9,203 research outputs found
Local properties of patterned vegetation: quantifying endogenous and exogenous effects
Dryland ecosystems commonly exhibit periodic bands of vegetation, thought to
form due to competition between individual plants for heterogeneously
distributed water. In this paper, we develop a Fourier method for locally
identifying the pattern wavenumber and orientation, and apply it to aerial
images from a region of vegetation patterning near Fort Stockton, Texas. We
find that the local pattern wavelength and orientation are typically coherent,
but exhibit both rapid and gradual variation driven by changes in hillslope
gradient and orientation, the potential for water accumulation, or soil type.
Endogenous pattern dynamics, when simulated for spatially homogeneous
topographic and vegetation conditions, predict pattern properties that are much
less variable than the orientation and wavelength observed in natural systems.
Our local pattern analysis, combined with ancillary datasets describing soil
and topographic variation, highlights a largely unexplored correlation between
soil depth, pattern coherence, vegetation cover and pattern wavelength. It
also, surprisingly, suggests that downslope accumulation of water may play a
role in changing vegetation pattern properties
Protocol-Dependence and State Variables in the Force-Moment Ensemble
Stress-based ensembles incorporating temperature-like variables have been
proposed as a route to an equation of state for granular materials. To test the
efficacy of this approach, we perform experiments on a two-dimensional
photoelastic granular system under three loading conditions: uniaxial
compression, biaxial compression, and simple shear. From the interparticle
forces, we find that the distributions of the normal component of the
coarse-grained force-moment tensor are exponential-tailed, while the deviatoric
component is Gaussian-distributed. This implies that the correct stress-based
statistical mechanics conserves both the force-moment tensor and the
Maxwell-Cremona force-tiling area. As such, two variables of state arise: the
tensorial angoricity () and a new temperature-like quantity
associated with the force-tile area which we name {\it keramicity} ().
Each quantity is observed to be inversely proportional to the global confining
pressure; however only exhibits the protocol-independence expected of
a state variable, while behaves as a variable of process
Evolution of Network Architecture in a Granular Material Under Compression
As a granular material is compressed, the particles and forces within the system arrange to form complex and heterogeneous collective structures. Force chains are a prime example of such structures, and are thought to constrain bulk properties such as mechanical stability and acoustic transmission. However, capturing and characterizing the evolving nature of the intrinsic inhomogeneity and mesoscale architecture of granular systems can be challenging. A growing body of work has shown that graph theoretic approaches may provide a useful foundation for tackling these problems. Here, we extend the current approaches by utilizing multilayer networks as a framework for directly quantifying the progression of mesoscale architecture in a compressed granular system. We examine a quasi-two-dimensional aggregate of photoelastic disks, subject to biaxial compressions through a series of small, quasistatic steps. Treating particles as network nodes and interparticle forces as network edges, we construct a multilayer network for the system by linking together the series of static force networks that exist at each strain step. We then extract the inherent mesoscale structure from the system by using a generalization of community detection methods to multilayer networks, and we define quantitative measures to characterize the changes in this structure throughout the compression process. We separately consider the network of normal and tangential forces, and find that they display a different progression throughout compression. To test the sensitivity of the network model to particle properties, we examine whether the method can distinguish a subsystem of low-friction particles within a bath of higher-friction particles. We find that this can be achieved by considering the network of tangential forces, and that the community structure is better able to separate the subsystem than a purely local measure of interparticle forces alone. The results discussed throughout this study suggest that these network science techniques may provide a direct way to compare and classify data from systems under different external conditions or with different physical makeup
Gamma-Rays Produced in Cosmic-Ray Interactions and the TeV-band Spectrum of RX J1713.7-3946
We employ the Monte Carlo particle collision code DPMJET3.04 to determine the
multiplicity spectra of various secondary particles (in addition to 's)
with 's as the final decay state, that are produced in cosmic-ray
('s and 's) interactions with the interstellar medium. We derive an
easy-to-use -ray production matrix for cosmic rays with energies up to
about 10 PeV. This -ray production matrix is applied to the GeV excess
in diffuse Galactic -rays observed by EGRET, and we conclude the
non- decay components are insufficient to explain the GeV excess,
although they have contributed a different spectrum from the -decay
component. We also test the hypothesis that the TeV-band -ray emission
of the shell-type SNR RX J1713.7-3946 observed with HESS is caused by hadronic
cosmic rays which are accelerated by a cosmic-ray modified shock. By the
statistics, we find a continuously softening spectrum is strongly
preferred, in contrast to expectations. A hardening spectrum has about 1%
probability to explain the HESS data, but then only if a hard cutoff at 50-100
TeV is imposed on the particle spectrum.Comment: 3 pages; 4 figures; Contribution to the First GLAST Symposium,
Standord, 200
Nucleation at the DNA supercoiling transition
Twisting DNA under a constant applied force reveals a thermally activated
transition into a state with a supercoiled structure known as a plectoneme.
Using transition state theory, we predict the rate of this plectoneme
nucleation to be of order 10^4 Hz. We reconcile this with experiments that have
measured hopping rates of order 10 Hz by noting that the viscosity of the bead
used to manipulate the DNA limits the measured rate. We find that the intrinsic
bending caused by disorder in the base-pair sequence is important for
understanding the free energy barrier that governs the transition. Both
analytic and numerical methods are used in the calculations. We provide
extensive details on the numerical methods for simulating the elastic rod model
with and without disorder.Comment: 18 pages, 15 figure
Islands of conformational stability for Filopodia
Filopodia are long, thin protrusions formed when bundles of fibers grow outwardly from a cell surface while remaining closed in a membrane tube. We study the subtle issue of the mechanical stability of such filopodia and how this depends on the deformation of the membrane that arises when the fiber bundle adopts a helical configuration. We calculate the ground state conformation of such filopodia, taking into account the steric interaction between the membrane and the enclosed semiflexible fiber bundle. For typical filopodia we find that a minimum number of fibers is required for filopodium stability. Our calculation elucidates how experimentally observed filopodia can obviate the classical Euler buckling condition and remain stable up to several tens of . We briefly discuss how experimental observation of the results obtained in this work for the helical-like deformations of enclosing membrane tubes in filopodia could possibly be observed in the acrosomal reactions of the sea cucumber Thyone, and the horseshoe crab Limulus. Any realistic future theories for filopodium stability are likely to rely on an accurate treatment of such steric effects, as analysed in this work
Photoelastic force measurements in granular materials
Photoelastic techniques are used to make both qualitative and quantitative
measurements of the forces within idealized granular materials. The method is
based on placing a birefringent granular material between a pair of polarizing
filters, so that each region of the material rotates the polarization of light
according to the amount of local of stress. In this review paper, we summarize
past work using the technique, describe the optics underlying the technique,
and illustrate how it can be used to quantitatively determine the vector
contact forces between particles in a 2D granular system. We provide a
description of software resources available to perform this task, as well as
key techniques and resources for building an experimental apparatus
System and market failures: the unavailability of magnesium sulphate for the treatment of eclampsia and pre-eclampsia in Mozambique and Zimbabwe.
Low cost and effective drugs, such as magnesium sulphate, need to be included in initiatives to improve access to essential medicines in Afric
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