75 research outputs found
Model for Glass Transition in a Binary fluid from a Mode Coupling approach
We consider the Mode Coupling Theory (MCT) of Glass transition for a Binary
fluid. The Equations of Nonlinear Fluctuating Hydrodynamics are obtained with a
proper choice of the slow variables corresponding to the conservation laws. The
resulting model equations are solved in the long time limit to locate the
dynamic transition. The transition point from our model is considerably higher
than predicted in existing MCT models for binary systems. This is in agreement
with what is seen in Computer Simulation of binary fluids. fluids.Comment: 9 Pages, 3 Figure
Clusters in weighted macroeconomic networks : the EU case. Introducing the overlapping index of GDP/capita fluctuation correlations
GDP/capita correlations are investigated in various time windows (TW), for
the time interval 1990-2005. The target group of countries is the set of 25 EU
members, 15 till 2004 plus the 10 countries which joined EU later on. The
TW-means of the statistical correlation coefficients are taken as the weights
(links) of a fully connected network having the countries as nodes. Thereafter
we define and introduce the overlapping index of weighted network nodes. A
cluster structure of EU countries is derived from the statistically relevant
eigenvalues and eigenvectors of the adjacency matrix. This may be considered to
yield some information about the structure, stability and evolution of the EU
country clusters in a macroeconomic sense.Comment: 6 pages, 8 figures, 1 table, 17 references, submitted to Physica A;
proceedings of APFA
Fluctuation-Dissipation relations in Driven Granular Gases
We study the dynamics of a 2d driven inelastic gas, by means of Direct
Simulation Monte Carlo (DSMC) techniques, i.e. under the assumption of
Molecular Chaos. Under the effect of a uniform stochastic driving in the form
of a white noise plus a friction term, the gas is kept in a non-equilibrium
Steady State characterized by fractal density correlations and non-Gaussian
distributions of velocities; the mean squared velocity, that is the so-called
{\em granular temperature}, is lower than the bath temperature. We observe that
a modified form of the Kubo relation, which relates the autocorrelation and the
linear response for the dynamics of a system {\em at equilibrium}, still holds
for the off-equilibrium, though stationary, dynamics of the systems under
investigation. Interestingly, the only needed modification to the equilibrium
Kubo relation is the replacement of the equilibrium temperature with an
effective temperature, which results equal to the global granular temperature.
We present two independent numerical experiment, i.e. two different observables
are studied: (a) the staggered density current, whose response to an impulsive
shear is proportional to its autocorrelation in the unperturbed system and (b)
the response of a tracer to a small constant force, switched on at time ,
which is proportional to the mean-square displacement in the unperturbed
system. Both measures confirm the validity of Kubo's formula, provided that the
granular temperature is used as the proportionality factor between response and
autocorrelation, at least for not too large inelasticities.Comment: 11 pages, 7 figures, submitted for publicatio
Density functional theory of phase coexistence in weakly polydisperse fluids
The recently proposed universal relations between the moments of the
polydispersity distributions of a phase-separated weakly polydisperse system
are analyzed in detail using the numerical results obtained by solving a simple
density functional theory of a polydisperse fluid. It is shown that universal
properties are the exception rather than the rule.Comment: 10 pages, 2 figures, to appear in PR
Crossover between Equilibrium and Shear-controlled Dynamics in Sheared Liquids
We present a numerical simulation study of a simple monatomic Lennard-Jones
liquid under shear flow, as a function of both temperature and shear rate. By
investigating different observables we find that i) It exists a line in the
(temperature-shear) plane that sharply marks the boarder between an
``equilibrium'' and a ``shear-controlled'' region for both the dynamic and the
thermodynamic quantities; and ii) Along this line the structural relaxation
time, is proportional to the inverse shear rate, i.e. to the typical time-scale
introduced by the shear flow. Above the line the liquid dynamics is unaffected
by the shear flow, while below it both temperature and shear rate control the
particle motion.Comment: 14 pages, 5 figure
Attractive Interactions Between Rod-like Polyelectrolytes: Polarization, Crystallization, and Packing
We study the attractive interactions between rod-like charged polymers in
solution that appear in the presence of multi-valence counterions. The
counterions condensed to the rods exhibit both a strong transversal
polarization and a longitudinal crystalline arrangement. At short distances
between the rods, the fraction of condensed counterions increases, and the
majority of these occupy the region between the rods, where they minimize their
repulsive interactions by arranging themselves into packing structures. The
attractive interaction is strongest for multivalent counterions. Our model
takes into account the hard-core volume of the condensed counterions and their
angular distribution around the rods. The hard core constraint strongly
suppresses longitudinal charge fluctuations.Comment: 4 figures, uses revtex, psfig and epsf. The new version contains a
different introduction, and the bibliography has been expande
Charge Fluctuations on Membrane Surfaces in Water
We generalize the predictions for attractions between over-all neutral
surfaces induced by charge fluctuations/correlations to non-uniform systems
that include dielectric discontinuities, as is the case for mixed charged lipid
membranes in an aqueous solution. We show that the induced interactions depend
in a non-trivial way on the dielectric constants of membrane and water and show
different scaling with distance depending on these properties. The generality
of the calculations also allows us to predict under which dielectric conditions
the interaction will change sign and become repulsive
Offsprings of a point vortex
The distribution engendered by successive splitting of one point vortex are
considered. The process of splitting a vortex in three using a reverse
three-point vortex collapse course is analysed in great details and shown to be
dissipative. A simple process of successive splitting is then defined and the
resulting vorticity distribution and vortex populations are analysed
Shear yielding of amorphous glassy solids: Effect of temperature and strain rate
We study shear yielding and steady state flow of glassy materials with
molecular dynamics simulations of two standard models: amorphous polymers and
bidisperse Lennard-Jones glasses. For a fixed strain rate, the maximum shear
yield stress and the steady state flow stress in simple shear both drop
linearly with increasing temperature. The dependence on strain rate can be
described by a either a logarithm or a power-law added to a constant. In marked
contrast to predictions of traditional thermal activation models, the rate
dependence is nearly independent of temperature. The relation to more recent
models of plastic deformation and glassy rheology is discussed, and the
dynamics of particles and stress in small regions is examined in light of these
findings
Scaling properties in off equilibrium dynamical processes
In the present paper, we analyze the consequences of scaling hypotheses on
dynamic functions, as two times correlations . We show, under general
conditions, that must obey the following scaling behavior , where the scaling variable is
and , two
undetermined functions. The presence of a non constant exponent
signals the appearance of multiscaling properties in the dynamics.Comment: 6 pages, no figure
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