Vicinity Of Boundary Research Articles

Characterizing twist grain boundaries in BCC Nb by molecular simulation: Structure and shear deformation

Atomic scale modeling was used to study the structure, energy and shear behaviors of (110) twist grain boundaries (TWGBs) in body-centered cubic Nb. The relation between grain boundary energy (GBE) and the twist angle θ agrees well with the Read-Shockley equation in low-angle range. At higher angles, the GBEs show no distinct trend with the variation of the twist angle or the density of coincident lattice sites. All (110) twist boundaries can be classified into two types: low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). LAGBs contain a hexagonal dislocation network (HDN) which is composed of 12[111], 12[1¯1¯1] and [001] screw dislocations. HAGBs can be classified into three sub-types further: special boundaries with low Σ, boundaries in the vicinity of special boundaries with similar structures and ordinary HAGBs consisting of periodic patterns. Besides, a dependence of grain boundary shear response vs the twist angle over the entire twist angle range is obtained. Pure sliding behavior is found at all TWGBs. When θ<12°, the flow stress of LAGBs is found to be correlated with the HDNs and decreases with the increasing twist angle. For ordinary HAGBs, the magnitude of flow stress is around 0.8–1.0GPa and the twist angle has little effect on the anisotropy mobility. For special grain boundaries with low Σ, the boundary structures govern the GBEs and shear motion behavior significantly.

Extending the range of validity of Fourier's law into the kinetic transport regime via asymptotic solution of the phonon Boltzmann transport equation

We derive the continuum equations and boundary conditions governing phonon-mediated heat transfer in the limit of small but finite mean free path from asymptotic solution of the linearized Boltzmann equation in the relaxation time approximation. Our approach uses the ratio of the mean free path to the characteristic system lengthscale, also known as the Knudsen number, as the expansion parameter to study the effects of boundaries on the breakdown of the Fourier descrition. We show that, in the bulk, the traditional heat conduction equation using Fourier's law as a constitutive relation is valid at least up to second order in the Knudsen number for steady problems and first order for time-dependent problems. However, this description does not hold within distances on the order of a few mean free paths from the boundary; this breakdown is a result of kinetic effects that are always present in the boundary vicinity and require solution of a Boltzmann boundary-layer problem to be determined. Matching the inner, boundary layer, solution to the outer, bulk, solution yields boundary conditions for the Fourier description as well as additive corrections in the form of universal kinetic boundary layers; both are found to be proportional to the bulk-solution gradients at the boundary and parametrized by the material model and the phonon-boundary interaction model (Boltzmann boundary condition). Our derivation shows that the traditional no-jump boundary condition for prescribed temperature boundaries and no-flux boundary condition for diffusely reflecting boundaries are appropriate only to zeroth order in the Knudsen number; at higher order, boundary conditions are of the jump type. We illustrate the utility of the asymptotic solution procedure by demonstrating that it can be used to predict the Kapitza resistance (and temperature jump) associated with an interface between two materials.

Analysis of the Boundary Problem with the Preference of Mass Flow

We work with the numerical solution of the turbulent compressible gas flow, and we focus on the numerical solution of these equations, and on the boundary conditions, particularly on the outlet boundary condition with the preference of given mass flow. Usually, the boundary problem is being linearized, or roughly approximated. The inaccuracies implied by these simplifications may be small, but it has a huge impact on the solution in the whole studied area, especially for the non-stationary flow. The boundary condition with the preference of mass flow is sometimes being implemented with the use of some iterative process, guessing the correct values (for the pressure, density, velocity) in order to match the given mass flow through the boundary. In our approach we try to be as exact as possible, using our own original procedures. We follow the exact solution of the initial-value problem for the system of hyperbolic partial differential equations. This complicated problem is modified at the close vicinity of boundary, where the conservation laws are supplied with the additional boundary conditions. We complement the boundary problem suitably, and we show the analysis of the resulting uniquely-solvable modified Riemann problem.The resulting algorithm was coded and used within our own developed code for the solution of the compressible gas flow (the Euler, NS, and RANS equations). The examples show good behaviour of the analyzed boundary condition.

Refined sectional analysis with shear center prediction for nonhomogeneous anisotropic beams with nonuniform warping

A two-dimensional (2D) finite element (FE) sectional analysis system is developed for nonhomogeneous anisotropic beams established on a refined displacement-based elasticity theory. The classical effects due to elastic couplings, and nonclassical effects pertaining to three-dimensional (3D) warping displacements are incorporated in the formulation. The formulation allows for a generalized refined model with 12 × 12 stiffness matrix which subsequently encapsulates the Timoshenko model, Vlasov model for restrained torsion, and refined model for fully nonuniform warping (NUW). In addition, the shear center and tension center offsets are computed as cross-sectional properties based on the extended Trefftz’ theory for anisotropic beams. The accuracy of the sectional analysis is substantiated for isotropic as well as anisotropic, closed and open section beams with and without end restraints. The results indicate reliable predictions of the elastic properties for isotropic as well as anisotropic beams compared with the analytical solution, 3D FE solution, and other state-of-the-art methods. The static behavior of the beams is shown to be significantly influenced by the NUW effects especially in the vicinity of beam boundaries.

Improved segmentation of low-contrast lesions using sigmoid edge model.

The intensity profile of an image in the vicinity of a tissue's boundary is modeled by a step/ramp function. However, this assumption does not hold in cases of low-contrast images, heterogeneous tissue textures, and where partial volume effect exists. We propose a hybrid algorithm for segmentation of CT/MR tumors in low-contrast, noisy images having heterogeneous/homogeneous or hyper-/hypo-intense abnormalities. We also model a smoothed noisy intensity profile by a sigmoid function and employ it to find the true location of boundary more accurately. A novel combination of the SVM, watershed, and scattered data approximation algorithms is employed to initially segment a tumor. Small and large abnormalities are treated distinctly. Next, the proposed sigmoid edge model is fitted to the normal profile of the border. The estimated parameters of the model are then utilized to find true boundary of a tissue. We extensively evaluated our method using synthetic images (contaminated with varying levels of noise) and clinical CT/MR data. Clinical images included 57 CT/MR volumes consisting of small/large tumors, very low-/high-contrast images, liver/brain tumors, and hyper-/hypo-intense abnormalities. We achieved a Dice measure of [Formula: see text] and average symmetric surface distance of [Formula: see text]mm. Regarding IBSR dataset, we fulfilled Jaccard index of [Formula: see text]. The average run-time of our code was [Formula: see text]s. Individual treatment of small and large tumors and boundary correction using the proposed sigmoid edge model can be used to develop a robust tumor segmentation algorithm which deals with any types of tumors.

Research on superconductivity of epitaxial FeSe films

The newly discovered iron-based superconductors in 2008 have become the second class of high temperature superconductor besides the cuprates. Among all these iron-based compounds, β-FeSe has the simplest chemical structure and can be an archetype system for unraveling the mechanism of superconductivity. By using state-of-the-art molecular beam epitaxy (MBE) technique, high-quality superconducting FeSe single crystalline films with finely controllable topography and composition have been successfully prepared, with their superconducting properties extensively investigated. The low-temperature scanning tunneling spectra reveal that the superconducting gap in the quasiparticle density of states is visible down to two unit cells of FeSe films, and evidence for a gap function with nodal lines. Electron pairing with two-fold symmetry has also been demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms and Se vacancies. The two-fold pairing symmetry is supported by our observation of striped electronic nanostructures in the slightly Se-doped samples, primarily due to the orbital ordering. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap and pin magnetic vortices. This is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity. Furthermore, we reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films, whose energy reduces with decreasing gap magnitude Ω . Recently, the growth recipe has been further extended to grow FeSe film on SrTiO3(001) substrate, leading to the high- T c superconductivity in the very FeSe/SrTiO3 interface. A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy suggest that the superconductivity of single-unit-cell FeSe films could occur around 77 K. Our subsequent transport and measurements of the one-unit-cell thick FeSe films on insulating SrTiO3 substrates with non-superconducting FeTe protection layers provide definitive evidence for high temperature superconductivity with an onset T c above 40 K which are much higher than T c~9.4 K for bulk FeSe, respectively. The finding provides a completely new avenue and method to reveal the pairing mechanism of high- T c superconductors, and to further enhance T c, stirring up worldwide increasing attention in the field of high- T c superconductors and material sciences.

Magnetic resonance elastography of the brain: An in silico study to determine the influence of cranial anatomy

Magnetic resonance elastography (MRE) of the brain has demonstrated potential as a biomarker of neurodegenerative disease such as dementia but requires further evaluation. Cranial anatomical features such as the falx cerebri and tentorium cerebelli membranes may influence MRE measurements through wave reflection and interference and tissue heterogeneity at their boundaries. We sought to determine the influence of these effects via simulation. MRE-associated mechanical stimulation of the brain was simulated using steady state harmonic finite element analysis. Simulations of geometrical models and anthropomorphic brain models derived from anatomical MRI data of healthy individuals were compared. Constitutive parameters were taken from MRE measurements for healthy brain. Viscoelastic moduli were reconstructed from the simulated displacement fields and compared with ground truth. Interference patterns from reflections and heterogeneity resulted in artifacts in the reconstructions of viscoelastic moduli. Artifacts typically occurred in the vicinity of boundaries between different tissues within the cranium, with a magnitude of 10%-20%. Given that MRE studies for neurodegenerative disease have reported only marginal variations in brain elasticity between controls and patients (e.g., 7% for Alzheimer's disease), the predicted errors are a potential confound to the development of MRE as a biomarker of dementia and other neurodegenerative diseases. Magn Reson Med 76:645-662, 2016. © 2015 Wiley Periodicals, Inc.

Analysis of Interfacial Processes at the SOFC Electrodes by I n-Situ Raman Spectroscopy

Kinetics of redox reactions and the variations of real charge-carrier concentrations across the SOFC electrode / electrolyte interfaces have critical importance for the fuel cell performance. The microscopic mechanisms of electrochemical processes in the vicinity of triple-phase boundary (TPB) can be assessed employing impedance spectroscopy, current-voltage measurements and various pulse techniques. Except for the micro-electrode approaches, however, these methods are cumulative. Moreover, unambiguous interpretation of the electrochemical measurement results requires, as a rule, to introduce complimentary experimental methods and/or to use simplified models. The present work is centered on the developments of a new combined technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions. The Raman spectroscopy was already successfully used to investigate both cathodic [1,2] and anodic [3] processes in SOFC, but the resultant information was mainly related to outer boundaries of the model electrochemical cells, primarily electrode surface, due to low penetration depth of the excitation radiation. The most important zones of the electrode systems, where the electrochemical reactions occur and ionic charge carriers are generated, cannot be achieved viewing the surface and edge areas. In the present work, this problem was solved by employing optically transparent, single-crystal membranes made of 10 mol.% Sc2O3 and 1% mol.% Y2O3 stabilized zirconia (10Sc1YSZ). In order to provide simultaneous Raman and electrochemical measurements of the cell placed under an oxygen chemical potential gradient, such as air/H2, a special controlled-atmosphere chamber was elaborated and tested. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the TPB zone, passing the beam through single-crystal solid electrolyte onto the interface, as a function of temperature, atmosphere, current density and/or overpotential. The results of case studies focused on redox kinetics of Ni-containing cermet anodes, are presented. This work was supported by grant 14-29-04031 of Russian Foundation for Basic Research. References S. Loridant, L. Abello, E. Siebert, G. Lucazeau, Solid State Ion. 78 (1995) 249-258K. Blinn, H. Abernathy, M.Liu, Advances in Solid Oxide Fuel Cells V. (2010) 63-73W. Bessler, M.Vogler, H. Stoermer, D. Gerthsen, A. Utz, A. Weber, E. Ivers-Tiffee, Phys. Chem. Chem. Phys., 12 (2010) 13888-13903

Inhibition of cell–cell adhesion impairs directional epithelial migration on micropatterned surfaces

The development of the vertebrate body plan with left-right (LR) asymmetry (also known as handedness and chirality) requires the emerging chiral morphogenesis of epithelial cells at specific embryonic stages. In this process, cell-cell adhesions coordinate cellular organization and collective cell migration, and are critical for the directional looping of developing embryonic organs. However, the underlying biophysical mechanism is not yet well understood. Here we modeled normal and delayed epithelial LR symmetry breaking with patterned epithelial chiral morphogenesis on microscale lines with various widths. The patterned cells exhibited biased migration wherein those on opposing boundaries migrated in different directions. Disrupting adherens junctions with ethylene glycol tetraacetic acid (EGTA) resulted in a decrease in velocity difference in opposing boundaries as well as the associated biased cell alignment, along with an increase in the overall random motion. Altering the distance between the opposing boundaries did not significantly alter alignment, but significantly disturbed the velocity profile of the cell migration field. Further examination of cell polarity indicated that disruption of adherens junctions did not affect cell polarization on the boundaries, but decreased the transmission of chiral bias into the interior region of the epithelial cell sheet. Overall, our results demonstrated the dependence of the scale of collective cell migration on the strength of cell-cell adhesion, and its effects on the chirality of a multicellular structure through mediating cell polarity in the vicinity of geometric boundaries. This study demonstrated that our 2D microscale system provides a simple yet effective tool for studying the influence of collective cell migration on LR symmetry breaking, and possibly for fetal drug screening to prevent birth defects related to alteration in cell-cell adhesion.

Mechanical Behavior of Light Alloys with Bimodal Grain Size Distribution

Deformation and damage at the meso-scale level in representative volumes (RVE) of light ultrafine grained (UFG) alloys with distribution of grain size were simulated in wide loading conditions. The computational models of RVE were developed using the data of structure researches aluminum and magnesium UFG alloys on meso-, micro -, and nanoscale levels. The critical fracture stress on meso-scale level depends not only probabilistic of grain size distribution in RVE but relative volumes of coarse grains. Microcracks nucleation is associated with strain localization in UFG partial volumes in alloys with bimodal grain size distribution. Microcracks branch in the vicinity of coarse and ultrafine grains boundaries. It is revealed that the occurrence of bimodal grain size distributions causes the increasing of UFG alloys ductility, but decreasing of the tensile strength. The distribution the shear stress and the local particle velocity takes place at mesoscale level under dynamic loading of UFG alloys with bimodal grain size. The increasing of fine precipitations concentration not only causes the hardening but increasing of ductility of UFG alloys with bimodal grain size distribution.

Moving particle semi-implicit method for fluid simulation with implicitly defined obstacles

We developed a particle-based fluid simulation method with obstacles defined in implicit form. Our fluid simulation is based on the Moving Particle Semi-implicit (MPS) method, a typical particle-based algorithm achieving incompressible flow. In general, the particle-based fluid simulation is performed with obstacles that are defined as a set of particles located along the boundaries. We employ the implicit representation for the geometric information of obstacles which requires a new formulation of particle motion at the vicinity of boundaries. The main difficulty of MPS-based simulation with implicit obstacles is the lack of boundary particles that are required for the computation of two quantities: particle number density and particle force determined by pressure field. In our formulation, new definitions of the two quantities giving good estimation of the original ones is developed and incorporated in the MPS algorithm. In addition, we provide a modified algorithm for the construction of discrete linear system specific to the implicit representation for the computation of particle pressure. Our numerical tests show that the proposed approximation techniques provide adequate particle motion at the vicinity of the implicitly defined obstacles.

Lift Force Acting on Body in Liquid in the Vicinity of Boundary Executing Tangential Oscillations

The average force acting on a cylindrical or spherical body placed near the boundary of cylindrical cavity filled with liquid and executing rotational vibrations is experimentally studied. The repulsive lift force acting on solid is found and measured using the method of body suspension in the gravity field. In horizontal cavity the repulsive force provides steady state of light cylinder near the upper boundary at a distance comparable with the thickness of the Stokes layer. The heavy sphere which hangs on thread repulses from the wall of vertical cylindrical cavity under torsional vibration. The dependence of the average lift force on the amplitude and frequency of vibrations and on distance between the body and the boundary has been investigated. Results are presented on the plane of dimensionless parameters. In the examined frequency range the lift force acting on cylinder decreases exponentially with distance from the boundary. Lift force acting on sphere decreases with distance and strongly depends on dimensionless frequency of vibration. Conclusion that lift force is generated by body oscillations excited by its viscous interaction with the oscillating boundary is done. The found phenomenon is important for control of inclusions under microgravity conditions.

Diffusion in low-dimensional lipid membranes.

The diffusion behavior of biological components in cellular membranes is vital to the function of cells. By collapsing the complexity of planar 2D membranes down to one dimension, fundamental investigations of bimolecular behavior become possible in one dimension. Here we develop lipid nanolithography methods to produce membranes, under fluid, with widths as low as 6 nm but extending to microns in length. We find reduced lipid mobility, as the width is reduced below 50 nm, suggesting different lipid packing in the vicinity of boundaries. The insertion of a membrane protein, M2, into these systems, allowed characterization of protein diffusion using high-speed AFM to demonstrate the first membrane protein 1D random walk. These quasi-1D lipid bilayers are ideal for testing and understanding fundamental concepts about the roles of dimensionality and size on physical properties of membranes from energy transfer to lipid packing.

Robust Detection of Abandoned Objects Based on Modified Dual Background Segmentation

A robust real-time method for detection of abandoned objects in complex outdoor environments is presented. The system introduces a dual background segmentation method based on feedback modification. One background updates fast and the other updates slow. The slow background is then modified by the detection result to exclude the static foreground. They contribute to build the static foreground region masks which are used to update a counter. Static foreground regions are then extracted from the counter and further filtered by a light and shadow eliminator. The filtered regions are then classified into abandoned or removed objects by comparing the feature of its surroundings and the interior boundary vicinity of the fast background and slow background. The algorithm is simple and computationally less intensive. Experimental results on public video datasets and real image sequences acquired outdoor show its practicality and robustness to moving vegetation, shaking camera, illumination change and crowed scenes.

Multiphase dynamic flash simulations using entropy maximization and application to compressible flow with phase change

A isochoric‐isoenergetic flash solver using a direct entropy maximization principle for phase splitting and Gibbs free energy minimization for phase stability is developed. The solver searches for the global stable state in a rigorous and thermodynamically consistent way. The solver is demonstrated to be robust and efficient to handle multiphase flash, even in the vicinity of phase boundaries. Dynamic flash computations and gas dynamics simulations of shock waves are considered for pure ethylene and for binary ethylene‐nitrogen mixtures. The simulations show significantly different shock wave characteristics when phase separation is considered due to intensive energy exchange, compared to the frozen flow limiting single‐phase solution. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3013–3024, 2014

Hot forging characteristic of Ti–5Al–5V–5Mo–3Cr alloy with single metastable β microstructure

Hot deformation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700°C, subgrain formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deformation and affects the flow behavior. In testing at and above 800°C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.

A subgrid-scale eddy-viscosity model based on the volumetric strain-stretching

We propose a novel subgrid-scale (SGS) eddy-viscosity model for large eddy simulation (LES) to accurately reproduce the effect of subgrid stresses on the resolved scales. The developed SGS model is based on the second-order volumetric strain-stretching (VSS) tensor, which is constructed by the multiplication of diagonal components of the strain-rate tensor with its off-diagonal components. The proposed VSS-model is validated in typical flow cases: freely decaying isotropic turbulence, incompressible turbulent channel flow at Reτ = 395, compressible turbulent channel flows at Ma = 1.5 and Reτ = 221, and Ma = 3.0 and Reτ = 556. LESs with the dynamic Smagorinsky model and the Vreman model are also performed to compare the performance of the VSS-model. The proposed model correctly recovers cubic wall behavior in the vicinity of solid boundaries in incompressible flow regime, and it has no limitation in its application to geometrically complex flows.

Residence times and boundary-following behavior in animals.

Many animals in heterogeneous environments bias their trajectories displaying a preference for the vicinity of boundaries. Here we propose a criterion, relying on recent invariance properties of residence times for microreversible Boltzmann's walks, that permits detecting and quantifying boundary-following behaviors. On this basis we introduce a boundary-following model that is a nonmicroreversible Boltzmann's walk and that can represent all kinds of boundary-following distributions. This allows us to perform a theoretical analysis of field-resolved boundary following in animals. Two consequences are pointed out and are illustrated: A systematic procedure can now be used for extraction of individual properties from experimental field measurements, and boundary-curvature influence can be recovered as an emerging property without the need for individuals perceiving the curvature via complex physiological mechanisms. The presented results apply to any memoryless correlated random walk, such as the run-and-tumble models that are widely used in cell motility studies.

Demosaicking using geometric duality and dilated directional differentiation

This paper presents a new demosaicking algorithm which uses two cost terms: the interpolation error of a low resolution image based on geometric duality and the dilated directional differentiation of color differences. Since a given high resolution image and its low resolution image obtained by sampling have similar edge properties, the proposed algorithm computes the interpolation errors for the candidate directions in the low resolution image, and exploits them as a cost term for the direction. In addition, the interpolation direction can be determined accurately even in the vicinity of object boundaries by dilating the directional differentiation of the color difference values. Through dilation, some pixels, which are in the neighborhood of an edge but classified into a flat region by simple edge detection like the Sobel filter, are reclassified. By combining this edge classifier and the weighted sum of the estimates obtained by Taylor approximation, missing pixels are interpolated. Simulation results show that the proposed demosaicking algorithm is superior to other state-of-the-art algorithms in terms of visual and objective qualities. Furthermore, the computational complexity is comparable with the existing algorithms. Therefore, the proposed algorithm is suitable for real-time implementation.

Manipulation of lens-shaped objects in various materials to enhance photonic nanojet using MMP method

Lens-shaped particles with different sizes for various refractive indices have been investigated under plane wave illumination by using 2D-MMP simulation. Distribution of the power in the vicinity of opposite boundary of the particle has resulted in a tightly focused photonic nanojet in specific conditions. The results show that the beam waists are smaller than the diffraction limit, and they propagate over several optical wavelengths without significant diffraction while the beam reaches a high intensity. In this paper, it is argued the manipulation of various dielectric lens shapes with respect to a highly confined power distribution in the producing nanojet has been parameterized according to the beam waist and divergence with regards to particular refractive index (glass, plastic and patinal), and appropriate structure shapes. Optimized dimensions of the nanojets for different physical properties are calculated.