Alignment Of Nematic Liquid Crystals Research Articles

Self-Consistent Explanation of the Untwist Alignment of Ferroelectric Nematic Liquid Crystals with Decreasing Cell Thickness and Deviation of the Surface Easy Axis Experimented upon Using the Brewster Angle Reflection Method

The huge dielectric constant of ferroelectric nematic liquid crystals (FNLCs) seems to bring about a difficulty of molecular alignment control in exchange for a potential device application. To obtain a satisfactory level of uniform molecular alignment, it is essential to understand how the molecules near the alignment surface are anchored. In this study, bulk molecular alignment with an anti-parallel rubbing manner, which has not yet been investigated extensively, is explained using a conventional torque balance model introducing a polar anchoring function, and it is shown that the disappearance of the bulk twist alignment with decreasing cell thickness can be explained self-consistently. To validate this estimation for a room-temperature FNLC substance, the Brewster angle reflection method was attempted to confirm the surface director’s deviation from the rubbing direction caused by the polar surface anchoring.

Liquid Piezoelectric Materials: Linear Electromechanical Effect in Fluid Ferroelectric Nematic Liquid Crystals

AbstractThe first demonstration of converse piezoelectricity in 3D fluids is presented by measuring a linear electromechanical effect in ferroelectric nematic liquid crystals. The observed piezoelectric coupling constant below 6 kHz electric field is larger than 1 nC/N, comparable to, or better than, values for the strongest solid piezoelectric materials. Symmetry considerations indicate that the alignment of the ferroelectric nematic liquid crystal in the experimental study is not optimized, so the observed signal is likely only a fraction of the theoretically achievable signal. Understanding the electromechanical response of ferroelectric nematics will enable mechanical energy harvesting and open up a new avenue for developing fluid actuators, micro positioners, and electrically tunable optical lenses.

Surface alignment of nematic liquid crystals by direct laser writing of photopolymer alignment layers

ABSTRACT We demonstrate the fabrication of good quality surface alignment layers on glass by Direct Laser Writing method using a 2-photon polymerisation technique. We use commercially available photosensitive resins to print alignment layers by scanning the focal point of a femtosecond laser near the resin-glass interface. This results in down to ~ 100 nm thin alignment layers that provide good planar anchoring of 5CB and MLC13300, with the easy axis of alignment along the scanning direction. The azimuthal anchoring strength is ~ 5 × 10−6 J/m2 and is an order of magnitude weaker compared to commercial rubbed polyimide alignment layer. The threshold voltage for Fréedericksz transition in a 90° twisted nematic cell is slightly increased compared to conventional rubbed polyimide for printed alignment layers. The turn-on switching time is longer for printed layers compared to polyimide alignment layers, whereas the turn-off time is shorter for printed alignment layers. The advantage of this new method is in its flexibility, as we demonstrate printing of complex surface alignment patterns with alignment layer thickness below 100 nm.

Eco-friendly carbon dots induced thermally stable vertical alignment in planar anchored nematic liquid crystal

We report the induction of thermally stable vertical alignment of nematic liquid crystal, 4-cyano-4′-pentylbiphenyl (5CB), in a planar anchored cell by doping with eco-friendly carbon dots (CDs, diameter ∼7–8 nm), investigated through thermal, optical, electro-optical and dielectric spectroscopic techniques. Cross-polarized optical textures and voltage-dependent optical transmission measurements demonstrate the induction of vertical alignment of CDs-5CB composites with concentration of CDs (≥0.3 wt%) in planar anchored sample cells at room temperature. Moreover, the frequency-bias-concentration dependent dielectric studies done also confirms the induced vertical alignment. For instance, the relative dielectric permittivity measured on CDs-5CB composites (CDs ≥ 0.3 wt%) in planar anchored sample cells at bias voltage of 0 V and 30 V DC are almost equal and the presence of a short axis molecular relaxation at 0 V confirms the induced vertical alignment. The concentration and bias dependence of short axis molecular relaxation frequency are also discussed. It appears that CDs are able to align 5CB molecules vertically at the substrate by overcoming the pre-existing planar anchoring. Such induced alignment would propagate through the bulk of sample cell because of the coherence length of nematic liquid crystal molecules and eventually the whole sample obtains vertical alignment. We anticipate the great utility of these composites in the production of eco-friendly liquid crystal displays and other photonic devices, sensors, etc.

Analysis of morphological and electro-optical properties of silica nanoparticles induced vertically aligned liquid crystal - effect of doping and coating techniques

Conventionally, polyimide (PI) coating materials are used on the indium tin oxide (ITO) glass substrates to induce the vertical alignment (VA) of nematic liquid crystals (LCs). However, this coating technique requires a couple of steps such as spin coating, high-temperature baking and mechanical rubbing which limits their applicability to flexible LC display device applications. In the present work, VA of nematic LCs in a confined cell is induced using two different approaches: one via doping of silica nanoparticles (SNPs) as guest particles in LCs and the other with the deposition of SNPs layer on ITO glass substrates. Further, morphological and electro-optical (E-O) studies have been studied for the SNPs doped and SNPs coated vertically aligned liquid crystals (VALCs). The morphological study shows the identical degree of dark (black) and bright (white) textures of SNPs doped and coated VALC cells at OFF and ON states, respectively under the crossed polarizers. Also, the VA of nematic LC molecules in confined cells has been confirmed from the conoscopic behaviour. Consequently, morphological and E-O results revealed that the threshold (V th ) and operating (V o ) voltages required to switch the LCs from their initial VA (black state) towards minimum transition planar state and complete planar (bright) state, respectively, are reduced with better contrast ratio (CR) in SNPs coated VALC cell as compared with SNPs doped VALC cell.

Structural and Optical Characteristics of Flexible Optically Rewritable Electronic Paper

A comprehensive theory of light-reflective characteristics and experimental technique of liquid crystal layer thickness control for flexible optically rewritable electronic paper is presented. Cylindrical pillars were used to control the gap between flexible substrates. The introduced prototype of optically rewritable electronic paper has shown very promising performance. In this regard, we report theoretical results of structural photosensitive alignment of nematic liquid crystals on flexible substrate. The focus of theoretical study is on understanding the self-assembled complex structure, governed by the interplay between surface anchoring and liquid crystal elasticity. Mueller matrix spectroscopic ellipsometry was used to study light-reflecting characteristics and polarization properties of the twisted nematic film.

Manipulation of plasmonic random laser from dye-doped liquid crystals inside photonic crystal fiber by the electric field

The manipulation of plasmonic random lasers (PRLs) from dye-doped liquid crystals (DDLCs) inside the air holes of photonic crystal fiber (PCF) with silver nanoprisms has been investigated by means of applied voltage. In comparison to the sample without AgNPs, PRL exhibits multiple emission spikes and a lower lasing threshold owing to the recurrent light scattering and the localized surface plasmon resonance (LSPR) effect. Through the modulation of the applied voltage above 80 V, the alignment of nematic liquid crystals inside the air holes of PCF will become more disordered to produce photon trapping for improving the output intensity of PRLs and will induce strong light scattering for turning the incoherent feedback to coherent feedback. When the applied voltage exceeds a certain value, the emission spike of RL does not increase further because of the gain saturation effect. The enhancement of light scattering inside the air holes of PCF as voltage increases was demonstrated by the white light scattering spectrum from polarization optical microscopy.

Neutron reflectometry-based in situ structural analysis of an aligning agent additive for the alignment of nematic liquid crystals on solid substrates.

Surface aligning agents, such as amphiphilic surfactants, are widely used to control the initial alignment of nematic liquid crystals (NLCs) in liquid crystal displays (LCDs). Generally, these agents are first coated on a substrate prior to NLC introduction. When mixed with NLCs, long alkyl chain amphiphilic agent additives may control the NLC alignment without requiring pretreatment because they may spontaneously form an adsorbed layer at the solid-NLC interface. These self-assembled layers (SALs) appear promising in the effective control of the initial alignment of LCDs. However, direct observation of the adsorbed layer structure in contact with the NLCs is challenging due to probe limitations. Furthermore, the areal densities and alignments of the amphiphiles adsorbed from NLCs at the solid-NLC interface are not previously reported. Herein, the structure of the surface aligning agent n-hexadecyltrimethylammonium-d42 bromide (d-CTAB) was investigated at the silicon-NLC interface using in situ neutron reflectometry (NR), which indicated that the CTAB self-assembled as a monolayer, with its alignment dependent on the amphiphile concentration. At low amphiphile concentrations, the alignment of the SAL and NLCs was parallel to the substrate. With increasing amphiphile concentration, the number of amphiphiles attached to the substrate increased within the framework of the Gibbs monolayer, with the alignment of the amphiphiles and NLCs becoming perpendicular to the substrate. The experimental setup used here is comparable to those of more natural systems, such as those found in the alignment of NLCs in LCDs.

Influence of the flexoelectric effect on director alignment of nematic liquid crystals confined to the planar anchoring cylinder

ABSTRACT The influence of the flexoelectric effect on the director distribution of nematic liquid crystals confined to a cylinder with planar anchoring under an applied electric field is investigated by the continuum theory and the difference iterative method. Numerical analyses of the influence of the dielectric effect, flexoelectric effect and the coupling of these two effects on director alignment of nematic liquid crystal in a cylinder are obtained on the basis of the equilibrium equations using continuum theory. The analyses revealed that the change of director of nematic liquid crystals was caused by flexoelectric effect and dielectric effect in different directions. It provides a good theoretical basis for controlling the orientation of liquid crystal director in a cylinder and measuring of flexoelectric coefficient of liquid crystal materials.

Surface-anchored alkylated graphene oxide as a two-dimensional homeotropic alignment layer for nematic liquid crystals

Graphene oxide (GO) is one of the thinnest two-dimensional (2D) organic materials and has been investigated for application in various fields due to its excellent mechanical and optical properties. The unique planar structure and hydrophilic functional groups allow GO to spontaneously form various nanoarchitectures. Herein, we report an unprecedented methodology to achieve the homeotropic alignment of nematic liquid crystals (NLCs) using surface-anchored alkylated graphene oxide (AGO). The GO is functionalized by amino reactions with dodecylamine to impart amphiphilic characteristics. This AGO spontaneously forms a 2D-alignment layer on indium tin oxide surface via polar affinity, and the hydrophobic alkyl chains of AGO guide the constant direction of the NLCs. Furthermore, a study on the effect of the alkylation ratio of AGO on the NLC orientation was conducted, and the achievement of a 2D-alignment layer based on the optimal AGO successfully resulted in the initial homeotropic orientation of the NLCs without any defects. Compared to a conventional polymer alignment layer, it showed a comparable alignment ability, and exhibited an improved optical transmittance due to the ultrathin nature of the AGO alignment layer. This new modus of NLC vertical alignment based on the use of AGO can expand the utility of functionalized GO.

Two-fluid model for the breakdown of flow alignment in nematic liquid crystals.

We present a macroscopic two-fluid model to explain the breakdown of flow alignment in nematic liquid crystals under shear flow due to smectic clusters. We find that the velocity difference of the two fluids plays a key role to mediate the time-dependent behavior as soon as a large enough amount of smectic order is induced by flow. For the minimal model it is sufficient to keep the nematic degrees of freedom, the mass density of the smectic clusters and the degree of smectic order, the density, and two velocities as macroscopic variables. While frequently a smectic A or C phase arises at lower temperatures, this is not required for the applicability of the present model. Indeed, as pointed out before by Gähwiller, there are compounds showing a breakdown of flow alignment over a large temperature range and no smectic phase, but a solid phase at lower temperatures. We also demonstrate that, using a one velocity model, there is no coupling under shear flow between induced smectic order and the director orientation in stationary situations thus rendering such a model to be unsuitable to describe the breakdown of flow alignment. In a two-fluid description, flow alignment breaks down and becomes unstable with regard to a space- and time-dependent state due to an induced finite velocity difference. In an Appendix we outline a mesoscopic model to account for the sign change in the anisotropy of the electric conductivity observed in nematics with smectic clusters.

Surface alignment of ferroelectric nematic liquid crystals.

The success of nematic liquid crystals in displays and optical applications is due to the combination of their optical uniaxiality, fluidity, elasticity, responsiveness to electric fields and controllable coupling of the molecular orientation at the interface with solid surfaces. The discovery of a polar nematic phase opens new possibilities for liquid crystal-based applications, but also requires a new study of how this phase couples with surfaces. Here we explore the surface alignment of the ferroelectric nematic phase by testing different rubbed and unrubbed substrates that differ in coupling strength and anchoring orientation and find a variety of behaviors - in terms of nematic orientation, topological defects and electric field response - that are specific to the ferroelectric nematic phase and can be understood as a consequence of the polar symmetry breaking. In particular, we show that by using rubbed polymer surfaces it is easy to produce cells with a planar polar preferential alignment and that cell electrostatics (e.g. grounding the electrodes) has a remarkable effect on the overall homogeneity of the ferroelectric ordering.

Fringe field-tunable LC refractive index interface for in-plane beam steering applications

We report on the electrically tunable optical structure based on dual-domain nematic liquid crystal (LC) alignment for in-plane beam steering applications. The device operates due to the total internal reflection of an extraordinary beam at the LC refractive index interface that separates homeotropic and planar-aligned nematics. Patterned electrodes were used in order to switch on the refractive index interface in the bulk of a planar-aligned LC layer. An outstanding feature of the proposed device is the function of tuning the spatial position of the LC interface by means of a fringing electric field, which allowed one to implement wide range light beam microscanning, as well as to realize in-plane angular beam steering with a milliradian resolution.

Vertical Alignment of Nematic Liquid Crystals Based on Spontaneous Alignment Layer Formation between Silver Nanowire Networks and Nonionic Amphiphiles

The vertical arrangement of nematic liquid crystals (LCs) in displays can be generally achieved by introducing a polyimide material onto indium tin oxide electrodes. However, this method requires multiple coating and deposition processes as well as high curing temperature, restricting the potential applicability to flexible displays. Thus, we herein propose the facile approach for homeotropic alignment of nematic LCs based on spontaneous alignment layer formation between silver nanowire networks and nonionic amphiphiles. The silver nanowires as transparent electrode materials were spin-coated on glass substrate and 4-(4-heptylphenyl)benzoic acids as nonionic amphiphiles were doped into the LC medium. The nonionic amphiphiles were spontaneously bonded to the polyvinylpyrrolidone capping layer of silver nanowire networks through polar interactions, creating the self-assembled alignment layer of nonionic amphiphiles on silver nanowire electrodes. In addition, the alkyl chains of the amphiphiles interacted with the LC molecules, leading to stable directional LC alignment along vertical direction. The electro-optical characteristics of the manufactured LC cell were comparable to those of conventional device including polyimide layer and indium tin oxide electrode. Overall, the combination of silver nanowire electrode and nonionic amphiphiles presents a new way to achieve the vertical alignment of nematic LCs without polyimide layer.

Accurate determination on the pre-tilt angle of liquid crystal cell by combining optical and electrical measurement

ABSTRACT In order to accurately determine the pre-tilt angle, we proposed a systematic method by combining optical and electrical measurement. First, the pre-tilt angles of the parallel and vertical aligned nematic liquid crystal (LC) cell with a large thickness (>10 μm) were measured by the crystal rotation method. Then, the material parameters of E7, including the dielectric constant and the splay and bend elastic constant, were obtained by using the dual-cell capacitance model and comparing the capacitance-voltage (C-U) curves of the thick cells. Finally, the pre-tilt angles of two thin LC cells to be tested were determined by theoretical and experimental fitting of their normalised C-U curve, which are 1.80°±0.04° and 3.50°±0.02°, respectively. Our method not only has relatively high accuracy, but also is not limited by the thickness and mode of LC cell.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Ultra-fast liquid crystal lens with variable focal length

Conventional electrically driven lenses with an aligned nematic liquid crystal (NLC) as a responsive material show considerably wider range of refractive index and thereby the focal length variation in comparison with their solid counterparts, but are known to concede significantly by their operational speed. In this paper, we present electrically adaptive lenses in a cell assembled of two flat glass substrates with a plano-convex polymer lens fabricated of a photoresist seating on one substrate and a plano-concave lens formed by a shear-stressed LC polymer system, filling the remaining space in the cell. Under the voltage applied to the inner surfaces of the cell substrates, the NLC reorients, its refractive index changes and consequently, the focal lengths of such a plano-concave–convex lens decrease from about 360 mm at zero voltage down to 155 mm at high (about 60 V) voltages. The benefit of the stressed LC state allows one to reduce the switching (on–off) time down to 2–3 ms, which is more than an order of magnitude lower in comparison with conventional NLC lenses.

Difference in the interaction of nano-diameter rod and tubular particles with a disclination line in a nematic liquid crystal.

In the presence of a disclination line, inclusions within an aligned nematic liquid crystal (LC) are first attracted and ultimately trapped in it. The kind of orientational distortion created by the inclusions is fundamental in determining the trapping. In the present work, we observe differences in the trapping behaviour, onto a ½ defect line in a nematic LC, of two types of particles both elongated but different in their actual geometry. Even if both types have cylindrical shape, aggregates of Mo6S2I8 nanowires (rod-like shape) and multiwall carbon nanotubes (tubular shape, i.e. hollow) trap differently although still due to deformations induced in the LC director field. Attractive forces are stronger on elongated bundles of nanowires than on similarly sized bundles of multi-wall carbon nanotubes. The reason is the difference in the attraction forces originating from different types of distortions of the LCs. The hollow and the full cylinders are not homotopically equivalent and this inequivalence holds also for the liquid crystal around them. The nanowires induce defects in the LC close-by their surfaces as shown for microrods, topologically equivalent to spheres. In contrast, multi-wall carbon nanotubes, being hollow, do not form defects close to their ends. However, the tubes are strongly bent and the strong planar anchoring of LC at the surface induces deformation in the LC enabling attraction forces with the defect line. HiPco single wall carbon nanotubes could not be trapped because their bundles looked much straighter and smaller than the ones of MWCNTs and thus neither defects nor standard strong deformations are expected. In conclusion, even if the shape of both types of particles is cylindrical, the topological difference between rods and tubes has profound consequences on the physical behaviour and on the presence and type of defect-mediated nematic attraction forces.

Improvement of the electro-optical properties of nematic liquid crystals doped with strontium titanate nanoparticles at various doping concentrations

ABSTRACTIn this study, we doped homogenous aligned nematic liquid crystal (NLC) systems with strontium titanate (SrTiO3) nanoparticles (NPs), and investigated the impact of doping concentration on the NLC’s electro-optical (EO) properties. SrTiO3 NP-doped NLC cells maintained a high optical transmittance of 77.51% to 78.41% compared to pure NLC cells (78.09%). At a 0.1 wt.% SrTiO3 NP doping concentration, twisted-nematic (TN)-LC cells exhibited enhanced EO performance, with a reduced threshold voltage from 1.70V to 1.61V and a shorter response time from 17.03 ms to 10.66 ms without optical defects and degradations. We also observed an improvement in thermal endurance for doping concentrations of 0.05 and 0.1 wt.%.

Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment.

High-quality alignment control of liquid crystals (LCs) for ultrahigh-definition large-sized display is a challenging task. A conventional rubbing method has obvious limitations for fabricating large-sized displays with a small pixel size and an uneven inner surface. To comply with the current trend, we propose a simple and reliable polyimide-less in situ photoalignment. It was achieved using a visible-light-sensitive azo-dye and a mesogenic acrylate, both doped to host LCs. Without using a pretreated alignment layer, mono- and multidomain uniaxial alignments of LC molecules were induced by linearly polarized visible light (LPVL) and subsequently stabilized by unpolarized UV-light irradiation. The stepwise process was monitored by adopting a fluorescent indicator. By loading the mixture into a confined cell, azo-dyes were spontaneously adsorbed at inner surfaces of the cell, whereas reactive mesogens (RMs) were homogeneously dissolved in an LC host. The molecular orientational anisotropy of dyes at the surface, induced by LPVL, aligned the LC director perpendicular to the polarization direction. Upon the second step, UV-irradiation, the RMs in an LC host were photopolymerized into thin interfacial layers, stabilizing the aligned LC director. The overlaid cross-linked RM layers secured a thermal and a radiative stability of LC alignment. The RM layers completely screened the effect of azo-dyes, which can be easily randomized by heat and irradiation. The interfacial RM layer functioned as a permanently stable alignment layer. It provided sufficient azimuthal anchoring strength together with heat and light stabilities, which are essential for practical applications. Such sequential interfacial modifications through dual-wavelength processes can completely avoid interference between forming alignment and stabilization layers, inevitable if the same wavelength light is used. The proposed method provides a simple fabrication process and reliable alignment characteristics by employing effective in situ photoalignment and without using a traditional alignment layer. Therefore, it meets a current trend in the display market toward ultrahigh-resolution and large-area displays.