304 research outputs found
Coulomb friction driving Brownian motors
We review a family of models recently introduced to describe Brownian motors
under the influence of Coulomb friction, or more general non-linear friction
laws. It is known that, if the heat bath is modeled as the usual Langevin
equation (linear viscosity plus white noise), additional non-linear friction
forces are not sufficient to break detailed balance, i.e. cannot produce a
motor effect. We discuss two possibile mechanisms to elude this problem. A
first possibility, exploited in several models inspired to recent experiments,
is to replace the heat bath's white noise by a ``collisional noise'', that is
the effect of random collisions with an external equilibrium gas of particles.
A second possibility is enlarging the phase space, e.g. by adding an external
potential which couples velocity to position, as in a Klein-Kramers equation.
In both cases, non-linear friction becomes sufficient to achieve a
non-equilibrium steady state and, in the presence of an even small spatial
asymmetry, a motor effect is produced.Comment: 19 pages, 10 figures, Proceedings of the Conference "Small system
nonequilibrium fluctuations, dynamics and stochastics, and anomalous
behavior", KITPC, Beijing, Chin
Coarsening in granular systems
We review a few representative examples of granular experiments or models
where phase separation, accompanied by domain coarsening, is a relevant
phenomenon. We first elucidate the intrinsic non-equilibrium, or athermal,
nature of granular media. Thereafter, dilute systems, the so-called "granular
gases" are discussed: idealized kinetic models, such as the gas of inelastic
hard spheres in the cooling regime, are the optimal playground to study the
slow growth of correlated structures, e.g. shear patterns, vortices and
clusters. In fluidized experiments, liquid-gas or solid-gas separations have
been observed. In the case of monolayers of particles, phase coexistence and
coarsening appear in several different setups, with mechanical or electrostatic
energy input. Phenomenological models describe, even quantitatively, several
experimental measures, both for the coarsening dynamics and for the dynamic
transition between different granular phases. The origin of the underlying
bistability is in general related to negative compressibility from granular
hydrodynamics computations, even if the understanding of the mechanism is far
from complete. A relevant problem, with important industrial applications, is
related to the demixing or segregation of mixtures, for instance in rotating
tumblers or on horizontally vibrated plates. Finally, the problem of compaction
of highly dense granular materials, which has many important applications, is
usually described in terms of coarsening dynamics: there, bubbles of
mis-aligned grains evaporate, allowing the coalescence of optimally arranged
islands and a progressive reduction of total occupied volume.Comment: 12 pages, 10 figures, to appear in "Dynamics of coarsening" Comptes
Rendus Physique special issue,
https://sites.google.com/site/ppoliti/crp-special-issu
Irreversible dynamics of a massive intruder in dense granular fluids
A Generalized Langevin Equation with exponential memory is proposed for the
dynamics of a massive intruder in a dense granular fluid. The model reproduces
numerical correlation and response functions, violating the equilibrium
Fluctuation Dissipation relations. The source of memory is identified in the
coupling of the tracer velocity with a spontaneous local velocity field
in the surrounding fluid. Such identification allows us to measure the
intruder's fluctuating entropy production as a function of and ,
obtaining a neat verification of the Fluctuation Relation.Comment: 5 pages, 3 figures accepted for publication in EP
Nonequilibrium Brownian motion beyond the effective temperature
The condition of thermal equilibrium simplifies the theoretical treatment of
fluctuations as found in the celebrated Einstein's relation between mobility
and diffusivity for Brownian motion. Several recent theories relax the
hypothesis of thermal equilibrium resulting in at least two main scenarios.
With well separated timescales, as in aging glassy systems, equilibrium
Fluctuation-Dissipation Theorem applies at each scale with its own "effective"
temperature. With mixed timescales, as for example in active or granular fluids
or in turbulence, temperature is no more well-defined, the dynamical nature of
fluctuations fully emerges and a Generalized Fluctuation-Dissipation Theorem
(GFDT) applies. Here, we study experimentally the mixed timescale regime by
studying fluctuations and linear response in the Brownian motion of a rotating
intruder immersed in a vibro-fluidized granular medium. Increasing the packing
fraction, the system is moved from a dilute single-timescale regime toward a
denser multiple-timescale stage. Einstein's relation holds in the former and is
violated in the latter. The violation cannot be explained in terms of effective
temperatures, while the GFDT is able to impute it to the emergence of a strong
coupling between the intruder and the surrounding fluid. Direct experimental
measurements confirm the development of spatial correlations in the system when
the density is increased.Comment: 10 pages, 5 figure
Growing non-equilibrium length in granular fluids: from experiment to fluctuating hydrodynamics
Velocity correlations in a 2D granular fluid are studied in experiments and
numerical simulations. The transverse component of the velocity structure
factor reveals two well defined energy scales, associated with the external
"bath temperature" and with the internal granular one, ,
relevant at large and small wavelengths respectively. Experimental and
numerical data are discussed within a fluctuating hydrodynamics model, which
allows one to define and measure a non-equilibrium coherence length ,
growing with density, that characterizes order in the velocity field.Comment: 5 pages, 4 figure
Lattice models for granular-like velocity fields: Hydrodynamic limit
A recently introduced model describing -on a 1d lattice- the velocity field
of a granular fluid is discussed in detail. The dynamics of the velocity field
occurs through next-neighbours inelastic collisions which conserve momentum but
dissipate energy. The dynamics can be described by a stochastic equation in
full phase space, or through the corresponding Master Equation for the time
evolution of the probability distribution. In the hydrodynamic limit, equations
for the average velocity and temperature fields with fluctuating currents are
derived, which are analogous to those of granular fluids when restricted to the
shear modes. Therefore, the homogeneous cooling state, with its linear
instability, and other relevant regimes such as the uniform shear flow and the
Couette flow states are described. The evolution in time and space of the
single particle probability distribution, in all those regimes, is also
discussed, showing that the local equilibrium is not valid in general. The
noise for the momentum and energy currents, which are correlated, are white and
Gaussian. The same is true for the noise of the energy sink, which is usually
negligible
Ricerca-azione e analisi delle reti sociali in contesti di apprendimento linguistico online: una proposta metodologica
At the present time, talking about “distance” education means almost exclusively talking about education that takes place on the Web and through its use for educational purposes. More specifically, the last two years have shown a dramatic increase in the popularity of MOOCs (Massive Open Online Courses); this phenomenon, especially with reference to Language MOOCs, requires to be observed and studied adequately. Subsequently, it is necessary to shape and develop critical skills and tools to notice and confront issues in massive online learning contexts.This paper aims to present a methodology that encompasses action research and social network analysis to detect and address issues in LMOOCs. Relying on Moore’s transactional distance theory, the methodology involves a four-phases process in which two distinct analyses are conducted. On the one hand, a monitoring system gathers a large amount of data, regarding the social network in the LMOOC, to be examined; on the other hand, the LMOOC tutor makes use of different tools to reflect upon his/her teaching. All this leads, therefore, to the availability of a brand-new procedure (to be empirically validated) in studying learning interactions, in a broader sense, in massive online learning contexts
Single prazosin infusion in prelimbic cortex Fosters extinction of amphetamine-induced conditioned place preference
Exposure to drug-associated cues to induce extinction is a useful strategy to contrast cue-induced drug seeking. Norepinephrine (NE) transmission in medial prefrontal cortex has a role in the acquisition and extinction of conditioned place preference induced by amphetamine. We have reported recently that NE in prelimbic cortex delays extinction of amphetamine-induced conditioned place preference (CPP). A potential involvement of α1-adrenergic receptors in the extinction of appetitive conditioned response has been also suggested, although their role in prelimbic cortex has not been yet fully investigated. Here, we investigated the effects of the α1-adrenergic receptor antagonist prazosin infusion in the prelimbic cortex of C57BL/6J mice on expression and extinction of amphetamine-induced CPP. Acute prelimbic prazosin did not affect expression of amphetamine-induced CPP on the day of infusion, while in subsequent days it produced a clear-cut advance of extinction of preference for the compartment previously paired with amphetamine (Conditioned stimulus, CS). Moreover, prazosin-treated mice that had extinguished CS preference showed increased mRNA expression of brain-derived neurotrophic factor (BDNF) and post-synaptic density 95 (PSD-95) in the nucleus accumbens shell or core, respectively, thus suggesting that prelimbic α1-adrenergic receptor blockade triggers neural adaptations in subcortical areas that could contribute to the extinction of cue-induced drug-seeking behavior. These results show that the pharmacological blockade of α1-adrenergic receptors in prelimbic cortex by a single infusion is able to induce extinction of amphetamine-induced CPP long before control (vehicle) animals, an effect depending on contingent exposure to retrieval, since if infused far from or after reactivation it did not affect preference. Moreover, they suggest strongly that the behavioral effects depend on post-treatment neuroplasticity changes in corticolimbic network, triggered by a possible “priming” effect of prazosin, and point
to a potential therapeutic power of the antagonist for maladaptive memories
Non-equilibrium fluctuations in frictional granular motor: experiments and kinetic theory
We report the study of a new experimental granular Brownian motor, inspired
to the one published in [Phys. Rev. Lett. 104, 248001 (2010)], but different in
some ingredients. As in that previous work, the motor is constituted by a
rotating pawl whose surfaces break the rotation-inversion symmetry through
alternated patches of different inelasticity, immersed in a gas of granular
particles. The main novelty of our experimental setup is in the orientation of
the main axis, which is parallel to the (vertical) direction of shaking of the
granular fluid, guaranteeing an isotropic distribution for the velocities of
colliding grains, characterized by a variance . We also keep the
granular system diluted, in order to compare with Boltzmann-equation-based
kinetic theory. In agreement with theory, we observe for the first time the
crucial role of Coulomb friction which induces two main regimes: (i) rare
collisions (RC), with an average drift , and (ii)
frequent collisions (FC), with . We also study the
fluctuations of the angle spanned in a large time interval, ,
which in the FC regime is proportional to the work done upon the motor. We
observe that the Fluctuation Relation is satisfied with a slope which weakly
depends on the relative collision frequency.Comment: 7 pages, 6 figure
Fluctuation relations without uniform large deviations
We study the fluctuations of a stochastic Maxwell-Lorentz particle model
driven by an external field to determine the extent to which fluctuation
relations are related to large deviations. Focusing on the total entropy
production of this model in its steady state, we show that, although the
probability density of this quantity globally satisfies (by definition) a
fluctuation relation, its negative tail decays exponentially with time, whereas
its positive tail decays slower than exponentially with time because of long
collision-free trajectories. This provides an example of physical system for
which the fluctuation relation does not derive, as commonly thought, from a
probability density decaying everywhere exponentially with time or, in other
words, from a probability density having a uniform large deviation form.Comment: 12 pages, 1 figure; v2: published versio
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