Random Orientation Distribution Research Articles

The Spatial Orientation of Planetary Nebulae within the Milky Way

ABSTRACTWe analyze the spatial orientation of a homogenous sample of 440 elongated Planetary Nebulae (PNe) in order to determine the orientation of their apparent major axis with respect to the Milky Way plane. We present some important geometrical and statistical considerations that have been overlooked by previous works on the subject. The global distribution of galactic position angles (GPA) of PNe is quantitatively not very different from a random distribution of orientations in the Galaxy. Nevertheless, we find that there is at least one region on the sky, toward the Galactic center, where a weak correlation may exist between the orientation of the major axis of some PNe and the Galactic equator, with an excess of axes with GPA ∼100°. Therefore, we confirm that “extrinsic” phenomena (i.e., global Galactic magnetic fields, shell compression from motion relative to the Interstellar Medium) do not determine the morphology of PNe on most of the sky, with a possible exception toward the Galactic center.

Shear-Induced Alignment of Smectic Side Group Liquid Crystalline Polymers

Large amplitude oscillatory shear (LAOS) is frequently capable of generating macroscopic alignment from an initially random orientation distribution in ordered polymer fluids. Side-group liquid crystalline polymers are of special interest in that the flow field may couple differently to the polymer backbone and the mesogen ordering. We report combined rheological and in situ X-ray scattering investigations of LAOS-induced alignment in smectic side-group LCPs. Synchrotron X-ray scattering is used to study orientation development using a rotating disk shear cell in which orientation is tracked within the flow-vorticity (1−3) plane. In all cases, we find that shear promotes anisotropic orientation states in which the lamellar normal tends to align along the vorticity direction of the shear flow (“perpendicular” alignment). We examine the effects of shear strain amplitude and polymer backbone molecular weight on the ability of LAOS to induce alignment. Rheological measurements of the dynamic moduli reveal that large amplitude shearing in the smectic phase causes a notable decrease in the modulus. X-ray and rheological data demonstrate that increasing strain promotes higher degrees of orientation, while increasing molecular weight impedes development of smectic alignment.

Multiple angle acoustic classification of zooplankton

The use of multiple angle acoustic scatter to discriminate between two taxa of fluid-like zooplankton, copepods and euphausiids, is explored. Using computer modeling, feature extraction, and subsequent classification, the accuracy in discriminating between the two taxa is characterized via computer simulations. The model applies the distorted wave Born approximation together with a simple system geometry, a linear array, to predict a set of noisy training and test data. Three feature spaces are designed, exploiting the relationship between the shape of the scatterer and angularly varying scattering amplitude, to extract discriminant features from these data. Under the assumption of uniform random length and uniform three-dimensional orientation distributions for each class of scatterers, the performance of several classification algorithms is evaluated. Simulations reveal that the incorporation of multiple angle data leads to a marked improvement in classification performance over single angle methods. The improvement is more substantial using broadband scatter. The simulations indicate that under the stated assumptions, a low classification error can be obtained. The use of multiple angle scatter therefore holds promise to substantially improve the in situ acoustic classification of fluid-like zooplankton using simple observation geometries.

Computational Simulation of Composites Reinforced by Planar Random Fibers: Homogenization and Localization by Unit Cell and Mean Field Approaches

The linear thermoelastic and thermophysical behavior of a short fiber reinforced composite material with planar random fiber arrangement is investigated by advanced numerical and analytical micromechanical methods. On the one hand, finite element based multi-fiber unit cells are introduced that contain 40-50 short fibers in arrangements approximating 2D random orientation distributions. On the other hand, the same fiber arrangements are investigated by an extended Mori-Tanaka mean field approach that can handle both statistical and discrete descriptions of the fiber orientations. Within the Mori-Tanaka scheme average microfields are extracted for individual fibers, and finite-length cylindrical reinforcements are modeled via averaged dilute concentration tensors that are evaluated numerically by finite element analysis. Homogenization and localization are performed for a metal matrix composite consisting of copper, reinforced by 21 vol% of carbon fibers that closely approximate a planar random arrangement. Simulation results on the macroscopic and microscopic linear elastic, thermoelastic, and thermal conductivity responses obtained by the two approaches are compared and excellent agreement is found.

Spontaneous field potentials in the glomeruli of the olfactory bulb: The leading role of juxtaglomerular cells

Field potentials recorded in the olfactory bulb glomerular layer (GL) are thought to result mainly from activation of mitral and tufted cells. The contribution of juxtaglomerular cells (JG) is unknown. We tested the hypothesis that JG are the main driving force to novel spontaneous glomerular layer field potentials (sGLFPs), which were recorded in rat olfactory bulb slices maintained in an interface chamber. We found that sGLFPs have comparable magnitudes, durations and frequencies both in standard horizontal slices, where all layers with all cell types were present, and in isolated GL slices, where only JG cells were preserved. Hence, the impact of mitral and deep/medium tufted cells to sGLFPs turned out to be minor. Therefore, we propose that the main generators of sGLFPs are JG neurons. We further explored the mechanism of generation of sGLFPs using a neuronal ensemble model comprising all types of cells associated with a single glomerulus. Random orientation and homogenous distribution of dendrites in the glomerular neuropil along with surrounding shell of cell bodies of JG neurons resulted in substantial spatial restriction of the generated field potential. The model predicts that less than 20% of sGLFP can spread from one glomerulus to an adjacent one. The contribution of JG cells to the total field in the center of the glomerulus is estimated as ∼50% ( ∼34% periglomerular and ∼16% external tufted cells), whereas deep/medium tufted cells provide ∼39% and mitral cells only ∼10%. Occasionally, some sGLFPs recorded in adjacent or remote glomeruli were cross-correlated, suggesting involvement of interglomerular communication in information coding. These results demonstrate a leading role of JG cells in activation of the main olfactory bulb (MOB) functional modules. Finally, we hypothesize that the GL is not a set of independent modules, but it represents a subsystem in the MOB network, which can perform initial processing of odors.

Misorientations of garnet aggregate within a vein: an example from the Sanbagawa metamorphic belt, Japan

AbstractIn this study, the chemistry and microstructure of garnet aggregates within a metamorphic vein are investigated. Garnet‐bearing veins in the Sanbagawa metamorphic belt, Japan, occur subparallel to the foliation of a host mafic schist, but some cut the foliation at low angle. Backscattered electron image and compositional mapping using EPMA and crystallographic orientation maps from electron‐backscattered diffraction (EBSD) reveal that numerous small garnet (10–100 μm diameter) coalesce to form large porphyroblasts within the vein. Individual small garnet commonly exhibits xenomorphic shape at garnet/garnet grain boundaries, whereas it is idiomorphic at garnet/quartz boundaries. EBSD microstructural analysis of the garnet porphyroblasts reveals that misorientation angles of neighbour‐pair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnet in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation‐driven rotation. As garnet nucleated with random orientation, the difference in misorientation between the two studies is due to the difference in the extent of grain rotation. A simple kinetic model that assumes grain rotation of garnet is rate‐limited by grain boundary diffusion creep of matrix quartz, shows that (i) the substantial rotation of a fine garnet grain could occur for the conditions of the Sanbagawa metamorphism, but (ii) the rotation rate drastically decreased as garnet grains formed large clusters during growth. Therefore, the random misorientation distribution of garnet porphyroblasts in the Sanbagawa vein is interpreted as follows: (i) garnet within the vein grew so fast that substantial grain rotation did not occur through porphyroblast formation, and thus (ii) random orientations at the nucleation stage were preserved. The extent of misorientation‐driven rotation indicated by deviation from random orientation distribution may be useful to constrain the growth rate of constituent grains of porphyroblast that formed by multiple nucleation and coalescence.

Peptide–lipid interactions of the β-hairpin antimicrobial peptide tachyplesin and its linear derivatives from solid-state NMR

The peptide–lipid interaction of a β-hairpin antimicrobial peptide tachyplesin-1 (TP-1) and its linear derivatives are investigated to gain insight into the mechanism of antimicrobial activity. 31P and 2H NMR spectra of uniaxially aligned lipid bilayers of varying compositions and peptide concentrations are measured to determine the peptide-induced orientational disorder and the selectivity of membrane disruption by tachyplesin. The disulfide-linked TP-1 does not cause any disorder to the neutral POPC and POPC/cholesterol membranes but induces both micellization and random orientation distribution to the anionic POPE/POPG membranes above a peptide concentration of 2%. In comparison, the anionic POPC/POPG bilayer is completely unaffected by TP-1 binding, suggesting that TP-1 induces negative curvature strain to the membrane as a mechanism of its action. Removal of the disulfide bonds by substitution of Cys residues with Tyr and Ala abolishes the micellization of POPE/POPG bilayers but retains the orientation randomization of both POPC/POPG and POPE/POPG bilayers. Thus, linear tachyplesin derivatives have membrane disruptive abilities but use different mechanisms from the wild-type peptide. The different lipid–peptide interactions between TP-1 and other β-hairpin antimicrobial peptides are discussed in terms of their molecular structure.

Effect of cold rolling on microstructure and material properties of 5052 alloy sheet produced by continuous casting

The microstructure and material properties of continuous cast 5052 alloy sheet in as-received condition (as-received sheet) and laboratory cold rolled condition (lab-processed sheet) were investigated after annealing treatment. Microstructural examinations show that the cold rolling process increases the extent of centerline segregation of second-phase particles. The increase in rolling reduction results in a decrease of grain size and a more random distribution of crystallite orientations. Tensile test results indicate that all the sheets exhibit two-stage strain-hardening behavior. Cold rolling increases the total elongation and n 2-value in the transverse direction of the lab-processed sheet with a thickness of 1.6 mm. However, further increase in the rolling reduction leads to an increase of UTS with a decrease of elongation and n 2-value. Furthermore, from the determined forming limit curves (FLCs), it is found that the 1.6 mm thick lab-processed sheet has better stretchability than the as-received sheet, whereas further cold rolling reduces the level of FLC. The effects of cold rolling on the mechanical property and formability are discussed in terms of the microstructural changes.

Ligand−Receptor Interactions in Tethered Polymer Layers

The binding of small proteins to ligands that are attached to the free ends of polymers tethered to a planar surface is studied using a molecular theory. The effects of changing the intrinsic binding equilibrium constant of the ligand-receptor pair, the polymer surface coverage, the polymer molecular weight, and the protein size are studied. The results are also compared with the case where ligands are directly attached to the surface without a polymer acting as a spacer. We found that within the biological range of binding constants the protein adsorption is enhanced by the presence of the polymer spacers. There is always an optimal surface coverage for which ligand-receptor binding is a maximum. This maximum increases as the binding energy and/or the polymer molecular weight increase. The presence of the maximum is due to the ability of the polymer-bound proteins to form a thick layer by dispersing the ligands in space to optimize binding and minimize lateral repulsions. The fraction of bound receptors is unity for a very small surface coverage of ligands. The very sharp decrease in the fraction of bound ligand-receptor pairs with surface coverage depends on the polymer spacer chain length. We found that the binding of proteins is reduced as the size of the protein increases. The orientation of the bound proteins can be manipulated by proper choice of the grafted layer conditions. At high polymer surface coverage the bound proteins are predominantly perpendicular to the surface, while at low surface coverage there is a more random distribution of orientations. To avoid nonspecific adsorption on the surface, we studied the case where the surface is covered by a mixture of a relatively high molecular weight polymer with a ligand attached to its free end and a low molecular weight polymer without ligand. These systems present a maximum in the binding of proteins, which is of the same magnitude as when only the long polymer-ligand is present. Moreover, when the total surface coverage in the mixed layers of polymers is high enough, nonspecific adsorption of the proteins on the surface is suppressed. The use of the presented theoretical results for the design of surface modifiers with tailored abilities for specific binding of proteins and optimal nonfouling capabilities is discussed.

Adsorbed layers of oriented fibronectin: A strategy to control cell–surface interactions

Fibronectin (Fn) is a matrix protein known to induce cell attachment and spreading through its cell binding site and related synergy sites. Fn-coated surfaces are therefore useful in tissue engineering and other cell contacting applications, but a problem with many immobilization strategies is a random distribution of molecular orientations. We sought to control Fn orientation, and thus enhance the availability of its cell binding site, by immobilizing Fn via a carboxymethyl dextran layer onto which are chemically attached monoclonal antibodies specific to a region near to Fn's C terminus (and thus away from the cell binding site). Using optical waveguide lightmode spectroscopy, we show the presence of chemically coupled antibodies to yield a considerably denser and thicker Fn layer, consistent with a more vertically aligned protein. Human umbilical vein endothelial cells spread significantly faster, and in a more spherically symmetric way, on an oriented Fn layer (i.e., in the presence of immobilized monoclonal antibodies) as compared with a control Fn layer (i.e., in the absence of bound antibodies). However, we observe human umbilical vein endothelial cell spreading on the oriented Fn layer to be similar to that on a Fn layer in the absence of a carboxymethyl dextran layer, suggesting that although orienting Fn is a promising strategy, coupling strategies using linkers other than dextran may be needed.

Evolution of dislocation cell structures in plastically deformed metals

We use a recently developed microstructure evolution model with stochastic dynamics to study the evolution of ordered dislocation patterns. Dislocations are described as groups arranged into arrays, with an initially random distribution of orientations, moving in an isotropic viscous medium. Stochastic displacements of dislocation walls describe the rearrangement of small groups of dislocations to form minimum-dissipation cells structures while lowering the overall misorientation. The result of large mechanical deformation, as could occur in cold-working, is simulated by introducing a preferential direction along which misorientation gradients of various kind are superimposed. The simulations show the emergence of ordered microstructure patterns, with high-density dislocation boundaries running perpendicular to the initial gradient, and a low-density cellular band structure in between. The results are compared to experimental data in heavily cold-worked metals.

A crystal plasticity based estimate for forming limit diagrams from textural inhomogeneities

An alternative method to predict forming limits for sheet metal forming is presented. Deformation instabilities are considered as the natural result of strain field fluctuations caused by textural inhomogeneities. The biaxial stretching of a sheet material configuration is analysed using the finite element method in plane stress conditions. For the description of the material behaviour at the microscopic level, a viscous crystal plasticity model is used. To accommodate the simulation of polycrystallinity, each individual integration point of the finite element mesh is assumed to be composed of a discrete number of single crystals. The single crystals are coupled according to a Taylor interaction approach. The initial texture inhomogeneity is considered as a natural microstructure imperfection, which is present in any real metal, potentially triggering deformation instabilities. The occurrence of a nonuniform deformation field is the result of the textural inhomogeneity only; no other imperfections are introduced to generate the instabilities observed. The structural instability criterion is used as an approximate localisation criterion. Proportional loading of a metal sheet with an initially random distribution of crystal lattice orientations has been analysed for different uniform strain paths. The results demonstrate the ability of the method to describe the development of strain localisation bands, from which the formability can be assessed.

Mechanical instabilities of bubble clusters between parallel walls

We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w*, for which the foam surface energy E reaches its minimum, E*. The clusters under investigation consist of either a single bubble, or of "twin bubbles" of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E* of the unbuckled state. Finally, we have also predicted the critical w(t)* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be "too parallel" to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations.

Low-Knudsen-number photophoresis of aerosol spheroids

The photophoretic motion of a freely suspended aerosol spheroid exposed to a radiative heat flux that is oriented arbitrarily with respect to its axis of revolution is analytically studied. The Knudsen number is assumed to be so small that the fluid flow can be described by a continuum model with a thermal slip at the particle surface. In the limit of small Peclet and Reynolds numbers, the appropriate energy and momentum equations are solved using the bifocal-coordinate transformations. Expressions for the photophoretic velocity and force are obtained in closed form for various cases of prolate and oblate spheroidal particles. The average photophoretic velocity and force for an ensemble of identical, noninteracting spheroids with random orientation distribution are also determined. The results indicate that the aspect ratio and relative thermal conductivity of a spheroidal particle and its orientation with respect to the incident light can have significant effects on its photophoretic behavior.

Modeling of Nucleation Spectra for Primary Recrystallization and Application to Through-Process Texture Modeling

Schemes to model deformation inhomogeneities and nuclei distributions based on the grain cluster model for deformation texture simulation GIA are presented. The orientation distributions of nuclei in stable orientations, nuclei in grains with orientation gradients and nuclei due to subgrain growth at grain boundaries are predicted. Additionally, nuclei with a random orientation distribution are considered, reflecting nucleation at shear bands or large constituent particles. Furthermore, models for a quantitative assessment of the participating nucleation mechanisms are proposed. The resulting nucleation texture was input to the static recrystallization texture model StaRT. The through-process texture development during a sequence of several hot rolling, cold rolling and annealing steps in industrial production of the aluminum alloy AA5182 is presented.

Creep of multidirectionally fibre-reinforced composites

A model for the creep of metal matrix composites multidirectionally reinforced by short fibres is proposed. The reinforcement is described by the effective stiffness tensor of a multidirectional arrangement of continuous fibres and the internal damage of the composite during creep due to fibre fragmentation is introduced by assigning a heuristic nonlinear stress–strain relationship to the fibres. Based on the model, the load partitioning between matrix and fibres is computed. The macroscopic creep behaviour is simulated for composites exhibiting different fibre orientation distributions and different heuristic nonlinear stress–strain functions. The computational results rationalize the creep behaviour of multidirectional fibre-reinforced composites. For a two-dimensional random orientation distribution, a good qualitative match between simulation and experimental results is obtained for compressive loading and for in-plane tensile loading. For loading normal to the reinforcement plane, the model overestimates the creep resistance. In this case, the formation and growth of cavities seems to govern the creep deformation of the composite.

Thermophoresis of Aerosol Spheroids

An analytical study is presented for the thermophoretic motion of a freely suspended aerosol spheroid in a uniform prescribed temperature gradient that is oriented arbitrarily with respect to its axis of revolution. The Knudsen number is assumed to be small so that the fluid flow is described by a continuum model with a thermal slip at the particle surface. In the limit of small Peclet and Reynolds numbers, the appropriate energy and momentum equations are solved in the quasisteady situation using the bifocal-coordinate transformations. Explicit expressions for the thermophoretic velocity and force are obtained for various cases of prolate and oblate spheroidal particles. The average thermophoretic velocity and force for an ensemble of identical, noninteracting spheroids with random orientation distribution are also determined. The results indicate that the shape and relative thermal conductivity of a spheroidal particle and its orientation with the thermal gradient can have significant effects on its the...

Cast NiTi Shape-Memory Alloys

ABSTRACTThe purpose of this study is to investigate the structure and properties of polycrystalline NiTi in its cast form. Although it is commonly stated in the literature that cast NiTi has poor shape-memory behavior, this study demonstrates that with appropriate nano/micro structural design, cast NiTi possesses excellent shape-memory properties. Cast NiTi shape-memory alloys may give rise to a new palette of low-cost, complex-geometry components. Results from two different nominal compositions of cast NiTi are presented: 50.1 at.%Ni and 50.9 at.%Ni. The cast NiTi showed a spatial variance in grain size and a random grain orientation distribution throughout the cast material. However, small variances in the thermo-mechanical response of the cast material resulted. Transformation temperatures were slightly influenced by the radial location from which the material was extracted from the casting, showing a change in Differential Scanning Calorimetry peak diffuseness as well as a change in transformation sequence for the 50.9 at.%Ni material. Mildly aged 50.9 at.%Ni material was capable of full shape-memory strain recovery after being strained to 5% under compression, while the 50.1 at.%Ni demonstrated residual plastic strains of around 1.5%. The isotropic and symmetric response under tensile and compressive loading is a result of the measured random grain orientation distribution. The favorable recovery properties in the cast material are primarily attributed to the presence of nanometer scale precipitates, which inhibit dislocation motion and favor the martensitic transformation.

Effect of the local anisotropy on the structural and dynamical properties in liquid chloroform: a molecular dynamics analysis

We report results of an investigation of the effect of the local anisotropy on the structural and dynamical properties of liquid chloroform at room temperature and ambient pressure using molecular dynamics simulations. First, we evaluated the radial distribution functions, finding good agreement between our simulated and the experimental radial distribution functions from previously reported neutron scattering results. Second, based on the analysis of the orientational distribution between neighboring molecules, we find that each chloroform molecule is within an environment of neighbors that constitute a combination of (i) anti-parallel states, (ii) nearly perpendicularly oriented configurations, and (iii) a random orientational distribution. Third, through the analysis of the force autocorrelation function, we find that the Lennard-Jones interactions are the largest contribution to the intermolecular interactions. As a consequence, the values of the diffusion and friction constants are mainly controlled by these interactions. An analysis of the total force autocorrelation function along the principal axes of the chloroform molecule shows that along the CH direction (i) this function has a particular behavior and (ii) the value of the mean square force is the largest. This is expressed in a local anisotropy which should affect the dynamical (translation and rotation) properties.

Collisional orientation of the benzene molecular plane in supersonic seeded expansions, probed by infrared polarized laser absorption spectroscopy and by molecular beam scattering

The large number of elastic and inelastic collisions which take place during supersonic gaseous expansions produce not only acceleration and internal cooling of molecules, but also their alignment or orientation. The collisional alignment of the rotational angular momentum, corresponding to the orientation of the benzene molecular plane, in supersonic seeded expansions with lighter carrier gases is demonstrated via two complementary experiments: one interrogating benzene via polarized laser light IR absorption the other one probing its orientation via molecular beam scattering on rare gas targets. Typical seeding gases are helium, neon and their mixtures, and molecular hydrogen. Total stagnation pressures are of the order of ∼1 bar and ∼0.1 mm nozzle. A propensity is demonstrated for benzene molecules in seeded molecular beams to fly with the molecular plane preferentially oriented parallel to the molecular beam propagation direction. The analysis of the experimental results has been carried out using a phenomenological model which provides the fraction of molecules traveling in such a “frisbee” mode. A frisbee propensity function is defined and found to range between 0.71 and 0.85, corresponding to IR and to scattering experiments, respectively. These values are significantly higher than the value 2/3, expected for a random distribution of molecular plane orientations. The trend in the measured values is discussed in terms of different angular cones of the supersonic expansion sampled in the two experiments and evidence is provided that the orientation is higher the narrower is the sampled cone aperture.