784 research outputs found
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
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
Cages and anomalous diffusion in vibrated dense granular media
A vertically shaken granular medium hosts a blade rotating around a fixed
vertical axis, which acts as a mesorheological probe. At high densities,
independently from the shaking intensity, the blade's dynamics show strong
caging effects, marked by transient sub-diffusion and a maximum in the velocity
power density spectrum (vpds), at a resonant frequency Hz.
Interpreting the data through a diffusing harmonic cage model allows us to
retrieve the elastic constant of the granular medium and its collective
diffusion coefficient. For high frequencies , a tail in the vpds
reveals non-trivial correlations in the intra-cage micro-dynamics. At very long
times (larger than s), a super-diffusive behavior emerges, ballistic in
the most extreme cases. Consistently, the distribution of slow velocity
inversion times displays a power-law decay, likely due to persistent
collective fluctuations of the host medium.Comment: 5 pages + 4 page of supplemental material, 6 figures, to be published
on Phys. Rev. Let
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
Exploring logic-in-memory architectures with skyrmion technology
L'abstract è presente nell'allegato / the abstract is in the attachmen
Dissipative lateral walls are sufficient to trigger convection in vibrated granular gases
Buoyancy-driven (thermal) convection in dilute granular media, fluidized by a
vibrating base, is known to appear without the need of lateral boundaries in a
restricted region of parameters (inelasticity, gravity, intensity of energy
injection). We have recently discovered a second buoyancy-driven convection
effect which occurs at any value of the parameters, provided that the impact of
particles with the lateral walls is inelastic (Pontuale et al., Phys. Rev.
Lett. 117, 098006 (2016)). It is understood that this novel convection effect
is strictly correlated to the existence of perpendicular energy fluxes: a
vertical one, induced by both bulk and wall inelasticity, and a horizontal one,
induced only by dissipation at the walls. Here we first review those previous
results, and then present new experimental and numerical data concerning the
variations of box geometry, intensity of energy injection, number of particles
and width of the box.Comment: 4 pages, 4 figures, conference Powders and Grains 201
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