154 research outputs found
Dynamics of a massive intruder in a homogeneously driven granular fluid
A massive intruder in a homogeneously driven granular fluid, in dilute
configurations, performs a memory-less Brownian motion with drag and
temperature simply related to the average density and temperature of the fluid.
At volume fraction the intruder's velocity correlates with the
local fluid velocity field: such situation is approximately described by a
system of coupled linear Langevin equations equivalent to a generalized
Brownian motion with memory. Here one may verify the breakdown of the
Fluctuation-Dissipation relation and the presence of a net entropy flux - from
the fluid to the intruder - whose fluctuations satisfy the Fluctuation
Relation.Comment: 6 pages, 2 figures, to be published on "Granular Matter" in a special
issue in honor of the memory of Prof. Isaac Goldhirsc
Gamma-burst emission from neutron-star accretion
A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without a magnetic field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required to drag electrons into the star with the nuclei against the flux of photons leaking out through the accreting matter. The photon number is not increased substantially by Bremsstrahlung or any other process. It is suggested that instability in an accretion disc might provide the infalling matter required
Diffusion and subdiffusion of interacting particles on comb-like structures
We study the dynamics of a tracer particle (TP) on a comb lattice populated
by randomly moving hard-core particles in the dense limit. We first consider
the case where the TP is constrained to move on the backbone of the comb only,
and, in the limit of high density of particles, we present exact analytical
results for the cumulants of the TP position, showing a subdiffusive behavior
. At longer times, a second regime is observed, where standard
diffusion is recovered, with a surprising non analytical dependence of the
diffusion coefficient on the particle density. When the TP is allowed to visit
the teeth of the comb, based on a mean-field-like Continuous Time Random Walk
description, we unveil a rich and complex scenario, with several successive
subdiffusive regimes, resulting from the coupling between the inhomogeneous
comb geometry and particle interactions. Remarkably, the presence of hard-core
interactions speeds up the TP motion along the backbone of the structure in all
regimes.Comment: 5 pages, 3 figures + supplemental materia
Fluctuations in partitioning systems with few degrees of freedom
We study the behavior of a moving wall in contact with a particle gas and
subjected to an external force. We compare the fluctuations of the system
observed in the microcanonical and canonical ensembles, at varying the number
of particles. Static and dynamic correlations signal significant differences
between the two ensembles. Furthermore, velocity-velocity correlations of the
moving wall present a complex two-time relaxation which cannot be reproduced by
a standard Langevin-like description. Quite remarkably, increasing the number
of gas particles in an elongated geometry, we find a typical timescale, related
to the interaction between the partitioning wall and the particles, which grows
macroscopically.Comment: 10 pages, 12 figure
Scaling properties of field-induced superdiffusion in Continous Time Random Walks
We consider a broad class of Continuous Time Random Walks with large
fluctuations effects in space and time distributions: a random walk with
trapping, describing subdiffusion in disordered and glassy materials, and a
L\'evy walk process, often used to model superdiffusive effects in
inhomogeneous materials. We derive the scaling form of the probability
distributions and the asymptotic properties of all its moments in the presence
of a field by two powerful techniques, based on matching conditions and on the
estimate of the contribution of rare events to power-law tails in a field.Comment: 17 pages, 8 figures, Proceedings of the Conference "Small system
nonequilibrium fluctuations, dynamics and stochastics, and anomalous
behavior", KITPC, Beijing, Chin
Rare events and scaling properties in field-induced anomalous dynamics
We show that, in a broad class of continuous time random walks (CTRW), a
small external field can turn diffusion from standard into anomalous. We
illustrate our findings in a CTRW with trapping, a prototype of subdiffusion in
disordered and glassy materials, and in the L\'evy walk process, which
describes superdiffusion within inhomogeneous media. For both models, in the
presence of an external field, rare events induce a singular behavior in the
originally Gaussian displacements distribution, giving rise to power-law tails.
Remarkably, in the subdiffusive CTRW, the combined effect of highly fluctuating
waiting times and of a drift yields a non-Gaussian distribution characterized
by long spatial tails and strong anomalous superdiffusion.Comment: 11 pages, 3 figure
Brownian ratchet in a thermal bath driven by Coulomb friction
The rectification of unbiased fluctuations, also known as the ratchet effect,
is normally obtained under statistical non-equilibrium conditions. Here we
propose a new ratchet mechanism where a thermal bath solicits the random
rotation of an asymmetric wheel, which is also subject to Coulomb friction due
to solid-on-solid contacts. Numerical simulations and analytical calculations
demonstrate a net drift induced by friction. If the thermal bath is replaced by
a granular gas, the well known granular ratchet effect also intervenes,
becoming dominant at high collision rates. For our chosen wheel shape the
granular effect acts in the opposite direction with respect to the
friction-induced torque, resulting in the inversion of the ratchet direction as
the collision rate increases. We have realized a new granular ratchet
experiment where both these ratchet effects are observed, as well as the
predicted inversion at their crossover. Our discovery paves the way to the
realization of micro and sub-micrometer Brownian motors in an equilibrium
fluid, based purely upon nano-friction.Comment: main paper: 4 pages and 4 figures; supplemental material joined at
the end of the paper; a movie of the experiment can be viewed
http://www.youtube.com/watch?v=aHrdY4BC71k ; all the material has been
submitted for publication [new version with substantial changes in the order
of the presentation of the results; differences with previous works have been
put in evidence
Non-equilibrium fluctuations in a driven stochastic Lorentz gas
We study the stationary state of a one-dimensional kinetic model where a
probe particle is driven by an external field E and collides, elastically or
inelastically, with a bath of particles at temperature T. We focus on the
stationary distribution of the velocity of the particle, and of two estimates
of the total entropy production \Delta s_tot. One is the entropy production of
the medium \Delta s_m, which is equal to the energy exchanged with the
scatterers, divided by a parameter \theta, coinciding with the particle
temperature at E=0. The other is the work W done by the external field, again
rescaled by \theta. At small E, a good collapse of the two distributions is
found: in this case the two quantities also verify the Fluctuation Relation
(FR), indicating that both are good approximations of \Delta s_tot.
Differently, for large values of E, the fluctuations of W violate the FR, while
\Delta s_m still verifies it.Comment: 6 pages, 4 figure
Fluctuating hydrodynamics and correlation lengths in a driven granular fluid
Static and dynamical structure factors for shear and longitudinal modes of
the velocity and density fields are computed for a granular system fluidized by
a stochastic bath with friction. Analytical expressions are obtained through
fluctuating hydrodynamics and are successfully compared with numerical
simulations up to a volume fraction . Hydrodynamic noise is the sum
of external noise due to the bath and internal one due to collisions. Only the
latter is assumed to satisfy the fluctuation-dissipation relation with the
average granular temperature. Static velocity structure factors
and display a general non-constant behavior with two plateaux
at large and small , representing the granular temperature and the
bath temperature respectively. From this behavior, two different
velocity correlation lengths are measured, both increasing as the packing
fraction is raised. This growth of spatial order is in agreement with the
behaviour of dynamical structure factors, the decay of which becomes slower and
slower at increasing density.Comment: 24 pages, 8 figure
On anomalous diffusion and the out of equilibrium response function in one-dimensional models
We study how the Einstein relation between spontaneous fluctuations and the
response to an external perturbation holds in the absence of currents, for the
comb model and the elastic single-file, which are examples of systems with
subdiffusive transport properties. The relevance of non-equilibrium conditions
is investigated: when a stationary current (in the form of a drift or an energy
flux) is present, the Einstein relation breaks down, as is known to happen in
systems with standard diffusion. In the case of the comb model, a general
relation, which has appeared in the recent literature, between the response
function and an unperturbed suitable correlation function, allows us to explain
the observed results. This suggests that a relevant ingredient in breaking the
Einstein formula, for stationary regimes, is not the anomalous diffusion but
the presence of currents driving the system out of equilibrium.Comment: 10 pages, 4 figure
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