Nematic Liquid Crystal Mixture Research Articles

Mid-wave infrared liquid crystal shutter for breathalyzer applications

There exists a problem with an in situ diagnostics of contamination of ethyl alcohol in a human being exhaled air. When ethyl alcohol in a mouth blowing (in a gaseous state) exists, the characteristic C H stretch absorption bands in CH 3 and CH 2 functional groups in ethanol (CH 3 CH 2 OH) appear at a wavelength of λ = 3.42 μm. To investigate the presence of ethyl alcohol in exhaled human air, the light beam of λ = 3.42 μm is passing through an air sample. If one alternately measures the intensity of the investigated beam and the reference, a percentage of ethanol in the air sample can be estimated using a sensitive nondispersive infrared (NDIR) system with a stable operating flow mass detector. To eliminate a mechanical chopper and noise generating stepper motors, a photonic chopper as a liquid crystal shutter for λ = 3.42 μm has been designed. For this purpose, an innovative infrared nematic liquid crystal mixture was intentionally prepared. The working mixture was obtained by a selective removal of C H bonds and its exchange by heavier polar substituents, what ensures a lack of absorption band of C H bonds. The paper presents theory, concept and final experimental results of the infrared nematic liquid crystals mixture and the liquid crystal shutter for breathalyzer applications.

An experimental study of single-film wide gamut multi-twist retarders

ABSTRACTMulti-Twist Retarders (MTRs) are a class of complex retarders created by stacking multiple layers of various polymerizable nematic liquid crystal mixtures, where a chiral dopant may be added to each layer to control its twisting power. MTRs have a set of twist angles and thicknesses for each layer that are selected by use of a merit function. In this work, we utilize these degrees of freedom to create a single retarder film that can generate a variety of highly-chromatic retardation spectra, each selectable by the relative orientation of the MTR to the input polarization. Our goal in tuning is generation of multiple distinct color primaries, which when combined with patterning can generate full-color images in a single-film of liquid crystal.

Segregation of liquid crystal mixtures in topological defects

The structure and physical properties of liquid crystal (LC) mixtures are a function of composition, and small changes can have pronounced effects on observables, such as phase-transition temperatures. Traditionally, LC mixtures have been assumed to be compositionally homogenous. The results of chemically detailed simulations presented here show that this is not the case; pronounced deviations of the local order from that observed in the bulk at defects and interfaces lead to significant compositional segregation effects. More specifically, two disclination lines are stabilized in this work by introducing into a nematic liquid crystal mixture a cylindrical body that exhibits perpendicular anchoring. It is found that the local composition deviates considerably from that of the bulk at the interface with the cylinder and in the defects, thereby suggesting new assembly and synthetic strategies that may capitalize on the unusual molecular environment provided by liquid crystal mixtures.

Bistable Smart Window Based on Ionic Liquid Doped Cholesteric Liquid Crystal

We develop a bistable smart window based on mixture of negative dielectric nematic liquid crystal, chiral dopant, and chiral ionic liquid (CIL). The developed smart window is driven by dual-frequency modulation, where the transparent state is turned on under an electric field with high frequency (4 kHz), and the scattering state is switched on by an electric field with low frequency (20 Hz). The dual-frequency characteristics of the proposed smart window have been extensively investigated. This smart window has intrinsic features including dual-frequency modulation, simple fabrication process, and excellent stability. This smart window is capable of practical use in modern life as a building wall, sunroof, and curtain-free window.

Nematic liquid crystal mixtures for 3D active glasses application

ABSTRACTMethods of 3D imaging present on the market can be divided into two basic groups: autostereoscopic and stereoscopic. At the stereoscopic methods of 3D imaging it is necessary to use special glasses for separation of images constituting stereo-pair. The most commonly used active glasses consist of two electro-optical transducers based on twisted nematic or other electro-optical effects. Here the evaluation of liquid crystal mixtures is designed intentionally for 3D active glasses in order to eliminate drawbacks of active glasses available on the contemporary market. Material parameters of new designed liquid crystal mixtures such as: elastic constants, viscosity, refractive indices, dielectric and spectral properties as well as response times have been determined. In addition to the symmetrisation of the switching times of 3D active glasses, a dual frequency nematic liquid crystal has been proposed.

Optical measurement of flexoelectric polarisation change in liquid crystals doped with bent-core molecules using hybrid-aligned structure

ABSTRACTWe proposed an optical measurement method for determination of flexoelectric polarisation change in liquid crystals (LCs), which can be induced in highly distorted LC geometries. A hybrid-aligned nematic LC (NLC) mode was introduced to evaluate the direction and magnitude of the flexoelectric polarisation. We measured the DC offset amounts for equivalent brightness levels between forward and reverse bias vertical electric fields to estimate the sign and magnitude of es−eb of flexoelectric coefficients. Additionally, the optical incident angle (αmax) for the maximum effective birefringence was investigated to predict the depth distribution of the LC director affecting the magnitude of the flexoelectric polarisation. The relationship between the variations of the DC offset and αmax by the flexoelectric polarisation changes was examined using the NLC mixtures doped with three selected bent-core LCs.

Submillisecond-response nematic liquid crystals for augmented reality displays

We report a new nematic liquid crystal (LC) mixture with an ultra-low rotational viscosity (γ1 = 53.4 mPas @ 35°C), relatively high birefringence (Δn ≈0.15), and moderate dielectric anisotropy (Δe = −2.80 @ 35°C). When employed in a liquid-crystal-on-silicon (LCoS) projector with RGB light-emitting diodes (LEDs), a sub-millisecond response time is obtained without the need for complicated overdrive circuitry. Such a fast response time enables field sequential color display, which not only triples the optical efficiency and resolution density, but also greatly suppresses the image blur and color breakup. Moreover, the required cell gap is 1.2 µm, which is still manageable for high-yield manufacturing. We believe this mixture would find widespread applications for the emerging augmented reality displays.

Self-assembled liquid crystalline nanotemplates and their incorporation in dye-sensitised solar cells

Liquid junction dye-sensitised solar cells (DSSCs) suffer from solvent evaporation and leakage which limit their large-scale production. Here, we have prepared DSSC using a simple and cheap fabrication process with improved photovoltaic parameters and stability. A binary mixture of Smectic A (SmA) and Nematic Liquid Crystal (NLC) was used to provide a self-assembled template for a polymerisable reactive mesogen LC. The layered structure of SmA combined with a low viscosity NLC forms a polygonal structure that provides an ordered and continuous template for reactive mesogens. Once the reactive mesogen is polymerised under UV light, the SmA:NLC mixture is washed away, resulting in a polymer network template containing nanochannels. We demonstrate the incorporation of these templates into DSSCs and find that DSSCs containing these nanochannels show improved open-circuit voltage (VOC) (0.705V) and short-circuit current (JSC) (13.25mAcm−2) compared to that of the liquid electrolyte (VOC=0.694V and JSC=10.46mAcm−2). The highest obtained power conversion efficiency with Sm-PE was 5.94% which is higher than that of the reference solar cell (5.51%). These can be attributed to the improved ionic conductivity and ionic diffusion of Sm-PE where the presence of the nanochannels aided the ionic conduction in the polymer electrolyte. In addition, it is hypothesized that the light scattering effect of the polymerised reactive mesogen also contributed to the improved performance of the photovoltaic devices. This finding is important because it is known fact that when a polymer is added to liquid electrolyte, the ionic conductivity will decrease although the stability is improved.

High contrast switching of transmission due to electrohydrodynamic effect in stacked thin systems of liquid crystals.

We study the opportunity of using electrohydrodynamic instabilities in a nematic liquid crystal mixture with negative dielectric anisotropy for controlling laser beams. Switching between naturally transparent and diffuse light scattering states is achieved by application of low frequency, low amplitude voltages. The specifics of diffuse light scattering state depending on the orientation and thickness of the liquid crystal layer are revealed. The switching occurs on a milliseconds time scale. Combination of thin, flexible liquid crystal cells allows polarization independent, high contrast, fast switching in a broad band of visible wavelengths.

Blue phase liquid crystal phase transition for cyano compound chiral nematic liquid crystal mixtures with three two-ring core structures and chiral dopant concentrations

ABSTRACTBlue phase (BP) temperature range of a chiral nematic liquid crystal (LC) mixture is dependent upon the host nematic LC chemical structure and chiral dopant concentration. In this study, we investigated BP phase transition behaviour and helical twisting power (HTP) using three chiral dopant concentrations of cyano compound chiral nematic LC mixtures incorporating three two-ring core structures in the host nematic LCs. The effect of the host nematic LC core structure, HTP and chiral dopant concentrations were considered on BP temperature ranges, for two types of complete BPI and BPII without isotropic phase (Iso) and two types of coexistence state of BPI+Iso and BPII+Iso.

Phase and structural order in mixture of nematic liquid crystals and anisotropic nanoparticles

ABSTRACTWe studied the structural order in randomly perturbed nematic liquid crystals using a random anisotropic nematic-type lattice model. Simulations were carried out at zero temperature and at various other temperatures, for which we varied the concentration of impurities p, anchoring strength W and imposed external strength fields B at two different histories: temperature-quenched history (TQH) and field-quenched history. The Haller approximation method utilized the temperature dependence of the order parameter to determine the pseudo-critical exponent β at different values of p and W. Consequently, the values of β increased when p and W were increased. Additionally, applying an external field B for investigating the parameters revealed that the TQH established long-range order for high magnitudes of B at zero temperature (T* = 0) and T* = 0.5.

Improvement of the blue phase stability in chiral nematic liquid crystal mixtures

ABSTRACTThe method of widening of the blue phase (BP) temperature range is presented. By means of polarising optical microscopy, we demonstrate that the temperature regime in which BP is stable is greatly enhanced by the cell thickness changes in mixtures of nematic liquid crystal (LC) and chiral dopant.

Viscosity of Liquid Crystal Mixtures in the Presence of Electroconvection

We have experimentally investigated the viscosity of nematic liquid crystal mixtures of p-methoxybenzylidene-p′-n-butylaniline (MBBA) and p-ethoxybenzylidene-p′-cyanoaniline (EBCA) in the presence of electroconvection under an ac electric field with 60 Hz. Although the viscosity of the mixtures with negative dielectric anisotropy shows a characteristic decrease in the high-voltage regime, that with positive dielectric anisotropy shows a monotonic increase as the applied voltage is increased. The experimental results suggest that the decrease in viscosity observed only for the mixtures with negative dielectric anisotropy is attributed to the negative contribution of electric stress caused by the anisotropic director distribution of the turbulent state.

Pattern Formation in a Nematic Liquid Crystal Mixture with Negative Anisotropy of the Electric Conductivity-A Long-Known System with "Inverse" Light Scattering Revisited.

Pattern formation in binary calamitic liquid crystal mixtures with positive dielectric anisotropy and negative conductivity anisotropy, which attracted attention owing to field-induced light scattering under unusual conditions many years ago, is reinvestigated in the conductive regime. Homeotropic cells with these mixtures exhibit a direct transition to isotropic electroconvection, while planar cells show a Fréedericksz transition to the quasi-homeotropic state and subsequent electroconvection at higher voltages. A temperature-induced change from normal and oblique convection rolls to longitudinal rolls reveals a transition from standard electroconvection to nonstandard electroconvection, which can be attributed to a sign inversion of the conductivity anisotropy. In summary, this system shows an unusually large variety of patterns and effects, which were observed and theoretically considered more recently in other systems of quite different types.

Numerical approximations to a new phase field model for two phase flows of complex fluids

We derive a new phase field theory for immiscible mixtures of nematic liquid crystals and viscous fluids using the variational principle coupled with the generalized Onsager principle. A novel phase transition mechanism is implemented to couple the nematic liquid crystal phase with the viscous fluid phase to arrive at the dissipative hydrodynamic model for incompressible fluid mixtures. Through a delicate explicit–implicit numerical discretization, we develop a decoupled, linear scheme for a simplified version of the phase field model, as well as a coupled, nonlinear scheme for the full model. Both schemes are shown as unconditionally energy stable with consistent, discrete dissipative energy laws. Several numerical examples are presented to show the effectiveness of the new model and the new numerical schemes developed for it.

Experimental studies on the phase diagram and physical properties of mixture of calamitic and discotic nematic liquid crystals

ABSTRACTPhysical studies on mixture of calamitic and discotic nematic liquid crystals are meagre although they are potential for optimising physical properties. Here, we report experimental studies on the phase diagram and physical properties of mixtures of ambient temperature discotic and calamitic nematic liquid crystals. A substantial decrease in several physical properties such as birefringence, dielectric anisotropy and elastic constants are observed with increasing wt% of discotic compound. On the other hand a large increase in the rotational viscosity is observed. Based on the experimental results a simple model of mutual orientation of the rod-like and disc-like molecules is proposed.

Polar flexoelectric in-plane and out-of-plane switching in bent core nematic mixtures

Polar electro-optic response, arising from the coupling between an applied in-plane and out-of-plane dc electric field, respectively, and the flexoelectric polarization of bent core nematic liquid crystal mixtures with hybrid alignment is studied in conventional sandwich cells with homeotropic anchoring at one of the cell substrates and planar at the other. Such a hybrid alignment, however, results in a splay/bend elastic deformation of the nematic giving rise of a flexoelectric polarization. It was found that a pronounced polar electro-optic response, both in-plane and out of plane, took place in the bent core nematic mixtures at very low voltages due to the high flexoelectric polarization in these mixtures, compared with the one observed in calamitic liquid crystals.

The effects of carbon nanotubes on the clearing transition of the antiferroelectric liquid crystal MHPOBC

abstractWe present a first attempt to study the effect of carbon nanotubes (CNTs) on the clearing transition, from SmA* to isotropic liquid phase, of the antiferroelectric liquid crystal 4'-Octyloxy-biphenyl-4-carboxylic acid 4-(1-methyl-heptyloxycarbonyl)-phenyl ester (MHPOBC), following two methods to introduce the CNTs. On the one hand, they are dispersed before in the nematic liquid crystal mixture E7 and then mixed with MHPOBC. While it allows decent quality of the CNT dispersion, the effects of E7 itself on the MHPOBC phase sequence is very strong. As a complementary approach to avoid this effect, we therefore also dispersed dry CNT powder directly in MHPOBC.

Temperature variation of effective geometry parameter, αg, refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

ABSTRACTOrientational order parameter, S and effective geometry, αg are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is αg = no/ne. (no and ne are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of αg as a function of temperature and order parameter has also been reported. The S value calculated from αg is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dne/dT and dno/dT are obtained and the crossover temperatures, TCO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dno/dT.

Thermodynamic and acoustical study of zinc oxide-nematic liquid crystals mixtures

Thermodynamical and acoustical studies have been carried out in nematic liquid crystal (8CB) and zinc oxide (ZnO) nanoparticle doped 8CB mixtures. The effect of nanoparticle concentration on the ultrasonic velocity and density has been studied using density and sound velocity analyzer. Adiabatic compressibility, molar compressibility, sound velocity, acoustic impedance and intermolecular free length have been estimated. Density and sound velocity decrease with the increase of the temperature and ZnO concentration, however intermolecular free length increases with increasing the temperature and ZnO concentration. The results have been compared with pure nematic liquid crystal mixture.