Thin-film Liquid Crystal Research Articles

Mechanism of the effect of gel doping on the electro-optical properties of PDLC

ABSTRACT In this study, the gel molecules were further mixed with liquid crystal polymerisable monomers and introduced into the polymer matrix, and by selecting the ratios of three kinds of gel molecules to polymer monomers, namely, DMDBS, MDBS, and DBS, the liquid crystal/physical gel/polymer composite dispersed liquid crystal thin films with low driving voltage was prepared, and the transitions of transparency and scattering of the thin films were realised and theoretically analysed by the concentration of the gels, polymerisation temperature and polymerisation light intensity on the mechanism of electro-optical properties of the composite system. The results show that the doping of gel leads to a significant improvement in the electro-optical performance of PDLC, unfortunately, the contrast also decreases at the same time.

Membrane Proteins in Action Monitored by pH-Responsive Liquid Crystal Biosensors.

Liquid crystal (LC) biosensors have received significant attention for their potential applications for point-of-care devices due to their sensitivity, low cost, and easy read-out. They have been employed to detect a wide range of important biological molecules. However, detecting the function of membrane proteins has been extremely challenging due to the difficulty of integrating membrane proteins, lipid membranes, and LCs into one system. In this study, we addressed this challenge by monitoring the proton-pumping function of bacteriorhodopsin (bR) using a pH-sensitive LC thin film biosensor. To achieve this, we deposited purple membranes (PMs) containing a 2D crystal form of bRs onto an LC-aqueous interface. Under light, the PM patches changed the local pH at the LC-aqueous interface, causing a color change in the LC thin film that is observable through a polarizing microscope with crossed polarizers. These findings open up new opportunities to study the biofunctions of membrane proteins and their induced local environmental changes in a solution using LC biosensors.

Ultrasensitive Biomimetic Skin with Multimodal and Photoelectric Dual-Signal Sensing.

Mimicking biological skin enabling direct, intelligent interaction between users and devices, multimodal sensing with optical/electrical (OE) output signals is urgently required. Owing to this, this work aims to logically design a stretchable OE biomimetic skin (OE skin), which can sensitively sense complex external stimuli of pressure, strain, temperature, and localization. The OE skin consists of elastic thin polymer-stabilized cholesteric liquid crystal films, an ion-conductive hydrogel layer, and an elastic protective membrane formed with thin polydimethylsiloxane. The as-designed OE skin exhibits customizable structural color on demand, good thermochromism, and excellent mechanochromism, with the ability to extend the full visible spectrum, a good linearity of over 0.99, fast response speed of 93 ms, and wide temperature range of 119 °C. In addition, the conduction resistance variation of ion-conductive hydrogel exhibits excellent sensing capabilities under pressure, stretch, and temperature, endowing a good linearity of 0.99998 (stretching from 0 to 150%) and high thermal sensitivity of 0.86% per °C. Such an outstanding OE skin provides design concepts for the development of multifunctional biomimetic skin used in human-machine interaction and can find wide applications in intelligent wearable devices and human-machine interactions.

Surface effect on the phase separation in thin liquid crystal film as seen from a mean-field approach

The effect of the interplay between attractive nonlocal surface interactions and attractive pair long-range intermolecular couplings on molecular structures of liquid crystals confined in thin cells with flat solid surfaces has been studied. It has been shown, in the framework of the extended McMillan′s mean field theory, that in the confined LC system a complex orientational and translational ordering of molecules can be induced. Several scenarios of the formation of local smectic A, nematic, and isotropic phases have been shown to coexist in a certain temperature range. Due to the nonlocality of the surface interaction, an unexpected complexity of the spatial arrangement of these local phases along the direction of the normal to the surface may exhibit itself.

Ionic Liquid Crystal Thin Film as Switching Layer in Nonvolatile Resistive Memory.

In this study, we propose the use of an ionic liquid crystal (ILC) as a new resistive switching layer in nonvolatile resistive random-access memory (ReRAM) devices. The high-quality vacuum-deposited ILC films of 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]) enabled to demonstrate the first operation of ReRAM devices with a low set voltage of ∼1 V and stable switching behavior for up to ∼44 cycles. The key to the successful operation is that the ILC layer is in the liquid crystal phase (smectic A), where the electric double layers formed at the electrode-ILC interfaces play a significant role. The results of basic electrical properties and I-V curve fittings suggested the following operation principle: the formation and rupture of charge-composed filaments within the ILC film, where the current conduction is primarily governed by the trap charge limited current (TCLC) mechanism. These achievements will pave the way for advanced studies of ILC-based electronic devices.

Jetting and Droplet Formation Driven by Interfacial Electrohydrodynamic Effects Mediated by Solitons in Liquid Crystals.

Solitons are highly confined, propagating waves that arise from nonlinear feedback in natural (e.g., shallow and confined waters) and engineered systems (e.g., optical wave propagation in fibers). Solitons have recently been observed in thin films of liquid crystals (LCs) in the presence of ac electric fields, where localized LC director distortions arise and propagate due to flexoelectric polarization. Here we report that collisions between LC solitons and interfaces to isotropic fluids can generate a range of interfacial hydrodynamic phenomena. We find that single solitons can either generate single droplets or, alternatively, form jets of LC that subsequently break up into organized assemblies of droplets. We show that the influence of key parameters, such as electric field strength, LC film thickness, and LC-oil interfacial tension, map onto a universal state diagram that characterizes the transduction of soliton flexoelectric energy into droplet interfacial energy. Overall, we reveal that solitons in LCs can be used to focus the energy of nonlocalized electric fields to generate a new class of nonlinear electrohydrodynamic effects at fluid interfaces, including jetting and emulsification.

The directed self-assembly of reflective liquid crystalline polymer films to form polarization-independent diffractive optical elements

The directed self-assembly of liquid crystal polymers enables significantly enhanced optical properties compared to conventional isotropic optical materials. The transmissive properties of these thin polymeric films have been extensively studied, but the exploration of reflective liquid crystalline polymeric materials films has received little attention due to challenges in producing large, high-quality self-assembled films. We explore here for the first time stacked polymer cholesteric liquid crystal thin films as a pathway to generate reflective diffractive optics that act on both hands of circularly polarized light independently. We present 3 unique exemplar cases with increasing design complexity to communicate the widespread potential of these unique thin polymer films.

Energetics of topographically designed Smectic-A oily streaks.

A thin Smectic-A liquid crystal (LC) film is deposited on a polymer vinyl alcohol-coated substrate that had been scribed with a uniform easy axis pattern over a square of side length L ≤ 85 μm. The small size of the patterned region facilitates material distribution to form either a hill (for a thin film) or divot (for a thick film) above the scribed square and having an oily streak (OS) texture. Optical profilometry measurements vs. film thickness suggest that the OS structure aims to adopt a preferred thickness z0 that depends on the nature of the molecule, the temperature, and the surface tension at the air interface. We present a phenomenological model that estimates the energy cost of the OS layer as its thickness deviates from z0.

Continuous generation of topological defects in a passively driven nematic liquid crystal

Synthetic active matter is emerging as the prime route for the realisation of biological mechanisms such as locomotion, active mixing, and self-organisation in soft materials. In particular, passive nematic complex fluids are known to form out-of-equilibrium states with topological defects, but their locomotion, activation and experimental realization has been developed and understood to only a limited extent. Here, we report that the concentration-driven flow of small molecules triggers turbulent flow in the thin film of a nematic liquid crystal that continuously generates pairs of topological defects with an integer topological charge. The diffusion results in the formation of counter-rotating vortex rolls in the liquid crystal, which above a velocity threshold transform into a turbulent flow with continuous generation and annihilation of the defect pairs. The pairs of defects are created by the self-amplifying splay instability between the vortices, until a pair of oppositely charged defects is formed.

Computational Analysis to Optimize the Performance of Thin Film Liquid Crystal Biosensors

A nonlinear unsteady-state mathematical model employing torque balance and Frank free energy according to the Leslie-Ericksen continuum theory is developed and implemented to simulate the performance of nematic liquid crystal biosensor films with aqueous interfaces. A transient liquid crystal-aqueous interface realignment is modeled using the Euler–Lagrange equation by changing the easy axis when the surfactant molecules at the interface are introduced. In our study, we evaluated the dynamics between bulk and interface by controlling surface properties of the interface, such as homeotropic anchoring energy and surface viscosity. In addition, transient optical interference and response time have been examined in this study. Our parametric study results indicated that both homeotropic anchoring energy and surface viscosity at the interface contribute to bulk reorientation. Furthermore, the obtained numerical results indicate that as homeotropic anchoring strength increases, the effective birefringence decreases more gradual due to the increasing surfactant concentration at the aqueous interface, consistent with available experimental observations. Our results have been validated and compared to experimental results from thin-film liquid crystal biosensors in this study.

Electrotunable 180° achromatic linear polarization rotator based on a dual-frequency liquid crystal.

Linear polarization rotators have been widely used in optical systems. Commonly used polarization rotators are still beset by strong dispersion and thus restricted spectral bandwidth of operation. This leads to the development of achromatic or broadband alternatives, but most of them incorporate multiple waveplates for retardation compensation, which comes at the cost of increased complexity and reduced flexibility in operation and system design. Here, we demonstrate a single-element achromatic polarization rotator based on a thin film of dual-frequency chiral liquid crystal. The angle of polarization rotation is electrically tunable from 0° to 180° with low dispersion (±3°) in the entire visible spectrum, and a high degree of linear polarization (>95%) at the output.

Liquid Crystalline Half‐Skyrmions and Their Optical Properties

AbstractLiquid crystals have intrigued physicists as a platform that facilitates direct visualization of topological concepts. Here, the theoretical and experimental studies demonstrating that a thin film of a chiral liquid crystal forms topological entities known as half‐Skyrmions, swirl‐like orientational order that spans the hemisphere of the order parameter space , are reviewed. They appear in the form of a hexagonal lattice or in an isolated manner, accompanied by topological line defects to satisfy topological constraints. Under an optical microscope, half‐Skyrmions appear as dark spots, and their hexagonal lattice yields a Kossel diagram comprising a hexagonal arrangement of circular arcs. These experimental observations are corroborated by numerical calculations of the orientational order based on the Landau‐de Gennes theory with an orientational order parameter of a second‐rank tensor, and construction of its optical images and Kossel diagrams by directly solving Maxwell equations for light wave. A thin film of a chiral liquid crystal thus provides an interesting system for the investigation of topological concepts, and spontaneous formation of a half‐Skyrmion lattice whose periodicity is of the order of the wavelength of visible light has a potential for photonics applications as well as poses a challenging problem on optics.

Nontrivial electrophoresis of silica nano and microrods in a nematic liquid crystal.

We study DC and AC electrophoresis of silica nano and microrods in a thin film of a nematic liquid crystal. These particles induce virtual topological defects and demonstrate nontrivial electrophoresis. We measure several electrophoretic mobility coefficients and compare with those calculated theoretically. We demonstrate a competing effect of elastic and electrostatic torques that arises due to tilting of the rods in the liquid crystal. A simple theory describing this effect allows us to measure the effective polarisability of the rods. Our approach is simple and applicable to a wide variety of asymmetric and polarisable particles.

Electric Double Layer Action of High-quality Ionic Liquid Crystal Thin Films

Abstract We successfully prepared high-quality vacuum-deposited thin films of ionic liquid crystal 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C16mim][PF6]). The key point is to anneal the films in vacuum at the liquid crystal temperature of [C16mim][PF6], thereby much improving their crystallinity, surface smoothness, and air-exposure stability. The differential capacitance measurement of such a high-quality thin film revealed that an electric double layer was formed in the liquid crystalline phase, while it behaves like a normal dielectric in the crystal phase.

Role of homeotropic alignment strength at the air interface of polymerized liquid crystal layers

Liquid crystal thin films obtained by spincoating and photopolymerization are widely used nowadays for geometric phase optical components. The liquid crystal-air interface during photopolymerization plays a crucial role as it should not disturb the photoalignment induced by the other interface. When photopolymerizing the liquid crystal layer in vacuum, processing parameters need to be optimized carefully to avoid alignment issues at the air interface. In this work we demonstrate that domains are formed due to a tilt angle which is induced at the air interface under certain conditions. A measurement method is developed to quantify the tilt angle at the interface. The method consists of optical analysis of the domains using crossed-polarizer transmission microscopy with an oblique sample holder. By careful analysis of the obtained tilt angles for different parameters, it is assumed that a non-negligible homeotropic anchoring strength at the air interface is responsible for the domain formation. The results are explained with a theoretical model that assumes a finite anchoring strength at both the photoaligned interface and the air interface.

A Single-Substrate Biosensor with Spin-Coated Liquid Crystal Film for Simple, Sensitive and Label-Free Protein Detection.

A liquid crystal (LC)-based single-substrate biosensor was developed by spin-coating an LC thin film on a dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP)-decorated glass slide. Compared with the conventional sandwiched cell configuration, the simplified procedure for the preparation of an LC film allows the film thickness to be precisely controlled by adjusting the spin rate, thus eliminating personal errors involved in LC cell assembly. The limit of detection (LOD) for bovine serum albumin (BSA) was lowered from 10−5 g/mL with a 4.2-μm-thick sandwiched cell of the commercial LC E7 to 10−7 g/mL with a 4.2-μm-thick spin-coated E7 film and further to 10−8 g/mL by reducing the E7 film thickness to 3.4 μm. Moreover, by exploiting the LC film of the highly birefringent nematic LC HDN in the immunodetection of the cancer biomarker CA125, an LOD comparable to that determined with a sandwiched HDN cell was achieved at 10−8 g/mL CA125 using a capture antibody concentration an order of magnitude lower than that in the LC cell. Our results suggest that employing spin-coated LC film instead of conventional sandwiched LC cell provides a more reliable, reproducible, and cost-effective single-substrate platform, allowing simple fabrication of an LC-based biosensor for sensitive and label-free protein detection and immunoassay.

Phase separation and dewetting of polymer dispersed liquid crystal (PDLC) thin films on flat and patterned substrates

The morphology, phase separation, instability and dewetting of Polymer dispersed liquid crystal (PDLC) thin film comprising of nematic liquid crystal, 4-n-pentyl-4ʹ-cyanobiphenyl (5CB) and Polystyrene (PS) is reported in this paper. Thin film of PS-5CB PDLC is spin coated on flat and topographically patterned silica substrates from a common solvent (toluene). Films with two different thickness h ≈ 35 nm and h ≈ 85 nm are used in the study. The composition of PDLC is chosen as RB (PS: 5CB) = 1:2 and 1:1. On flat substrate, the thinner films (h ≈ 35 nm) dewet at room temperature (T ≈ 25 °C) over time, forming isolated droplets for both RB. In contrast, the h ≈ 85 nm thick films are stable at room temperature though they exhibit gradual phase evolution of the 5CB domains with time. These films dewet only when they are subject to thermal annealing at T ≈ 70 °C right after spin coating and results in an isotropic collection of nearly equal sized droplets, each of which comprises of an inner polymer core surrounded by a 5CB ring. Finally, we show that the instability patterns as well as the phase separated LC domains can be aligned by casting the films on topographically patterned substrate. While the 35 nm thin film results in spin dewetted array of aligned droplets along the substrates grooves, the 85 nm thick film remains continuous and undergoes progressive phase evolution, resulting in alternating domain of phase separated LC (within the grooves) and PS domains align along the contours of the substrate patterns.

Modeling the combined photo-chemo/thermo-mechanical actuation in azobenzene-doped liquid crystal thin films

Azo-doped liquid crystal polymer network (LCN) thin films are promising candidates for the development of soft-robots. These azo-doped LCNs exhibit either photo-chemo-mechanical or photo-thermo-mechanical or both deformations based on the type of azo-molecule used for doping, the type of external stimulus, and the surrounding environment. This paper presents a computational model capable of simulating this combined photo-chemo/thermo-mechanical deformation phenomena. Moreover, the influence of attenuation of light through the thickness and the influence of variation in the through-thickness alignment of mesogens are considered in the model. A detailed investigation of the influence of isomerization parameters, light intensity, and thermal parameters on light-induced temperature rise and the resultant deformations is also presented. The developed model shows good agreement with the experimental measurements available in the literature for the temperatures and deformations attained by azo-doped LCN films under illumination while placed in air/water.

Polymer dispersed liquid crystal thin films for sensing toluene

ABSTRACT In this paper, we prepare a polymer-dispersed liquid crystal (PDLC) thin film on glass substrates by the solvent- and polymerisation induced phase separation of a silicone adhesive (Platsil Gel) and a nematic liquid crystal, coupled with soft lithography. We show that the PDLC thin film can be used as a facile and portable sensor platform for the detection of toluene with the detection limit of 2.66 ppm by simply observing the director configuration change of the LC droplets dispersed in the polymer thin film. The sensor platform shows a rapid response time and can be repeatedly used for the detection of toluene.

Label-Free, Smartphone-Based, and Sensitive Nano-Structural Liquid Crystal Aligned by Ceramic Silicon Compound-Constructed DMOAP-Based Biosensor for the Detection of Urine Albumin.

IntroductionThe sensitive interfacial interaction of liquid crystals (LC) holds potential for precision biosensors. In the past, the developments of LC biosensors were limited by the complicated manufacturing process, which hinders commercialization and wider applications of such devices. In this report, we demonstrate the first nano-structural polymeric stabilized-cholesteric LC (PSCLC) thin films to be a new label-free biosensing technology.MethodsThe transmission spectra of PSCLC devices were measured by the fiber-optic spectrometer with high-resolution. In addition, a smartphone was set on the stage, and the camera of smartphone was placed and aligned with a set of lenses embedded in the designed stage. To decrease the chromatic and spherical aberrations, an achromatic lens set composition, consisting of both dual-convex lens and concave-plane lens, was applied for measuring and imaging the PSCLC texture. The average and the estimated standard deviation (SD) were used to present quantitative experimental results. The test BSA was immobilized and fulfilled by the ceramic silicon-constructed DMOAP-coated glass in aqueous BSA solutions at 1 mg/mL, 1 µg/mL, and 1 ng/mL.ResultsThe fabrication process of PSCLC is much simplified compared to previous LC biosensors. The color of PSCLC biosensor altered with the BSA concentration, making detection result easy to read. The detection limit of 1 ng/mL is achieved for label-free PSCLC biosensor. The PSCLC biosensor was able to successfully detect due to the albumin concentration’s alteration, with a linear range of 1 ng/mL–2 mg/mL. Thus, the label-free-proposed design-integrated nanoscale PSCLCs smartphone-based biosensor could successfully detect BSA in a preclinical urine sample.ConclusionFinally, we propose a design to integrate the PSCLC biosensor with a smartphone. The PSCLC owns potential for high performance, low cost for detecting various disease biomarkers in home use. Owing to its great potential for high performance and low cost, the PSCLC biosensors can be used as a label-free point-of-care for detecting various disease biomarkers for patients in care homes.