Nematic Mixtures Research Articles

Widely tunable reflection by a hybrid ion-doped positive-negative chiral nematic liquid crystal under an electric field.

In this study, a new methodology, in which the cation and chiral agent are doped in positive and negative nematic liquid crystal mixture to form cation-doped positive-negative chiral nematic liquid crystals (PN-CNLCs), is proposed. These PN-CNLCs are capable of achieving a wide wavelength tuning range close to 90 nm under the action of a high-frequency electric field. The dielectric interaction between these two liquid crystals with different dielectric anisotropies in the electric field allows for the effective change of the pitch of the CLCs. We discovered the tilting of the pitch in the PN-CNLCs by a series of well-designed validation experiments. The experimental results prove the symmetrical helix is formed along the alignment direction in PN-CNLCs. Furthermore, the experiments demonstrated that an increase in voltage does not directly affect the pitch, but it increases the tilt angle of the helix axis with respect to the normal of the LC cell. This study offers, to our knowledge, a novel perspective on the tuning performance of CLCs under electric field conditions, by integrating the properties of positive and negative liquid crystals. This finding not only contributes to the understanding of the electric field response mechanism of CLCs but also provides new avenues for the application of CLCs in fields such as wavelength tuning.

Controlling the dielectric constant values of nematic liquid crystals using ferroelectric and multiferroic nanoparticles

Notably, the simultaneous manifestation of ferromagnetic and ferroelectric properties at ambient temperature has spurred significant interest in incorporating multiferroic bismuth ferrite nanoparticles (BiFeO3) into liquid crystal matrices. In this study, the parallel and perpendicular dielectric components ( and ) and dielectric anisotropy () of two different nanoparticles, barium titanate (BaTiO3) and BiFeO3, doped into E7, were measured at various temperatures and frequencies. Nematic mixture E7, a widely utilized nematic liquid crystal in research and display technologies, is a eutectic mixture composed of four cyanobiphenyl derivatives i.e. 5CB, 7CB, 8OCB, and 5CT. The findings indicate a substantial reduction in dielectric anisotropy upon introducing nanoparticles into the nematic liquid crystal environment. This research highlights the significant influence of barium titanate nanoparticles, which are smaller (approximately 12 nm), on the dielectric constant. This effect was analyzed based on varying concentrations and compared to a nanoparticle with ferroic properties. Moreover, the dielectric constant can be modulated through an alternative approach. Specifically, applying an electric field during the filling process of liquid crystal cells leads to changes in the dielectric constant. These changes, observed for both nanoparticles, were particularly evident at a precise concentration of 0.1 wt%, resulting in a decreased dielectric constant.

Topological transition to rest in the electrohydrodynamics of nematics

ABSTRACT Some nematic liquid crystals undergo electro-convective instabilities if forced by a low-frequency oscillating electric field. When the forcing amplitude is increased, a series of bifurcations occurs, which correspond to increasing disorder and reduction of the size of the electro-convective rolls. At a threshold, a transition to an electrohydrodynamic state characterised by a high density of topological defects occurs. Here, we show that in some nematic mixtures this transition is replaced by a topological transition to rest, that is, the electrohydrodynamics are quenched by the nucleation of a state that is topologically non-equivalent to the initial texture of the nematic.

Binary Nematic Liquid Crystals Mixture with Enhanced Electro-Optics Properties for Photonic Applications

Purpose: In this work, we mix two simple nematic liquid crystals (NLCs) and investigated the binaryNLCs mixtures of 7CB/PCH5 of different mixing ratios. Methodology: The pure liquid crystals 7CB and PCH5 and binary mixtures of them of high temperature stability were thermally analyzed by differential scanning calorimetry. The mixture 7CB/PCH5:30/70 wt% has the highest thermal stability with a nematic-isotropic (N-I) transition temperature at 50oC. The electrooptic properties of 7CB, PCH5, and the mixture 7CB/PCH5:30/70 wt% at room temperature were also investigated using an amplitude modulated electric signal (1 kHz - 100 Hz) by increasing diving peak voltage from 0 V to 10 V. The threshold volage is relatively reduced for the binary mixture in comparison to that value for PCH5. In comparison to the pure LCs, the mixture 7CB/PCH5:30/70 wt% has the fastest response times of values 2.36 ms total time response, 0.41 ms rise time, and 1.95 ms fall time. It has also the highest contrast ratio. Moreover, it has a maximum measured transmission that is higher than those for PCH5 and 7CB by about 17 % and 8%, respectively, at a field strength of 2V/mm. Findings: The obtained results indicate that the electrooptic properties of PCH5 was improved when mixed with a proper ratio of 7CB, of lower cost, more stablity , and higher potential for photonic applications. Unique Contriburibution to Theory, Practice and Policy: This expermental study shows that simply by mixing two relatively low cost NLCs materials, one of high thermal stability and low electro-optic properties with other one of low thermal stability and better electro-optic properties; this would improve the stability, response, and transmition of the binary mixture. If the a suitable driving method is applied, without doping with other orgnic or inorganic matrial.

Dual-frequency-addressable photochromic cholesteric mixture with photo- and electro-switchable optical properties

Cholesteric liquid crystalline mixture with photo-, electro-controllable optical properties was prepared and the features of its switching behavior were demonstrated. The working principle of the obtained switchable cell is based on using dual-frequency-addressable nematic mixture that enables effective control of selectively reflected planar and scattered focal conics texture by applied field frequency modulation. The nematic mixture was doped with the chiral substance in order to induce a cholesteric structure formation with selective light reflection in visible spectral range. Azobenzene-containing photochrome was also introduced into this mixture in order to stimulate photo-optical response. An application of low-frequency electric field as well as UV irradiation induces short-wavelength shift of the selective light reflection. Under action of an electric field, a transition to scattered focal conics texture takes place. Amplitude of the light-induced shift is ca. 50 nm, while the electric field leads to a shift ca. 120 nm. On the contrary, high-frequency electric field induces fast (less than 1 s) restoration of perfect highly reflected planar texture. UV irradiation leads to not only shift of the selective light reflection band, but alters the electrooptic response by increase of frequency necessary for the restoration of the planar state and considerably changes the switching kinetics. The possibility of photopatterning of the obtained electrooptic cells by UV irradiation through a mask is demonstrated. It is shown, that application of electric field of different strength and frequencies enables smart control of the photorecorded image contrast.

Estimation of Ionic Impurities in Poly(propylene Glycol) Diacrylate Monomers/Liquid Crystal E7 Mixtures Using Dielectric Spectroscopy

The study investigated the effect of the molecular weight of three difunctional poly(propylene glycol) diacrylates on the temperature-dependent ionic conductivity of these monomers and their blends with an eutectic nematic liquid crystal mixture (E7). The results revealed two distinct regions. At low temperatures, ionic conduction can be described by the Vogel–Tamman–Fulcher (VTF) equation, while at high temperatures, the conductivity data follow the prediction of the Arrhenius model. The Arrhenius and VTF parameters and their corresponding activation energies were determined using the least squares method. In addition, a conductivity analysis based on an ionic hopping model is proposed. Estimates of ion concentrations and diffusion constants were calculated. It was found that both the ionic concentration and the diffusion constant decrease with the increase in the molecular weight of the monomers. The static dielectric permittivity decreases in the following order: TPGDA, PPGDA540, and PPGDA900. This can be explained by the higher dipole moment of TPGDA, which is caused by an enhanced volume density of carbonyl groups.

Size-dependent optical and dielectric anisotropy of CdS nanowire doped high-birefringent nematic liquid crystalline compound

Highly birefringent liquid crystals have grabbed ample attention because of their potential applications as an active medium for devices in visible, infrared and other spectral regions. Here, we have attempted to study the effect of the size and concentration of CdS nanowires on a nematic liquid crystal. Anisotropy in liquid crystals is one of those distinguishing traits that gain their superiority over other materials. We hereby have analyzed how optical and dielectric anisotropy respond to variations in the size and concentration of the dopant. The study is of significant scientific value because there is very little literature available on how dopant size can alter liquid crystalline properties and also introduces a method to tune the properties of pure liquid crystals (LCs) by varying the size and concentration of the dopant. The study explains how the size of the nanowires changes the analyzed parameters and up to what extent. The obtained results yield a significant increase in both optical and dielectric anisotropy by doping CdS nanowires making them an excellent choice for dopant selection. CdS-doped nematic LC mixtures have proved to be highly birefringent and possess a high positive dielectric anisotropy as well. Such a compound can be of great application in device fabrications such as laser beam steerers, electronic lenses, attenuators and light shutters, etc.

Dynamic processes in the liquid crystal-water emulsion under shear flow and electric field

We present the results of experimental investigation of dynamical processes arising in liquid crystal-water emulsion under action of a decay shear flow and ac electric field. The 10 : 1 emulsion, consisting of the well-studied nematic mixture E7 and ionized water was prepared by usage of ultrasonic (29 kHz) mixer. It made possible to get a number of water droplets with the distribution of diameters in the range 2… 20μm. So, the slow decreasing of sizes of these instabilities resulted in corresponding slow motion of the attracted water droplets. The strong electric filled, which stabilized the initial homeotropic orientation of LC suppressed the flow induced instabilities and the slow motion of water droplets mentioned above. It also induced the aggregation of droplets in the vicinity of boundary between regions with strong field and free of field. So, combined action of shear flow and electric field can be effectively used for manipulation of the motion and interaction of water droplets. The obtained results are discussed within the existing theory of liquid crystals.

Crystal Smectic G Phase Retarder for the Real-Time Spatial-Temporal Modulation of Optical Information

We have manufactured and investigated a novel phase retarder based on a rare and less studied liquid crystal phase, such as the Crystal Smectic G-phase prepared by mixing two certified nematic mixtures. The phase retarder is transparent in the visible and near-infrared parts of the optical spectrum. The temperature stability over a wide temperature range, high birefringence, and high strength, allow the production of various types of phase retarders that can be used in optics, optical chemical analysis,and polarimetry.

Novel properties of high-performance multi-component mixture for Vertically Aligned mode LCDs

ABSTRACT Vertically Aligned mode Liquid Crystal Displays (VALCDs) which have generated huge research interest due to their excellent contrast and wide viewing angle require materials with negative dielectric anisotropy. This work highlights the systematic development of a fifteen-component mixture from pure liquid crystalline compounds comprising laterally fluorinated bi-, tri- phenyl, tolane, bicyclohexane compounds and non-mesogenic compounds exhibiting negative dielectric anisotropy. The temperature dependence of birefringence (Δn), dielectric anisotropy (Δε), threshold voltage (Vth), bend elastic constant (K33), relaxation time (τ0) and rotational viscosity (γ1) as a function of temperature of this fifteen-component nematic mixture have been reported. This mixture exhibits a high Figure of Merit (FoM) which is desirable for reducing the device response time. Additionally, the orientational order parameter (OOP) values, activation energy (Ea), visco-elastic co-efficient (γ1/K33) and material parameters (A 0 ) have also been reported. The effect of pretilt angle in the alignment layers on the threshold voltage (Vth) and switching time (τ0) to ascertain the applicability of this material in Vertically Aligned mode Liquid Crystal Displays (VALCDs) have also been discussed.

Optical properties and zeta potential of polyvinyl pyrrolidone capped gold nanoparticles dispersed nematic liquid crystal mixture E7

The effect of dispersion of polyvinyl pyrrolidone (PVP) capped gold nanoparticles in nematic mixture E7 on the optical properties (absorbance and photoluminescence, fluorescence life time and Raman imaging) and zeta potential is studied. The gold nanoparticles of 0.25, 0.5 and 1.0 wt% concentrations are dispersed into E7. It has been found that the absorbance and photoluminescence intensity of dispersed systems are increased and depend on the GNPs concentration. These increase correspond to the coupling between the radiation field (electromagnetic waves of the light source) and the GNPs low-energy phonons and constructive coupling of emissions between the E7 and GNPs, respectively. The change in the fluorescence lifetime of pristine and GNPs dispersed E7 systems has confirmed the energy transfer between E7 and GNPs molecules. The study of Raman spectrum shows the variations in the intensity of the Raman peaks with different orientations. Due to dispersion of GNPs in E7 the values of zeta potential are increased. This ensures the good stability of the dispersed systems. The conductivity of dispersed system has increased drastically. In conclusion, the dispersion of GNPs in host nematic mixture E7 has influenced significantly its properties, thereby making the dispersed material more promising for the potential applications.

Millisecond-Response Nematic Liquid Crystal for Augmented Reality Displays

Developing fast-response liquid crystals (LCs) is an essential way to achieve low cost, high resolution, and good visual experience for augmented reality (AR) displays. In this paper, we optimized one new nematic LC mixture SNUP01 to meet the requirements of fast-response phase-only liquid crystal on silicon (LCoS) devices in AR displays. The photoelectric performance of this new LC mixture and three commercial LC mixtures were further comparatively evaluated, and the 2π phase-change response speed of this new LC mixture was extrapolated. The research results indicate that SNUP01 possesses high birefringence, moderate dielectric anisotropy, low viscoelastic coefficient, low activation energy, and high figure of merit values. When using this LC mixture at 25 °C @ λ = 633 nm, a 2π phase change can be achieved at 5 V with a total response time of up to millisecond response. Widespread applications of this LC mixture for AR displays are foreseeable.

Numerical deconvolution approaches for dielectric characteristics of complex composite materials based on liquid crystals and oxide nanopowders

Dielectric spectroscopy is a well-known method to characterize different liquid or solid (nano) materials such as liquid crystals, polymers, composites, ceramics etc. More the material structure is complex, more the molecular dynamics are put in evidence with difficulty. In many such cases, the analysis of the spectra with the classic Havriliak-Negami (H–N) functions is hard to apply or even fails. Therefore, we propose a simple numerical approach (with two different procedures) for the deconvolution of complex spectra in dielectric spectroscopy (DS). The final goal is to obtain the frequency of maximum loss (fmax) depending on the temperatures, Arrhenius diagram, and the activation energy/energies. The developed procedures have as their starting point the “logarithmic derivative of the permittivity”. To our knowledge, these procedures have not yet been presented in the literature. The proposed first numerical processing allows a better separation of the different relaxation processes, a decrease in the contribution of the electrical conductivity and a better localization of the frequencies where the dielectric loss has maximum values. The second of the procedures is applied in certain particular situations, and the other procedure is more general. Where the former can be applied, it is equivalent to using the classical H–N functions in terms of obtaining the fmax, but it is simpler to apply. The first procedure uses an “artificial” and simpler H–N function, in which the fewer fitting parameters have no physical meaning, but allow a good localization of the frequency of maximum dielectric loss. The proposed procedure was applied on experimental data of dielectric spectroscopy obtained on pristine materials (nematic mixture E7 and ZnO) and composites E7-ZnO, obtained by a “green technology”. The complicated spectra of the dielectric permittivity presented by these samples represent the suitable test to highlight the advantages but also the limitations of our approach. Comparing the values of the activation energy obtained for the two procedures is the criterion for verifying their correctness. The volume of processed experimental data is large, for this reason we present only the results obtained based on numerical approaches. The differences between the results obtained with the two procedures are small. The procedures can be considered as alternatives to the application of fitting with H–N functions. One procedure replaces H–N by eliminating the use of functions with complex variables and the method of least squares and has the advantage that it can apply non-specialized software like Origin. The other allows a better analysis of spectra being an improved-modified H–N approach. Proposed procedures are sufficiently effective to be used in the dielectric spectroscopy data analysis of a wide variety of liquid crystals, composites, ceramics, nanomaterials, etc.

Electrochromic reverse-mode PDLCs as smart windows

ABSTRACT The reduction of energy consumption is according to the EU Parliament one of the most important actions to reduce the use of fossil fuels and the emission of carbon dioxide to contrast climate changes. Smart windows, and in particular PDLCs, are an easy way to increase the energy efficiency of buildings and promote energy saving. They have found application as large area surfaces able to change on-demand their transmittance in a continuous manner from an opaque to a transparent state in many fields such as interior design, architecture, and automotive. In this work, we report the preparation and characterisation of a reverse mode PDLC, in which the nematic mixture of a liquid crystal and an ionic liquid allows the on-demand modification of both the transmittance and colour of the device. The transparent and uncoloured OFF state, which is the suggested state from a safety and energy saving point of view, can be modulated into an opaque and/or coloured state by the application of an AC and/or DC electric field.

Sensitive liquid crystal composites for optical sensors

The paper presents a comparative analysis of liquid crystal cholesteric-nematic mixtures for their use as sensitive elements of gas sensors. Liquid crystal mixtures based on nematic mixture E7 and cholesteric impurity CB15 were considered. In order to determine the optimal mixture, a study of the spectral characteristics of the mixtures was carried out, and the main transmission peaks were determined for different concentrations of the optically active impurity. The temperature characteristics were measured to determine the threshold values for the transition of the mixtures to the isotropic state and stable temperature areas. As a result of the conducted research, we can state that as a sensitive element of optical sensors, it is advisable to use liquid crystal mixtures with impurity concentrations of CB15 of 38% and 44%. Their main transmission peaks are in the visible region of the spectrum of 500-600 nm. The temperature characteristics are slightly better in the 38% mixture, but we can claim a slightly higher sensitivity to organic substances in the 44% mixture. Further research of mixtures can be carried out during the direct development of optical sensors.

ZnO nanoparticles dispersed cholesteric liquid crystal based smart window for energy saving application

Electrically tunable smart window has been fabricated by using green synthesized Zinc oxide nanoparticles (ZnO NPs) dispersed in cholesteric liquid crystals (CLC). The fabricated device is semi-transparent in the planar state, opaque in the focal conic state and fully transparent in the homeotropic state. Interestingly, switching among these states is possible by just varying the applied voltage. The proposed device can even regulate the ambient temperature by controlling the sunlight. This type of smart window is developed by doping of ZnO NPs into Chiral nematic LC mixture, interestingly, as the concentration of ZnO NPs increases the transmittance decreases in Focal conic and it increases in Homeotropic state. The transmittance of NPs doped CLC is decreased from ∼87 to 4 % in the focal conic state (2.4 V/μm) and increased 4 % to ∼ 90 % in the homeotropic state (7 V/μm) with a short response time (< 1 ms). The versatility exhibited by the device showed good stability even after repeatedly turning the electric field on and then off for 170 s. Quantitatively, after 20 cycles of repeated switching, the spectral transmittance in each state remained essentially constant. We have demonstrated the operation of privacy windows alongside smart windows by exploiting the capabilities of NPs and CLCs to reduce reliance on artificial lighting, heating and cooling building operation costs. The indoor temperature-regulation effects of Pure CLC and ZnO doped CLC devices were investigated using a solar simulator device. In order to regulate interior temperature and to conserve energy, it is anticipated that this technology may be used as windows for automobiles and buildings to control temperature and save energy.

52‐3: Field sequential color LCD for enabling 60‐PPD and 100°‐FoV VR displays

To achieve 60 PPD and 100° FoV while keeping a reasonably high aperture ratio for AMLCDs, field sequential color (FSC) is a promising approach. We evaluate the physical properties of a high birefringence nematic LC mixture and then use these data in simulation. Results indicate that such a fringe‐field switching LCD exhibits a fast average gray‐to‐gray response time (~ 1.5 ms) to enable FSC operation for VR applications.

Electrooptical switching and photopatterning of luminescence in nematic liquid crystals doped with mesogenic europium complex

Stimuli-responsive self-organized soft matter such as liquid crystals, polymers, gels attracts great attention due to a variety of possible applications in modern technologies. Electroinduced hydrodynamic (EHD) instability in liquid crystals is one of long-discovered effects that shows a great potential for future applications. However, this effect is insufficiently studied and its practical implementation has yet to be found. In the present paper, we applied an EHD instability to tuning luminescent properties of nematic mixtures doped with a highly emissive europium complex, which has good solubility in a liquid crystalline (LC) matrix and high quantum yield of luminescence. Nematic mixtures studied in this work possess negative anisotropy of dielectric constant and positive anisotropy of conductivity that allow for observing convective EHD patterns induced by ionic conductivity. The influence of field frequency on the threshold voltage was studied and optimal conditions for induction of EHD patterns were determined. The luminescence intensity was found to substantially decrease or increase under an applied electric field depending on the alignment behavior of LC molecules inside the cells and the angle of observation. In a tilted cell (60°), the emission intensity gradually increases under applied electric field and reaches values ca. 2.5 times higher than in zero field conditions. This phenomenon is associated with alignment of the europium complex molecules along electric field direction (cooperatively with LC molecules) and EHD-induced light scattering. Kinetics of electrooptic switching was studied and different approaches to luminescence photopatterning of the studied nematic mixture with polymerizable monoacrylate and diacrylate were demonstrated.

Surface species of the nematic mixture E7 obtained by electrochemical insertion of Li+ ions.

We studied here the influence of Li+ ions on the benzene rings of nematic mixture E7, which is electrochemically adsorbed onto gold electrode surface, to highlight the ability of this mixture for the applications in the field of the rechargeable Li+-ion batteries. Raman spectra support the changes observed in electrochemical analyses while contact angle measurements show that wetting properties of E7 layer were modified after deposition of this mixture onto gold support and the doping with Li+ ions. Contact angle of acetonitrile drops and Raman spectrum of the film of E7 nematic mixture are two main properties which show deep modifications into the film by electro chemical deposition.

Surface species of nematic mixture E7 in hard confinement: spectroscopic investigations cannot distinguish among the E7 components interacting with the support surface

ABSTRACTProperties of surface species of the nematic mixture E7 in contact with inorganic oxides (especially those containing more or less aluminium ions) are in detail investigated by infrared spectroscopy and thermogravimetry. The absorption infrared peaks were decomposed into Gaussian components, the procedure allowing an easier comparison with the behaviour of related composites. We worked at enough small concentrations of the E7 relatively to support this ratio permitting the (partial) elimination of the species belonging to the bulk. Species put here in evidence are species that are hydrogen bonded to the surface of OH groups, molecules randomly oriented on the surface, some species bonded by involvement of π electrons of the CN group to the aluminium ions, thus implying a strong interaction to the support. Supramolecular assemblies of E7 molecules are thus followed up.