On the existence of stationary states during granular compaction.

When submitted to gentle mechanical taps a granular packing slowly compacts until it reaches a stationary state that depends on the tap characteristics. The properties of such stationary states are experimentally investigated. The influence of the initial state, taps properties and tapping protocol are studied. The compactivity of the packings is determinated. Our results strongly support the idea that the stationary states are genuine thermodynamic states.

Experimental compaction of anisotropic granular media.

We report on experiments to measure the temporal and spatial evolution of packing arrangements of anisotropic and weakly confined granular material, using high-resolution gamma-ray adsorption. In these experiments, the particle configurations start from an initially disordered, low-packing-fraction state and under vertical solicitations evolve to a dense state. We find that the packing fraction evolution is slowed by the grain anisotropy but, as for spherically shaped grains, can be well fitted by a stretched exponential. For a given type of grains, the characteristic times of relaxation and of convection are found to be of the same order of magnitude. On the contrary, compaction mechanisms in the media strongly depend on the grain anisotropy.

Granular medium under vertical tapping: change of compaction and convection dynamics around the liftoff threshold.

A granular medium submitted to vertical tapping reveals simultaneously compaction and convection. The two phenomena are directly coupled and their dynamics can be quantified by a characteristic compaction time and by an estimation of the convective downhill speed along the wall. A remarkable change of behavior is observed around the liftoff acceleration threshold of the whole packing, with a drastic slowing down of both dynamics below this threshold. Above it, a collective shock wave densifies the packing at each tap, whereas, below it, cumulative localized rearrangements will compact the entire system in the long time range.

Instabilities in slowly driven granular packing.

In this work, a digital imaging technique is used to study the superficial fluctuations observed when a granular packing is slowly driven to the threshold of instability. The experimental results show the presence of three types of events. Small superficial rearrangements of grains are observed during all the experiments. They present a power-law behavior although the system is not in a critical state as predicted by self-organized criticality models. In thick granular piles, large rearrangements are detected at regular angular intervals. They are related to the threshold of instability of the contact network that relaxes to stable configurations producing internal rearrangements of the grains. Finally, an avalanche is triggered when the superficial beads that are set in motion acquire enough momentum to destabilize grains from layers below.

Numerical model for granular compaction under vertical tapping.

A simple numerical model is used to simulate the effect of vertical taps on a packing of monodisperse hard spheres. Our results are in good agreement with an experimental work done in Chicago and with other previous models, especially concerning the dynamics of the compaction, the influence of the excitation strength on the compaction efficiency, and some aging effects. The principal asset of the model is that it allows a local analysis of the packings. Vertical and transverse density profiles are used, as well as size and volume distributions of the pores. An interesting result concerns the appearance of a vertical gradient in the density profiles during compaction. Furthermore, the volume distribution of the pores suggests that the smallest pores, ranging in size between tetrahedral and octahedral sites, are not strongly affected by the tapping process, in contrast to the largest pores which are more sensitive to the compaction of the packing.

Effect of boundary conditions on diffusion in two-dimensional granular gases.

We analyze the influence of boundary conditions on numerical simulations of the diffusive properties of a two-dimensional granular gas. We show in particular that periodic boundary conditions introduce unphysical correlations in time that cause the coefficient of diffusion to be strongly dependent on the system size. On the other hand, in large enough systems with hard walls at the boundaries, diffusion is found to be independent of the system size. We compare the results obtained in this case with Langevin theory for an elastic gas. Good agreement is found. We then calculate the relaxation time and the influence of the mass for a particle of radius R(s) in a sea of particles of radius R(b). As granular gases are dissipative, we also study the influence of an external random force on the diffusion process in a forced dissipative system. In particular, we analyze differences in the mean-square velocity and displacement between the elastic and inelastic cases.

Diffusion as a mixing mechanism in granular materials.

We present several numerical results on granular mixtures. In particular, we examine the efficiency of diffusion as a mixing mechanism in these systems. The collisions are inelastic and to compensate the energy loss, we thermalize the grains by adding a random force. Starting with a segregated system, we show that uniform agitation (heating) leads to a uniform mixture of grains of different sizes. We define a characteristic mixing time tau(mix), and study theoretically and numerically its dependence on other parameters like the density. We examine a model for bidisperse systems for which we can calculate some physical quantities. We also examine the effect of a temperature gradient and demonstrate the appearance of an expected segregation.

Influence of the number of layers on the equilibrium of a granular packing

This paper reports an experimental study on avalanches in a granular material contained in a confined geometry. The granular packing is made of monosize glass beads initially poured into a box that is slowly inclined until an avalanche takes place at a critical angle straight theta(M) (maximum angle of stability). The avalanche involves a decrease of the surface slope until a second critical angle straight theta(r) (angle of repose) is reached. Both angles and the mass displaced out of the box during the avalanche are studied as a function of the height of the granular packing. In order to avoid cohesion effects, experiments are carried out in a humidity controlled environment. For small packings, up to approximately ten layers, the stability of the system is significantly affected by the rough surface at the bottom. In contrast, for thicker systems, critical angles do not depend on the height.

Experimental study of the collision process of a grain on a two-dimensional granular bed

We report an experimental study on the collision of a bead on a two-dimensional hexagonal granular packing. This collision process is of crucial importance in aeolian transport of grains. We have investigated the kinematic properties of the incident bead before and after the collision, and the resulting deformation of the packing. A typical collision is characterized by the rebound of the impacting bead and the ejection of a few beads of the packing. We have shown that the properties of the rebound bead depend weakly on the impact speed and that the rebound process involves only a few bead layers of the packing. On the contrary, the ejection mechanism depends strongly on the impact speed. In particular, it is found that the number of ejected grains increases with the impact speed whereas the most likely value of their energy is practically independent of the impact speed. Furthermore, we have given evidences that the ejection process involves a great number of packing layers and therefore is extremely sensitive to the height of the packing. For small packing heights, one observes additional ejected grains which can be interpreted as being produced by the reflection of the shock wave on the bottom of the pile.

Competition between correlated buoyancy and uncorrelated capillary effects during drainage.

We study drainage in a horizontally oriented rough fracture joint filled with glass beads. The shape and structure of the drained areas is the result of competition between two effects: (1) variations in the capillary thresholds necessary to be overcome in order to drain the pores and (2) the height variations due to the roughness of the fracture joint. These height variations have long range correlations due to the self-affine nature of the fracture. The capillary thresholds are uncorrelated. We tune the relative strength of these two effects by performing experiments in a centrifuge and thus changing the "strength of gravity." As gravity is increased, the structure of the drained areas change from that of invasion percolation to a structure composed of compact blobs linked together by threadlike links. We study both the geometry and the effect of trapping while changing acceleration of gravity from zero to 6g(0). At high centrifugal acceleration we further observe fragmentation, migration and coalescence of bubbles of fluid inside the drained areas.

Motion of grains down a bumpy surface.

We summarize in this article an extensive experimental and theoretical effort carried out to understand the behavior of a single ball when rolling down a bumpy surface. This may appear to be a simple problem but in fact is one that displays a rich variety of different behaviors which allow us to understand better dissipative systems such as granular media. Studies performed previously have shown that the motion of the single ball on the rough surface can be characterized by three different dynamic regimes according to the different values of the two control parameters, the inclination angle theta and the ratio Phi=R/r, where R is the radius of the rolling ball and r the radius of the glass beads which make up the rough surface. The three regimes are a decelerated regime A, a stationary regime B, characterized by a constant average velocity and a jumping regime C. This result was found to be independent of the composition of the rolling ball and the rough surface. It has been demonstrated that regime B is characterized by a viscous-like friction force that appears for specific parameter values. This friction force can be explained by a model whose central ingredient is the geometry of the surface. The trajectory of the ball in regime B can be pictured as a driven random walk motion where the fluctuations of the local velocities are due to collisions of the moving sphere and the surface grains. A detailed analysis of diffusive properties of the motion is discussed. (c) 1999 American Institute of Physics.