Isotropic Crystals Research Articles

Fast and large refractive index switching of optically isotropic liquid crystal films

ABSTRACT Polymer Dispersed Liquid Crystals (PDLCs) consist of low molecular weight liquid crystal (LC) droplets embedded in a polymer matrix. Nano-PDLCs are a special type of PDLCs where the LC droplet sizes are in the nanometre range. For normal incidence of light, the nano-PDLC has a voltage dependent, isotropic refractive index that could be used to make polarisation independent tunable lenses. Here, we report the effect of chiral dopant on haze, switching time and field-induced variation of the refractive index δn E of PDLC materials having a high birefringence ( Δn = 0.38 at 550 nm ) nematic liquid crystal dispersed in a polymer matrix. As an effect of the chiral dopant, δn decreases slightly from 0.05 to 0.037 at E = 10 V / μm , however the relaxation time and the haze decrease to a much larger extent from 2.5 ms to 0.3 m s, and from 50% to less than 10% at 550 nm, respectively. These improvements are due to the sub-micrometer helical pitch in the LC droplets that reduces the size of the LC droplets well below 1 µm, leading to nano-PDLC with sub-millisecond switching time and low haze.

Study on Wear and Corrosion Resistance of Ni60/WC Coating by Laser Cladding on Reciprocating Pump Plunger: Comparison with Flame-Sprayed Plungers.

Reciprocating pumps are widely used in the current oil extraction process, and the plunger is a vulnerable part of these pumps that directly determines the service life of the reciprocating pump. To improve the service life of plungers, Ni60/WC coatings were applied to the surface of 45-steel plungers via laser cladding technology to improve wear and corrosion resistance. Defect-free and dense Ni60/WC coatings were successfully applied to the plunger surface with strong metallurgical bonding between the coating and the substrate. The coating consists mainly of a γ-(Ni, Fe) phase, which contains isotropic and isotropic-like crystals, dendritic crystals, and columnar crystals in the top, middle, and bottom regions of the coating, respectively. The service performance of the laser cladding coating was compared to the flame-sprayed plunger, which is widely used, and the laser cladding coating has a microhardness of up to 821.8 HV0.5, which is higher than that of the flame-sprayed coating (545.5 HV0.5) and the 45-steel substrate (200 HV0.5). The laser cladding coating has a lower friction coefficient and a smaller volumetric wear rate, and the corrosion current density and corrosion rate in the NaCl solution are 2.52 × 10-7 A/cm2 and 2.96 × 10-3 mmPY, respectively, which indicates superior corrosion resistance to the flame-sprayed coating and the substrate. The laser cladding of reciprocating pump plunger surfaces has a significantly improved comprehensive performance and is a promising way to increase the service life of reciprocating pumps.

Anisotropic Laser Performance in an Optically Isotropic Crystal by Phonon Engineering

Symmetry is an eternal motif in understanding the characteristics of nearly all the laws and courses of science. The symmetry‐breaking in the optical crystal will generate a rich variety of unprecedented physical phenomena and create new functional properties. Herein, the symmetry‐breaking in a photon–phonon collaboratively pumped laser, dominated by the anisotropic lattice vibrations is investigated. Using an optically isotropic Nd:YAG crystal as an example, the phonon‐assisted electronic transitions accompanied by anisotropic fluorescence emission are observed, beyond the intrinsic structural symmetry ruled by Neumann's principle. For the first time, the phonon‐assisted new‐wavelength lasers at 1151 and 1166 nm are achieved in Nd:YAG with divergent light polarization, determined by the vibrational direction of involved phonons. These results provide a flexible degree of freedom for photonics by phonon engineering and pave the way to new frontiers in the field of laser generation and manipulation.

Electro-Optic Kerr Response in Optically Isotropic Liquid Crystal Phases

The results of an experimental investigation of the temperature and wavelength dependence of the Kerr constant (K) of mixtures with an increasing amount of chiral dopant in an isotropic liquid crystal phase are reported. The material was composed of a nematic liquid crystal (5CB) and a chiral dopant (CE2), which formed non-polymer-stabilized liquid crystalline blue phases with an exceptionally large value of K∼2 × 10−9 mV−2. The measurements were performed on liquid and blue phases at several concentrations covering a range of temperatures and using three wavelengths: 532 nm, 589 nm and 633 nm. The work focused on changes caused by concentration and their impact on the increase in the value of K, and it was found that in the case of the 5CB/CE2 mixture these changes were significant and quite systematic with temperature and wavelength. It is shown that the dispersion relation based on the single-band birefringence model described K well in isotropic liquid crystal phases at all of the measured concentrations. In an isotropic fluid, both temperature-dependent parameters in the dispersion relation had a simple linear form and, therefore, the K-surface could be described by only four constants. In the blue phase, the expression reproducing the temperature variation of K depended on concentration, which could vary from being almost linear to quasi-linear and could be represented well by an inverse exponential analytic expression.

Superdiffusion and heterogeneous dynamics in liquid crystals by microrheology

Microrheology (MR) has emerged as a powerful tool for unraveling the intricate local viscoelastic properties of various soft materials. By tracking the free (passive MR) or forced (active MR) diffusion of a tracer, valuable insights into the mechanical characteristics of the host system can be obtained. In this study, we investigate the forced diffusion of a spherical tracer within isotropic and smectic liquid crystal phases of hard rod-like particles. Our findings reveal superdiffusive behaviour induced by external forces, particularly pronounced when these are aligned parallel to the nematic director. Analysis of the dynamical susceptibility unveils heterogeneities strongly correlated with the magnitude and orientation of applied forces, highlighting the system's critical dependence on structural ordering. Intriguingly, we observe that tracer superdiffusion, driven by external forces and evident across all relevant system directions, does not demonstrate a strong correlation with resulting dynamical heterogeneities.

The Modified Guyer-Krumhansl Equations Derived From the Linear Boltzmann Transport Equation

Abstract Phonon hydrodynamics originated from the macroscopic energy and momentum balance equations (called Guyer-Krumhansl equations) proposed by Guyer and Krumhansl by solving the linearized Boltzmann transport equation for studying second sound in the normal-process collision dominated phonon transports in an isotropic nonmetallic crystal with a dispersionless frequency spectrum. However, the low-dimensional dielectric materials and semiconductors are anisotropic, and the different branches in their phonon frequency spectrum usually have different group velocities. For such materials, we derive the macroscopic energy and momentum balance equations from the linear Boltzmann transport equation to describe the phonon hydrodynamic transport, and solve the longstanding debate about whether the energy balance equation contains the second-order spatial derivatives of temperature. Finally, by solving the modified Guyer–Krumhansl equations, we find the minimum and maximum values of the length required by the occurrence of second sound in suspended single-layer graphene with the rectangular shape.

Growth Behaviors of Bubbles and Intermetallic Compounds in Solidifying Al-5 wt.% Mn Alloy

The growth behaviors of hydrogen bubbles and intermetallic compounds (IMCs) during solidification of an Al-5 wt.% Mn alloy was investigated by synchrotron radiography. Results show that bubble collapse can increase hydrogen concentration in nearby Al melt, thus facilitating the formation and growth of new bubbles. Under the interference of Al6Mn IMCs, the growth method of an individual bubble is changed from a Gaussian distribution to a linear model. Al6Mn crystal growth can be divided into three stages: first an isotropic spherical crystal appears, then it evolves into primary branches, and eventually forms an irregular octahedron.

Surface-induced interaction of colloidal particles in isotropic liquid crystals

In this work, we study the interaction between two colloidal particles in a liquid crystal that is in the isotropic phase. The interaction is caused by surface-induced polarization of the liquid crystal molecules in the vicinity of the particles. We find that the interaction is short-ranged in both 2D and 3D geometry. Under symmetric homeotropic anchoring conditions, the interaction is repulsive. While under anti-symmetric homeotropic anchoring conditions, the interaction is repulsive at short distances but attractive at intermediate distances. The particle size has a strong impact on the effective interaction force.

Modelling the Deformation of Polydomain Liquid Crystal Elastomers as a State of Hyperelasticity

A hyperelasticity modelling approach is employed for capturing various and complex mechanical behaviours exhibited by macroscopically isotropic polydomain liquid crystal elastomers (LCEs). These include the highly non-linear behaviour of nematic-genesis polydomain LCEs, and the soft elasticity plateau in isotropic-genesis polydomain LCEs, under finite multimodal deformations (uniaxial and pure shear) using in-house synthesised acrylate-based LCE samples. Examples of application to capturing continuous softening (i.e., in the primary loading path), discontinuous softening (i.e., in the unloading path) and auxetic behaviours are also demonstrated on using extant datasets. It is shown that our comparatively simple model, which breaks away from the neo-classical theory of liquid crystal elastomers, captures the foregoing behaviours favourably, simply as states of hyperelasticity. Improved modelling results obtained by our approach compared with the existing models are also discussed. Given the success of the considered model in application to these datasets and deformations, the simplicity of its functional form (and thereby its implementation), and comparatively low(er) number of parameters, the presented isotropic hyperelastic strain energy function here is suggested for: (i) modelling the general mechanical behaviour of LCEs, (ii) the backbone in the neo-classical theory, and/or (iii) the basic hyperelastic model in other frameworks where the incorporation of the director, anisotropy, viscoelasticity, temperature, softening etc parameters may be required.

Optically isotropic longitudinal piezoelectric resonant photoelastic modulator for wide angle polarization modulation at megahertz frequencies

Polarization modulators have a broad range of applications in optics. The acceptance angle of a free-space polarization modulator is crucial for many applications. Polarization modulators that can achieve a wide acceptance angle are constructed by attaching a piezoelectric transducer to an isotropic material, and utilizing a resonant transverse interaction between light and acoustic waves. Since their demonstration in the 1960s, the design of these modulators has essentially remained the same with minor improvements in the following decades. In this work, we show that a suitable single crystal with the correct crystal orientation, functioning as both the piezoelectric transducer and the acousto-optic interaction medium, could be used for constructing a highly efficient free-space resonant polarization modulator operating at megahertz frequencies and exhibiting a wide acceptance angle. We construct the modulator using gallium arsenide, an optically isotropic and piezoelectric crystal, and demonstrate polarization modulation at 6 MHz with an input aperture of 1 cm in diameter, acceptance angle reaching ±30∘, and modulation efficiency exceeding 50%. Compared to state-of-the-art resonant photoelastic modulators, the modulator reported in this work exhibits greater than 50-fold improvement in modulation frequency for the same input aperture, while simultaneously reducing the thickness by approximately a factor of 80. Increasing the modulation frequency of photoelastic modulators from the kilohertz to the megahertz regime and substantially reducing their thickness lead to significant performance improvements for various use cases. This technological advancement also creates opportunities for utilizing these devices in new applications.

Liquid Crystal Electrolyte with Lamellar Ionic Channels for All-In-One Gel Flexible Supercapacitor.

Liquid crystalline hydrogels with nanoscale order are an attractive soft material to transport ions or electrons with high efficiency. By employing noncovalent interactions between amphiphiles and solvents, defined anisotropic ordered structures can assemble that serve as interior transmissible channels. Herein, the phase behaviors of a polymerizable amphiphile of 1-vinyl-3-alkylimidazolium bromide (VCn IMBr, n=12, 14, 16) are investigated at different concentrations in a deep eutectic solvent. The aggregation such as micelle, hexagonal, and lamellar liquid crystal phase is created. Through in-phase polymerization, the lamellar structures within an an isotropic liquid crystal can be well solidified to obtain a conductive gel electrolyte. A sandwich-structured all-in-one gel flexible supercapacitor is then built with this specific gel electrolyte. With greatly increased adhesion and minimized interfacial resistance between electrode and electrolyte, the approach will be able to create energy-storage devices with anisotropic ionic and electronic charge transportations envisioned for various electrochemical applications.

Stability, structure, compressibility factor, excess free energy and density profile of hard prolate pear shape fluids

ABSTRACT In this work, the Kerr coefficients of isotropic and uniaxial nematic liquid crystals consisting of hard ellipsoid and hard prolate pear-shaped molecules were theoretically studied using classical density functional theory. A new analytical approximate expression for the direct correlation function of hard pear- shaped molecules is applied. Using the expansion coefficients of this direct correlation function, we calculated the structural parameters, compressibility factor, and excess free energy of the hard pear-shaped liquid crystals. In addition, using the restricted orientation model, one particle density of a confined hard pear- shaped liquid crystal is calculated. The limiting stabilities and estimated transition densities are in qualitative agreement with available data. The results confirm the validity of this approximate direct correlation function.

Influence of mechanical vibration actions on the memory effect of PDLC films doped with SiO<sub>2</sub> nanoparticles

The effect of mechanical vibration in the frequency range from 200 Hz to 2500 Hz on the memory effect in 5CB polymer dispersed liquid crystals doped with SiO2 nanoparticles has been studied. As a result of capacitance-voltage measurements, the dependences of the dielectric properties of liquid crystal cells with 0.1 wt% SiO2 on the bias voltage. It has been established that the memory effect detected by dielectric hysteresis in polymer dispersed liquid crystals with silicon dioxide nanoparticles monotonically decreases with increasing oscillation frequency and completely disappears at a frequency of 2000 Hz due to the destruction of the network of silica nanoparticles and, respectively, the appearance of an isotropic orientation liquid crystals molecules.

Self-force of high-speed dislocation in anisotropic media based on configurational mechanics

Subsonic, transonic, and supersonic dislocations play a critical role during super high strain rate (>105/s) deformation. For these high-speed dislocations, the self-force is history dependent, which is different from and much more complex than the quasi-static dislocations. Solutions to the self-force of high-speed dislocation in continuum media are only known for some special cases, i.e., subsonic dislocation moving in isotropic infinite crystals with a constant speed. However, metals are generally anisotropic, and high-speed dislocations may have complicated histories and interact with the free surface. How crystal anisotropy, complicated motion history and free surface influence their self-force has so far received little attention. In the current work, we proposed an effective calculation method of self-force on high-speed dislocation based on the discrete-continuous model of three-dimensional dislocation elastodynamics and the dynamic J-integral of configurational mechanics. It is applicable to arbitrarily moving subsonic, transonic and supersonic dislocation, in both isotropic and anisotropic crystals, and can automatically consider the image force if the dislocation is close to the free surface. The effectiveness of the method is carefully verified by comparing it with the existing theoretical solutions and the molecular dynamics results. The effect of crystal anisotropy on the elastodynamic stress field, as well as the self-force of high-speed dislocation close to or far from free surface are studied.

Einstein–Podolsky–Rosen steering and coherence in structured environments

We examine the quantum steering and coherence for two qubits that are immersed in an isotropic (I) and anisotropic (A) photonic band-gab (PBG) crystal. We show how the steerability and coherence of the qubits’ state depends on the essential parameters of the model and initial state setting of the qubits. We find that the prevention of the amount of quantum steering and coherence can be achieved in the presence of PBG materials by a considerable choice of the system parameters. Furthermore, we show that the quantum steering and coherence can resist the degradation during the dynamics in IPBG more than in APBG according to the same physical conditions. Finally, we illustrate the relationship between the quantum steering and coherence with respect to the system parameters under the decoherence effect. These obtained results reveal that quantum systems in PBG materials are very useful to implement diverse applications of quantum technology with optimal performance.

Testing different supervised machine learning architectures for the classification of liquid crystals

ABSTRACTDifferent convolutional neural network (CNN) and inception network architectures were trained for the classification of isotropic, nematic, cholesteric and smectic liquid crystal phase textures to test the prediction accuracy for each one of these models. Varying the number of layers and inception blocks, as well as the regularisation, and application to different phase transitions and classification tasks, it is shown that in general the architecture of an inception network with two blocks leads to the best classification results. Regularisation, such as image flipping, and dropout layers additionally somewhat increase the classification accuracy. Even for simple tasks like the isotropic-nematic transition, which is of importance for applications in the automatic readout of sensors, convolutional neural networks need more than one layer. Care must be taken to not apply architectures of too large complexity, as this will again reduce the classification accuracy due to overfitting. Architecture complexity needs to be adjusted to the given classification task

Properties of mid-infrared interface modes at the interface of two isotropic dielectric photonic crystals

Properties of mid-infrared interface modes at the interface of two isotropic dielectric photonic crystals

Stationary Charge Radiation in Anisotropic Photonic Time Crystals.

Time metamaterials offer a great potential for wave manipulation, drawing increasing attention in recent years. Here, we explore the exotic wave dynamics of an anisotropic photonic time crystal (APTC) formed by an anisotropic medium whose optical properties are uniformly and periodically changed in time. Based on a temporal transfer matrix formalism, we show that a stationary charge embedded in an APTC emits radiation, in contrast to the case of isotropic photonic time crystals, and its distribution in momentum space is controlled by the APTC band structure. Our approach extends the functionalities of time metamaterials, offering new opportunities for radiation generation and control, with implications for both classical and quantum applications.

A Novel Molecular Assembly of a Cobalt-Sulfate Coordination Polymer and Melamine: A Manifestation of Magnetic Anisotropy.

A novel molecular assembly of a cobalt-sulfate coordination polymer and melamine is synthesized under acidic conditions. Bar-shaped pink monocrystals as long as 1 mm are found to align along magnetic field lines in the proximity of a strong magnet. Magnetometry shows no hysteresis at temperatures down to 2 K but instead magnetic anisotropy and antiferromagnetic coupling. X-ray diffraction on a single crystal reveals that the cobalt-sulfate chains are along the shortest lattice vector or the crystal's long axis. The crystal alignment along the magnetic flux can be attributed to single-ion anisotropy that results in longitudinal antiferromagnetic coupling along the chain. Both structurally and magnetically isotropic crystals of metal-organic hybrid materials can be highly useful as elemental components in magneto-optics.

The impact of ionic contribution to dielectric permittivity in 11CB liquid crystal and its colloids with BaTiO3 nanoparticles

The report shows the temperature behavior of the real part of dielectric permittivity in the static (dielectric constant) and low-frequency (LF) domains in bulk samples of 11CB and its BaTiO3-based nanocolloids. The study covers the isotropic liquid (I), nematic (N), smectic A (SmA), and solid crystal (Cr) phases. For each phase, the dominance of pretransitional fluctuations, significantly moderated by nanoparticles, is shown. The authors consider separate focuses on the dielectric constant varepsilon left( {varvec{T}} right) evolution in the static domain, yielding mainly response from permanent dipole moment and its arrangement, and in the low-frequency (LF) domain Delta varepsilon^{prime } left( f right) = varepsilon^{prime } left( f right) - varepsilon (where varepsilon^{prime } left( {varvec{f}} right) is for the real part of dielectric permittivity in the LF domain), which is associated solely with ionic-related polarization mechanisms. All of these led to new experimental evidence concerning I–N, N–SmA, and SmA–solid transitions, focusing on the strength and extent of pretransitional effects, critical exponents, and phase transitions discontinuities. The strong evidence for pretransitional effects near the SmA–Cr transition is notable, particularly regarding Delta varepsilon^{user2{^{prime}}} left( {{varvec{f}},{varvec{T}}} right). Studies are supplemented by the discussion of DC electric conductivity—a parameter also related to the LF domain. Finally, the validity of the relation varepsilon^{prime } left( {varvec{f}} right) = {varvec{Af}}^{ - 3/2} + varepsilon (where f stands for frequency, and A is a constant parameter), often used for discussing dielectric spectra in LC compound and its nanocolloids in the LF domain, is examined.Graphical abstract