Nicolas Taberlet

On axial segregation in a tumbler: an experimental and numerical study

We report experimental and discrete element method (DEM) simulation results on size segregation of granular materials in a rotating drum. We first define the degree of segregation which allows us to quantify the time evolution of the formation of axial segregation bands. We then focus on the interplay between axial and radial segregation. In experiments, surface particle velocities are measured by particle tracking. There is no significant jump in velocity or velocity gradient at the interface between small to large grains in the radial direction. On the other hand, there is a large difference in flow velocity between large and small particles along the axial direction of the drum, and strong gradients at interfaces between large and small particle bands. Measurements of the diffusion of a localized axial band of small grains show the formation of a radial core from an axial band. This process is not sub-diffusive (nor super-diffusive) in our simulation. The time evolution of the axial segregation pattern is strongly affected by radial segregation. Coarsening and oscillatory motion of axial bands involve flows of small particles through the radial core, in particular when the band position or width oscillates. Coarsening of the axial segregation pattern is significantly retarded or eliminated in the absence of a radial core.

Diffusion of a granular pulse in a rotating drum.

The diffusion of a pulse of small grains in an horizontal rotating drum is studied through discrete elements methods simulations. We present a theoretical analysis of the diffusion process in a one-dimensional confined space in order to elucidate the effect of the confining end-plate of the drum. We then show that the diffusion is neither subdiffusive nor superdiffusive but normal. This is demonstrated by rescaling the concentration profiles obtained at various stages and by studying the time evolution of the mean squared deviation. Finally we study the self-diffusion of both large and small grains, and we show that it is normal and that the diffusion coefficient is independent of the grain size.

Understanding the dynamics of segregation bands of simulated granular material in a rotating drum

Axial segregation of a binary mixture of grains in a rotating drum is studied using Molecular Dynamics (MD) simulations. A force scheme leading to a constant restitution coefficient is used and shows that axial segregation is possible between two species of grains made of identical material differing by size. Oscillatory motion of bands is investigated and the influence of the frictional properties elucidated. The mechanism of bands merging is explained using direct imaging of individual grains.

Melting studies of indium: determination of the structure and density of melts at high pressures and high temperatures

The melting behaviour, structure, and density of molten indium at high pressures have been studied in an externally heated diamond anvil cell (DAC) using x-ray diffraction/scattering measurements. Melting at high pressure was identified by the appearance of diffuse scattering from the melt. Analysis of the diffuse scattering shows that at 710(3) K the coordination number at the nearest neighbour increases from 10.1(4) at 1.0 GPa to 12.1(5) at 6.3 GPa. A method for measuring the density of amorphous materials is introduced for DAC studies and the first result on molten indium is presented.

Time dependence in large amplitude oscillatory shear: A rheo-ultrasonic study of fatigue dynamics in a colloidal gel

We report on the response of a yield stress material, namely, a colloidal gel made of attractive carbon black particles, submitted to large amplitude oscillatory shear stress (LAOStress) in a Couette geometry. At a constant stress amplitude well below its apparent yield stress, the gel displays fatigue and progressively turns from an elastic solid to a viscous fluid. The time-resolved analysis of the strain response, of the Fourier components, and of Lissajous plots allows one to define two different timescales τw<τf associated with the yielding and fluidization of the gel. Coupling rheology to ultrasonic imaging further leads to a local picture of the LAOStress response in which the gel first fails at the walls at τw and then undergoes a slow heterogeneous fluidization involving solid–fluid coexistence until the whole sample is fluid at τf. Spatial heterogeneities are observed in both the gradient and vorticity directions and suggest a fragmentation of the initially solidlike gel into macroscopic domains...

Washboard Road: The Dynamics of Granular Ripples Formed by Rolling Wheels

We report laboratory experiments on rippled granular surfaces formed under rolling wheels. Ripples appear above a critical speed and drift slowly in the driving direction. Ripples coarsen as they saturate and exhibit ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft-particle simulations in which a disk rolls over smaller disks in a periodic box. The simulations show that compaction and segregation are inessential to the ripple phenomenon. We describe a simplified scaling model which gives some insight into the mechanism of the instability.

Two- and three-dimensional confined granular chute flows: experimental and numerical results

We present experimental and numerical results on 2D and 3D confined granular chute flows. We address the issue of the role of the lateral boundaries. In particular, we find that the presence of flat frictional lateral walls greatly alters the flow features as soon as the width of the flowing layer is of the order of the spacing between the walls or greater. First, steady and fully developed (SFD) flows are observed up to very large inclination angles where accelerated flows would have been expected. Second, at given inclination angle, there exists an upper bound on the flow rate for SFD flows to occur. When one approaches this critical flow rate, a static heap forms along the chute base, on which is the flowing layer. The heap is stabilized by the flow atop it and was named a sidewall-stabilized heap (SSH) since its angle is much greater than those usually exhibited by granular heaps. Both kinds of flow have been studied in 2D and 3D confined configurations. In particular, it is found that these flows exhibit either a Bagnold velocity profile or an exponential one. Moreover, we identify a dimensionless parameter, depending crucially on the sidewall friction, that is expected to drive the transition between these two regimes. We also point out the differences between purely 2D flows and 3D confined flows.

The effect of sidewall friction on dense granular flows

We present novel results on flow properties using molecular dynamics simulation as well as @c-densitometry experiments. We show that fundamentally different types of granular flow can be observed in one unique system. Using both frictional and frictionless confining walls, we show that the differences existing between these regimes originate in the frictional properties of the walls. The salient features of each regime are described in detail. In particular, the packing fraction and streamwise velocity profiles are shown to hinge on the frictional properties of the confining walls. Finally, we propose a dimensionless number, independent of the grain size, which helps to delineate the different regimes.

Flow instabilities in large amplitude oscillatory shear: a cautionary tale

In recent years, large amplitude oscillatory shear (LAOS) has become a prime tool to investigate the nonlinear rheology of complex fluids. More specifically, most studies use LAOS data as a macroscopic probe for hypothetical microscopic changes, sometimes successfully. Nevertheless, we would like to raise awareness on the potential impact of secondary flows triggered by instabilities in LAOS performed in curved shear flows exemplified by the Taylor–Couette geometry. First, we show that even for Newtonian fluids, where no micro-structural changes are expected, complex flow patterns can emerge in the LAOS data. Second, we stress the potential impact of similar effects in complex fluids by studying LAOS flows of a well-known shear-banding surfactant solution. Our hope is that our thorough study of the Newtonian case together with preliminary experiments on a complex fluid will reinforce the already successful analogy existing between inertial and elastic instabilities, suggest caution for future LAOS studies focused solely on a structural perspective, and open new research avenues combining flow instabilities and unsteady flows in complex fluids.

Lift and drag forces on an inclined plow moving over a granular surface.

We studied the drag and lift forces acting on an inclined plate while it is dragged on the surface of a granular media, both in experiment and in numerical simulation. In particular, we investigated the influence of the horizontal velocity of the plate and its angle of attack. We show that a steady wedge of grains is moved in front of the plow and that the lift and drag forces are proportional to the weight of this wedge. These constants of proportionality vary with the angle of attack but not (or only weakly) on the velocity. We found a universal effective friction law that accounts for the dependence on all the above-mentioned parameters. The stress and velocity fields are calculated from the numerical simulations and show the existence of a shear band under the wedge and that the pressure is nonhydrostatic. The strongest gradients in stress and shear occur at the base of the plow where the dissipation rate is therefore highest.