496 research outputs found
Invited review: Effect of temperature on a granular pile
As a fragile construction, a granular pile is very sensitive to minute
external perturbations. In particular, it is now well established that a
granular assembly is sensitive to variations of temperature. Such variations
can produce localized rearrangements as well as global static avalanches inside
a pile. In this review, we sum up the various observations that have been made
concerning the effect of temperature on a granular assembly. In particular, we
dwell on the way controlled variations of temperature have been employed to
generate the compaction of a granular pile. After laying emphasis on the key
features of this compaction process, we compare it to the classic
vibration-induced compaction. Finally, we also review other granular systems in
a large sense, from microscopic (jammed multilamellar vesicles) to macroscopic
scales (stone heave phenomenon linked to freezing and thawing of soils) for
which periodic variations of temperature could play a key role in the dynamics
at stake.Comment: 16 pages, 14 figures, Commentary from the reviewer available in
Papers in Physic
Spin-models of granular compaction: From one-dimensional models to random graphs
We discuss two athermal types of dynamics suitable for spin-models designed
to model repeated tapping of a granular assembly. These dynamics are applied to
a range of models characterised by a 3-spin Hamiltonian aiming to capture the
geometric frustration in packings of granular matter.Comment: Contribution to "Challenges in Granular Media", ICTP Trieste; to
appear in 'Advances in Complex Systems
Memory effects in vibrated granular systems
Granular materials present memory effects when submitted to tapping
processes. These effects have been observed experimentally and are discussed
here in the context of a general kind of model systems for compaction
formulated at a mesoscopic level. The theoretical predictions qualitatively
agree with the experimental results. As an example, a particular simple model
is used for detailed calculations.Comment: 12 pages, 5 figures; to appear in Journal of Physics: Condensed
Matter (Special Issue: Proceedings of ESF SPHINX Workshop on ``Glassy
behaviour of kinetically constrained models.''
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
On the existence of stationary states during granular compaction
When submitted to gentle mechanical taps a granular packing slowly compacts
until it reaches a stationary state that depends on the tap characteristics.
The properties of such stationary states are experimentally investigated. The
influence of the initial state, taps properties and tapping protocol are
studied. The compactivity of the packings is determinated. Our results strongly
support the idea that the stationary states are genuine thermodynamic states.Comment: to be published in EPJE. The original publication will be available
at www.europhysj.or
Stick-slip instabilities in sheared granular flow: the role of friction and acoustic vibrations
We propose a theory of shear flow in dense granular materials. A key
ingredient of the theory is an effective temperature that determines how the
material responds to external driving forces such as shear stresses and
vibrations. We show that, within our model, friction between grains produces
stick-slip behavior at intermediate shear rates, even if the material is
rate-strengthening at larger rates. In addition, externally generated acoustic
vibrations alter the stick-slip amplitude, or suppress stick-slip altogether,
depending on the pressure and shear rate. We construct a phase diagram that
indicates the parameter regimes for which stick-slip occurs in the presence and
absence of acoustic vibrations of a fixed amplitude and frequency. These
results connect the microscopic physics to macroscopic dynamics, and thus
produce useful information about a variety of granular phenomena including
rupture and slip along earthquake faults, the remote triggering of
instabilities, and the control of friction in material processing.Comment: 12 pages, 8 figure
Phenomenological glass model for vibratory granular compaction
A model for weakly excited granular media is derived by combining the free
volume argument of Nowak et al. [Phys. Rev. E 57, 1971 (1998)] and the
phenomenological model for supercooled liquids of Adam and Gibbs [J. Chem.
Phys. 43, 139 (1965)]. This is made possible by relating the granular
excitation parameter \Gamma, defined as the peak acceleration of the driving
pulse scaled by gravity, to a temperature-like parameter \eta(\Gamma). The
resulting master equation is formally identical to that of Bouchaud's trap
model for glasses [J. Phys. I 2, 1705 (1992)]. Analytic and simulation results
are shown to compare favourably with a range of known experimental behaviour.
This includes the logarithmic densification and power spectrum of fluctuations
under constant \eta, the annealing curve when \eta is varied cyclically in
time, and memory effects observed for a discontinuous shift in \eta. Finally,
we discuss the physical interpretation of the model parameters and suggest
further experiments for this class of systems.Comment: 2 references added; some figure labels tweaked. To appear in PR
Compaction and mobility in randomly agitated granular assemblies
We study the compaction and mobility properties of a dense granular material
under weak random vibration. By putting in direct contact millimetric glass
beads with piezoelectric transducers we manage to inject energy to the system
in a disordered manner with accelerations much smaller than gravity, resulting
in a slow compaction dynamics and no convection. We characterize the mobility
inside the medium by pulling through it an intruder grain at constant velocity.
We present an extensive study of the relation between drag force and velocity
for different vibration conditions and sizes of the intruder.Comment: 4 pages, 6 figures, to appear in the proceedings of Powders and
Grains 200
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