Isotropic Domain Research Articles

Thermally Excited Spin Waves of NiFe Nano-ring Arrays on Anodic Aluminum Oxide Templates

The patterned substrates have been widely developed for various applications in the nanotechnology. In particular, the magnetic ring arrays could be utilized to not only the magnetic devices, such as memories and logics, but also magnetic stealth films using spin wave excitations. In this works, the magnetic switching behaviors and the thermally excited spin wave properties of the NiFe (permalloy) ring arrays on the anodic aluminum oxide template with carbon nanotubes were investigated. With increasing the diameter of the rings, the larger dipole interactions induced the larger coercive field of the array films in the plane directions. At the magnetically saturation conditions, the smaller diameter of the rings produced the single and broad spin wave absorptions due to the isotropic magnetic domains in both the horizontal and the vertical directions. When the size of the ring increased, each component felt different magnetic anisotropy field so that the two spin wave excitation peaks revealed. In terms of the mass production of the magnetic nano-ring arrays on the anodic aluminum oxide template, the various size and the shapes of the magnetic ring patterns could be good candidates for the rf stealth films of the defense industries.

Local Polarization Reversal by Ion Beam Irradiation in SBN Single Crystals Covered by Dielectric Layer.

Formation of the domain structure by ion beam irradiation was studied in thermally depolarized Ce-doped strontium barium niobate single crystals covered by a dielectric layer. Three types of irradiation regimes were used: dot exposure, stripe exposure, and line exposure. The dependences of the domain size and depth on the irradiated dose were measured. The circular shape of the isolated domains with partially switched broad domain boundary was obtained. Isotropic domain growth was attributed to the step generation at the wall by merging with the residual nanodomains that appeared after thermal depolarization. The obtained linear dose dependence of the switched area was attributed to the screening of the depolarization field by the injected charge. The shape distortion of the domains growing in the neighborhood with already created ones was attributed to the electrostatic interaction of the approaching charged domain walls. The obtained results can be applied for the creation of precise domain patterns with arbitrary orientation and shape to produce nonlinear optical devices with improved characteristics, including electrically tunable diffractive optical elements.

Extracting stress intensity factors for isotropic cracked domains having stochastic material properties

Material properties are inevitably stochastic due to the manufacturing process and the measurement procedure. In case of a cracked domain, the stochasticity of material properties (as stochastic variables) may manifest in the stress intensity factors (SIFs). Having the stochastic representation of the material properties, Young modulus and Poisson ratio (isotropic material), we approximate the SIFs for 2D cracked domains using the generalized polynomial chaos (gPC). The approximated SIF consists of two families of orthogonal polynomials, selected by the probability distribution function of the material properties. The polynomials are multiplied by a deterministic coefficient, consisting of deterministic SIFs, extracted from a finite element model according to the stochastic properties of both Young modulus and Poisson ratio. Numerical example problems are provided where the stochastic approximation of the SIF is computed. The obtained approximation of the SIF is compared with results obtained using the Monte Carlo method. The results demonstrate the efficiency and accuracy of the proposed method.

Design of Chemoresponsive Soft Matter Using Hydrogen-Bonded Liquid Crystals.

Soft matter that undergoes programmed macroscopic responses to molecular analytes has potential utility in a range of health and safety-related contexts. In this study, we report the design of a nematic liquid crystal (LC) composition that forms through dimerization of carboxylic acids and responds to the presence of vapors of organoamines by undergoing a visually distinct phase transition to an isotropic phase. Specifically, we screened mixtures of two carboxylic acids, 4-butylbenzoic acid and trans-4-pentylcyclohexanecarboxylic acid, and found select compositions that exhibited a nematic phase from 30.6 to 111.7 °C during heating and 110.6 to 3.1 °C during cooling. The metastable nematic phase formed at ambient temperatures was found to be long-lived (>5 days), thus enabling the use of the LC as a chemoresponsive optical material. By comparing experimental infrared (IR) spectra of the LC phase with vibrational frequencies calculated using density functional theory (DFT), we show that it is possible to distinguish between the presence of monomers, homodimers and heterodimers in the mixture, leading us to conclude that a one-to-one heterodimer is the dominant species within this LC composition. Further support for this conclusion is obtained by using differential scanning calorimetry. Exposure of the LC to 12 ppm triethylamine (TEA) triggers a phase transition to an isotropic phase, which we show by IR spectroscopy to be driven by an acid-base reaction, leading to the formation of ammonium carboxylate salts. We characterized the dynamics of the phase transition and found that it proceeds via a characteristic spatiotemporal pathway involving the nucleation, growth, and coalescence of isotropic domains, thus amplifying the atomic-scale acid-base reaction into an information-rich optical output. In contrast to TEA, we determined via both experiment and computation that neither hydrogen bonding donor or acceptor molecules, such as water, dimethyl methylphosphonate, ethylene oxide or formaldehyde, disrupt the heterodimers formed in the LC, hinting that the phase transition (including spatial-temporal characteristics of the pathway) induced in this class of hydrogen bonded LC may offer the basis of a facile and chemically selective way of reporting the presence of volatile amines. This proposal is supported by exploratory experiments in which we show that it is possible to trigger a phase transition in the LC by exposure to volatile amines emitted from rotting fish. Overall, these results provide new principles for the design of chemoresponsive soft matter based on hydrogen bonded LCs that may find use as the basis of low-cost visual indicators of chemical environments.

Boundary Integral Formulations for Transient Linear Thermoelasticity with Combined-Type Boundary Conditions

We study boundary integral formulations for an interior/exterior initial boundary value problem arising from the thermo-elasto-dynamic equations in a homogeneous and isotropic domain. The time depe...

Inhomogeneous nematic-isotropic phase transition of a thermotropic liquid crystal doped with iron oxide nanoparticles

We elucidate the local distribution and aggregated structure of iron oxide nanoparticles (IONPs; 6 nm in diameter) doped in the matrix of a nematic liquid crystal (LC), 4-cyano-4'-pentylbiphenyl, utilizing in situ cryogenic transmission electron microscopy and polarized light microscopy. We show that tens of IONPs aggregate into a sphere-like morphology, and the aggregates combine into elongated clusters with a length of hundreds of nm. With the IONP-doped LC matrix confined to a thin glass cell, we study the nematic-isotropic (N-I) phase transition, and suggest that local heterogeneity of LC textures as seen in polarized microscopy is caused by the existence of IONP aggregate clusters. These clusters act also as nuclei for the formation of isotropic domains upon heating.

Conformal scale factor inversion for domain walls and holography

We describe a correspondence between domain wall solutions of Einstein gravity with a single scalar field and self-interaction potential. The correspondence we call conformal scale factor inversion (CSFI) is a map comprising the inversion of the scale factor in conformal coordinates, and a transformation of the field and its potential which preserves the form of the Einstein equations for static and isotropic domain walls. By construction, CSFI maps the asymptotic anti--de Sitter boundary to the vicinity of a naked singularity in a theory with a special Liouville (exponential) scalar field potential; it is also a map in the parameter space of exponential potentials. The correspondence can be extended to linear fluctuations, being akin to an S-duality, and can be interpreted in terms of ``SUSY quantum mechanics'' for the fluctuation modes. The holographic implementation of CSFI relates the UV and IR regimes of a pair of holographic renormalization group flows; in particular, it is a symmetry of the GPPZ flow.

Predictive modelling of structure formation in semiconductor films produced by meniscus-guided coating.

Meniscus-guided coating methods, such as zone casting, dip coating and solution shearing, are scalable laboratory models for large-area solution coating of functional materials for thin-film electronics. Unfortunately, the general lack of understanding of how the coating parameters affect the dry-film morphology upholds trial-and-error experimentation and delays lab-to-fab translation. We present herein a model that predicts dry-film morphologies produced by meniscus-guided coating of a crystallizing solute. Our model reveals how the interplay between coating velocity and evaporation rate determines the crystalline domain size, shape anisotropy and regularity. If coating is fast, evaporation drives the system quickly past supersaturation, giving isotropic domain structures. If coating is slow, depletion due to crystallization stretches domains in the coating direction. The predicted morphologies have been experimentally confirmed by zone-casting experiments of the organic semiconductor 4-tolyl-bithiophenyl-diketopyrrolopyrrole. Although here we considered a small molecular solute, our model can be applied broadly to polymers and organic-inorganic hybrids such as perovskites.

Chiral domain formation and spontaneous de-racemization in the Dark Conglomerate phase of a bent-core liquid crystal

One of the more recent additions to the wealth of thermotropic liquid crystals are the various phases of bent-core molecules. In particular the Dark Conglomerate phase is still quite intriguing, as it displays the relatively uncommon phenomena of spontaneous de-racemization, the formation of two types of chiral domains in the absence of overall chirality of the constituent molecules or the macroscopic phase. This domain formation was investigated in detail with respect to cooling rate and applied electric fields. It was found that any effects of domain coarsening or aging are practically absent and that only the cooling rate influences the domain sizes and thus their number density. The presence of an electric field during the domain formation results in a modification from shape-anisotropic domains with an aspect ratio of length to width of approximately L/W ≈ 3 at zero field, to shape isotropic domains with an aspect ratio of about L/W ≈ 1 at non-zero field. The overall chirality of the investigated materials remains unaltered in all cases.

Mechanisms of Directional Polymer Crystallization.

Zone annealing, a directional crystallization technique originally used for the purification of semiconductors, is applied here to crystalline polymers. Tight control over the final lamellar orientation and thickness of semicrystalline polymers can be obtained by directionally solidifying the material under optimal conditions. It has previously been postulated by Lovinger and Gryte that, at steady state, the crystal growth rate of a polymer undergoing zone annealing is equal to the velocity at which the sample is drawn through the temperature gradient. These researchers further implied that directional crystallization only occurs below a critical velocity, when crystal growth rate dominates over nucleation. Here, we perform an analysis of small-angle X-ray scattering, differential scanning calorimetry, and cross-polarized optical microscopy of zone-annealed poly(ethylene oxide) to examine these conjectures. Our long period data validate the steady-state ansatz, while an analysis of Herman’s orientation function confirms the existence of a transitional region around a critical velocity, vcrit, where there is a coexistence of oriented and isotropic domains. Below vcrit, directional crystallization is achieved, while above vcrit, the mechanism more closely resembles that of conventional isotropic isothermal crystallization.

Formation of Antihyperlipidemic Nano-Ezetimibe from Volatile Microemulsion Template for Enhanced Dissolution Profile.

Nanostructures play an important role in targeting sparingly water-soluble drugs to specific sites. Because of the structural flexibility and stability, the use of template microemulsions (μEs) can produce functional nanopharmaceuticals of different sizes, shapes, and chemical properties. In this article, we report a new volatile oil-in-water (o/w) μE formulation comprising ethyl acetate/ethanol/brij-35/water to obtain the highly water-dispersible nanoparticles of an antihyperlipidemic agent, ezetimibe (EZM-NPs), to enhance its dissolution profile. A pseudoternary phase diagram was delineated in a specified brij-35/ethanol ratio (1:1) to describe the transparent, optically isotropic domain of the as-formulated μE. The water-dilutable μE formulation, comprising an optimum composition of ethyl acetate (18.0%), ethanol (25.0%), brij-35 (25.0%), and water (32.0%), showed a good dissolvability of EZM around 4.8 wt % at pH 5.2. Electron micrographs showed a fine monomodal collection of EZM-loaded μE droplets (∼45 nm) that did not coalesce even after lyophilization, forming small spherical EZM-NPs (∼60 nm). However, the maturity of nanodrug droplets observed through dynamic light scattering suggests the affinity of EZM to the nonpolar microenvironment, which was further supported through peak-to-peak correlation of infrared analysis and fluorescence measurements. Moreover, the release profile of the as-obtained EZM-nanopowder increased significantly >98% in 30 min, which indicates that a reduced drug concentration will be needed for capsules or tablets in the future and can be simply incorporated into the multidosage formulation of EZM.

Statistical Volume Elements for the Characterization of Angle-Dependent Fracture Strengths in Anisotropic Microcracked Materials

Abstract Statistical volume elements (SVEs) are used to homogenize fracture strength of rock, based on the microcrack statistics of a real-world Yuen-Long marble sample. The small size of SVEs enables maintaining inhomogeneities in fracture properties with lower computational cost compared to methods that explicitly model microcracks at macroscale. Maintaining inhomogeneity is important to capture realistic fracture patterns in rock as a quasi-brittle material. Uniaxial tensile, uniaxial compressive, and shear strengths are derived for arbitrary angle for loading and orientation of a single crack by using the linear elastic fracture mechanics (LEFM) method and incorporating frictional effects. Mesoscopic fracture strength fields are generated for different strengths and angle of loading by traversing the spatial domain with circular SVEs. Increasing the SVE size smoothens the spatial inhomogeneity and angular anisotropy of homogenized strengths. Spatial and angular covariance functions of the random fields are obtained to demonstrate how fracture strength varies in space and by changing the angle of loading. Two isotropic and anisotropic rock domains are studied and shown to have very different single- and two-point statistics. Macroscopic fracture simulations by an asynchronous spacetime discontinuous Galerkin (aSDG) method demonstrate that most macroscopic cracks for the anisotropic domain are aligned with the weakest strength planes.

Establishment of an optical trapping curve for prediction of trapping parameters

Optical trapping is widely used to manipulate a small-sized particle freely suspended in the isotropic fluidic domain. Trapping is done by means of optical forces developed by conversing light beam. The active gradient forces, Fgrad depends upon parameters like light wavelength, particle size, and refractive index of medium and particle. The viscous drag forces, Fvisc depends upon parameter like viscosity of fluid, relative velocity of particle with respect to medium. The necessary condition for particle trapping is to maintain Fgrad, max > Fvisc, ( Fgrad, max is a maximum value of Fgrad). In this paper, a graphical approach is applied to predetermine the value of specific parameter such as relative velocity of particle and light wavelength under the necessary condition for optical trapping, while keeping other parameters are as fixed. Software simulation is performed with set of relative velocities on polystyrene small-sized particle in water channel to validate the graphical approach.

Reconfigurable Multicompartment Emulsion Drops Formed by Nematic Liquid Crystals and Immiscible Perfluorocarbon Oils.

Liquid crystalline (LC) oils offer the basis of stimuli-responsive LC-in-water emulsions. Although past studies have explored the properties of single-phase LC emulsions, few studies have focused on complex multicompartment emulsions containing co-existing isotropic and LC domains. In this paper, we report a study of multiphase emulsions using LCs and immiscible perfluoroalkanes dispersed in water or glycerol (the latter continuous phase is used to enable characterization). We found that the nematogen 4'-pentyl-4-biphenylcarbonitrile (5CB) anchors homeotropically (perpendicularly) and weakly at liquid perfluorononane (F9) interfaces, consistent with the smectic layering of 5CB molecules. The proposed role of smectic layering is supported by experiments performed with 4-(trans-4-pentylcyclohexyl)benzonitrile, a nematogen that possesses a cyclohexyl group that frustrates the smectic packing and leads to tilted orientations at the F9 interface. By employing perfluorocarbon and hydrocarbon surfactants in combination with multiphase 5CB and F9 emulsion droplets dispersed in a continuous water or glycerol phase, we observe a range of emulsion droplet morphologies to form, including core-shell and Janus structures, with internal organizations that reflect an interplay of interfacial (anchoring energies; F9 and glycerol) and elastic energies within the confines of the geometry of the emulsion droplet. By comparing experimental observations to simulations of the LC-perfluorocarbon droplets based on a Landau-de Gennes model of the free energy, we place bounds on the orientation-dependent interfacial energies that underlie the internal ordering of these complex emulsions. Additionally, by forming core-shells emulsion droplets from 5CB (shell) and perfluoroheptane (cores), we demonstrate how a liquid-to-vapor phase transition in the perfluorocarbon core can be used to actuate the droplet and rapidly thin the nematic shell. Overall, the results reported in this paper demonstrate that multiphase LC emulsions formed from mixtures of perfluoroalkanes and LCs provide new opportunities to engineer hierarchical and stimuli-responsive emulsion systems.

Modified element stacking method for multi-material topology optimization with anisotropic materials

This article presents a modified element stacking method for anisotropic multi-material topology optimization. This method can transform standard multi-material topology optimization formulations into a series of equivalent single-material topology optimization ones to overcome the various limitations inherent to conventional techniques. First, typical multi-material topology optimization methods utilize material interpolation schemes that restrict the study to the isotropic domain with a constant Poisson’s ratio, limiting practical applications and solution accuracy. Additionally, in attempts to further extend these classical multi-material topology optimization methods to anisotropic materials, preparing the necessary element information matrices becomes increasingly laborious or even impossible for complex models, preventing the application of sensitivity analysis for robust gradient-based optimization methods. To directly address these limitations, the element interpolation method replaces material interpolation schemes with an element interpolation framework, where each design cell property is determined using a weighted sum of various coincident single-material elements. This paper provides a thorough description of element interpolation and presents several numerical examples demonstrating successful implementation.

Domain Switching by Electron Beam Irradiation in SBN61:Ce Single Crystals Covered by Dielectric Layer.

The formation of the domain structure by electron beam irradiation in thermally depolarized Ce-doped strontium barium niobate single crystals with free surface and surface covered by a dielectric layer has been studied. The dependences of the domain sizes and domain depth on the irradiated dose have been measured. The circular shape of the isolated domains was obtained. The isotropic domain growth was attributed to step generation at the wall as a result of merging with the residual nanodomains which existed after thermal depolarization. The linear dose dependence of the switched area was attributed to the screening of the depolarization field by the injected charge. The electrostatic interaction of the approaching charged domain walls was revealed. The better quality of the domain patterns was achieved in the samples with electron localization in the dielectric layer. The obtained results can be applied for the creation of precise domain patterns with arbitrary orientation and shape to produce nonlinear optical devices with improved characteristics.

Interaction of Vanadyl Complexes in Atmospheric Residue with Their Matrixes: An ESR Study in a Temperature Range up to 170 °C

It is important to understand the interaction of vanadyl complexes in atmospheric residue (AR) with their matrixes at different conditions for demetallization processes of AR and heavy oil. An ESR study in rotational mobility of vanadyl complexes in AR was conducted at a temperature range from 20 to 170 °C with and without 1-methylnaphthalene as solvent. A complete transformation of the ESR spectrum from anisotropy to isotropy was observed at a temperature of 170 °C with decreasing AR concentration in 1-methylnaphthalene from 100 to 20 wt %, which corresponds to a transformation of the vanadyl complexes from a rigid status constrained by the surrounding matrixes to a free status. Both increase in the temperature and decrease in the AR concentration are beneficial to such transformation. The study showed that dependency of the rotational mobility of vanadyl complexes on the temperature and concentration can be classified into three domains: isotropic, transitional, and anisotropic domains. In the anisotrop...

Buckling instability of smectic lamellar phase affected by the constituent molecular shape

Buckling instabilities and their collapse are major issues for the lamellar materials, and its mechanism at the molecular scale has not been fully addressed. We study the evolution of the lamellar structure under the lateral compression through the coarse-grained molecular dynamics approach. We investigate the influence of the molecular shape on their collapse based on explicit representation, and find that the critical strain of their collapse depends on the molecular shape. When the molecular length is smaller than the tip width of the chevron-like collective structure, the directors can be aligned in two possible directions parallel and perpendicular to the layers. This variation of molecular orientation causes the generation of isotropic domain wall. On the other hand, when the length of the molecules is longer than the tip width, the parallel misalignment is suppressed and the lamellar structure is more stable. Therefore, the ratio of the molecular length to the tip width affects the buckling instability of the lamellar structures.

Failure analysis for the cement with radial cracking in HPHT wells based on stress intensity factors

Wells are often cemented to provide zonal isolation and structural support through their entire service lifetime. Even if a cement sheath is set properly, it may inevitably contain small cracks which tend to propagate under harsh environmental and operational conditions. This work presents a mechanical model to predict the stress intensity factor at a radial crack tip in the cement under high pressure/high temperature (HPHT) conditions. The well section comprised of casing-cement-formation is modeled as three isotropic material domains under plane strain condition. The model calculates the stress field in the well section and estimates the stress intensity factor by a distributed dislocation technique. Two failure factors corresponding to the material strength criterion and fracture propagation criterion are introduced to evaluate the cement integrity. The influence of loading conditions and cement's mechanical and thermal properties on the failure factors is also investigated. The results indicate the predominant failure mode under HPHT conditions. The proposed model provides guidelines for the assessment of cement sheath integrity based on fracture mechanics.

Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Rapid bulk assembly of nanoparticles into microstructures is challenging, but highly desirable for applications in controlled release, catalysis, and sensing. We report a method to form hollow microstructures via a two-stage nematic nucleation process, generating size-tunable closed-cell foams, spherical shells, and tubular networks composed of closely packed nanoparticles. Mesogen-modified nanoparticles are dispersed in liquid crystal above the nematic-isotropic transition temperature (TNI). On cooling through TNI, nanoparticles first segregate into shrinking isotropic domains where they locally depress the transition temperature. On further cooling, nematic domains nucleate inside the nanoparticle-rich isotropic domains, driving formation of hollow nanoparticle assemblies. Structural differentiation is controlled by nanoparticle density and cooling rate. Cahn-Hilliard simulations of phase separation in liquid crystal demonstrate qualitatively that partitioning of nanoparticles into isolated domains is strongly affected by cooling rate, supporting experimental observations that cooling rate controls aggregate size. Microscopy suggests the number and size of internal voids is controlled by second-stage nucleation.