

Circularly polarized luminescence (CPL) of chiral materials responsive to external stimuli has drawn widespread attention due to their important applications in smart photonic materials. Here, a CPL‐active chiral emitter derived from guanidine‐substituted 1,8‐naphthalimide (R/S‐1) is reported, which could emit tunable circularly polarized light through regulating the situation of protonation/deprotonation in aqueous, as well as the solvent polarity. Tunable fluorescence emission from the protonated compound can be observed, which is contributed to excited‐state proton transfer of guanidine moiety, as well as the intramolecular charge transfer of naphthalimide group. Subsequently, color‐tunable CPL is obtained, enabling a single‐molecule‐white‐color circularly polarized light emission. Additionally, by introducing R/S‐1 into chiral nematic liquid crystal film, CPL emission is adjusted by exposing to acid/alkaline environment, which can be directly distinguished by naked eyes due to the extremely large dissymmetry factor (|glum| = 1.1). This work opens new venues for photonic applications of CPL active materials.

Self-diffusion method for broadband reflection in polymer-stabilized cholesteric liquid crystal films

ABSTRACT Chiral nematic liquid crystal (N*-LCs) films with wide pitch gradients covering the near infrared (NIR) region are facilely obtained by a simple controlling diffusion method. The polymer network can not only fix the orientation of N*-LCs, but also serves as a diffusion channel for substances, and has excellent compatibility. First, in the design of the pre-polymerisation (Incomplete polymerisation) layer, mainly by controlling the content of polymerisable monomer and UV-absorbing dyes in N*-LCs, a polymer network with density gradient distribution and suitable for material diffusion is formed, and the substrate of the oil repellent layer on the side of the low-density polymer network is easier to be peeled off. Then, nematic liquid crystals are injected between the pre-polymerisation layer and another oriented glass substrate. Due to the large concentration difference of chiral compounds and UV-327 between the two liquid crystals, the gradient distribution of spacing is formed by diffusion. Because of the gradient distribution of the UV-327, the gradient distribution of the pitch is further increased after polymerisation again. Finally, the effect of the UV-327 on the polymer network of the pre-polymerised layer is observed using SEM. Thus, a new method for reflective broadband in the near-infrared region is proposed.

Light-induced umbilical defects due to temperature gradients in nematic liquid crystal with a free surface

We report the light beam action on the nematic liquid crystal film with a free surface. It was found that a weak light absorption by the liquid crystal substrate dramatically changes the orienting properties of the light beam; in particular, a thermal gradient field induces an umbilical defect formation.

Curvature Potential Unveiled Topological Defect Attractors

We consider the theoretical and positional assembling of topological defects (TDs) in effectively two-dimensional nematic liquid crystal films. We use a phenomenological Helfrich–Landau–de Gennes-type mesoscopic model in which geometric shapes and nematic orientational order are expressed in terms of a curvature tensor field and a nematic tensor order parameter field. Extrinsic, intrinsic, and total curvature potentials are introduced using the parallel transport concept. These potentials reveal curvature seeded TD attractors. To test ground configurations, we used axially symmetric nematic films exhibiting spherical topology.

Dielectrowetting of a thin nematic liquid crystal layer.

We consider a mathematical model that describes the flow of a nematic liquid crystal (NLC) film placed on a flat substrate, across which a spatially varying electric potential is applied. Due to their polar nature, NLC molecules interact with the (nonuniform) electric field generated, leading to instability of a flat film. Implementation of the long wave scaling leads to a partial differential equation that predicts the subsequent time evolution of the thin film. This equation is coupled to a boundary value problem that describes the interaction between the local molecular orientation of the NLC (the director field) and the electric potential. We investigate numerically the behavior of an initially flat film for a range of film heights and surface anchoring conditions.

Size effect of topological charge in disc-like nematic liquid crystal films

拓扑现象对于病毒颗粒的空间分布、高分子聚合物纳米囊泡的成型以及玻色-爱因斯坦凝聚物等方面都发挥着重要作用. 本文利用Landau-de Gennes理论, 构建模型来模拟液晶中拓扑荷分布及其他现象. 通过对数值模型序参量场的演化, 以及模拟液晶薄膜中所生成的拓扑荷之间的相互作用来分析液晶(Lqc)薄膜的尺寸对拓扑荷的影响. 研究结果表明,随着液晶盘半径增大, 拓扑荷间最优距离与半径之比渐增并趋于稳定. 此研究结论对利用拓扑荷凝聚颗粒物效应设计分离容器有指导意义, 有助于进一步理解拓扑胶体和液晶以及液晶共聚物等软物质中的拓扑现象.