1,698 research outputs found
First-order transition in small-world networks
The small-world transition is a first-order transition at zero density of
shortcuts, whereby the normalized shortest-path distance undergoes a
discontinuity in the thermodynamic limit. On finite systems the apparent
transition is shifted by . Equivalently a ``persistence
size'' can be defined in connection with finite-size
effects. Assuming , simple rescaling arguments imply that
. We confirm this result by extensive numerical simulation in one to
four dimensions, and argue that implies that this transition is
first-order.Comment: 4 pages, 3 figures, To appear in Europhysics Letter
Inter- and Intra-Chain Attractions in Solutions of Flexible Polyelectrolytes at Nonzero Concentration
Constant temperature molecular dynamics simulations were used to study
solutions of flexible polyelectrolyte chains at nonzero concentrations with
explicit counterions and unscreened coulombic interactions. Counterion
condensation, measured via the self-diffusion coefficient of the counterions,
is found to increase with polymer concentration, but contrary to the prediction
of Manning theory, the renormalized charge fraction on the chains decreases
with increasing Bjerrum length without showing any saturation. Scaling analysis
of the radius of gyration shows that the chains are extended at low polymer
concentrations and small Bjerrum lengths, while at sufficiently large Bjerrum
lengths, the chains shrink to produce compact structures with exponents smaller
than a gaussian chain, suggesting the presence of attractive intrachain
interactions. A careful study of the radial distribution function of the
center-of-mass of the polyelectrolyte chains shows clear evidence that
effective interchain attractive interactions also exist in solutions of
flexible polyelectrolytes, similar to what has been found for rodlike
polyelectrolytes. Our results suggest that the broad maximum observed in
scattering experiments is due to clustering of chains.Comment: 12 pages, REVTeX, 15 eps figure
Gender homophily from spatial behavior in a primary school: a sociometric study
We investigate gender homophily in the spatial proximity of children (6 to 12
years old) in a French primary school, using time-resolved data on face-to-face
proximity recorded by means of wearable sensors. For strong ties, i.e., for
pairs of children who interact more than a defined threshold, we find
statistical evidence of gender preference that increases with grade. For weak
ties, conversely, gender homophily is negatively correlated with grade for
girls, and positively correlated with grade for boys. This different evolution
with grade of weak and strong ties exposes a contrasted picture of gender
homophily
Voter models on weighted networks
We study the dynamics of the voter and Moran processes running on top of
complex network substrates where each edge has a weight depending on the degree
of the nodes it connects. For each elementary dynamical step the first node is
chosen at random and the second is selected with probability proportional to
the weight of the connecting edge. We present a heterogeneous mean-field
approach allowing to identify conservation laws and to calculate exit
probabilities along with consensus times. In the specific case when the weight
is given by the product of nodes' degree raised to a power theta, we derive a
rich phase-diagram, with the consensus time exhibiting various scaling laws
depending on theta and on the exponent of the degree distribution gamma.
Numerical simulations give very good agreement for small values of |theta|. An
additional analytical treatment (heterogeneous pair approximation) improves the
agreement with numerics, but the theoretical understanding of the behavior in
the limit of large |theta| remains an open challenge.Comment: 21 double-spaced pages, 6 figure
Compaction dynamics of a granular media under vertical tapping
We report new experimental results on granular compaction under consecutive
vertical taps. The evolution of the mean volume fraction and of the mean
potential energy of a granular packing presents a slow densification until a
final steady-state, and is reminiscent to usual relaxation in glasses via a
stretched exponential law. The intensity of the taps seems to rule the
characteristic time of the relaxation according to an Arrhenius's type relation
>. Finally, the analysis of the vertical volume fraction profile reveals an
almost homogeneous densification in the packing.Comment: 7 pages, 4 figures, to appear in Europhysics Letter
Soft effective interactions between weakly charged polyelectrolyte chains
We apply extensive Molecular Dynamics simulations and analytical
considerations in order to study the conformations and the effective
interactions between weakly charged, flexible polyelectrolyte chains in
salt-free conditions. We focus on charging fractions lying below 20%, for which
case there is no Manning condensation of counterions and the latter can be thus
partitioned in two states: those that are trapped within the region of the
flexible chain and the ones that are free in the solution. We examine the
partition of counterions in these two states, the chain sizes and the monomer
distributions for various chain lengths, finding that the monomer density
follows a Gaussian shape. We calculate the effective interaction between the
centers of mass of two interacting chains, under the assumption that the chains
can be modeled as two overlapping Gaussian charge profiles. The analytical
calculations are compared with measurements from Molecular Dynamics
simulations. Good quantitative agreement is found for charging fractions below
10%, where the chains assume coil-like configurations, whereas deviations
develop for charge fraction of 20%, in which case a conformational transition
of the chain towards a rodlike configuration starts to take place.Comment: 38 pages, 12 figures, 2 tables. Revised version of the manuscript.
Selected for publication in the V\irtual Journal of Biological Physics
Research, issue of 1 september, 200
Spatially heterogeneous dynamics and dynamic facilitation in a model of viscous silica
Performing molecular dynamics simulations, we find that the structural
relaxation dynamics of viscous silica, the prototype of a strong glass former,
are spatially heterogeneous and cannot be understood as a statistical bond
breaking process. Further, we show that high particle mobility predominantly
propagates continuously through the melt, supporting the concept of dynamic
facilitation emphasized in recent theoretical work.Comment: 4 pages, 4 figure
Dynamics of simple liquids at heterogeneous surfaces : Molecular Dynamics simulations and hydrodynamic description
In this paper we consider the effect of surface heterogeneity on the slippage
of fluid, using two complementary approaches. First, MD simulations of a
corrugated hydrophobic surface have been performed. A dewetting transition,
leading to a super-hydrophobic state, is observed for pressure below a
``capillary'' pressure. Conversely a very large slippage of the fluid on this
composite interface is found in this superhydrophobic state. Second, we propose
a macroscopic estimate of the effective slip length on the basis of continuum
hydrodynamics, in order to rationalize the previous MD results. This
calculation allows to estimate the effect of a heterogeneous slip length
pattern on the composite interface. Comparison between the two approaches are
in good agreement at low pressure, but highlights the role of the exact shape
of the liquid-vapor interface at higher pressure. These results confirm that
small variations in the roughness of a surface can lead to huge differences in
the slip effect. On the basis of these results, we propose some guidelines to
design highly slippery surfaces, motivated by potential applications in
microfluidics.Comment: submitted to EPJ
Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field
We study the local disorder in the deformation of amorphous materials by
decomposing the particle displacements into a continuous, inhomogeneous field
and the corresponding fluctuations. We compare these fields to the commonly
used non-affine displacements in an elastically deformed 2D Lennard-Jones
glass. Unlike the non-affine field, the fluctuations are very localized, and
exhibit a much smaller (and system size independent) correlation length, on the
order of a particle diameter, supporting the applicability of the notion of
local "defects" to such materials. We propose a scalar "noise" field to
characterize the fluctuations, as an additional field for extended continuum
models, e.g., to describe the localized irreversible events observed during
plastic deformation.Comment: Minor corrections to match the published versio
Universal velocity distributions in an experimental granular fluid
We present experimental results on the velocity statistics of a uniformly
heated granular fluid, in a quasi-2D configuration. We find the base state, as
measured by the single particle velocity distribution , to be universal
over a wide range of filling fractions and only weakly dependent on all other
system parameters. There is a consistent overpopulation in the distribution's
tails, which scale as . More
importantly, the high probability central region of , at low velocities,
deviates from a Maxwell-Boltzmann by a second order Sonine polynomial with a
single adjustable parameter, in agreement with recent theoretical analysis of
inelastic hard spheres driven by a stochastic thermostat. To our knowledge,
this is the first time that Sonine deviations have been measured in an
experimental system.Comment: 13 pages, 15 figures, with minor corrections, submitted to Phys. Rev.
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