3,271 research outputs found

### Granular Brownian motion

We study the stochastic motion of an intruder in a dilute driven granular
gas. All particles are coupled to a thermostat, representing the external
energy source, which is the sum of random forces and a viscous drag. The
dynamics of the intruder, in the large mass limit, is well described by a
linear Langevin equation, combining the effects of the external bath and of the
"granular bath". The drag and diffusion coefficients are calculated under few
assumptions, whose validity is well verified in numerical simulations. We also
discuss the non-equilibrium properties of the intruder dynamics, as well as the
corrections due to finite packing fraction or finite intruder mass.Comment: 19 pages, 4 figures, in press on Journal of Statistical Mechanics:
Theory and Experiment

### Temperature in and out of equilibrium: a review of concepts, tools and attempts

We review the general aspects of the concept of temperature in equilibrium
and non-equilibrium statistical mechanics. Although temperature is an old and
well-established notion, it still presents controversial facets. After a short
historical survey of the key role of temperature in thermodynamics and
statistical mechanics, we tackle a series of issues which have been recently
reconsidered. In particular, we discuss different definitions and their
relevance for energy fluctuations. The interest in such a topic has been
triggered by the recent observation of negative temperatures in condensed
matter experiments. Moreover, the ability to manipulate systems at the micro
and nano-scale urges to understand and clarify some aspects related to the
statistical properties of small systems (as the issue of temperature's
"fluctuations"). We also discuss the notion of temperature in a dynamical
context, within the theory of linear response for Hamiltonian systems at
equilibrium and stochastic models with detailed balance, and the generalised
fluctuation-response relations, which provide a hint for an extension of the
definition of temperature in far-from-equilibrium systems. To conclude we
consider non-Hamiltonian systems, such as granular materials, turbulence and
active matter, where a general theoretical framework is still lacking.Comment: Review article, 137 pages, 12 figure

### Ratchet effect driven by Coulomb friction: the asymmetric Rayleigh piston

The effect of Coulomb friction is studied in the framework of collisional
ratchets. It turns out that the average drift of these devices can be expressed
as the combination of a term related to the lack of equipartition between the
probe and the surrounding bath, and a term featuring the average frictional
force. We illustrate this general result in the asymmetric Rayleigh piston,
showing how Coulomb friction can induce a ratchet effect in a Brownian particle
in contact with an equilibrium bath. An explicit analytical expression for the
average velocity of the piston is obtained in the rare collision limit.
Numerical simulations support the analytical findings.Comment: 5 pages, 2 figure

### Entropy production for velocity-dependent macroscopic forces: the problem of dissipation without fluctuations

In macroscopic systems, velocity-dependent phenomenological forces $F(v)$ are
used to model friction, feedback devices or self-propulsion. Such forces
usually include a dissipative component which conceals the fast energy
exchanges with a thermostat at the environment temperature $T$, ruled by a
microscopic Hamiltonian $H$. The mapping $(H,T) \to F(v)$ - even if effective
for many purposes - may lead to applications of stochastic thermodynamics where
an $incomplete$ fluctuating entropy production (FEP) is derived. An
enlightening example is offered by recent macroscopic experiments where
dissipation is dominated by solid-on-solid friction, typically modelled through
a deterministic Coulomb force $F(v)$. Through an adaptation of the microscopic
Prandtl-Tomlinson model for friction, we show how the FEP is dominated by the
heat released to the $T$-thermostat, ignored by the macroscopic Coulomb model.
This problem, which haunts several studies in the literature, cannot be cured
by weighing the time-reversed trajectories with a different auxiliary dynamics:
it is only solved by a more accurate stochastic modelling of the thermostat
underlying dissipation.Comment: 6 pages, 3 figure

### Granular Brownian motion with dry friction

The interplay between Coulomb friction and random excitations is studied
experimentally by means of a rotating probe in contact with a stationary
granular gas. The granular material is independently fluidized by a vertical
shaker, acting as a 'heat bath' for the Brownian-like motion of the probe. Two
ball bearings supporting the probe exert nonlinear Coulomb friction upon it.
The experimental velocity distribution of the probe, autocorrelation function,
and power spectra are compared with the predictions of a linear Boltzmann
equation with friction, which is known to simplify in two opposite limits: at
high collision frequency, it is mapped to a Fokker-Planck equation with
nonlinear friction, whereas at low collision frequency, it is described by a
sequence of independent random kicks followed by friction-induced relaxations.
Comparison between theory and experiment in these two limits shows good
agreement. Deviations are observed at very small velocities, where the real
bearings are not well modeled by Coulomb friction.Comment: 7 pages, 6 figure

### PASSATA - Object oriented numerical simulation software for adaptive optics

We present the last version of the PyrAmid Simulator Software for Adaptive
opTics Arcetri (PASSATA), an IDL and CUDA based object oriented software
developed in the Adaptive Optics group of the Arcetri observatory for
Monte-Carlo end-to-end adaptive optics simulations. The original aim of this
software was to evaluate the performance of a single conjugate adaptive optics
system for ground based telescope with a pyramid wavefront sensor. After some
years of development, the current version of PASSATA is able to simulate
several adaptive optics systems: single conjugate, multi conjugate and ground
layer, with Shack Hartmann and Pyramid wavefront sensors. It can simulate from
8m to 40m class telescopes, with diffraction limited and resolved sources at
finite or infinite distance from the pupil. The main advantages of this
software are the versatility given by the object oriented approach and the
speed given by the CUDA implementation of the most computational demanding
routines. We describe the software with its last developments and present some
examples of application.Comment: 9 pages, 2 figures, 3 tables. SPIE conference Astronomical Telescopes
and Instrumentation, 26 June - 01 July 2016, Edinburgh, Scotland, United
Kingdo

### Anomalous mobility of a driven active particle in a steady laminar flow

We study, via extensive numerical simulations, the force-velocity curve of an
active particle advected by a steady laminar flow, in the nonlinear response
regime. Our model for an active particle relies on a colored noise term that
mimics its persistent motion over a time scale $\tau_A$. We find that the
active particle dynamics shows non-trivial effects, such as negative
differential and absolute mobility (NDM and ANM, respectively). We explore the
space of the model parameters and compare the observed behaviors with those
obtained for a passive particle ($\tau_A=0$) advected by the same laminar flow.
Our results show that the phenomena of NDM and ANM are quite robust with
respect to the details of the considered noise: in particular for finite
$\tau_A$ a more complex force-velocity relation can be observed.Comment: 12 pages, 9 figures, paper submitted for the Special Issue of Journal
of Physics: Condensed Matter, "Transport in Narrow Channels", Guest Editors
P. Malgaretti, G. Oshanin, J. Talbo

### Anomalous force-velocity relation of driven inertial tracers in steady laminar flows

We study the nonlinear response to an external force of an inertial tracer
advected by a two-dimensional incompressible laminar flow and subject to
thermal noise. In addition to the driving external field $F$, the main
parameters in the system are the noise amplitude $D_0$ and the characteristic
Stokes time $\tau$ of the tracer. The relation velocity vs force shows
interesting effects, such as negative differential mobility (NDM), namely a
non-monotonic behavior of the tracer velocity as a function of the applied
force, and absolute negative mobility (ANM), i.e. a net motion against the
bias. By extensive numerical simulations, we investigate the phase chart in the
parameter space of the model, $(\tau,D_0)$, identifying the regions where NDM,
ANM and more common monotonic behaviors of the force-velocity curve are
observed.Comment: 5 pages, 13 figures. Contribution to the Topical Issue "Fluids and
Structures: Multi-scale coupling and modeling", edited by Luca Biferale,
Stefano Guido, Andrea Scagliarini, Federico Toschi. The final publication is
available at Springer via http://dx.doi.org/10.1140/epje/i2017-11571-

### Entropy production and coarse-graining in Markov processes

We study the large time fluctuations of entropy production in Markov
processes. In particular, we consider the effect of a coarse-graining procedure
which decimates {\em fast states} with respect to a given time threshold. Our
results provide strong evidence that entropy production is not directly
affected by this decimation, provided that it does not entirely remove loops
carrying a net probability current. After the study of some examples of random
walks on simple graphs, we apply our analysis to a network model for the
kinesin cycle, which is an important biomolecular motor. A tentative general
theory of these facts, based on Schnakenberg's network theory, is proposed.Comment: 18 pages, 13 figures, submitted for publicatio

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