760 research outputs found
Power injected in a granular gas
A granular gas may be modeled as a set of hard-spheres undergoing inelastic
collisions; its microscopic dynamics is thus strongly irreversible. As pointed
out in several experimental works bearing on turbulent flows or granular
materials, the power injected in a dissipative system to sustain a steady-state
over an asymptotically large time window is a central observable. We describe
an analytic approach allowing us to determine the full distribution of the
power injected in a granular gas within a steady-state resulting from
subjecting each particle independently either to a random force (stochastic
thermostat) or to a deterministic force proportional to its velocity (Gaussian
thermostat). We provide an analysis of our results in the light of the
relevance, for other types of systems, of the injected power to fluctuation
relations.Comment: 9 pages, 4 figures. Contribution to Proceedings of "Work,
Dissipation, and Fluctuations in Nonequilibrium Physics", Brussels, 200
Injected power and entropy flow in a heated granular gas
Our interest goes to the power injected in a heated granular gas and to the
possibility to interpret it in terms of entropy flow. We numerically determine
the distribution of the injected power by means of Monte-Carlo simulations.
Then, we provide a kinetic theory approach to the computation of such a
distribution function. Finally, after showing why the injected power does not
satisfy a Fluctuation Relation a la Gallavotti-Cohen, we put forward a new
quantity which does fulfill such a relation, and is not only applicable in a
variety of frameworks outside the granular world, but also experimentally
accessible.Comment: accepted in Europhys. Let
Fluctuation relation for a L\'evy particle
We study the work fluctuations of a particle subjected to a deterministic
drag force plus a random forcing whose statistics is of the L\'evy type. In the
stationary regime, the probability density of the work is found to have ``fat''
power-law tails which assign a relatively high probability to large
fluctuations compared with the case where the random forcing is Gaussian. These
tails lead to a strong violation of existing fluctuation theorems, as the ratio
of the probabilities of positive and negative work fluctuations of equal
magnitude behaves in a non-monotonic way. Possible experiments that could probe
these features are proposed.Comment: 5 pages, 2 figures, RevTeX4; v2: minor corrections and references
added; v3: typos corrected, new conclusion, close to published versio
Fluctuations of power injection in randomly driven granular gases
We investigate the large deviation function pi(w) for the fluctuations of the
power W(t)=w t, integrated over a time t, injected by a homogeneous random
driving into a granular gas, in the infinite time limit. Starting from a
generalized Liouville equation we obtain an equation for the generating
function of the cumulants mu(lambda) which appears as a generalization of the
inelastic Boltzmann equation and has a clear physical interpretation.
Reasonable assumptions are used to obtain mu(lambda) in a closed analytical
form. A Legendre transform is sufficient to get the large deviation function
pi(w). Our main result, apart from an estimate of all the cumulants of W(t) at
large times t, is that pi(w) has no negative branch. This immediately results
in the failure of the Gallavotti-Cohen Fluctuation Relation (GCFR), that in
previous studies had been suggested to be valid for injected power in driven
granular gases. We also present numerical results, in order to discuss the
finite time behavior of the fluctuations of W(t). We discover that their
probability density function converges extremely slowly to its asymptotic
scaling form: the third cumulant saturates after a characteristic time larger
than 50 mean free times and the higher order cumulants evolve even slower. The
asymptotic value is in good agreement with our theory. Remarkably, a numerical
check of the GCFR is feasible only at small times, since negative events
disappear at larger times. At such small times this check leads to the
misleading conclusion that GCFR is satisfied for pi(w). We offer an explanation
for this remarkable apparent verification. In the inelastic Maxwell model,
where a better statistics can be achieved, we are able to numerically observe
the failure of GCFR.Comment: 23 pages, 15 figure
Modeling the dynamics of a tracer particle in an elastic active gel
The internal dynamics of active gels, both in artificial (in-vitro) model
systems and inside the cytoskeleton of living cells, has been extensively
studied by experiments of recent years. These dynamics are probed using tracer
particles embedded in the network of biopolymers together with molecular
motors, and distinct non-thermal behavior is observed. We present a theoretical
model of the dynamics of a trapped active particle, which allows us to quantify
the deviations from equilibrium behavior, using both analytic and numerical
calculations. We map the different regimes of dynamics in this system, and
highlight the different manifestations of activity: breakdown of the virial
theorem and equipartition, different elasticity-dependent "effective
temperatures" and distinct non-Gaussian distributions. Our results shed light
on puzzling observations in active gel experiments, and provide physical
interpretation of existing observations, as well as predictions for future
studies.Comment: 11 pages, 6 figure
Non Poissonian statistics in a low density fluid
Our interest goes to the collisional statistics in an arbitrary interacting
fluid. We show that even in the low density limit and contrary to naive
expectation, the number of collisions experienced by a tagged particle in a
given time does not obey Poisson law, and that conversely, the free flight time
distribution is not a simple exponential. As an illustration, the hard sphere
fluid case is worked out in detail. For this model, we quantify analytically
those deviations and successfully compare our predictions against molecular
dynamics simulations.Comment: 4 pages, 2 figure
Collisional statistics of the hard-sphere gas
We investigate the probability distribution function of the free flight time
and of the number of collisions in a hard sphere gas at equilibrium. At
variance with naive expectation, the latter quantity does not follow Poissonian
statistics, even in the dilute limit which is the focus of the present
analysis. The corresponding deviations are addressed both numerically and
analytically. In writing an equation for the generating function of the
cumulants of the number of collisions, we came across a perfect mapping between
our problem and a previously introduced model: the probabilistic ballistic
annihilation process [Coppex et al., Phys. Rev. E 69 11303 (2004)]. We exploit
this analogy to construct a Monte-Carlo like algorithm able to investigate the
asymptotically large time behavior of the collisional statistics within a
reasonable computational time. In addition, our predictions are confronted
against the results of Molecular Dynamics simulations and Direct Simulation
Monte Carlo technique. An excellent agreement is reported.Comment: 13 pages, 12 figure
Dark Matter searches using gravitational wave bar detectors: quark nuggets and newtorites
Many experiments have searched for supersymmetric WIMP dark matter, with null
results. This may suggest to look for more exotic possibilities, for example
compact ultra-dense quark nuggets, widely discussed in literature with several
different names. Nuclearites are an example of candidate compact objects with
atomic size cross section. After a short discussion on nuclearites, the result
of a nuclearite search with the gravitational wave bar detectors Nautilus and
Explorer is reported. The geometrical acceptance of the bar detectors is 19.5
sr, that is smaller than that of other detectors used for similar
searches. However, the detection mechanism is completely different and is more
straightforward than in other detectors. The experimental limits we obtain are
of interest because, for nuclearites of mass less than g, we find a
flux smaller than that one predicted considering nuclearites as dark matter
candidates. Particles with gravitational only interactions (newtorites) are
another example. In this case the sensitivity is quite poor and a short
discussion is reported on possible improvements.Comment: published on Astroparticle Physics Sept 25th 2016 replaced fig 1
The effect of a temporary absence of target velocity information on visual tracking
Experiments with the Rashbass ‘step-ramp’ paradigm have revealed that the initial catchup saccade that occurs near pursuit onset uses target velocity as well as position information in its programming. Information about both position and motion also influences smooth pursuit. To investigate the timing of velocity sampling near the initiation of saccades and smooth pursuit, we analyzed the eye movements made in nine ‘step-ramp’ conditions, produced by combining –2, 0 and +2 deg steps with –8, 0 and +8 deg/s ramps. Each trial had either no temporal gap or a 50-ms gap during which the laser target was extinguished, beginning 25, 50, 75 or 100 ms after the step. Six subjects repeated each of the resulting 45 conditions 25 times. With no temporal gap, saccades were larger in the step-ramp-away’ than the ‘step-only’ condition, confirming that saccade programming incorporates ramp velocity information. A temporal gap had no effect on the accuracy of saccades on ‘step-only’ trials, but often caused undershoots in ‘step-ramp’ trials. A 50-ms gap within the first 100 ms also increased the latency of the initial saccade. Although initial pursuit velocity was unaffected by a temporal gap, a gap that started at 25 ms reliably delayed pursuit onset for ramp motion of the target toward the fovea. Later gaps had a minimal effect on initial pursuit latency. The similar timing of the temporal gaps in target motion information that affect the initiation of saccades and pursuit provides further behavioral evidence that the two types of eye movements share pre-motor neural mechanisms
How Far from Equilibrium Is Active Matter?
Active matter systems are driven out of thermal equilibrium by a lack of generalized Stokes-Einstein relation between injection and dissipation of energy at the microscopic scale. We consider such a system of interacting particles, propelled by persistent noises, and show that, at small but finite persistence time, their dynamics still satisfy a time-reversal symmetry. To do so, we compute perturbatively their steady-state measure and show that, for short persistent times, the entropy production rate vanishes. This endows such systems with an effective fluctuation-dissipation theorem akin to that of thermal equilibrium systems. Last, we show how interacting particle systems with viscous drags and correlated noises can be seen as in equilibrium with a viscoelastic bath but driven out of equilibrium by nonconservative forces, hence providing energetic insight into the departure of active systems from equilibrium.JT was supported by ANR project Bactterns. CN was supported by EPSRC grant Nr. EP/J007404.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by the American Physical Society
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