Nematic Liquid Crystal Phase Research Articles

Polarization singularities in the self-focusing of an elliptically polarized laser beam in an isotropic phase of nematic liquid crystal close to the temperature of phase transition

ABSTRACTNumerical simulations of initially homogenously polarized Gaussian light beam self-focusing in isotropic phase of nematic liquid crystal, the temperature of which is close to the temperature of nematic-isotropic phase transition, were carried out. Here, we show that light polarization singularities (C-lines) of both polarization handednesses are nucleated near these transversal cross-sections of the beam, in which local extrema of peak intensities of corresponding circularly polarized components of the beam are attained. These lines are closed loops, surrounding the beam axis. They appear in wide range of beam and nonlinear medium parameters. Minor fluctuations of beam polarization and amplitude profiles do not destroy these singularities.

Mesogens' orientation at cylindrical and spherical surfaces during transition from isotropic phase to nematic liquid crystal phase

ABSTRACTStructure of an oriented layer of mesogens on the interface at the isotropic phase (IP) – nematic liquid crystal (NLC) transition has been studied by polarization microscopy method. The thickness of the oriented layer was found to be equal to 50 ÷1000 µm, it decreases with a reduction of the surface curvature radius. Analytical expressions for the phase delay difference arising at passing of plane-polarized light through the mesogen layer oriented towards cylindrical and spherical surfaces have been obtained. The order parameter in mesogens at the IP-NLC transition was determined.

Electric-field-induced transport of microspheres in the isotropic and chiral nematic phase of liquid crystals.

The application of an electric field to microspheres suspended in a liquid crystal causes particle translation in a plane perpendicular to the applied field direction. Depending on applied electric field amplitude and frequency, a wealth of different motion modes may be observed above a threshold, which can lead to linear, circular, or random particle trajectories. We present the stability diagram for these different translational modes of particles suspended in the isotropic and the chiral nematic phase of a liquid crystal and investigate the angular velocity, circular diameter, and linear velocity as a function of electric field amplitude and frequency. In the isotropic phase a narrow field amplitude-frequency regime is observed to exhibit circular particle motion whose angular velocity increases with applied electric field amplitude but is independent of applied frequency. The diameter of the circular trajectory decreases with field amplitude as well as frequency. In the cholesteric phase linear as well as circular particle motion is observed. The former exhibits an increasing velocity with field amplitude, while decreasing with frequency. For the latter, the angular velocity exhibits an increase with field amplitude and frequency. The rotational sense of the particles on a circular trajectory in the chiral nematic phase is independent of the helicity of the liquid crystalline structure, as is demonstrated by employing a cholesteric twist inversion compound.

Label-free protein sensing by employing blue phase liquid crystal.

Blue phases (BPs) are mesophases existing between the isotropic and chiral nematic phases of liquid crystals (LCs). In recent years, blue phase LCs (BPLCs) have been extensively studied in the field of LC science and display technology. However, the application of BPLCs in biosensing has not been explored. In this study, a BPLC-based biosensing technology was developed for the detection and quantitation of bovine serum albumin (BSA). The sensing platform was constructed by assembling an empty cell with two glass slides coated with homeotropic alignment layers and with immobilized BSA atop. The LC cells were heated to isotropic phase and then allowed to cool down to and maintained at distinct BP temperatures for spectral measurements and texture observations. At BSA concentrations below 10-6 g/ml, we observed that the Bragg reflection wavelength blue-shifted with increasing concentration of BSA, suggesting that the BP is a potentially sensitive medium in the detection and quantitation of biomolecules. By using the BPLC at 37 °C and the same polymorphic material in the smectic A phase at 20 °C, two linear correlations were established for logarithmic BSA concentrations ranging from 10-9 to 10-6 g/ml and from 10-6 to 10-3 g/ml. Our results demonstrate the potential of BPLCs in biosensing and quantitative analysis of biomolecules.

Lyotropic liquid crystal behavior of carboxylated cellulose nanocrystals

In this work, the lyotropic liquid crystal (LC) behavior of carboxylated cellulose nanocrystals (CNs) has been studied for cotton cellulose prepared by TEMPO-mediated oxidation of cellulose nanocrystals (TOCNs). It is found that suspensions of TOCNs at a critical concentration of 4.1wt% form anisotropic phase with clear optical birefringence. Furthermore, fingerprint texture is clearly observed when the concentration is 9.0wt% under a polarizing optical microscope. Fingerprint texture is characteristic of chiral nematic liquid crystal (N*-LC) phase. The band-spacing corresponds to half the helical pitch, and thus, the helical pitch of N*-LCs (9.0wt%) is about 6.0μm. The degree of oxidation (DO) and zeta potential of TOCNs in suspension are 0.15 and −59.5mV, respectively. Relatively larger aspect ratio and higher surface charges density of TOCNs make the critical concentration of LC formation decreased and the helical pitch increased compared to that of the LC prepared by H2SO4 hydrolysis system. The interparticle electrostatic repulsion between CNs with different surface charges influence the helical pitch of N*-LCs.

Formation of periodic zigzag patterns in the twist-bend nematic liquid crystal phase by surface treatment

ABSTRACTZigzag patterns were successfully generated in the twist-bend nematic (NTB) phase of 1-(4-cyanobiphenyl-4′-yl)-6-(4-cyanobiphenyl-4′-yloxy)hexane (CB6OCB) via simple surface treatment. A detailed microscopy study using polarised optical microscopy and fluorescence confocal polarising microscopy was performed to observe the director arrangement in the zigzags, where distinctive periodic patterns were found to be aligned perpendicular to the rubbing direction. These patterns originate from the structural instability and generation of splay deformation with focal conic domain-like structures that are typically found in smectic phases, revealing that the NTB phase has physical properties similar to those of the smectic phase. Observation of these unusual zigzag patterns in the NTB phase opens an avenue for use of this phase in potential applications such as optical modulators and gratings.

Tuning Coulombic interactions to stabilize nematic and smectic ionic liquid crystal phases in mixtures of charged soft ellipsoids and spheres.

We have investigated the effect of electrostatic interactions in mixtures of soft ellipsoids and spheres based on the well-known Gay-Berne (GB) and Lennard-Jones (LJ) potential, respectively. These model systems, in their original version, that is without any electrostatic charge, have been thoroughly investigated in the literature both as pure components and mixtures. In particular, mixtures of particles of different shapes, such as spheres and ellipsoids, tend to phase separate because of the excluded volume effects. Common ionic liquid crystals, based on imidazolium or other quaternary ammonium salts, are usually composed of roughly elongated (although flexible) cations and roughly spherical anions, that is, particles with a similar shape such as the GB and LJ models. Therefore, in this work, we present the results of molecular dynamics simulations of mixtures of positively charged GB and negatively charged LJ particles as models of ionic liquid crystals. Interestingly, by modulating the charge of the particles it is possible to stabilize isotropic, nematic, smectic and crystalline ionic phases. The relative weight of Coulomb (a radial, therefore isotropic interaction) and van der Waals (an anisotropic interaction) contributions is a key parameter to tune the stability of various mesophases.

High-density equilibrium phases of colloidal ellipsoids by application of optically enhanced, direct current electric fields.

We use direct current (DC) electric fields in conjunction with ultraviolet light to self-assemble highly dense structures of colloidal ellipsoids with three-dimensional order and volume fraction as large as 67%. Ellipsoidal phases of colloids are of fundamental interest because novel packing structures are predicted to occur at high volume fractions; the symmetries of these crystal unit cells can also contribute to a variety of applications, including structural color materials. Previously, the very high volume fraction range of ellipsoidal phases has been inaccessible because of limitations such as vitrification and kinetic trapping. Here we report that the coupling of light to DC electric fields causes electrophoretic deposition that yields ellipsoid phases that are significantly denser than previous reports. The applied voltage across the capacitor-like device used for self-assembly was varied from 1.75-2.3 V and the power density of incident UV light was varied between 75-400 W m-2. As the coupled field strengths were increased, the assembled colloids underwent a phase transition from an isotropic fluid to a nematic liquid crystal phase consistent with previous reports. When the voltage and light intensity were between 1.9-2.1 V and 100-200 W m-2 respectively, the assembly had a high degree of orientational ordering and a degree of positional order along axes both parallel and perpendicular to the plane of the electrode surface. For the densest assembly achieved, the interlaying spacing is 0.9D, where D is the ellipsoid minor axis.

Why chiral tartaric imide derivatives give large helical twisting powers in nematic liquid crystal phases: substituent-effect approach to investigate intermolecular interactions between dopant and liquid crystalline molecules

ABSTRACTNovel C2-symmetric chiral dopant derivatives, namely, N-substituted (2R, 3R)-2,3-bis(4-(4-octyloxyphenyl)benzoyloxy)succinimides1a-h, were synthesised, and the effects of the N-substituent and imide-carbonyl groups on the helical twisting powers (HTPs) were investigated in two nematic liquid crystalline compounds, 4-n-pentyl-4ʹ-cyanobiphenyl (5CB) and N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA). As a result, it was clarified that the bulkiness of the N-substituents has a significant correlation with the HTPs, and the imide-carbonyl group interacts strongly with the cyano group of 5CB to give high HTPs in the nematic phases. However, it is assumed that the imide-carbonyl groups of the dopants do not have strong electrostatic attractive interactions with EBBA molecules in the nematic phase to afford the moderate HTPs.

The NTB phase in an achiral asymmetrical bent-core liquid crystal terminated with symmetric alkyl chains

ABSTRACTThe characteristics of the twist-bend nematic (NTB) phase of an achiral asymmetrical rigid bent-core liquid crystal (LC), the ends of which are terminated by symmetric alkyl chains, are reported. The nematic–nematic phase transition and its properties are studied by differential scanning calorimetry (DSC), polarising microscopy and the electro-optic techniques. Large domains of opposite handedness are observed in the absence of the external field in the NTB phase. Another set of periodic striped pattern consisting of domains with sharp boundaries is formed when a high-frequency electric field with a magnitude above its threshold is applied across a planarly aligned cell. The neighbouring domains are of opposite chirality. The temperature dependence of the heliconical angle θ0 is determined from the birefringence measurements using Haller’s extrapolation technique. This material shows lower values of the heliconical angle (~9.3° at a temperature of 155°C within the NTB phase) when compared with the previously reported dimer-based twist-bend nematic LCs (31°±3°).

Wavelength Tuning of the Photonic Band Gap of an Achiral Nematic Liquid Crystal Filled into a Chiral Polymer Scaffold

We demonstrate electric-field-induced wavelength tuning of the entire photonic band gap of an achiral nematic liquid crystal (LC) filled into a chiral polymer scaffold. This chiral polymer scaffold has been formed by creating a template of a chiral nematic LC phase, which remarkably does not compromise the optical finesse of the band gap when compared to that of a conventional, polymer-stabilized chiral nematic LC. We present results on the spectral shift and temporal evolution of the photonic band gap in the presence of an external ac electric field. It is shown that initially there is a rapid (τ ≈ 1 ms) blue-shift of the long-wavelength band-edge followed by a considerably slower blue-shift (τ ≈ 6.5 s) of the entire band gap. We compare the results with those obtained for a polymer-stabilized chiral nematic LC where only a blue-shift of the long-wavelength band-edge is observed. Consequently, we find that for the templated sample the tuning range is more than a factor of 2 greater than that observed for...

Thermodynamic evidence for nematic superconductivity in CuxBi2Se3

Unconventional superconductivity is characterized by the spontaneous symmetry breaking of the macroscopic superconducting wavefunction in addition to the gauge symmetry breaking, such as rotational-symmetry breaking with respect to the underlying crystal-lattice symmetry. Particularly, superconductivity with spontaneous rotational-symmetry breaking in the wavefunction amplitude and thus in bulk properties, not yet reported previously, is intriguing and can be termed "nematic" superconductivity in analogy to nematic liquid-crystal phases. Here, based on specific-heat measurements of the single-crystalline Cu$_x$Bi$_2$Se$_3$ under accurate magnetic-field-direction control, we report thermodynamic evidence for nematic superconductivity, namely, clear two-fold-symmetric behavior in a trigonal lattice. The results indicate realization of an "odd-parity nematic" state, feasible only by macroscopic quantum condensates and distinct from nematic states in liquid crystals. The results also confirm topologically non-trivial superconductivity in Cu$_x$Bi$_2$Se$_3$.

Dielectric spectroscopy of isotropic liquids and liquid crystal phases with dispersed graphene oxide.

Graphene oxide (GO) flakes of different sizes were prepared and dispersed in isotropic and nematic (anisotropic) fluid media. The dielectric relaxation behaviour of GO-dispersions was examined for a wide temperature (25–60 oC) and frequency range (100 Hz–2 MHz). The mixtures containing GO flakes exhibited varying dielectric relaxation processes, depending on the size of the flakes and the elastic properties of the dispersant fluid. Relaxation frequencies of the GO doped isotropic media, such as isopropanol IPA, were observed to be much lower than the GO doped thermotropic nematic medium 5CB. It is anticipated that the slow relaxation frequencies (~10 kHz) could be resulting from the relaxation modes of the GO flakes while the fast relaxation frequencies (~100 kHz) could indicate strongly slowed down molecular modes of the nematogenic molecules, which are anchored to the GO flakes via dispersion interactions. The relaxation frequencies decreased as the size of the GO flakes in the isotropic solvent was increased. Polarizing microscopy showed that GO flakes with a mean diameter of 10 μm, dispersed in water, formed a lyotropic nematic liquid crystal phase. This lyotropic nematic exhibited the slowest dielectric relaxation process, with relaxation frequencies in the order of 2 kHz, as compared to the GO-isotropic suspension and the GO-doped 5CB.

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.

Aspect Ratio Dependence of Isotropic-Nematic Phase Separation of Nanoplates in Gravity

Abstract We studied isotropic-nematic (I-N) phase separation via gravity sedimentation in suspensions of plate-like colloidal particles of identical thickness but different lateral sizes (diameters). It is well-known that I-N phase transition occurs at a higher concentration for particles with larger aspect ratio (thickness/diameter) than for particles with smaller aspect ratio. Here we report that for the larger aspect ratios of nanoplates, gravity-driven I-N phase separation is faster. In a homogenously mixed I-N biphasic suspension of nanoplates, nematic tactoids nucleate, grow, and then undergo sedimentation in gravity, leading to the formation of a clear horizontal interface between the I and N phase. For I-N coexistent suspension of nanoplates with different aspect ratios but the same amount of nematic fractions, the larger the aspect ratio, the faster the formation of nematic tactoids and interface between isotropic liquid and nematic liquid crystal phase. The tactoid formation rate is governed by the rotational and translational diffusion rates, which are faster at larger aspect ratios. The time required for I-N separation (t*, seconds) varies inversely with the mean aspect ratio (< ξ >) of nanoplates and follows the relation, t * = α < ξ > n , where α = 0.97 ± 1.30 s and n = −2.1 ± 0.2. The phase separation kinetics studied in our experiments offers guidance for the selection of aspect ratio of nanoplates for samples to be studied at the International Space Station (ISS).

Soft ellipsoid potential for biaxial molecules

A soft ellipsoid contact pair potential model for biaxial molecules is proposed considering the configuration dependent energy anisotropy explicitly alongwith their geometrical aspects. Analytic expressions of forces and torques for this coarse-grained potential are provided to make it convenient for molecular dynamics simulation study. Additionally, some molecular dynamics simulations are performed to generate biaxial smectic and nematic liquid crystal phases.

Synthesis and Exfoliation of Discotic Zirconium Phosphates to Obtain Colloidal Liquid Crystals.

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and engineers. Growth of such anisotropic nanocrystals through a simple chemical method is a challenging task. In this study, we fabricate discotic nanodisks of zirconium phosphate [Zr(HPO4)2·H2O] as a model material using hydrothermal, reflux and microwave-assisted methods. Growth of crystals is controlled by duration time, temperature, and concentration of reacting species. The novelty of the adopted methods is that discotic crystals of size ranging from hundred nanometers to few micrometers can be obtained while keeping the polydispersity well within control. The layered discotic crystals are converted to monolayers by exfoliation with tetra-(n)-butyl ammonium hydroxide [(C4H9)4NOH, TBAOH]. Exfoliated disks show isotropic and nematic liquid crystal phases. Size and polydispersity of disk suspensions is highly important in deciding their phase behavior.

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

ABSTRACTSecond-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.

Influence of Proton and Salt Concentration on the Chromonic Liquid Crystal Phase Diagram of Disodium Cromoglycate Solutions: Prospects and Limitations of a Host for DNA Nanostructures.

Lyotropic chromonic liquid crystals have recently been suggested for use as a self-organized host for dispersing and aligning self-organized DNA origami nanostructures. However, an appropriate pH value and a suitable cation concentration are necessary to stabilize such nanostructures and to avoid unfolding of the DNA. The present study shows that the nematic and columnar liquid crystal phases appearing in aqueous solutions of disodium cromoglycate are robust against the replacement of deionized water by a neutral or alkaline buffer solution. However, disodium cromoglycate precipitates when an acidic buffer is used or when the concentration of magnesium cations exceeds a critical concentration of about 0.6-0.7 mmol/L.

Remote‐Controllable Molecular Knob in the Mesomorphic Helical Superstructures

A programed light‐responsive chiral liquid crystal (LC) containing four photochromic azobenzene moieties covalently connected to a central bicyclic chiral core (abbreviated as AZ4ICD) is newly designed, precisely synthesized, and efficiently applied as a remote‐controllable molecular knob for the optically tunable thin film. First of all, phase evolutions and ordered structures of AZ4ICD are systematically investigated by a combination of thermal, microscopic, scattering, and simulation techniques. Wide‐angle X‐ray diffractions of oriented AZ4ICD samples indicate that the AZ4ICD molecule itself basically forms layer structures: one is a low‐ordered chiral smectic A LC phase (SmA*) with 5.61 nm layer periodicity at high temperatures, and two highly ordered smectic crystal (SmCr1 and SmCr2) phases are subsequently formed at lower temperatures with the anticlinically tilted molecular packing structures. The helical superstructures of chiral nematic LC phase (N*) can be spontaneously constructed by doping AZ4ICD chiral agents into the achiral nematic molecules. Due to the bent conformational geometry of AZ4ICD, the thermal window of blue LC phase (BP) is expanded by stabilizing the double twisted cylindrical building blocks. Remote‐controllable phase transformations in the mesomorphic helical superstructures are demonstrated by tuning the wavelength of light.