Orientation Of Nematic Liquid Crystals Research Articles

Influence of bipyridine isomers and some structurally related compounds on nematic liquid crystal orientation at the aqueous interface

In this paper, orientational anchoring transition of nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) at the LC-aqueous interface in the presence of bipyridine isomers (2,2′-bipyridine (BPY22) and 4,4′-bipyridine (BPY44)) and some structurally related compounds (1,10-phenanthroline (PANTH110) and aldrithiol-4 (ALD4)) in the aqueous phase is presented. The addition of BPY22, BPY44 and PANTH110 into the aqueous phase causes 5CB to undergo concentration dependent planar-to-homeotropic transition at the LC-aqueous interface. In contrast, ALD4 on addition into the aqueous phase depending on its concentration diminishes the interface between 5CB and the aqueous phase. Furthermore, the LC ordering transitions are related to pH of the solutions containing BPY22 or BPY44 or PANTH110 as revealed by the cross-polarized optical images and the changes in the tilt angle. Density functional theory (DFT) has been used to evaluate stabilization energy and hydrogen bonding strength for understanding the mechanism of LC ordering at the aqueous interface. Our findings provide a new set of compounds that cause homeotropic alignment of 5CB at the LC-aqueous interface, which is a prerequisite for the development LC based biosensors.

Designing Biological Microsensors with Chiral Nematic Liquid Crystal Droplets.

Biosensing using liquid crystals has a tremendous potential by coupling the high degree of sensitivity of their alignment to their surroundings with clear optical feedback. Many existing set-ups use birefringence of nematic liquid crystals, which severely limits straightforward and frugal implementation into a sensing platform due to the sophisticated optical set-ups required. In this work, we instead utilize chiral nematic liquid crystal microdroplets, which show strongly reflected structural color, as sensing platforms for surface active agents. We systematically quantify the optical response of closely related biological amphiphiles and find unique optical signatures for each species. We detect signatures across a wide range of concentrations (from micromolar to millimolar), with fast response times (from seconds to minutes). The striking optical response is a function of the adsorption of surfactants in a nonhomogeneous manner and the topology of the chiral nematic liquid crystal orientation at the interface requiring a scattering, multidomain structure. We show that the surface interactions, in particular, the surface packing density, to be a function of both headgroup and tail and thus unique to each surfactant species. We show lab-on-a-chip capability of our method by drying droplets in high-density two-dimensional arrays and simply hydrating the chip to detect dissolved analytes. Finally, we show proof-of-principle in vivo biosensing in the healthy as well as inflamed intestinal tracts of live zebrafish larvae, demonstrating CLC droplets show a clear optical response specifically when exposed to the gut environment rich in amphiphiles. Our unique approach shows clear potential in developing on-site detection platforms and detecting biological amphiphiles in living organisms.

Influence of Surface Relief on Orientation of Nematic Liquid Crystals: Polyimide Doped with WS2 Nanotubes

Among the different methods for orienting liquid crystal (LC) molecules, adding nanoparticles into the matrix of the substrate material towards modifying its surface, is actively pursued. In this context, the influence of the nanoparticle content on the texture of the surface of polymer film used as the substrate for the LC orientation is of particular interest. Thus, in the current paper, WS2 nanotubes were used to dope the polyimide (PI) substrate-film in order to modify and control its surface morphology/roughness and properties. The modified organic surface structure is applied in order to achieve a new means for controlling the orientation of the LC molecules. This tool adds to the classical methods for controlling the orientation of the LC molecules, such as the display technique.

LCOS Spatial Light Modulator for Digital Holography

Liquid crystal on silicon (LCOS) spatial light modulator (SLM) is the most widely used optical engine for digital holography. This paper aims to provide an overview of the applications of phase-only LCOS in two-dimensional (2D) holography. It begins with a brief introduction to the holography theory along with its development trajectory, followed by the fundamental operating principle of phase-only LCOS SLMs. Hardware performance of LCOS SLMs (in terms of frame rate, phase linearity and flicker) and related experimental results are presented. Finally, potential improvements and applications are discussed for futuristic holographic displays. Full Text: PDF ReferencesM. Wolfke, Physikalische Zeitschrift 21, 495 (1920). DirectLink D. Gabor, "A New Microscopic Principle", Nature 161, 777 (1948). CrossRef H. Haken, "Laser Theory", Light and Matter 5, 14 (1970). CrossRef S. Benton, "Selected Papers on Three-dimensional displays", SPIE Press (2001). DirectLink X. 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Sign-alternating optical reorientation in nematic liquid crystals with low-molar-mass and polymeric absorbing bis-azobenzene dopants

A comparative study of optical orientation in nematic liquid crystals (NLCs) doped with a low-molar-mass bis-azobenzene dye (monomer) and a comb-shaped polymer with side fragments similar in structure to the monomer has been carried out. Both types of bis-azobenzene dopants induced a sign-alternating nonlinearity in the NLC: the positive when the angle Ψ between the light field and the director is less than a certain critical value Ψc and the negative in the opposite case of Ψc < Ψ ≤ 90°. The transition from the monomer to polymer led to a decrease in the critical angle Ψc, i.e., to an expansion of the region of negative nonlinearity. At the same concentration of chromophores, an increase in both negative and positive nonlinearities occurs.The magnitudes of the optical-torque enhancement factors due to trans- and cis-isomers (ηT and ηC), and the ratios ηT/ηC for the low- and high-molar-mass dopants are compared.The results obtained show the possibilities of increasing the orientational optical nonlinearity when passing from low-molar bis-azobenzene dye dopants to the corresponding polymers and are important for elucidating the optimal architecture of absorbing additives.

Continuous modulation of geometric phase via electrically tuning the in-plane orientation of nematic liquid crystal

ABSTRACT Holographic display requires thinner liquid crystal cell for smaller pixels and fast response. In this paper, a continuous geometric phase modulation method realised by electrically tuning a dual in-plane-switching (IPS) LCD is proposed. Firstly, the function of geometric phase modulation and the driving voltage is obtained via numerical simulation. Secondly, a dual IPS liquid crystal cells is designed and fabricated, and by using it, the experimental data is measured. Lastly, the simulation and test results are matched, which show good consistence with each other, and a phase modulation range of near is achieved. By using the method, the phase modulation quantity is not only independent of the thickness of liquid crystal layer, but can be achieved almost 360° continuous modulation of the geometric phase. This method paves an avenue towards dynamic geometric phase modulation of light, which is meaningful for large field of view and colourful holographic display.

Low-temperature properties of nematic liquid crystal materials for display

ABSTRACT The orientation of Nematic liquid crystal (NLC) and order degree of LC molecules are easily affected by ambient temperature, so it is of great significance to explore the low-temperature properties of NLC materials to improve the low-temperature performance of LC devices. The dielectric and elastic constant of the NLC material 1M10500-000 under the high and low-temperature conditions were obtained by the dual-cell capacitance method, linear fitting method and numerical fitting, and were compared. The dynamic response of the NLC at different temperatures was tested, and the rotational viscosity was calculated. The experimental results show that the dielectric constant is still proportional to the order parameter at low temperature. The elastic constants k 11 and k 33 and the square of order parameter no longer conform to the proportional relationship at low temperature, and the lower the temperature, the greater the deviation of elastic constants from the proportional relationship. As the temperature decreases, the ratio of k 33/k 11 changes from 2.62 to 2.71, and the rotational viscosity coefficient changes from 5.66 mPa.s to 177.64 mPa.s. Finally, the transient hysteresis phenomenon at low temperature was proposed and explained by the residual direct current voltage.

Selection rules and a new model for stable topological defect arrays in nematic liquid crystal

ABSTRACT Orientation of nematic liquid crystal (NLC) around a topological defect is expressed by the equation, , where , , and are the director spatial phase, topological charge, azimuthal angle and a spatial phase shift, respectively. Conventionally, defect array is modelled by simply adding the fields of all the defects. The resultant array is a combination of the defects imposed by design () and the defects due to the lattice geometry (). However, defect arrays generated in homeotropic confinement show hyperbolic hedgehogs () between the s, which are out of the scope of the conventional model. In this research, two-dimensional defect ‘crystals’ with various lattice structures (square and hexagonal), various defect shapes (radial or circular) and various lattice constants (pixel size) are generated by pixelated patterned electrodes in homeotropic NLC cells. The results verify the two selection rules, 1) The total must be zero, 2) must be a constant throughout the array, and a new model composed of , and is established. Calculation proves that the selected modelled array has minimum free energy. The -- model is versatile and easy to apply, which benefits the design of new topological defect arrays.

Computer simulation of the pulse-periodic electric field effect on the 2D director orientation of nematic liquid crystal. Experimental research of multimode nematic liquid crystal waveguides.

In this paper, we numerically investigate a two-dimensional differential equation describing the motion of a director of a nematic liquid crystal for the case of an alternating external electric field. The presence of the previously discovered accumulation effect has been confirmed by numerical modeling. A comparison is made with the case of a constant electric field, and also a qualitative comparison with an experiment is given. Incomplete agreement with experimental data indicates the need for further research. However, it should be noted that the constructed mathematical model of the phenomenon allows at this stage to obtain estimates that are sufficiently acceptable for experiment and correctly predict the dynamics of processes in liquid crystals. An analysis of the features of the propagation of quasi-waveguide modes in a liquid crystal waveguide showed that, in the case of dynamic processes, such effects as power exchange between coupled modes, leakage of modes, re-emission of modes into modes of a different order, etc., can be observed. The programs for numerical solution and computer modeling of two-dimensional parabolic partial differential equation were developed both in FORTRAN and C/C++. The results obtained are important for further investigation of dynamic processes inside non-stationary liquid crystal layers, both from a theoretical point of view for understanding kinetic processes in liquid crystals and from a practical point of view when organizing and conducting different experimental research.

Optofluidic platform using liquid crystals in lithium niobate microchannel

We demonstrate the all optical control of the molecular orientation of nematic liquid crystals confined in microfluidic channels engraved in lithium niobate. Microchannels are obtained by a novel approach based on femtosecond pulse laser micromachining carried on in controlled atmosphere. The combined effect of photovoltaic and pyroelectric fields generated by light in lithium niobate crystals on the liquid crystal orientation, is reported for the first time. The total space charge field and its dependence on the incident light intensity can be controlled by changing the direction of pump light propagation through the microfluidic chip. The results reported in this manuscript demonstrate that liquid crystals and lithium niobate can efficiently be combined in microfluidic configuration, in order to push forward a novel class of optofluidic devices.

Simulation of the Static Electric Field Effect on the Director Orientation of Nematic Liquid Crystal in the Transition State

The effect of static electric field on a nematic cell with side electrodes in the transition state (which occurs in the initial stage of field effect on the cell) has been investigated within the two-dimensional dynamic model. The dynamic model is compared with the previously used static one. The two-dimensional dynamic model is found to yield better agreement with the experiment.

Phosphorylation status of peptide monolayers modulates hydrogen bonding and orientations of nematic liquid crystals

ABSTRACTWe report on the azimuthal orientations of nematic liquid crystals (LCs) on monolayers of dipeptides and tripeptides containing cysteine (C) and either tyrosine (Y), serine (S) or threonine (T). Measurements performed with 5CB and TL205 (with and without doping with trimethylamine), and dipeptide monolayers in which we removed the –OH groups of CY, CS or CT, led us to conclude that hydrogen bonding strongly influences LC orientations on CY but not CS or CT monolayers. Chemical force microscopy and cyclic voltammetry support the hypothesis that intra-monolayer hydrogen bonding within monolayers of CS and CT occurs preferentially to hydrogen bonding with 5CB. In contrast, the orientations of the LCs on tripeptide monolayers of CAY, CAS or CAT, where A is alanine, were all influenced by hydrogen bonding, consistent with a decrease in surface density of tripeptides (as compared to dipeptides; supported by x-ray photoelectron spectroscopy) and increase in hydrogen bonding with the LC overlayer. Phosphorylation of monolayers that oriented 5CB through hydrogen bonding led to easily measured changes in LC orientation. Overall, our results advance the rational design of LC materials with internal order that is coupled to biologically relevant chemical modifications of oligopeptides and proteins.

The effect of surface polarity of glass on liquid crystal alignment

ABSTRACTThe effects of the surface polarity of a glass substrate on the orientation of nematic liquid crystals (LCs) were studied using the polarised optical microscope and Fourier-transform infrared spectroscopy. On the surface of oxygen plasma treated glass, a homeotropic alignment of LCs was induced for LCs with negative dielectric anisotropy. This suggests that vertical orientation of LCs could be induced on a polar glass substrate without using an LC alignment layer. Upon cooling towards the isotropic–nematic transition, E7 with positive dielectric anisotropy changes its LC arrangement to isotropic, homeotropic, planar orientations in order. The nematic LC anchoring transition of E7 was interpreted by considering the competition between van der Waals forces and dipole interactions that control the alignment of LC molecules on a polar glass surface.

UV-induced homeotropic orientation of nematic liquid crystals dissolving two species of monomers carrying liquid-crystalline side-chain group and anthracene group

ABSTRACTWe propose the technique for formation of the polymer orientation film (POF) by polymerisation of the monomers being dissolved in the liquid crystal (LC) material for controlling the orientation of the LC molecules. For obtaining the homeotropic orientation, combination of the two monomers, 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-anthracene (DMAAnth), was found to be useful. The monomer DMAAnth initiates the polymerisation under ultraviolet (UV) light exposure, and the AOBBE unit induces the homeotropic orientation. The monomer DMAAnth is useful for maintaining the high voltage holding ratio and low residual direct-current voltage after UV light exposure because the molecules of DMAAnth do not remain in the LC layer.

A finite volume method for computing flow induced orientation of nematic liquid crystals

ABSTRACTIn this work, we propose a new method for studying dynamics in a liquid crystal arising from torques due to an externally imposed shear flow or other sources of torque. Here a spatially homogeneous liquid crystal system governed by a Smoluchowski equation for the orientational probability density function is investigated. To analyze the PDE model, we develop a novel direct computational algorithm based on a Voronoi cell-based finite volume scheme. This method has the capability of describing abrupt changes in the density function, using fluxes through cell boundaries. We first validate our approach by capturing prior results from Maier-Saupe theory illustrating a phase transition in the system due to nematic interactions as well as the effects of an externally imposed electric field. We then investigate the coupling of an external flow field with the Smoluchowski equation describing the full orientational dynamics of the system.

A Blow-up Criterion for 2D Compressible Nematic Liquid Crystal Flows in Terms of Density

This paper is concerned with the Cauchy problem for compressible nematic liquid crystal flows in the two-dimensional space (2D). We establish a blow-up criterion in terms of the density only, provided the macroscopic average of the nematic liquid crystal orientation field satisfies a geometric condition. In particular, the initial vacuum is allowed and the compatibility condition is removed.

Control of liquid crystal molecular orientation using ultrasound vibration

We propose a technique to control the orientation of nematic liquid crystals using ultrasound and investigate the optical characteristics of the oriented samples. An ultrasonic liquid crystal cell with a thickness of 5–25 μm and two ultrasonic lead zirconate titanate transducers was fabricated. By exciting the ultrasonic transducers, the flexural vibration modes were generated on the cell. An acoustic radiation force to the liquid crystal layer was generated, changing the molecular orientation and thus the light transmission. By modulating the ultrasonic driving frequency and voltage, the spatial distribution of the molecular orientation of the liquid crystals could be controlled. The distribution of the transmitted light intensity depends on the thickness of the liquid crystal layer because the acoustic field in the liquid crystal layer is changed by the orientational film.

Electric and magnetic fields in photoalignment of liquid crystals

AbstractThe results of investigation of slow surface dynamics (easy axis gliding), induced by secondary illumination of photoaligned substrates with linearly polarized UV or blue light in the presence of electric (magnetic) fields, are presented for the first time. The initial surface orientation of nematic liquid crystal (NLC) was provided, in accordance with the standard photoalignment technique, via preliminary UV treatment of the glass substrates coated by sulfonic azo‐dye SD1 film. The experiments fulfilled for the two geometries, A and B, corresponding to the normal (A) and parallel (B) orientation of fields relatively to liquid crystal (LC) layer, revealed different effects induced by electric (magnetic) fields. For geometry A, strong electric field applied, in combination with a linearly polarized blue light, to the planar layer in a sandwich‐like LC cell with two photoaligned substrates results in simultaneous reorientation of easy axis on both substrates. It demonstrates the possibility of an azimuthal rotation of monodomain planar samples of LCs. For geometry B, usage of “in‐plane” electric (magnetic) fields in the cell, composed of photoaligned and rubbed substrates, speeds up reorientation process of NLC easy axis respectively to pure light‐induced reorientation. It provides electric control of operating times in previously proposed optical rewritable technology.

Planar Alignment of Liquid Crystal in Silica-Based Photonic Structure

The planar orientation of Nematic Liquid crystal (NLC) molecules on the surface of (111) plane of silica photonic structure has been investigated. Nano voids of silica photonic crystal has been obtained by vertical deposition technique on indium tin oxide-coated substrate. NLC was infiltrated into the channels of voids in the photonic structures. Optical texture studies under with and without electric field were applied to investigate the NLC orientation. It is observed that NLC molecules in the nano voids of silica photonic structure exhibit an excellent planar alignment. This obtained hybrid composite structure on infiltration of NLC will be a promising candidate for electro optic applications.

Terahertz characterization of tunable metamaterial based on electrically controlled nematic liquid crystal

The terahertz time domain spectroscopy (THz-TDS) system is used to determine the effects of an AC bias voltage on the tunable response of a metamaterial transducer. The tunability of the metamaterial structure, which is based on the rod-split-square resonator, is demonstrated at terahertz frequencies through electrical control of the nematic liquid crystal orientation. Experimental results show that the metamaterial device can be tuned effectively (with transmittance change of up to 19%) by changing the magnitude of the AC bias voltage from 0 to 300 V. This type of tunable metamaterial could find application in the development of devices operating in the THz frequency region for filtering, modulating, and switching of the electromagnetic signals.