285 research outputs found
Time asymmetry of the Kramers equation with nonlinear friction: fluctuation-dissipation relation and ratchet effect
We show by numerical simulations that the presence of nonlinear
velocity-dependent friction forces can induce a finite net drift in the
stochastic motion of a particle in contact with an equilibrium thermal bath and
in an asymmetric periodic spatial potential. In particular, we study the
Kramers equation for a particle subjected to Coulomb friction, namely a
constant force acting in the direction opposite to the particle's velocity. We
characterize the nonequilibrium irreversible dynamics by studying the
generalized fluctuation-dissipation relation for this ratchet model driven by
Coulomb friction.Comment: 6 pages, 4 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
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
Heat fluctuations of Brownian oscillators in nonstationary processes: fluctuation theorem and condensation transition
We study analytically the probability distribution of the heat released by an
ensemble of harmonic oscillators to the thermal bath, in the nonequilibrium
relaxation process following a temperature quench. We focus on the asymmetry
properties of the heat distribution in the nonstationary dynamics, in order to
study the forms taken by the Fluctuation Theorem as the number of degrees of
freedom is varied. After analysing in great detail the cases of one and two
oscillators, we consider the limit of a large number of oscillators, where the
behavior of fluctuations is enriched by a condensation transition with a
nontrivial phase diagram, characterized by reentrant behavior. Numerical
simulations confirm our analytical findings. We also discuss and highlight how
concepts borrowed from the study of fluctuations in equilibrium under symmetry
breaking conditions [Gaspard, J. Stat. Mech. P08021 (2012)] turn out to be
quite useful in understanding the deviations from the standard Fluctuation
Theorem.Comment: 16 pages, 7 figure
Nonequilibrium fluctuation-dissipation theorem and heat production
We use a relationship between response and correlation function in
nonequilibrium systems to establish a connection between the heat production
and the deviations from the equilibrium fluctuation-dissipation theorem. This
scheme extends the Harada-Sasa formulation [Phys. Rev. Lett. 95, 130602
(2005)], obtained for Langevin equations in steady states, as it also holds for
transient regimes and for discrete jump processes involving small entropic
changes. Moreover, a general formulation includes two times and the new
concepts of two-time work, kinetic energy, and of a two-time heat exchange that
can be related to a nonequilibrium "effective temperature". Numerical
simulations of a chain of anharmonic oscillators and of a model for a molecular
motor driven by ATP hydrolysis illustrate these points.Comment: 5 pages, 3 figure
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 . 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 () 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
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 , the main
parameters in the system are the noise amplitude and the characteristic
Stokes time 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, , 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-
Nonlinear Response of Inertial Tracers in Steady Laminar Flows: Differential and Absolute Negative Mobility
We study the mobility and the diffusion coefficient of an inertial tracer advected by a two-dimensional incompressible laminar flow, in the presence ofthermal noise and under the actionof an external force. We show, with extensive numerical simulations, that the force-velocity rela-tion for the tracer, in the nonlinear regime, displays complex and rich behaviors, including negativedifferential and absolute mobility. These effects rely upon asubtle coupling between inertia andapplied force which induce the tracer to persist in particular regions of phase space with a velocityopposite to the force. The relevance of this coupling is revisited in the framework of non-equilibriumresponse theory, applying a generalized Einstein relationto our system. The possibility of experi-mental observation of these results is also discussed
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
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
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