Normal impact of a ball rotating around its linear velocity.

We report on an experimental study of the normal impact on a solid surface of a table tennis ball that rotates around its linear velocity vector. We observe that the ratio of the reflected velocity to the incident velocity does not depend on the initial spin. In contrast, the reflected spin depends not only on the incident spin but also on the incident velocity.

Oblique impact of a buckling table-tennis ball on a rigid surface.

We report on the rebound of a table-tennis ball impinging without any initial spin in oblique incidence on a rigid surface. We show that, below a critical incidence angle, the ball rolls without sliding when bouncing back from the surface. In that case, the reflected angular velocity acquired by the ball can be predicted without any knowledge of the properties of the contact between the ball and the solid surface.

Dynamical buckling of a table-tennis ball impinging normally on a rigid target: Experimental and numerical studies.

We report on the dynamical buckling of a spherical shell (a table-tennis ball) impinging in normal incidence on a rigid surface (a glass plate). Experimentally, we observe and decipher the geometrical characteristics of the shell profile in the contact region along with global metrics such as the contact duration and the coefficient of restitution of the linear velocity. We determine, in particular, the onset of the ball buckling instability.

Sediment motion induced by Faraday waves in a Hele-Shaw cell.

The interaction between the oscillatory boundary-layer flow induced by Faraday waves and a sedimentary granular layer was studied in a Hele-Shaw cell vertically vibrated. The experimental parameters were the vibration frequency f and acceleration a and the particle diameter d_{p}. At a critical value for the depth of the supernatant fluid layer Δh_{c}, a transition between a flat motionless granular layer and a second regime in which the granular layer undulates and oscillates periodically was observed.

Nonlinear denoising for characterization of solid friction under low confinement pressure.

The present work investigates paper-paper friction dynamics by pulling a slider over a substrate. It focuses on the transition between stick-slip and inertial regimes. Although the device is classical, probing solid friction with the fewest contact damage requires that the applied load should be small.

Faraday waves over a permeable rough substrate.

We report on an experimental study of the Faraday instability in a vibrated fluid layer situated over a permeable and rough substrate, consisting either of a flat solid plate or of woven meshes having different openings and wire diameters, open or closed (by a sealing paint). We measure the critical acceleration and the wavelength (on the images from top) at the onset of the instability for vibration frequencies between 28 and 42 Hz. We observe that, in comparison with the flat plate, a mesh leads to an increase of the critical acceleration, whereas the wavelength is not significantly altered in none of the explored cases.

Patterning of a cohesionless granular layer under pure shear.

The response of a thin layer of granular material to an external pure shear imposed at its base is investigated. The experiments show that, even for noncohesive materials, the resulting deformation of the material is inhomogeneous. Indeed, a novel smooth pattern, consisting of a periodic modulation of the shear deformation of the free surface, is revealed by an image-correlation technique.

Avalanche precursors in a frictional model.

We present a one-dimensional numerical model based on elastically coupled sliders on a frictional incline of variable tilt. This very simple approach makes it possible to study the precursors to the avalanche and to provide a rationalization of different features that have been observed in experiments. We provide a statistical description of the model leading to master equations describing the state of the system as a function of the angle of inclination.

Gas-induced fluidization of mobile liquid-saturated grains.

Gas invasion in liquid-saturated sands exhibits different morphologies and dynamics. For mobile beds, the repeated rise of gas through the layer leads to the growth of a fluidized zone, which reaches a stationary shape. Here, we present experimental results characterizing the evolution of the fluidized region as a function of the gas-flow rate and grain size.

Granular friction: Triggering large events with small vibrations.

Triggering large-scale motion by imposing vibrations to a system can be encountered in many situations, from daily-life shaking of saltcellar to silo unclogging or dynamic earthquakes triggering. In the well-known situation of solid or granular friction, the acceleration of imposed vibrations has often been proposed as the governing parameter for the transition between stick-slip motion and continuous sliding. The threshold acceleration for the onset of continuous slip motion or system unjamming is usually found of the order of the gravitational acceleration.

On-and-off dynamics of a creeping frictional system.

We report on the dynamics of a model frictional system submitted to minute external perturbations. The system consists of a chain of sliders connected through elastic springs that rest on an incline. By introducing cyclic expansions and contractions of the rest length of the springs, we induce the reptation of the chain.

Interstitial gas effect on vibrated granular columns.

Vibrated granular materials have been intensively used to investigate particle segregation, convection, and heaping. We report on the behavior of a column of heavy grains bouncing on an oscillating solid surface. Measurements indicate that, for weak effects of the interstitial gas, the temporal variations of the pressure at the base of the column are satisfactorily described by considering that the column, despite the observed dilation, behaves like a porous solid.

Intrinsic creep of a granular column subjected to temperature changes.

Minute external perturbations, such as temperature variations, can lead to a creep of the fragile structures that are the granular piles. We report a study, resolved in space and time, of the dynamics associated with the slow compaction of a granular column submitted to thermal cycles. Avoiding the thermal dilations of the container, we observe that the material still creeps, even in the absence of external mechanical perturbations.

Oscillating gas flow induces reptation of granular droplets.

We report on the reptation of vertically vibrated droplets of fine particles lying on a solid incline. On the one hand, time-resolved measurements show that the gas pressure in the gap between the droplet bottom and the solid surface can be accounted for by a Darcy law. The cumulative effect of the viscous drag is responsible for the droplet formation.

Effect of cohesion and shear modulus on the stability of a stretched granular layer.

The main mechanism of the cellular pattern which forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching has been identified as the "strain softening" arising from the features of grain-grain interactions. We perform measurements of the strain field associated with such structures by using a correlation image technique and additionally characterize the cohesion and shear modulus of the samples. We show that for high cohesion, the layer is fragile and the surface deformation is highly nonlinear, whereas at low cohesion, a smooth and linearly growing structure is observed as a function of external stretching.

Evolution of pressure profiles during the discharge of a silo.

We report measurements of the pressure profile in the outlet plane of a discharging silo. We observe that, whatever the preparation of the granular system, a dynamic Janssen effect is at play: the apparent mass of the grains (i.e.

Flexural fracturing of a cohesive granular layer.

We report on the fracturing of cohesive granular materials subjected to a flexural deformation. A thin layer of glass beads or of flour is deposited on an unstretchable membrane to which flexion is imposed. We observe the formation of a periodic fracturing pattern whose characteristics are discussed in comparison with results previously obtained for an in-plane stretching [Alarcon, Ramos, Vanel, Vittoz, Melo, and Géminard, Phys.

Degassing cascades in a shear-thinning viscoelastic fluid.

We report the experimental study of the degassing dynamics through a thin layer of shear-thinning viscoelastic fluid when a constant air flow is imposed at its bottom. The fluid is an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). Over a large range of parameters, the air is periodically released through a series of successive bubbles, hereafter named cascades.

Creep motion of a model frictional system.

We report on the dynamics of a model frictional system submitted to minute external perturbations. The system consists of a chain of sliders connected through elastic springs that rest on an incline. By introducing cyclic expansions and contractions of the springs we observe a reptation of the chain.

Granular flow through an aperture: pressure and flow rate are independent.

We simultaneously measure the flow rate and the normal force on the base, near the outlet, during the discharge through an orifice of a dense packing of monosized disks driven by a conveyor belt. We find that the normal force on the base decreases even when a constant flow rate is measured. In addition, we show, by changing the mass of the disks, that pressure can be changed while the flow rate remains constant.

Morphology of air invasion in an immersed granular layer.

We report a study of the paths formed by a finite volume of air gently injected at the base of an immersed granular material. A two-dimensional model, based on experimental observations, shows that the typical height and width of the region explored by the branched path depends not only on the injected volume V, but also on a dimensionless parameter χ which accounts for the relative effects of the gravity and capillarity. For a given injected volume V, larger gravity effects lead to taller and narrower structures; for a given χ, the typical height and width of the structure scale like V(1/2) and V(1/4), respectively, while the typical gaseous fraction in the corresponding region increases accordingly like V(1/4).

Podosome rings generate forces that drive saltatory osteoclast migration.

Podosomes are dynamic, actin-containing adhesion structures that collectively self-organize as rings. In this study, we first show by observing osteoclasts plated on bead-seeded soft substrates that podosome assemblies, such as rings, are involved in tension forces. During the expansion of a podosome ring, substrate displacement is oriented outward, suggesting that podosomal structures push the substrate away.

Venting dynamics of an immersed granular layer.

Air is injected locally at the base of an immersed granular bed. The gas, which is forced to flow gently through the material, creates several paths between the grains. We observe that the latter gas venting results in the emission of bubbles in a localized region at the free surface.

Softening induced instability of a stretched cohesive granular layer.

We report on a cellular pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching. We present a simple model in which the mechanism responsible of the instability is the "strain softening" exhibited by humid granular materials above a typical strain. Our analysis indicates that such a type of instability should be observed in any system presenting a negative stress sensitivity to strain perturbations.

Aging of the frictional properties induced by temperature variations.

The dry frictional contact between two solid surfaces is well known to obey Coulomb friction laws. In particular, the static friction force resisting the relative lateral (tangential) motion of solid surfaces, initially at rest, is known to be proportional to the normal force and independent of the area of the macroscopic surfaces in contact. Experimentally, the static friction force has been observed to slightly depend on time.