153 research outputs found
Journey of an intruder through the fluidisation and jamming transitions of a dense granular media
We study experimentally the motion of an intruder dragged into an amorphous
monolayer of horizontally vibrated grains at high packing fractions. This
motion exhibits two transitions. The first transition separates a continuous
motion regime at comparatively low packing fractions and large dragging force
from an intermittent motion one at high packing fraction and low dragging
force. Associated to these different motions, we observe a transition from a
linear rheology to a stiffer response. We thereby call "fluidisation" this
first transition. A second transition is observed within the intermittent
regime, when the intruder's motion is made of intermittent bursts separated by
long waiting times. We observe a peak in the relative fluctuations of the
intruder's displacements and a critical scaling of the burst amplitudes
distributions. This transition occurs at the jamming point characterized in a
previous study and defined as the point where the static pressure (i.e. the
pressure measured in the absence of vibration) vanishes. Investigating the
motion of the surrounding grains, we show that below the fluidisation
transition, there is a permanent wake of free volume behind the intruder. This
transition is marked by the evolution of the reorganization patterns around the
intruder, which evolve from compact aggregates in the flowing regime to
long-range branched shapes in the intermittent regime, suggesting an increasing
role of the stress fluctuations. Remarkably, the distributions of the kinetic
energy of these reorganization patterns also exhibits a critical scaling at the
jamming transition.Comment: 12 pages, 11 figure
The glass transition in a nutshell: a source of inspiration to describe the subcritical transition to turbulence
The starting point of the present work is the observation of possible
analogies, both at the phenomenological and at the methodological level,
between the subcritical transition to turbulence and the glass transition.
Having recalled the phenomenology of the subcritical transition to turbulence,
we review the theories of the glass transition at a very basic level, focusing
on the history of their development as well as on the concepts they have
elaborated. Doing so, we aim at attracting the attention on the above mentioned
analogies, which we believe could inspire new developments in the theory of the
subcritical transition to turbulence. We then briefly describe a model inspired
by one of the simplest and most inspiring model of the glass transition, the
so-called Random Energy Model, as a first step in that direction.Comment: 9 pages, 1 figure; to appear in a topical issue of Eur. Phys. J. E
dedicated to Paul Mannevill
Experimental evidences of the Gardner phase in a granular glass
Analyzing the dynamics of a vibrated bi-dimensional packing of bidisperse
granular discs below jamming, we provide evidences of a Gardner phase deep into
the glass phase. To do so we perform several independent compression cycles
within the same glass and show that the particles select different average
vibrational positions at each cycle, while the neighborhood structure remains
unchanged. We compute the mean square displacement as a function of the packing
fraction and compare it with the average separation between the cages obtained
for different compression cycles. Our results are fully compatible with recent
numerical observations obtained for a mean field model of glass as well as for
hard spheres in finite dimension. We also characterize the distribution of the
cage order parameters. Here we note several differences from the numerical
results, which could be attributed to activated processes and cage
heterogeneities.Comment: 4 pages, 3 figure
Collective Motion in Active Materials: Model Experiments
In these lecture notes from the Les Houches School, we discuss collective
motion in model experiments of active systems. We specifically discuss walking
grains and colloidal rollers experiments. In both cases, we focus on the
theoretical tools one can use to relate the knowledge of the dynamics at the
particle scale to the large scale physics.Comment: 28 pages, published as Ch3 in Active Matter and Nonequilibrium
Statistical Physics Lecture Notes of the Les Houches Summer School: Volume
112, September 201
Bulk elastic fingering instability in Hele-Shaw cells
We demonstrate experimentally the existence of a purely elastic fingering
instability which arises when air penetrates into an elastomer confined in a
Hele-Shaw cell. Fingers appear sequentially and propagate within the bulk of
the material as soon as a critical strain, independent of the elastic modulus,
is exceeded. Their width depends non-linearly on the distance between the
confining glass plates. A key element in the driving force of the instability
is the adhesion of layers of gels to the plates, which results in a
considerable expense of elastic energy during the growth of the air bubble.Comment: Submitted to Physical Review letters; 5 pages; 6 figure
Shear modulus and Dilatancy Softening in Granular Packings above Jamming
We investigate experimentally the mechanical response of a monolayer of
bi-disperse frictional grains to an inhomogeneous shear perturbation across the
jamming transition. We inflate an intruder inside the packing and use
photo-elasticity and tracking techniques to measure the induced shear strain
and stresses at the grain scale. We quantify experimentally the constitutive
relations for strain amplitudes as low as 0.001 and for a range of packing
fractions within 2% variation around the jamming transition. At the transition
strong nonlinear effects set in : both the shear modulus and the dilatancy
shear-soften at small strain until a critical strain is reached where effective
linearity is recovered. The dependencies of the critical strain and the
associated critical stresses on the distance from jamming are extracted via
scaling analysis. We check that the constitutive laws, when applied to the
equations governing mechanical equilibrium, lead to the observed stress and
strain profiles. These profiles exhibit a spatial crossover between an
effective linear regime close to the inflater and the truly nonlinear regime
away from it. The crossover length diverges at the jamming transition.Comment: 5 pages, 5 figure
Collective Motion of Vibrated Polar Disks
We experimentally study a monolayer of vibrated disks with a built-in polar
asymmetry which enables them to move quasi-balistically on a large persistence
length. Alignment occurs during collisions as a result of self-propulsion and
hard core repulsion. Varying the amplitude of the vibration, we observe the
onset of large-scale collective motion and the existence of giant number
fluctuations with a scaling exponent in agreement with the predicted
theoretical value.Comment: 4 pages, 4 figure
Intensive thermodynamic parameters in nonequilibrium systems
Considering a broad class of steady-state nonequilibrium systems for which
some additive quantities are conserved by the dynamics, we introduce from a
statistical approach intensive thermodynamic parameters (ITPs) conjugated to
the conserved quantities. This definition does not require any detailed balance
relation to be fulfilled. Rather, the system has to satisfy a general
additivity property, which holds in most of the models usually considered in
the literature, including those described by a matrix product ansatz with
finite matrices. The main property of these ITPs is to take equal values in two
subsystems, making them a powerful tool to describe nonequilibrium phase
coexistence, as illustrated on different models. We finally discuss the issue
of the equalization of ITPs when two different systems are put into contact.
This issue is closely related to the possibility of measuring the ITPs using a
small auxiliary system, in the same way as temperature is measured with a
thermometer, and points at one of the major difficulties of nonequilibrium
statistical mechanics. In addition, an efficient alternative determination,
based on the measure of fluctuations, is also proposed and illustrated.Comment: 17 pages, 5 figures; final version, with minor change
Two-phase flow in a chemically active porous medium
We study the problem of the transformation of a given reactant species into
an immiscible product species, as they flow through a chemically active porous
medium. We derive the equation governing the evolution of the volume fraction
of the species -- in a one-dimensional macroscopic description --, identify the
relevant dimensionless numbers, and provide simple models for capillary
pressure and relative permeabilities, which are quantities of crucial
importance when tackling multiphase flows in porous media. We set the domain of
validity of our models and discuss the importance of viscous coupling terms in
the extended Darcy's law. We investigate numerically the steady regime and
demonstrate that the spatial transformation rate of the species along the
reactor is non-monotonous, as testified by the existence of an inflection point
in the volume fraction profiles. We obtain the scaling of the location of this
inflection point with the dimensionless lengths of the problem. Eventually, we
provide key elements for optimization of the reactor.Comment: 13 pages, 10 figure
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