Nematic Liquid Crystal Mixture Research Articles

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.

Sorption and selective chromatographic properties of isomer-selective composite sorbent based on a eutectic mixture of nematic liquid crystals and perbenzoylated β-cyclodextrin

Mesomorphic, sorption, and selective properties of a three-component sorbent based on a mixture of nematic (N) liquid crystals of 4-methoxy-4′-ethoxyazoxybenzene (MEAB) and 4,4′-diethoxyazoxybenzene (azoxyphenetol, AOP) of an eutectic composition and heptakis-(2,3,6-tri-O-benzoyl)-β-cyclodextrin (Bz-β-CD) are studied. For 30 organic compounds of different classes with linear and cyclic molecular structures, including optical isomers of limonene, pinene, camphene, and butanediol-2,3, thermodynamic functions are determined for their gas-phase sorption using a three-component MEAB-AOP-Bz-β- CD sorbent (62: 28: 10 wt %). It is found that the investigated sorbent possesses high structural selectivity (αp/m = 1.128–1.059, 100–130°C, N) and moderate enantioselectivity (1.07–1.02) within a broad temperature range (95–170°C) including both mesomorphic and isotropic phases of the sorbent. It is shown that the enantioselectivity of the sorbent is apparent under conditions of both increasing retention when a chiral Bz-β-CD additive is introduced into the MEAB–AOP system (limonenes, pinenes, camphenes) and decreasing retention (butanediols-2,3).

Two-step switching in dual-frequency nematic liquid crystal mixtures

The so-called dual-frequency nematic mixtures are very promising components for applications in fast operating devices. Compared with classical nematics, they exhibit positive or negative anisotropy of the electric permittivity depending on the applied frequencies of an external electric field. Owing to this property, an overall switching process from planar to homeotropic orientations, and vice versa, can be shortened by using the electric field with two different frequencies. Electro-optical switching characteristics of transmission versus time as a function of applied voltage were obtained for two different dual-frequency mixtures in twisted nematic cells. For one of the investigated mixtures, unusual decrease in the light transmission at switching from the homeotropic to planar orientation at threshold voltage was observed. The switching process apparently occurs in two steps. The mechanism of the two-step switching at twisted dual-frequency nematic structures was discussed. The explanation of the switching mechanism takes into account the influence of the electric field with different frequencies on molecules with transverse and longitudinal dipole moments. Moreover, molecular structure of compounds constituting the mixtures was analyzed. Additionally, response times of the switching driven with low and high frequency pulses were shown. This work helps to understand the molecular interaction and electro-optical switching in the dual-frequency nematic liquid crystals.

A random laser made of nematic liquid crystal doped with a laser dye

We report on random laser emission obtained in 5CB and E7 nematic liquid crystal (LC) mixtures doped with 1% weight to weight ratio of DCM laser dye. The LC cell was used as asymmetric planar waveguide were emission was collected from the edge of the sample. Variable stripe length method was utilized to estimate the gain and the losses coefficients. Both systems have shown the threshold energy fluence in order of several mJ/cm2. In both cases above Fredericks potential, significant increase of emission intensity was observed due to the increase of light scattering on liquid crystalline domains. Moreover the use of fifth order of diffraction grating covered with thin alumina film resulted in strong multimode and directional laser emission.

A decoupled energy stable scheme for a hydrodynamic phase-field model of mixtures of nematic liquid crystals and viscous fluids

We develop a linear, first-order, decoupled, energy-stable scheme for a binary hydrodynamic phase field model of mixtures of nematic liquid crystals and viscous fluids that satisfies an energy dissipation law. We show that the semi-discrete scheme in time satisfies an analogous, semi-discrete energy-dissipation law for any time-step and is therefore unconditionally stable. We then discretize the spatial operators in the scheme by a finite-difference method and implement the fully discrete scheme in a simplified version using CUDA on GPUs in 3 dimensions in space and time. Two numerical examples for rupture of nematic liquid crystal filaments immersed in a viscous fluid matrix are given, illustrating the effectiveness of this new scheme in resolving complex interfacial phenomena in free surface flows of nematic liquid crystals.

Light-controllable linear dichroism in nematics.

We report a method to obtain a light-controllable dichroism. The main effect is achieved using spiropyran-doped (SP-doped) nematic liquid crystal mixtures. SP molecules exhibit a high solubility in the liquid crystal host, which can vary between 1% and 4% in weight, without destroying the liquid crystalline phase. Due to their elongated shape, SP molecules are oriented along the nematic liquid crystal director. The obtained linear dichroism was measured to be 1.08 with a dichroic ratio of 7.12. Further, a two-direction linear dichroism was obtained by adding a dichroic dye to the mixture. The angle between the two dichroic axes was found to be 11°. Two-direction linear dichroism is also light controllable and can be switched back to one-direction dichroism.

Effect of nanoparticle chain formation on dielectric anisotropy of nematic composites

A general theory of the dielectric constant of nematic liquid crystal mixtures is presented including the particular case of nematics doped with polar nanoparticles. The results are used to estimate the contribution of chains of polar nanoparticles to the static dielectric anisotropy and birefringence of the nematic composite taking into account contributions from chains of different lengths. The dependence of the dielectric anisotropy on the dipolar interaction strength is considered in detail and it is shown that formation of polar chains of nanoparticles enables one to explain a significant increase of the dielectric constant of the composite as observed experimentally.

Asymmetric director structures and flexoelectricity in nematic pi-cell devices

The sum of the flexoelectric coefficients in a liquid crystal material has been measured in nematic pi-cell devices, based on a method that exploits the asymmetry in the director configurations of the different states in a pi-cell, the uniform surface alignment polarities, and the influence of ions. A value of |e1 + e3| = 10 pC m−1 was measured from data-theory comparisons in the standard commercial eutectic E7 nematic liquid crystal mixture.

‘Blue phases’ of highly chiral thermotropic liquid crystals with a wide range of near-room temperature

The phase transitions of pure cholesteric liquid crystals, based on right- or left-handed chiral dopants (ChDs) (enantiomers R-811 and S-811, Merck, Darmstadt, Germany) with various concentrations dissolving in the nematic liquid crystal mixture CHCA (ZhK-805, Russia, NIOPIK) were experimentally studied. For the first time, blue phases of cholesterics were found at the concentration range of ChDs 32–36 wt.%. Experimentally it was observed that during the cooling process the blue phase of cholesterics is stable over a wide temperature range about ~15°C including human body and near-room temperatures. Thermal phase transitions, spectral characteristics and electro-optical features of blue phases were examined. Planar and homeotropic alignment layers were used to study the influence of various boundary conditions on platelet textures of blue phases.

Properties of two-component nematic liquid crystal mixtures in the range of 0.3–3.0 THz

In this work, we present the results of terahertz measurements of five nematic liquid crystal materials. Optical properties like refractive indices, birefringence and absorption coefficients of liquid crystals (LCs) for ordinary and extraordinary polarisation were compared. The aim of this study was to check the impact of the structure of LC molecules on above properties. The compounds with different core structure and various number of laterally substituted fluorine atoms were chosen. A standard terahertz Time-Domain Spectrometer to measurements from 0.3 THz to 3 THz was used.

Photoalignment and resulting holographic vector grating formation in composites of low molecular weight liquid crystals and photoreactive liquid crystalline polymers

Polarization holographic gratings were formed in liquid crystal (LC) cells fabricated from a mixture of low molecular weight nematic LC and a photoreactive liquid crystalline polymer (PLCP) with 4-(4-methoxycinnamoyloxy)biphenyl side groups. The diffraction properties of the gratings were analyzed using theoretical models which were determined based on the polarization patterns of the polarization holography. The results demonstrated that vector gratings comprised of periodic orientation distributions of the LC molecule were induced in the cells based on the axis-selective photoreaction of the PLCP. The vector gratings were erased by applying a sufficiently high voltage to the cells and then were reformed with no hysteresis after the voltage was removed. This phenomenon suggested that the PLCP molecules were stabilized based on the axis-selective photocrosslink reaction and that the LC molecules were aligned by the photocrosslinked PLCP. This LC composite with axis-selective photoreactivity is useful for various optical applications, because of their stability, transparency, and response to applied voltage.

Helical Twisting Power Dependence of Blue Phase Stability for Chiral Nematic Liquid Crystal

In this study, we investigated blue phase (BP) temperature range and helical twisting power (HTP) using various types of chiral nematic liquid crystal (LC) mixtures composed of two kinds of chiral dopants and four kinds of host nematic LCs with different core structures. We found that chiral nematic LC mixtures containing three-ring core structure host nematic LCs such as 5CCB and 5CT presented small HTP values compared to those containing two-ring core structure host nematic LCs such as 5CB and 5CH regardless of chiral dopant species. We also found that the BP was not present when the HTP was of a magnitude less than 35.64 μm−1, that the BP temperature range increased when the HTP was of a magnitude between 35.64–49.03 μm−1, and that it decreased when the HTP was of a magnitude between 51.60 –63.03 μm−1 in cyano homologue chiral LC nematic LC mixtures.

Evolution of Microwave Nematic Liquid Crystal Mixtures and Development of Continuously Tuneable Micro- and Millimetre Wave Components

This work represents the evolution of micro- and millimetre wave optimized nematic liquid crystal mixtures and their applications. Starting with the well-known liquid crystal mixture K15 used in the display technology, microwave optimized LC mixtures have been developed. For that, novel characterization setups have been developed, which play an essential factor in the optimization process for the micro-, millimetre wave and THz regime. By using a specialized LC simulation tool, different tuneable waveguide topologies are compared in terms of tuneability, transmission loss and tuning speed. Based on that, a phase shifter for W-band frequencies has been fabricated, which reached a measured figure of merit for passive phase shifters of up to 148° /dB.

Molecular and Intramolecular Motions in Dual-Frequency Nematic Liquid Crystal, at Extra Low Temperatures, Observed by Means of Dielectric Spectroscopy

New dual frequency nematic liquid crystal mixture has been prepared and investigated by means of dielectric spectroscopy in a wide temperature range (from 120°C to −100°C). The molecular structures of compounds used to prepare the final mixture and mixture's composition influence its dielectric properties. Wide temperature investigations show how molecular and intramolecular motions change with temperature. It is possible at low temperatures to slow down intramolecular motions so much that they are visible in dielectric spectroscopy. At −80°C molecular motions disappear from dielectric response. At −100°C all thermally activated motions are too slow to be detected by means of dielectric spectroscopy.

Asymmetric Director Structures and Ions in the Measurement of the Flexoelectric Sum (e1 + e3)

Hybrid Aligned Nematic (HAN) devices filled with highly ionic liquid crystal materials screen out the internal bias due to surface polarization, making it possible to determine the flexoelectric sum, e1 + e3. A transient change is observed in the device response due to differing polarities of an external offset applied to an AC signal, with a time constant similar to the characteristic ion relaxation time in the material. Assuming ions screen out the internal bias, data-theory fits in the low frequency, low voltage perturbative regime gives a value of /verte1 + e3/vert = 10 pCm−1 for the standard commercial eutectic E70 nematic liquid crystal mixture.

Ultra-low viscosity liquid crystal materials

We report five ultra-low viscosity nematic liquid crystal mixtures with birefringence around 0.1, dielectric anisotropy in the range of 3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity mixtures is low activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm cell at −20°C is only 30 ms. Widespread application of these materials for display devices demanding a fast response time, especially at low temperatures, is foreseeable.

Hybrid aligned nematic based measurement of the sum (e1+e3) of the flexoelectric coefficients

A new method has been established for the measurement of the sum of the flexoelectric coefficients e1+e3 in liquid crystals by exploiting the properties of highly ionic materials in order to screen out the internal bias due to the different surface alignment polarities in a Hybrid Aligned Nematic (HAN) liquid crystal device. It has been shown that responses to pulses are independent of the external offset of a signal applied to a HAN device filled with a highly ionic material. Driving the device with step changes in the offset leads to either a transient increase or transient decrease in the response, depending on the polarity of the offset, while the equilibrium response remains the same. The time constant of the transient effect is consistent with the relaxation time of the ions present in the material. Assuming these ions screen out the internal bias completely, the remaining response can be used as a measure of the flexoelectric effect. Based on this approach, a value of (10 ± 2) pC m−1 was found for the modulus of the flexoelectric sum in the standard commercial eutectic E70 nematic liquid crystal mixture.

Immunoassays for the cancer biomarker CA125 based on a large-birefringence nematic liquid-crystal mixture.

The use of fluorescence is ubiquitously found in the detection of immunoreaction; though with good sensitivity, this technique requires labeling as well as other time-consuming steps to perform the measurement. An alternative approach involving liquid crystals (LCs) was proposed, based on the fact that an immunocomplex can disturb the orientation of LCs, leading to an optical texture different from the case when only antigen or antibody exists. This method is label-free, easy to manipulate and low-cost. However, its sensitivity was low for practical usage. In this study, we adopted a high-birefringence liquid crystal (LC) to enhance the sensitivity for the immunodetection. Experiments were performed, targeting at the cancer biomarker CA125. We showed that the larger birefringence (Δn = 0.33 at 20 °C) amplifies the detected signal and, in turn, dramatically improves the detection limit. To avoid signal loss from conventional rinsing steps in immunodetection, CA125 antigen and antibody were reacted before immobilized on substrates. We studied the specific binding events and obtained a detection limit as low as 1 ng/ml. The valid temperature ranges were compared by using the typical single-compound LC 5CB and the high-birefringence LC mixture. We further investigated time dependency of the optical textures and affirmed the capability of LC-based immunodetection in distinguishing between specific and nonspecific antibodies.

Enlargement of blue-phase stability for rod-like low-molecular-weight chiral nematic liquid crystal mixtures

In this study, we investigated the enlargement of liquid crystal (LC) blue-phase (BP) temperature range using the rod-like low-molecular-weight cyano phenyl-type chiral nematic LC with various core group and chiral dopant concentrations. Also, the electro-optic response time was investigated for them. We found that the BP temperature range was strongly dependent upon the core structure and the chiral dopant concentration for the chiral nematic LC mixtures having the same terminal group. Also, we found a stable BP with a wide temperature range (more than 6 K), including a BP-isotropic coexistence state over 13.5 K upon heating and cooling processes and very fast response time (less than 1 ms), by using the cyano phenyl-type chiral nematic LC mixture with a high molecular aspect ratio and a high chiral dopant concentration.

Dielectric properties of ternary composites of triblock polymer surfactant, liquid crystal and organic solvent

Dielectric spectroscopy, at room temperature (20°C), is used to study the dielectric response of ternary mixtures of commercial nematic liquid crystal mixtures E7 and E33, an organic solvent N-Methyl-2-Pyrrolidone (NMP) and a triblock polymers in the frequency range from 0.01 Hz to 1 MHz. The results indicate a dielectric relaxation in the hectohertz region. Individually, both E7 and NMP have rather large low frequency conductivities; however, the low frequency (0.01–10 Hz) behavior of the mixtures has no such behavior. We attribute this behavior to an ion getter effect of the triblock polymer surfactant. Optimized ternary mixtures obtain a real dielectric constant near 230, and loss tangent less than 0.05 at frequencies near 10 mHz.