31 research outputs found
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
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
Rescaling invariance and anomalous energy transport in a small vertical column of grains
It is well known that energy dissipation and finite size can deeply affect
the dynamics of granular matter, often making usual hydrodynamic approaches
problematic. Here we report on the experi-mental investigation of a small model
system, made of ten beads constrained into a 1-d geometry by a narrow vertical
pipe and shaken at the base by a piston excited by a periodic wave. Recording
the beads motion with high frame rate camera allows to investigate in detail
the microscopic dynamics and test hydrodynamic and kinetic models. Varying the
energy we explore different regimes from fully fluidized to the edge of
condensation, observing good hydrodynamic behavior down to the edge of
fluidization, despite the small system size. Density and temperature fields for
different system energies can be collapsed by suitable space and time
rescaling, and the expected constitutive equation holds very well when the
particle diameter is considered. At the same time the balance between
dissipated and fed energy is not well described by commonly adopted dependence,
due to the up-down symmetry breaking. Our observations, supported by the
measured particle velocity distributions, show a different phenomenological
temperature dependence, which yields equation solutions in agreement with
experimental results
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
Out-of-Equilibrium Non-Gaussian Behavior in Driven Granular Gases
The characterization of the distance from equilibrium is a debated problem in
particular in the treatment of experimental signals. If the signal is a
1-dimensional time-series, such a goal becomes challenging. A paradigmatic
example is the angular diffusion of a rotator immersed in a vibro-fluidized
granular gas. Here, we experimentally observe that the rotator's angular
velocity exhibits significative differences with respect to an equilibrium
process. Exploiting the presence of two relevant time-scales and non-Gaussian
velocity increments, we quantify the breakdown of time-reversal asymmetry,
which would vanish in the case of a 1d Gaussian process. We deduce a new model
for the massive probe, with two linearly coupled variables, incorporating both
Gaussian and Poissonian noise, the latter motivated by the rarefied collisions
with the granular bath particles. Our model reproduces the experiment in a
range of densities, from dilute to moderately dense, with a meaningful
dependence of the parameters on the density.Comment: 5 pages, 4 figure
Variable clinical expression of Stickler Syndrome: A case report of a novel COL11A1 mutation
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Unified rheology of vibro-fluidized dry granular media: From slow dense flows to fast gas-like regimes
Granular media take on great importance in industry and geophysics, posing a severe challenge to materials science. Their response properties elude known soft rheological models, even when the yield-stress discontinuity is blurred by vibro-fluidization. Here we propose a broad rheological scenario where average stress sums up a frictional contribution, generalizing conventional μ(I)-rheology, and a kinetic collisional term dominating at fast fluidization. Our conjecture fairly describes a wide series of experiments in a vibrofluidized vane setup, whose phenomenology includes velocity weakening, shear thinning, a discontinuous thinning transition, and gaseous shear thickening. The employed setup gives access to dynamic fluctuations, which exhibit a broad range of timescales. In the slow dense regime the frequency of cage-opening increases with stress and enhances, with respect to μ(I)-rheology, the decrease of viscosity. Diffusivity is exponential in the shear stress in both thinning and thickening regimes, with a huge growth near the transition
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