3,011 research outputs found
Conduction in jammed systems of tetrahedra
Control of transport processes in composite microstructures is critical to
the development of high performance functional materials for a variety of
energy storage applications. The fundamental process of conduction and its
control through the manipulation of granular composite attributes (e.g., grain
shape) are the subject of this work. We show that athermally jammed packings of
tetrahedra with ultra-short range order exhibit fundamentally different
pathways for conduction than those in dense sphere packings. Highly resistive
granular constrictions and few face-face contacts between grains result in
short-range distortions from the mean temperature field. As a consequence,
'granular' or differential effective medium theory predicts the conductivity of
this media within 10% at the jamming point; in contrast, strong enhancement of
transport near interparticle contacts in packed-sphere composites results in
conductivity divergence at the jamming onset. The results are expected to be
particularly relevant to the development of nanomaterials, where nanoparticle
building blocks can exhibit a variety of faceted shapes.Comment: 9 pages, 10 figure
Variable-cell method for stress-controlled jamming of athermal, frictionless grains
A new method is introduced to simulate jamming of polyhedral grains under
controlled stress that incorporates global degrees of freedom through the
metric tensor of a periodic cell containing grains. Jamming under
hydrostatic/isotropic stress and athermal conditions leads to a precise
definition of the ideal jamming point at zero shear stress. The structures of
tetrahedra jammed hydrostatically exhibit less translational order and lower
jamming-point density than previously described `maximally random jammed' hard
tetrahedra. Under the same conditions, cubes jam with negligible nematic order.
Grains with octahedral symmetry jam in the large-system limit with an abundance
of face-face contacts in the absence of nematic order. For sufficiently large
face-face contact number, percolating clusters form that span the entire
simulation box. The response of hydrostatically jammed tetrahedra and cubes to
shear-stress perturbation is also demonstrated with the variable-cell method.Comment: 10 pages, 8 figure
Isostaticity of Constraints in Jammed Systems of Soft Frictionless Platonic Solids
The average number of constraints per particle in
mechanically stable systems of Platonic solids (except cubes) approaches the
isostatic limit at the jamming point (), though
average number of contacts are hypostatic. By introducing angular alignment
metrics to classify the degree of constraint imposed by each contact,
constraints are shown to arise as a direct result of local orientational order
reflected in edge-face and face-face alignment angle distributions. With
approximately one face-face contact per particle at jamming chain-like
face-face clusters with finite extent form in these systems.Comment: 4 pages, 3 figures, 4 tabl
Basic Features of a Cell Electroporation Model: Illustrative Behavior for Two Very Different Pulses
Science increasingly involves complex modeling. Here we describe a model for cell electroporation in which membrane properties are dynamically modified by poration. Spatial scales range from cell membrane thickness (5 nm) to a typical mammalian cell radius (10 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\upmu\end{document}m), and can be used with idealized and experimental pulse waveforms. The model consists of traditional passive components and additional active components representing nonequilibrium processes. Model responses include measurable quantities: transmembrane voltage, membrane electrical conductance, and solute transport rates and amounts for the representative ālongā and āshortā pulses. The long pulseā1.5 kV/cm, 100 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\upmu\end{document}sāevolves two pore subpopulations with a valley at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}5 nm, which separates the subpopulations that have peaks at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}1.5 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}12 nm radius. Such pulses are widely used in biological research, biotechnology, and medicine, including cancer therapy by drug delivery and nonthermal physical tumor ablation by causing necrosis. The short pulseā40 kV/cm, 10 nsācreates 80-fold more pores, all small (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}3 nm; \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}\end{document}1 nm peak). These nanosecond pulses ablate tumors by apoptosis. We demonstrate the modelās responses by illustrative electrical and poration behavior, and transport of calcein and propidium. We then identify extensions for expanding modeling capability. Structure-function results from MD can allow extrapolations that bring response specificity to cell membranes based on their lipid composition. After a pulse, changes in pore energy landscape can be included over seconds to minutes, by mechanisms such as cell swelling and pulse-induced chemical reactions that slowly alter pore behavior. Electronic supplementary material The online version of this article (doi:10.1007/s00232-014-9699-z) contains supplementary material, which is available to authorized users
The Development of a Human Well-Being Index for the United States
The US Environmental Protection Agency (EPA) has developed a human well-being index (HWBI) that assesses the over-all well-being of its population at the county level. The HWBI contains eight domains representing social, economic and environmental well-being. These domains include 25 indicators comprised of 80 metrics and 22 social, economic and environmental services. The application of the HWBI has been made for the nation as a whole at the county level and two alternative applications have been made to represent key populations within the overall US populationāNative Americans and children. A number of advances have been made to estimate the values of metrics for counties where no data is available and one such estimatorāMERLINāis discussed. Finally, efforts to make the index into an interactive web site are described
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