72 research outputs found
Creep motion of a granular pile induced by thermal cycling
We report a time-resolved study of the dynamics associated with the slow
compaction of a granular column submitted to thermal cycles. The column height
displays a complex behavior: for a large amplitude of the temperature cycles,
the granular column settles continuously, experiencing a small settling at each
cycle; By contrast, for small-enough amplitude, the column exhibits a
discontinuous and intermittent activity: successive collapses are separated by
quiescent periods whose duration is exponentially distributed. We then discuss
potential mechanisms which would account for both the compaction and the
transition at finite amplitude.Comment: 4 pages, 5 figures, accepted for publication in Physical Review
Letters (05sep08
Pressure measurement in two-dimensional horizontal granular gases
International audienceA two-dimensional granular gas is produced by vibrating vertically a partial layer of beads on a horizontal plate. Measurements of the force applied by the granular gas to the sidewalls of the container, or granular pressure, are used to study the effect of the shaking strength, density, bead-plate restitution coefficient, and particle size on the steady properties of the gas
Anisotropic friction: assessment of force components and resulting trajectories
International audienceWe report on an experimental device that makes it possible to assess the frictional properties of the contact between a slider and a horizontal surface, and to study the resulting trajectories of the slider when pulled across the surface by means of a flexible link. First, we show experimentally that, when the frictional properties are anisotropic, the slider is subjected, in addition to the dissipative frictional force oriented along the trajectory, to a force, perpendicular to the trajectory, which thus does not contribute to energy dissipation. Therefore, the slider does not necessarily moves in the pulling direction. Second, we show that the trajectories of the slider, when in continuous motion, in absence of inertial effects, can be recovered by assuming that, at all time, the friction force compensates the pulling force. We point out that we prove experimentally that the normal component of the friction force is given as the derivative, with respect to the sliding direction, of the tangential component. This result is particularly interesting as the relation between the normal and the tangential components is compatible with " the maximum of energy release rate " criterion used in the theory of fracture
Observation of negative line tensions from Plateau border regions in dry foam films
International audienceWe measure the angles produced in the Plateau border region of " dry " soap films. In a simple experimental geometry, we demonstrate that a negative line tension can be attributed to these regions. This result has important consequences for the theoretical description of foams approaching the dry limit
Internal dynamics of actin structures involved in the cell motility and adhesion: Modeling of the podosomes at the molecular level
International audiencePodosomes are involved in the spreading and motility of various cells to a solid substrate. These dynamical structures, which have been proven to consist of a dense actin core surrounded by an actin cloud, nucleate when the cell comes in the vicinity of a substrate. During the cell spreading or motion, the podosomes exhibit collective dynamical behaviors, forming clusters and rings. We design a simple model aiming at the description of internal molecular turnover in a single podosome: actin filaments form a brush which grows from the cellular membrane whereas their size is regulated by the action of a severing agent, the gelsolin. In this framework, the characteristic sizes of the core and of the cloud, as well as the associated characteristic times are expressed in terms of basic ingredients. Moreover, the collocation of the actin and gelsolin in the podosome is understood as a natural result of the internal dynamics
Mechanisms for slow strengthening in granular materials
Several mechanisms cause a granular material to strengthen over time at low
applied stress. The strength is determined from the maximum frictional force
F_max experienced by a shearing plate in contact with wet or dry granular
material after the layer has been at rest for a waiting time \tau. The layer
strength increases roughly logarithmically with \tau -only- if a shear stress
is applied during the waiting time. The mechanisms of strengthening are
investigated by sensitive displacement measurements and by imaging of particle
motion in the shear zone. Granular matter can strengthen due to a slow shift in
the particle arrangement under shear stress. Humidity also leads to
strengthening, but is found not to be its sole cause. In addition to these time
dependent effects, the static friction coefficient can also be increased by
compaction of the granular material under some circumstances, and by cycling of
the applied shear stress.Comment: 21 pages, 11 figures, submitted to Phys. Rev.
Dynamics of soap bubble bursting and its implications to volcano acoustics
In order to assess the physical mechanisms at stake when giant gas bubbles
burst at the top of a magma conduit, laboratory experiments have been
performed. An overpressurized gas cavity is initially closed by a thin liquid
film, which suddenly bursts. The acoustic signal produced by the bursting is
investigated. The key result is that the amplitude and energy of the acoustic
signal strongly depend on the film rupture time. As the rupture time is
uncontrolled in the experiments and in the field, the measurement of the
acoustic excess pressure in the atmosphere, alone, cannot provide any
information on the overpressure inside the bubble before explosion. This could
explain the low energy partitioning between infrasound, seismic and explosive
dynamics often observed on volcanoes
Anomalous acoustic reflection on a sliding interface or a shear band
We study the reflection of an acoustic plane wave from a steadily sliding
planar interface with velocity strengthening friction or a shear band in a
confined granular medium. The corresponding acoustic impedance is utterly
different from that of the static interface. In particular, the system being
open, the energy of an in-plane polarized wave is no longer conserved, the work
of the external pulling force being partitioned between frictional dissipation
and gain (of either sign) of coherent acoustic energy. Large values of the
friction coefficient favor energy gain, while velocity strengthening tends to
suppress it. An interface with infinite elastic contrast (one rigid medium) and
V-independent (Coulomb) friction exhibits spontaneous acoustic emission, as
already shown by M. Nosonovsky and G.G. Adams (Int. J. Ing. Sci., {\bf 39},
1257 (2001)). But this pathology is cured by any finite elastic contrast, or by
a moderately large V-strengthening of friction.
We show that (i) positive gain should be observable for rough-on-flat
multicontact interfaces (ii) a sliding shear band in a granular medium should
give rise to sizeable reflection, which opens a promising possibility for the
detection of shear localization.Comment: 13 pages, 10 figure
Rheology of a confined granular material
We study the rheology of a granular material slowly driven in a confined
geometry. The motion is characterized by a steady sliding with a resistance
force increasing with the driving velocity and the surrounding relative
humidity. For lower driving velocities a transition to stick-slip motion
occurs, exhibiting a blocking enhancement whith decreasing velocity. We propose
a model to explain this behavior pointing out the leading role of friction
properties between the grains and the container's boundary.Comment: 9 pages, 3 .eps figures, submitted to PR
Wet Granular Materials
Most studies on granular physics have focused on dry granular media, with no
liquids between the grains. However, in geology and many real world
applications (e.g., food processing, pharmaceuticals, ceramics, civil
engineering, constructions, and many industrial applications), liquid is
present between the grains. This produces inter-grain cohesion and drastically
modifies the mechanical properties of the granular media (e.g., the surface
angle can be larger than 90 degrees). Here we present a review of the
mechanical properties of wet granular media, with particular emphasis on the
effect of cohesion. We also list several open problems that might motivate
future studies in this exciting but mostly unexplored field.Comment: review article, accepted for publication in Advances in Physics;
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