The weight of a falling chain, revisited

A vertically hanging chain is released from rest and falls due to gravity on a scale pan. We discuss the various experimental and theoretical aspects of this classic problem. Careful time-resolved force measurements allow us to determine the differences between the idealized and its implementation in the laboratory problem. We observe that, in spite of the upward force exerted by the pan on the chain, the free end at the top falls faster than a freely falling body. Because a real chain exhibits a finite minimum radius of curvature, the contact at the bottom results in a tensional force which pulls the falling part downward

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. Such an aging phenomenon has been accounted for either by the creep of the material or by the condensation of water bridges at the microscopic contacts points. Studying a toy-model, we show that the small uncontrolled temperature changes of the system can also lead to a significant increase of the static friction force.Comment: 8 pages, 5 figures, final version, to appear in Phys. Rev.

INSTABILITY REVEALS CLUSTERING IN COHESIVE GRANULAR MATTER

International audienceWe lately reported on a periodic pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching or to bending. The mechanism responsible for the instability is the strain softening exhibited by humid granular materials above a typical strain. The present short article gives the opportunity to review the potential physical origin of the cohesion in granular materials and, then, to discuss one experimental example which points out the fundamental role played by the clustering in the mechanical response of the cohesive materials

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

Friction and dilatancy in immersed granular matter.

The friction of a sliding plate on a thin immersed granular layer obeys Amonton-Coulomb law. We bring to the fore a large set of experimental results which indicate that, over a few decades of values, the effective dynamical friction-coefficient depends neither on the viscosity of the interstitial fluid nor on the size of beads in the sheared layer, which bears out the analogy with the solid-solid friction in a wide range of experimental parameters. We accurately determine the granular-layer dilatancy, which dependance on the grain size and slider velocity can be qualitatively accounted by considering the rheological behaviour of the whole slurry. However, additional results, obtained after modification of the grain surface by a chemical treatment, demonstrate that the theoretical description of the flow properties of dense granular matter, even immersed, requires the detailed properties of the grain surface to be taken into account

Intrinsic creep of a granular column subjected to temperature changes.

International audienceMinute external pertubations, 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 absence of external mechanical perturbations. The latter intrinsic creep of the material exhibits several surprizing features: the compaction of the material is not homogeneous along the column height and sudden collapses involve the entire system even if one could expect arches to screen the mechanical interaction between regions far apart one from another. In addition, the analysis of the dependence of the flow-rate on the frequency of the temperature cycles suggests that the creep is mainly induced by the propagation of the temperature pro file, slow temperature changes being less effi cient than rapid ones in making the material flow

Frictional properties of bidisperse granular matter: Effect of mixing ratio

International audienceThe frictional response of granular binary mixtures to an applied shear stress is studied experimentally by sliding a rough plate across a granular surface. The static friction force is found to be up to 25% larger than a linear interpolation between the frictional properties of each component. The dynamical friction coefficient can exhibit a maximum, a minimum, or an oscillatory behavior as a function of mixing ratio, depending on the size ratio or shape of the two components. In addition, visualization of the granular flow makes it possible to show that the shear layer thickness and the characteristic shear displacement, over which a steady state dilation is reached, change linearly with the mass concentration

Energy of a single bead bouncing on a vibrating plate: Experiments and numerical simulations

International audienceThe energy of a single bead bouncing on a vibrating plate is determined in simulations and experiments by tracking the bead-plate collision times. The plate oscillates sinusoidally along the vertical with the dimension-less peak acceleration ⌫, and the bead-plate collisions are characterized by the velocity restitution coefficient ⑀. Above the threshold dimensionless peak acceleration ⌫ s Ӎ0.85, which does not depend on the restitution coefficient, the bead energy is shown to initially increase linearly with the vibration amplitude A, whereas it is found to scale like v p 2 /(1Ϫ⑀), where v p is the peak velocity of the plate, only in the limit ⌫ӷ⌫ s. The threshold ⌫ s is shown to decrease when the bead is subjected, in simulations, to additional nondissipative collisions occurring with the typical frequency ␯ c. As a consequence, the bead energy scales like v p 2 /(1Ϫ⑀) for all vibration strengths in the limit ␯ c ӷ␯ c *. From the experimental and numerical findings, an analytical expression of the bead energy as a function of the experimental parameters is proposed. As they exhibit a wide range of unusual behaviors, granu-lar materials are the subject of intensive investigations ͓1–3͔. Because of the inelastic nature of the contacts between the grains, these systems are intrinsically dissipative. In order to explore experimentally the effects of the dissipation on the properties of granular systems, it is convenient to produce stationary states; they are achieved by continuously providing the system with energy, compensating the energy intrin-sically lost when the grains are in motion. Among these studies , one can note the experimental realization of two-dimensional (2D) granular gases, consisting of inelastic beads constituting less than one-layer coverage on a vertically shaken, horizontal plate ͓4 –8͔ ͑the experimental situation has been the framework of molecular-dynamics simulations by Nie et al. ͓9͔͒. The velocity distributions, granular temperature, pressure, as well as phase transitions have been studied as functions of the vibration strength, usually characterized by the peak plate acceleration ⌫. Nevertheless, detailed analysis of the clustering transition and of the pressure indicates that the vibrating boundary becomes inefficient to thermalize the system when the acceleration or the density of the gas are decreased. The energy input by the vibrating boundary has been the subject of several theoretical studies ͓10–13͔; the scaling law for the energy as a function of the vibration strength has been shown to depend on the shape of the boundary vibration ͑sinusoidal, sawtooth, etc.͒ and on the nature of the dissipation within the gas ͑viscous, inelastic͒. In these studies, the assumption was made that the bead impinges randomly on the boundary; we note that this is not the case when the bead collides more than once with the boundary between two collisions with another bead. One can easily show that the velocity is exponentially correlated between two successive collisions in this case. It is hence relevant to study the dynamical behavior of a single bead bouncing on a vibrating plate, and to determine the mean energy ͗E͘ of th

Fracture path in an anisotropic material in the light of a friction experiment

A slider is pulled by means of a flexible link on a flat solid surface which exhibits anisotropic frictional properties. The resulting trajectory of the slider is assessed experimentally. First, we check that the experimental results are in excellent agreement with a theoretical description of the problem based on an expression of the frictional forces. Second, we point out that the trajectory of the slider can be recovered by the use of a "maximum of energy release rate" criterion which is generally used to predict the path of a fracture even if the validity of the principle is difficult to verify in the latter complex systems