Lamellar Models Research Articles

Effect of interface type on deformation mechanisms of γ-TiAl alloy under different temperatures and strain rates by molecular dynamics simulation

Crystalline interface plays a significant role in strengthening lamellar γ-TiAl alloys through reconciling the strength and ductility. Herein, the effects of temperature and strain rate on the tensile deformation of three lamellar γ-TiAl interface models are investigated by molecular dynamics simulations. The three interfaces, pseudo twin (PT), rotational boundary (RB), and true twin (TT), exhibit different tensile responses due to the different interface effects: TT interface only acts as a barrier of dislocation traversing to facilitate crack extension; PT interface acts as both dislocation barrier and emission source and has a stronger release of strain energy than TT interface, retarding the crack extension; RB interface can retard and resist crack extension due to the blunting and deflection of the crack tip and the best interface geometry compatibility. The defect evolution indicates that the elevated temperature suppresses dislocation propagation at low strain rate, while the high strain rate causes small lamellar stacking faults and slit-shaped holes along tensile direction at low temperature. In addition, the dual conditions of high strain rate and low temperature induce the phase transition from FCC to BCC and then BCC to HCP. These findings provide a specific insight to understand the atomistic mechanism of interface-mediated deformation.

A Mechanistic Study on the Fluidization and Enhancement Effects of Cineole Toward Stratum Corneum Intercellular Lamellar Lipids: A Liquid Crystalline Model Approach.

The stratum corneum (SC) serves as the primary barrier for permeation in human skin. Penetration enhancers, such as 1,8-cineole, are utilized to enhance the permeation of drugs. Cineole increases the permeation of chemicals through different mechanisms. However, its mechanism, particularly at high concentrations, has not been well-studied and is the subject of the present investigation. In continuation of our previous studies, the present investigation aims to elucidate the mechanism of action and concentration dependency of the effects of 1,8-cineole on the structure, diffusional properties, and partitioning behavior of the SC at high concentrations. This will be achieved through lamellar liquid crystalline models and ex-vivo skin studies. A lamellar liquid crystalline lipid matrix model in the presence (25 - 90%, w/w) and absence of cineole was prepared from SC lipids and characterized by X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and polarized light microscopy (PLM) studies. Release of the model lipophilic drug (diazepam) from cineole and cineole-treated matrices and the permeation of the drug from cineole and cineole-containing matrices (as a vehicle similar to the stratum corneum lipids) through excised rat skin were studied. Drug assay was performed by HPLC. The PLM, DSC, and X-ray studies showed that the model matrix had a lamellar gel-liquid crystalline structure, and cineole fluidized the structure concentration-dependently and created other mesomorphic textures, such as myelinic figures. Release experiments showed that diffusion coefficients remained almost constant at high cineole concentrations of 40-90%, suggesting similar fluidization states. Skin permeation studies indicated that the diffusion coefficient (estimated from lag-time) increased concentration-dependently and played a role in permeability coefficient (Kp) increments alongside the increased partitioning of the model drug into the skin. Data suggest that high concentrations of cineole at the skin surface might not provide enough cineole in the skin for full fluidization, despite the similarity of the vehicle to SC lipids and even at high concentrations. The enhancement effect of cineole is concentration-dependent and might reach maximum fluidization at certain concentrations, but this maximum might not be easily achievable when cineole is used in formulations as pure or in a vehicle.

A Mechanistic Study on the Fluidization and Enhancement Effects of Cineole Toward Stratum Corneum Intercellular Lamellar Lipids: A Liquid Crystalline Model Approach

Background: The stratum corneum (SC) serves as the primary barrier for permeation in human skin. Penetration enhancers, such as 1,8-cineole, are utilized to enhance the permeation of drugs. Cineole increases the permeation of chemicals through different mechanisms. However, its mechanism, particularly at high concentrations, has not been well-studied and is the subject of the present investigation. Objectives: In continuation of our previous studies, the present investigation aims to elucidate the mechanism of action and concentration dependency of the effects of 1,8-cineole on the structure, diffusional properties, and partitioning behavior of the SC at high concentrations. This will be achieved through lamellar liquid crystalline models and ex-vivo skin studies. Methods: A lamellar liquid crystalline lipid matrix model in the presence (25 - 90%, w/w) and absence of cineole was prepared from SC lipids and characterized by X-ray diffraction, differential scanning calorimetry (DSC), Thermogravimetric Analysis (TGA), and Polarized Light Microscopy (PLM) studies. The release of the model lipophilic drug (diazepam) from cineole and cineole-treated matrices and the permeation of the drug from cineole and cineole-containing matrices (as a vehicle similar to the stratum corneum lipids) through excised rat skin were studied. A drug assay was performed by HPLC. Results: The PLM, DSC, and X-ray studies showed that the model matrix had a lamellar gel-liquid crystalline structure, and cineole fluidized the structure concentration dependently and created other mesomorphic textures, such as myelinic figures. Release experiments showed that diffusion coefficients remained almost constant at high cineole concentrations of 40-90%, suggesting similar fluidization states. Skin permeation studies indicated that the diffusion coefficient (estimated from lag-time) increased concentration-dependently and played a role in permeability coefficient (Kp) increments alongside the increased partitioning of the model drug into the skin. Data suggest that high concentrations of cineole at the skin surface might not provide enough cineole in the skin for full fluidization, despite the similarity of the vehicle to SC lipids and even at high concentrations. Conclusions: The enhancement effect of cineole is concentration-dependent and might reach maximum fluidization at certain concentrations, but this maximum might not be easily achievable when cineole is used in formulations as pure or in a vehicle.

Mixing in Porous Media: Concepts and Approaches Across Scales

This review provides an overview of concepts and approaches for the quantification of passive, non-reactive solute mixing in steady uniform porous media flows across scales. Mixing in porous media is the result of the interaction of spatial velocity fluctuations and diffusion or local-scale dispersion, which may lead to the homogenization of an initially segregated system. Velocity fluctuations are induced by spatial medium heterogeneities at the pore, Darcy or regional scales. Thus, mixing in porous media is a multiscale process, which depends on the medium structure and flow conditions. In the first part of the review, we discuss the interrelated processes of stirring, dispersion and mixing, and review approaches to quantify them that apply across scales. This implies concepts of hydrodynamic dispersion, approaches to quantify mixing state and mixing dynamics in terms of concentration statistics, and approaches to quantify the mechanisms of mixing. We review the characterization of stirring in terms of fluid deformation and folding and its relation with hydrodynamic dispersion. The integration of these dynamics to quantify the mechanisms of mixing is discussed in terms of lamellar mixing models. In the second part of this review, we discuss these concepts and approaches for the characterization of mixing in Poiseuille flow, and in porous media flows at the pore, Darcy and regional scales. Due to the fundamental nature of the mechanisms and processes of mixing, the concepts and approaches discussed in this review underpin the quantitative analysis of mixing phenomena in porous media flow systems in general.

Rod eutectic growth in bulk undercooled melts

This article proposes an analytical model to understand the rod growth of eutectic in the bulk undercooled melt. Based on the previous derivations of the lamellar eutectic growth models, relaxing the assumptions of small Péclet numbers, the model is derived by considering melt kinetic and thermal undercoolings. The intent of this model is to predict the transitions in eutectic pattern for conditions of the low and high growth velocities. In addition to investigation of the transition between lamellar and rod eutectic patterns, mathematical simplifications of solving Bessel function are presented as well, which is the most important priority to model calculation.

Mixing driven by transient buoyancy flows.II. Flow dynamics

The chaotic advection of the interface between two miscible liquids inside a closed cavity, generated by a damped oscillatory buoyancy-driven (BD) regular flow field, is investigated experimentally for BD mixing. The Lagrangian history of interface motion, determined using the planar laser-induced fluorescence and the photographic full-field view method, is contrasted against the Eulerian flow field measured from particle image velocimetry. Chaotic advection stretches and folds the interface at an early stage to produce an asymmetric pairwise Rayleigh–Taylor (RT) morphology (RTM) structure from long wavelength RT instability and short-time Richtmyer–Meshkov instability and its fractal interface structure at a high impulsive-Reynolds number. The mechanism of folding, from global bifurcation of the flow field, caused by a hyperbolic point, served as an organizing center for multiple vortex interactions. The intermediate-stage kinematics of the RTM structure exhibits RT mixing and shows unfolding of the lamellar structure from the net effect of stretching, folding, and molecular diffusion prior to its breakdown; and it has a probabilistic outcome of exhibiting topological transitions through a breakup of the RTM structure in phase space from necking singularity and pinch-off, indicating sensitivity to the initial conditions. The effectiveness of mixing determined from mixing efficiency is contrasted against mechanical and lamellar models of mixing. The determination of topological entropy, from an approximate Gaussian distribution of the interface length stretch, yields time scale for information decay comparable to time scale for which a low-order horseshoe map emerges from flow, indicating local chaos of the interface. The late-stage breakdown of the RTM structure from internal and wall collision drives the interaction between advection and diffusion, which indicates that critical mixing time scales as the logarithmic of Peclet number, comparable to time-periodic sine flow and blinking vortex flow chaotic mapping models.

Synthesis and properties of supramolecular gels based on tetrathiafulvalene and cyanobiphenyl units

ABSTRACT For the development of self-assembly donor-σ-acceptor (D-σ-A) molecules, a series of D-σ-A low-molecular mass organogelators, consisting of tetrathiafulvalene (D) and cyanobiphenyl (A) connected by a thioalkanoyloxy spacer of varying lengths modified with hydrophobic chains incorporating amide groups on the side, was designed and readily synthesized. Their gelation properties were studied. The results showed that the gelation capability of these compounds was highly dependent on the alkyl chain length in the spacer group and the TTF and cyanobiphenyl units. Derivative 1a with short alkyl chain (n = 4) efficiently gelated in some aromatic solvents, forming opaque organogels. These gelators reacted with F4TCNQ to form stable charge-transfer complex gels in ethanol. Clear evidence of self-assembled micro/nanoflower, fibrous, or tubular morphologies in gelation state was obtained by scanning electron microscopy. It was found that the molecules adopted lamellar and rectangular columnar molecular packing models in the gel phase by X-ray diffraction. The native gels underwent a reversible gel-sol phase transition upon exposure to external stimuli. The gel-sol transition of the organogel was stimulated by the addition of fluoride anions. Interestingly, all gels showed an irreversible gel-sol transition triggered by TFA and TEA. The formation of flower-like morphologies of TTF-based derivatives in the gel state was observed.

Modeling of Micro- to Mesoscopic Deformation Behavior of Semi-Crystalline Polymer Based on Laminar Composite Model

Since semi-crystalline polymer has very complex hierarchical structure, it is essential to construct a suitable multiscale mechanical model for the prediction of its mechanical characteristics. For the establishment of the triple scale mechanical model of semi-crystalline polymer, which can reflect the non-uniform deformations in micro-, meso- and macroscopic scales, computationally efficient model to relate structures and deformations in micro- to mesoscopic scales is indispensable. In this paper, to solve the problems, tangent modulus method was introduced into the constitutive equations for crystalline and amorphous phases consisting of the microstructure of semi-crystalline polymer. Furthermore, we proposed a new laminar composite model connecting deformations in micro- to mesoscopic scales based on the equilibrium of stress acting on the interface between the phases and the compatibility of deformation along the interface. Appropriate time steps for stable simulation and validity of the proposed model were examined through the computational simulation of uniaxial tension of single lamellar and spherulite models of high density polyethylene. It has been clarified that the proposed model could predict an elasto-viscoplastic deformation behavior of semi-crystalline polymer in markedly less computational time and memory.

Time-resolved small-angle neutron scattering as a lamellar phase evolves into a microemulsion

Time-resolved small-angle neutron scattering (SANS) was used to follow the formation of a microemulsion after rapid stopped-flow mixing of a lamellar phase (Lα), comprising the non-ionic surfactant TX-100 and water, with toluene. Initially after mixing, an oil-swollen Lα phase was formed. Between 4 and 160 s the Lα system evolved into a dense water-in-oil (w/o) microemulsion, during which time the two phases coexisted. After 160 s, the system was a pure single-phase microemulsion. SANS spectra were fitted using a linear combination of the phenomenological Teubner–Stey microemulsion and the Nallet lamellar models. The characteristic repeat distance of water domains in the microemulsion was seen to decrease over the first 70 s of the transformation as more of the microemulsion formed; conversely the domain correlation length appeared to increase.

Structural interpretation of the strain-rate, temperature and morphology dependence of the yield stress of injection molded semicrystalline polymers

The yield stress of polypropylene specimens with different initial microstructural states is investigated. These latter were obtained by systematic variations of the processing conditions in injection molding, resulting in different laminated skin-core structures. The morphology of the molded specimens was characterized by polarized light microscopy (full specimen), differential scanning microscopy (core layer) and wide-angle X-ray scattering (skin region). The yield stress was evaluated at different nominal strain-rates (1.67×10 −3 to 150 s −1) and temperatures (23, 40 and 60 °C). The experimental results are analyzed in the frame of Eyring's viscous flow and lamellar cluster models, being established the relationships between the activation volume and enthalpy upon the initial morphological state of the specimens. It is proposed that similar deformation mechanisms operate in both skin and core layers, although with distinct temperature and strain-rate sensitivities. The morphology dependence of the yield stress at different temperatures and strain-rates is established in terms of a laminate composite approach. Yielding is interpreted based on the deformation of crystalline lamellae by the pulling out action of both the molecular chains that are anchored in the inter-lamellar amorphous phase and the tie-molecules. The deformation mechanism operating at the skin and core layers are unified by an elastic-beam mechanical analogue that is able of explaining the morphology, temperature and strain-rate dependences of the yield stress.

Chaotic mixing of a competitive–consecutive reaction

The evolution of a competitive–consecutive chemical reaction is computed numerically in a two-dimensional chaotic fluid flow with initially segregated reactants. Results from numerical simulations are used to evaluate a variety of reduced models commonly adopted to model the full advection–reaction–diffusion problem. Particular emphasis is placed upon fast reactions, where the yield varies most significantly with Péclet number (the ratio of diffusive to advective time scales). When effects of the fluid mechanical mixing are strongest, we find that the yield of the reaction is underestimated by a one-dimensional lamellar model that ignores the effects of fluid mixing, but overestimated by two other lamellar models that include fluid mixing.

Finite element modeling of banded structures in Bombyx mori silk fibres

Transmission electron microscopy studies have shown curved bands of contrast in degummed Bombyx mori fibres. This contrast implies a molecular difference between the light and dark bands, which in turn implies a periodic variation in mechanical properties along the fibre axis. Finite element modeling was used to compare the mechanical behaviour of fibres with various banded geometries. Lateral, longitudinal and diagonal lamellae were compared with V-shaped and parabolic cupped lamellae. The simulation results from the lamellar models were compared to laminar composite theory. The cupped parabolic and V-shaped geometries both showed a redistribution of the axial stresses from the centre toward the edge of the fibre. This redistribution allowed for a substantial increase in the toughness compared to simple lateral lamella as well as a moderate increase in the initial modulus without a significant change in the yield strength.

The paper presents results of measuring radar cross section (RCS) vs frequency dependences for wire-grid, lamellar and bulk models of small-size radar targets at decameter wavelengths in the case of ground-wave sounding. It has been shown that the RCS may

The paper presents results of measuring radar cross section (RCS) vs frequency dependences for wire-grid, lamellar and bulk models of small-size radar targets at decameter wavelengths in the case of ground-wave sounding. It has been shown that the RCS may

The effect of the surface of simple geometric models upon children's drawings

It is generally assumed that a child drawing a geometric model will do so by depicting its edges, the surface of the model merely defining these edges as it undergoes rapid change, whilst also obscuring some other edges and thus making depiction difficult. The present study, in which drawings of three groups of about 100 children drawn from grades 3, 5 and 7 of Scottish urban schools were used, suggests that the relationship between perception of the crucial edges and the surfaces is more complex. It shows that children draw lamellar models differently from geometrically congruent wire models (which have no surfaces) and suggests that the changes in drawing with age and/or schooling are at least in part due to changes of perception and surface/edge relationships.

Lamellar Structure in Melted-Slowly Cooled Vinylidene Fluoride/Trifluoroethylene (86/14) mol% Copolymer

Lamellar structure, and its thermally induced changes in melted-slowly cooled vinylidene fluoride/trifluoroethylene (86/14) mol% copolymer were studied by scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS). SAXS intensity data observed at room temperature showed peaks at around s ≃7.5 ×10-3 Å-1 and s ≃1.35 ×10-3 Å-1, the latter of which is caused by thick lamellar stacks such as those observed by SEM (s is the magnitude of the scattering vector). During the heating process, the higher-angle peak disappeared at 147–155°C, while the lower-angle peak disappeared at around 170°C (the melting point of the sample). These SAXS data were analyzed by fitting to lamellar stack models. For the data in the range 25–147°C, a model in which thin and thick lamellar stacks coexist was used. For the data in the range 155–165°C, a model in which only thick lamellar stacks exist was used. The calculated intensities were in good agreement with the observed intensities, which demonstrates that in the range 25–147°C the copolymer is composed of stacks of lamellae about 85 Å thick and stacks of lamellae about 570 Å thick, and that in the range 155–165°C it is composed of only the thick lamellar stacks. The mean crystallinity of the thin lamellar stacks was found to decrease markedly with increasing temperature, which indicates that thin lamellar stacks disappear in the range 147–155°C due to the melting of the crystals within them.

Small‐angle neutron and x‐ray scattering study of swollen nylon‐6

AbstractResults of swelling and small‐angle scattering experiments on samples of nylon‐6 swollen with heavy water are discussed on the basis of the lamellar and switchboard models. The small‐angle neutron scattering (SANS) intensity is very sensitive to the distribution of water in swollen samples, while the small‐angle x‐ray scattering (SAXS) data characterize the dry samples. The observed values of the mean‐square fluctuation of scattering‐density can be explained by a model with assumed inhomogeneous swelling of the amorphous phase.

Lamellar and Grain Boundary Models for the Electrical Properties of Post-Oxidized ITO Films

A very distinctive feature of the resistivity change during the course of heat treatment in air and a rather weak dependence on the impurity concentration are described on the basis of the diffusion of oxygen and the formation of highly resistive grain boundary.

Small‐angle x‐ray scattering study of ionomer deformation

AbstractThe small‐angle x‐ray scattering (SAXS) pattern from a cesium salt of a 6.1 mole % ethylenemethacrylic acid (E‐MAA) copolymer is shown to become azimuthally dependent on sample elongation. SAXS was measured using the Oak Ridge National Laboratory (ORNL) spectrometer with pinhole collimation and a two‐dimensional position‐sensitive detector. The sample was quenched prior to deformation to avoid crystallization of the ethylene unit which would complicate the interpretation of scattering. The observed SAXS patterns are interpreted in terms of several proposed models for the structure of ionomers. A model in which ionic aggregates are arranged on a paracrystalline lattice is found to be largely in disagreement with the results for undeformed and deformed samples. Spherical and lamellar models incorporating local structure around a central ionic core are capable of predicting the observed SAXS for the undeformed sample. A model of ellipsoidal deformation of the spherical shell‐core model fails to predict the correct azimuthal dependence of scattering. However, a deformation scheme involving rotation of the lamellar model is more satisfactory.

Lamellar mixing models for structured chemical reactions and their relationship to statistical models; Macro- and micromixing and the problem of averages

In the lamellar mixing model, statistical treatments and average values of important variables are introduced near the end of the analysis, not at the beginning. A description based on a Lagrangian point of view of fluid mechanics, diffusion, and reaction in the neighborhood of a moving material particle is compared with other mixing approaches. Rational definitions for micro and macro mixing are specified. Usefulness of the lamellar model is demonstrated by (a) solution for concentrations field with multicomponent diffusion and n-linear chemical reactions in an arbitrary local flow field, (b) indication of criteria for the relative importance of molecular diffusion chemical reaction, local flow, and associated real time in each micro-flow element, (c) effect of real time in each micro-flow element. Potential uses include (a) perturbation schemes for analysis of mixing influence on complex chemical reactions, (b) use of chemical reactions to obtain average fluid mechanical information in complex motions.

Dimensions de micelles mixtes de p-alkylbenzènesulfonates par diffusion central des rayons X

The structure of micellar solutions is determined from X-ray small-angle scattering measurements on an absolute scale. The most probable structure is chosen by comparison with spherical cylindrical and lamellar models. This method is applied to two-component micelles and it is possible to follow the variation of micellar dimensions when the concentration of each component is varying.