146,830 research outputs found

    Increasing the Longevity of Tungsten Filaments in a Zone Refiner

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    Zone refining is used for its ability to manipulate impurities in crystals and growing single crystals. To produce these single crystals, a floating molten zone, generated from electron bombardment, passed over the polycrystalline. While zone refining like this, the filaments that produce the electron bombardment can blow out during a run. The longevity of tungsten filaments in a zone refiner was investigated. A new lower insert was constructed to prevent the filaments from blowing out. The new lower insert had a shield machined into it to protect the filaments from the impurities striking them. It was found that the new lower insert did not significantly increase the lifespan of the filaments. The longevity of the tungsten filaments was longer in a zone refiner that had a larger area of where the electron bombardment took place. It is thought that this increased room allows for more locations for the impurities to strike

    On the Properties of a Bundle of Flexible Actin Filaments in an Optical Trap

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    We establish the Statistical Mechanics framework for a bundle of Nf living and uncrosslinked actin filaments in a supercritical solution of free monomers pressing against a mobile wall. The filaments are anchored normally to a fixed planar surface at one of their ends and, because of their limited flexibility, they grow almost parallel to each other. Their growing ends hit a moving obstacle, depicted as a second planar wall, parallel to the previous one and subjected to a harmonic compressive force. The force constant is denoted as trap strength while the distance between the two walls as trap length to make contact with the experimental optical trap apparatus. For an ideal solution of reactive filaments and free monomers at fixed free monomers chemical potential, we obtain the general expression for the grand potential from which we derive averages and distributions of relevant physical quantities, namely the obstacle position, the bundle polymerization force and the number of filaments in direct contact with the wall. The grafted living filaments are modeled as discrete Wormlike chains, with Factin persistence length, subject to discrete contour length variations to model single monomer (de)polymerization steps. Rigid filaments, either isolated or in bundles, all provide average values of the stalling force in agreement with Hill's predictions, independent of the average trap length. Flexible filaments instead, for values of the trap strength suitable to prevent their lateral escape, provide an average bundle force and an average trap length slightly larger than the corresponding rigid cases (few percents). Still the stalling force remains nearly independent on the average trap length, but results from the product of two strongly L dependent contributions: the fraction of touching filaments and the single filament buckling force.Comment: 21 pages, 8 figure

    Influence of cardiac tissue anisotropy on re-entrant activation in computational models of ventricular fibrillation

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    The aim of this study was to establish the role played by anisotropic diffusion in (i) the number of filaments and epicardial phase singularities that sustain ventricular fibrillation in the heart, (ii) the lifetimes of filaments and phase singularities, and (iii) the creation and annihilation dynamics of filaments and phase singularities. A simplified monodomain model of cardiac tissue was used, with membrane excitation described by a simplified 3-variable model. The model was configured so that a single re-entrant wave was unstable, and fragmented into multiple re-entrant waves. Re-entry was then initiated in tissue slabs with varying anisotropy ratio. The main findings of this computational study are: (i) anisotropy ratio influenced the number of filaments Sustaining simulated ventricular fibrillation, with more filaments present in simulations with smaller values of transverse diffusion coefficient, (ii) each re-entrant filament was associated with around 0.9 phase singularities on the surface of the slab geometry, (iii) phase singularities were longer lived than filaments, and (iv) the creation and annihilation of filaments and phase singularities were linear functions of the number of filaments and phase singularities, and these relationships were independent of the anisotropy ratio. This study underscores the important role played by tissue anisotropy in cardiac ventricular fibrillation

    Growth and Properties of Carbon Microcoils and Nanocoils

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    Various types of coiled carbon filaments have been synthesized using chemical vapor deposition and other methods. These carbon filaments exhibit unique electrical and mechanical properties due to their versatile shapes and structures. To form coiled shapes, different types of catalyst compositions and reactive gases have been explored. Generally, coiled carbon filaments are classified by coil diameter and shape (e.g., microcoil and nanocoil). In this review, coiled carbon filaments are classified into three growth mechanism categories: (1) bidirectional double helical growth; (2) bidirectional twisted growth; and (3) tip single helical or twisted growth. Next, their synthesis methods and hypothetical growth mechanisms are discussed. Then, their electrical and mechanical properties are listed. Finally, potential applications and uses of coiled carbon filament are mentioned

    CARMA Large Area Star Formation Survey: Observational Analysis of Filaments in the Serpens South Molecular Cloud

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    We present the N2H+(J=1-0) map of the Serpens South molecular cloud obtained as part of the CARMA Large Area Star Formation Survey (CLASSy). The observations cover 250 square arcminutes and fully sample structures from 3000 AU to 3 pc with a velocity resolution of 0.16 km/s, and they can be used to constrain the origin and evolution of molecular cloud filaments. The spatial distribution of the N2H+ emission is characterized by long filaments that resemble those observed in the dust continuum emission by Herschel. However, the gas filaments are typically narrower such that, in some cases, two or three quasi-parallel N2H+ filaments comprise a single observed dust continuum filament. The difference between the dust and gas filament widths casts doubt on Herschel ability to resolve the Serpens South filaments. Some molecular filaments show velocity gradients along their major axis, and two are characterized by a steep velocity gradient in the direction perpendicular to the filament axis. The observed velocity gradient along one of these filaments was previously postulated as evidence for mass infall toward the central cluster, but these kind of gradients can be interpreted as projection of large-scale turbulence.Comment: 12 pages, 4 figures, published in ApJL (July 2014

    TRAO Survey of Nearby Filamentary Molecular clouds, the Universal Nursery of Stars (TRAO FUNS) I. Dynamics and Chemistry of L1478 in the California Molecular Cloud

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    "TRAO FUNS" is a project to survey Gould Belt's clouds in molecular lines. This paper presents its first results on the central region of the California molecular cloud, L1478. We performed On-The-Fly mapping observations using the Taedeok Radio Astronomy Observatory (TRAO) 14m single dish telescope equipped with a 16 multi-beam array covering ∼\sim1.0 square degree area of this region using C18^{18}O (1-0) mainly tracing low density cloud and about 460 square arcminute area using N2_{2}H+^{+} (1-0) mainly tracing dense cores. CS (2-1) and SO (32−21)(3_{2}-2_{1}) were also used simultaneously to map ∼\sim440 square arcminute area of this region. We identified 10 filaments by applying the dendrogram technique to the C18^{18}O data-cube and 8 dense N2_{2}H+^{+} cores by using {\sc FellWalker}. Basic physical properties of filaments such as mass, length, width, velocity field, and velocity dispersion are derived. It is found that L1478 consists of several filaments with slightly different velocities. Especially the filaments which are supercritical are found to contain dense cores detected in N2_{2}H+^{+}. Comparison of non-thermal velocity dispersions derived from C18^{18}O and N2_{2}H+^{+} for the filaments and dense cores indicates that some of dense cores share similar kinematics with those of the surrounding filaments while several dense cores have different kinematics with those of their filaments. This suggests that the formation mechanism of dense cores and filaments can be different in individual filaments depending on their morphologies and environments.Comment: 25 pages, 15 figures, accepted for publication in Ap

    An Extended Filament Based Lamellipodium Model Produces Various Moving Cell Shapes in the Presence of Chemotactic Signals

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    The Filament Based Lamellipodium Model (FBLM) is a two-phase two-dimensional continuum model, describing the dynamcis of two interacting families of locally parallel actin filaments (C.Schmeiser and D.Oelz, How do cells move? Mathematical modeling of cytoskeleton dynamics and cell migration. Cell mechanics: from single scale-based models to multiscale modeling. Chapman and Hall, 2010). It contains accounts of the filaments' bending stiffness, of adhesion to the substrate, and of cross-links connecting the two families. An extension of the model is presented with contributions from nucleation of filaments by branching, from capping, from contraction by actin-myosin interaction, and from a pressure-like repulsion between parallel filaments due to Coulomb interaction. The effect of a chemoattractant is described by a simple signal transduction model influencing the polymerization speed. Simulations with the extended model show its potential for describing various moving cell shapes, depending on the signal transduction procedure, and for predicting transients between nonmoving and moving states as well as changes of direction
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