569 research outputs found
Improved modelling of liquid GeSe: the impact of the exchange-correlation functional
The structural properties of liquid GeSe are studied by using
first-principles molecular dynamics in conjuncton with the Becke, Lee, Yang and
Parr (BLYP) generalized gradient approximation for the exchange and correlation
energy. The results on partial pair correlation functions, coordination
numbers, bond angle distributions and partial structure factors are compared
with available experimental data and with previous first-principle molecular
dynamics results obtained within the Perdew and Wang (PW) generalized gradient
approximation for the exchange and correlation energy. We found that the BLYP
approach substantially improves upon the PW one in the case of the short-range
properties. In particular, the GeGe pair correlation function takes a more
structured profile that includes a marked first peak due to homopolar bonds, a
first maximum exhibiting a clear shoulder and a deep minimum, all these
features being absent in the previous PW results. Overall, the amount of
tetrahedral order is significantly increased, in spite of a larger number of
GeGe homopolar connections. Due to the smaller number of miscoordinations,
diffusion coefficients obtained by the present BLYP calculation are smaller by
at least one order of magnitude than in the PW case.Comment: 6 figure
The changing trends of childhood poisoning at a tertiary childrenâs hospital in South Africa
Context. Information on childhood poisoning in the developingworld, including South Africa, is scarce, despite its contribution tomorbidity and mortality.Objective. We describe the profile of children with exposuresand poisonings presenting to Red Cross War Memorial Children’sHospital (RCWMCH) in Cape Town, South Africa, from 2003 to2008 and compare the trends of causative agents over the past twodecades.Methods. Cases were identified by review of the RCWMCH caserecords.Results. Of the total incidents (N=2 872), paraffin (kerosene)was the commonest agent (n=692, 24%) with 124 poisoningsincluding two deaths. Drugs were the most common toxin group(n=988, 34%), including 139 single-drug poisonings with 5deaths; 4 associated with traditional medicine use. Householdcleaning product incidents (n=302, 10%) resulted in 29 singleproductpoisonings with no deaths. Pesticide incidents (n=311,10%) included 6 deaths; 203 (65%) incidents were due toorganophosphates or carbamates. The suburban distribution ofthe main toxin groups varied. Comparing 1987 and 2008, thenumber of incidents decreased from 1 116 to 447; drug and paraffinincidents decreased respectively (from 673 to 150 and from 332 to87), household cleaning products and cosmetics increased (21 to69) and pesticide incidents increased (7 to 69).Conclusion. Despite a decrease in the overall number of incidentsover two decades at RCWMCH, paraffin and drugs remainthe principal agents responsible for paediatric exposures andpoisonings, with increasing incidents due to household cleaningproducts and pesticides. Identification of these toxin groups comingfrom specific suburbs allows for targeted prevention initiatives
Deformation and flow of a two-dimensional foam under continuous shear
We investigate the flow properties of a two-dimensional aqueous foam
submitted to a quasistatic shear in a Couette geometry. A strong localization
of the flow (shear banding) at the edge of the moving wall is evidenced,
characterized by an exponential decay of the average tangential velocity.
Moreover, the analysis of the rapid velocity fluctuations reveals self-similar
dynamical structures consisting of clusters of bubbles rolling as rigid bodies.
To relate the instantaneous (elastic) and time-averaged (plastic) components of
the strain, we develop a stochastic model where irreversible rearrangements are
activated by local stress fluctuations originating from the rubbing of the
wall. This model gives a complete description of our observations and is also
consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure
A Scaling Theory of Bifurcations in the Symmetric Weak-Noise Escape Problem
We consider the overdamped limit of two-dimensional double well systems
perturbed by weak noise. In the weak noise limit the most probable
fluctuational path leading from either point attractor to the separatrix (the
most probable escape path, or MPEP) must terminate on the saddle between the
two wells. However, as the parameters of a symmetric double well system are
varied, a unique MPEP may bifurcate into two equally likely MPEP's. At the
bifurcation point in parameter space, the activation kinetics of the system
become non-Arrhenius. In this paper we quantify the non-Arrhenius behavior of a
system at the bifurcation point, by using the Maslov-WKB method to construct an
approximation to the quasistationary probability distribution of the system
that is valid in a boundary layer near the separatrix. The approximation is a
formal asymptotic solution of the Smoluchowski equation. Our analysis relies on
the development of a new scaling theory, which yields `critical exponents'
describing weak-noise behavior near the saddle, at the bifurcation point.Comment: LaTeX, 60 pages, 24 Postscript figures. Uses epsf macros to include
the figures. A file in `uufiles' format containing the figures is separately
available at ftp://platinum.math.arizona.edu/pub/papers-rsm/paperF/figures.uu
and a Postscript version of the whole paper (figures included) is available
at ftp://platinum.math.arizona.edu/pub/papers-rsm/paperF/paperF.p
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
Blast Shock Wave Mitigation Using the Hydraulic Energy Redirection and Release Technology
A hydraulic energy redirection and release technology has been developed for mitigating the effects of blast shock waves on protected objects. The technology employs a liquid-filled plastic tubing as a blast overpressure transformer to transfer kinetic energy of blast shock waves into hydraulic energy in the plastic tubings. The hydraulic energy is redirected through the plastic tubings to the openings at the lower ends, and then is quickly released with the liquid flowing out through the openings. The samples of the specifically designed body armor in which the liquid-filled plastic tubings were installed vertically as the outer layer of the body armor were tested. The blast test results demonstrated that blast overpressure behind the body armor samples was remarkably reduced by 97% in 0.2 msec after the liquid flowed out of its appropriate volume through the openings. The results also suggested that a volumetric liquid surge might be created when kinetic energy of blast shock wave was transferred into hydraulic energy to cause a rapid physical movement or displacement of the liquid. The volumetric liquid surge has a strong destructive power, and can cause a noncontact, remote injury in humans (such as blast-induced traumatic brain injury and post-traumatic stress disorder) if it is created in cardiovascular system. The hydraulic energy redirection and release technology can successfully mitigate blast shock waves from the outer surface of the body armor. It should be further explored as an innovative approach to effectively protect against blast threats to civilian and military personnel
Stress-corrosion mechanisms in silicate glasses
The present review is intended to revisit the advances and debates in the
comprehension of the mechanisms of subcritical crack propagation in silicate
glasses almost a century after its initial developments. Glass has inspired the
initial insights of Griffith into the origin of brittleness and the ensuing
development of modern fracture mechanics. Yet, through the decades the real
nature of the fundamental mechanisms of crack propagation in glass has escaped
a clear comprehension which could gather general agreement on subtle problems
such as the role of plasticity, the role of the glass composition, the
environmental condition at the crack tip and its relation to the complex
mechanisms of corrosion and leaching. The different processes are analysed here
with a special focus on their relevant space and time scales in order to
question their domain of action and their contribution in both the kinetic laws
and the energetic aspects.Comment: Invited review article - 34 pages Accepted for publication in J.
Phys. D: Appl. Phy
Large-deviation principles, stochastic effective actions, path entropies, and the structure and meaning of thermodynamic descriptions
The meaning of thermodynamic descriptions is found in large-deviations
scaling of the fluctuations probabilities. The primary large-deviations rate
function is the entropy, which is the basis for both fluctuation theorems and
for characterizing the thermodynamic interactions of systems. Freidlin-Wentzell
theory provides a general formulation of large-deviations scaling for
non-equilibrium stochastic processes, through a representation in terms of a
Hamiltonian dynamical system. A number of related methods now exist to
construct the Freidlin-Wentzell Hamiltonian for many kinds of stochastic
processes; one method due to Doi and Peliti, appropriate to integer counting
statistics, is widely used in reaction-diffusion theory.
Using these tools together with a path-entropy method due to Jaynes, we show
how to construct entropy functions that both express large-deviations scaling
of fluctuations, and describe system-environment interactions, for discrete
stochastic processes either at or away from equilibrium. A collection of
variational methods familiar within quantum field theory, but less commonly
applied to the Doi-Peliti construction, is used to define a "stochastic
effective action", which is the large-deviations rate function for arbitrary
non-equilibrium paths.
We show how common principles of entropy maximization, applied to different
ensembles of states or of histories, lead to different entropy functions and
different sets of thermodynamic state variables. Yet the relations of among all
these levels of description may be constructed explicitly and understood in
terms of information conditions. The example systems considered introduce
methods that may be used to systematically construct descriptions with all the
features familiar from equilibrium thermodynamics, for a much wider range of
systems describable by stochastic processes
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