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; tex-style change