284 research outputs found
Theory of continuum percolation I. General formalism
The theoretical basis of continuum percolation has changed greatly since its
beginning as little more than an analogy with lattice systems. Nevertheless,
there is yet no comprehensive theory of this field. A basis for such a theory
is provided here with the introduction of the Potts fluid, a system of
interacting -state spins which are free to move in the continuum. In the limit, the Potts magnetization, susceptibility and correlation functions
are directly related to the percolation probability, the mean cluster size and
the pair-connectedness, respectively. Through the Hamiltonian formulation of
the Potts fluid, the standard methods of statistical mechanics can therefore be
used in the continuum percolation problem.Comment: 26 pages, Late
Theory of continuum percolation II. Mean field theory
I use a previously introduced mapping between the continuum percolation model
and the Potts fluid to derive a mean field theory of continuum percolation
systems. This is done by introducing a new variational principle, the basis of
which has to be taken, for now, as heuristic. The critical exponents obtained
are , and , which are identical with the mean
field exponents of lattice percolation. The critical density in this
approximation is \rho_c = 1/\ve where \ve = \int d \x \, p(\x) \{ \exp [-
v(\x)/kT] - 1 \}. p(\x) is the binding probability of two particles
separated by \x and v(\x) is their interaction potential.Comment: 25 pages, Late
Recommended from our members
The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS)
The CLASS experiment on Chandrayaan-2, the second Indian lunar mission, aims tomap the abundance of the major rock forming elements on the lunar surface using the technique of X-ray fluorescence during solar flare events. CLASS is a continuation of the successful C1XS [1] XRF experiment on Chandrayaan-1. CLASS is designed to provide lunar mapping of elemental abundances with a nominal spatial resolution of 25 km (FWHM) from a 200 km polar, circular orbit of Chandrayaan-2
Microscopic Model of Charge Carrier Transfer in Complex Media
We present a microscopic model of a charge carrier transfer under an action
of a constant electric field in a complex medium. Generalizing previous
theoretical approaches, we model the dynamical environment hindering the
carrier motion by dynamic percolation, i.e., as a medium comprising particles
which move randomly on a simple cubic lattice, constrained by hard-core
exclusion, and may spontaneously annihilate and re-appear at some prescribed
rates. We determine analytically the density profiles of the "environment"
particles, as seen from the stationary moving charge carrier, and calculate its
terminal velocity as the function of the applied field and other system
parameters. We realize that for sufficiently small external fields the force
exerted on the carrier by the "environment" particles shows a viscous-like
behavior and define an analog of the Stokes formula for such dynamic
percolative environments. The corresponding friction coefficient is also
derived.Comment: appearing in Chem. Phys. Special Issue on Molecular Charge Transfer
in Condensed Media - from Physics and Chemistry to Biology and
Nano-Engineering, edited by A.Kornyshev (Imperial College London), M.Newton
(Brookhaven Natl Lab) and J.Ulstrup (Technical University of Denmark
Treatment of poor-risk myelodysplastic syndromes and acute myeloid leukemia with a combination of 5-azacytidine and valproic acid
5-azacytidine (AZA) has become standard treatment for patients with higher-risk myelodysplastic syndrome (MDS). Response rate is about 50% and response duration is limited. Histone deactylase (HDAC) inhibitors are attractive partners for epigenetic combination therapy. We treated 24 patients with AZA (100 mg/m2, 5 days) plus valproate (VPA; continuous dosing, trough serum level 80–110 μg/ml). According to WHO classification, 5 patients had MDS, 2 had MDS/MPD, and 17 had acute myeloid leukemia (AML). Seven patients (29%) had previously received intensive chemotherapy, and five had previous HDAC inhibitor treatment. The overall response rate was 37% in the entire cohort but significantly higher (57%) in previously untreated patients, especially those with MDS (64%). Seven (29%) patients achieved CR (29%) and two PR (8%), respectively. Hematological CR was accompanied by complete cytogenetic remission according to conventional cytogenetics in all evaluable cases. Some patients also showed complete remission according to FISH on bone marrow mononuclear cells and CD34+ peripheral blood cells, as well as by follow-up of somatic mitochondrial DNA mutations. Four additional patients achieved at least marrow remissions. Factors influencing response were AML (vs. MDS), marrow blast count, pretreatment, transfusion dependency, concomitant medication with hydroxyurea, and valproic acid (VPA) serum level. This trial is the first to assess the combination of AZA plus VPA without additional ATRA. A comparatively good CR rate, relatively short time to response, and the influence of VPA serum levels on response suggest that VPA provided substantial additional benefit. However, the importance of HDAC inhibitors in epigenetic combination therapy can only be proven by randomized trials
Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions
: Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS2 , even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear-induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction- and wear-reducing impact
Theory of continuum percolation III. Low density expansion
We use a previously introduced mapping between the continuum percolation
model and the Potts fluid (a system of interacting s-states spins which are
free to move in the continuum) to derive the low density expansion of the pair
connectedness and the mean cluster size. We prove that given an adequate
identification of functions, the result is equivalent to the density expansion
derived from a completely different point of view by Coniglio et al. [J. Phys A
10, 1123 (1977)] to describe physical clustering in a gas. We then apply our
expansion to a system of hypercubes with a hard core interaction. The
calculated critical density is within approximately 5% of the results of
simulations, and is thus much more precise than previous theoretical results
which were based on integral equations. We suggest that this is because
integral equations smooth out overly the partition function (i.e., they
describe predominantly its analytical part), while our method targets instead
the part which describes the phase transition (i.e., the singular part).Comment: 42 pages, Revtex, includes 5 EncapsulatedPostscript figures,
submitted to Phys Rev
Generalized model for dynamic percolation
We study the dynamics of a carrier, which performs a biased motion under the
influence of an external field E, in an environment which is modeled by dynamic
percolation and created by hard-core particles. The particles move randomly on
a simple cubic lattice, constrained by hard-core exclusion, and they
spontaneously annihilate and re-appear at some prescribed rates. Using
decoupling of the third-order correlation functions into the product of the
pairwise carrier-particle correlations we determine the density profiles of the
"environment" particles, as seen from the stationary moving carrier, and
calculate its terminal velocity, V_c, as the function of the applied field and
other system parameters. We find that for sufficiently small driving forces the
force exerted on the carrier by the "environment" particles shows a
viscous-like behavior. An analog Stokes formula for such dynamic percolative
environments and the corresponding friction coefficient are derived. We show
that the density profile of the environment particles is strongly
inhomogeneous: In front of the stationary moving carrier the density is higher
than the average density, , and approaches the average value as an
exponential function of the distance from the carrier. Past the carrier the
local density is lower than and the relaxation towards may
proceed differently depending on whether the particles number is or is not
explicitly conserved.Comment: Latex, 32 pages, 4 ps-figures, submitted to PR
Transitions in the Horizontal Transport of Vertically Vibrated Granular Layers
Motivated by recent advances in the investigation of fluctuation-driven
ratchets and flows in excited granular media, we have carried out experimental
and simulational studies to explore the horizontal transport of granular
particles in a vertically vibrated system whose base has a sawtooth-shaped
profile. The resulting material flow exhibits novel collective behavior, both
as a function of the number of layers of particles and the driving frequency;
in particular, under certain conditions, increasing the layer thickness leads
to a reversal of the current, while the onset of transport as a function of
frequency occurs gradually in a manner reminiscent of a phase transition. Our
experimental findings are interpreted here with the help of extensive, event
driven Molecular Dynamics simulations. In addition to reproducing the
experimental results, the simulations revealed that the current may be reversed
as a function of the driving frequency as well. We also give details about the
simulations so that similar numerical studies can be carried out in a more
straightforward manner in the future.Comment: 12 pages, 18 figure
- …