182 research outputs found
Propagating Waves in a Monolayer of Gas-Fluidized Rods
We report on an observation of propagating compression waves in a
quasi-two-dimensional monolayer of apolar granular rods fluidized by an upflow
of air. The collective wave speed is an order of magnitude faster than the
speed of the particles. This gives rise to anomalously large number
fluctuations dN ~ , which are greater than ordinary number
fluctuations of N^{1/2}. We characterize the waves by calculating the
spatiotemporal power spectrum of the density. The position of observed peaks,
as a function of frequency w and wavevector k, yields a linear dispersion
relationship in the long-time, long-wavelength limit and a wavespeed c = w/k.
Repeating this analysis for systems at different densities and air speeds, we
observe a linear increase in the wavespeed with increasing packing fraction
with no dependence on the airflow. Although air-fluidized rods self-propel
individually or in dilute collections, the parallel and perpendicular
root-mean-square speeds of the rods indicate that they no longer self-propel
when propagating waves are present. Based on this mutual exclusivity, we map
out the phase behavior for the existence of waves vs self-propulsion as a
function of density and fluidizing airflow
Core Precession and Global Modes in Granular Bulk Flow
A transition from local to global shear zones is reported for granular flows
in a modified Couette cell. The experimental geometry is a slowly rotating drum
which has a stationary disc of radius R_s fixed at its bottom. Granular
material, which fills this cell up to height H, forms a wide shear zone which
emanates from the discontinuity at the stationary discs edge. For shallow
layers (H/R_s < 0.55), the shear zone reaches the free surface, with the core
of the material resting on the disc and remaining stationary. In contrast, for
deep layers (H/R_s > 0.55), the shear zones meet below the surface and the core
starts to precess. A change in the symmetry of the surface velocities reveals
that this behavior is associated with a transition from a local to a global
shear mode.Comment: 4 pages, 7 figures, submitte
Flow-induced Agitations Create a Granular Fluid
We fluidize a granular medium through localized stirring and probe the
mechanical response of quiescent regions far away from the main flow. In these
regions the material behaves like a liquid: high-density probes sink,
low-density probes float at the depth given by Archimedes' law, and drag forces
on moving probes scale linearly with the velocity. The fluid-like character of
the material is set by agitations generated in the stirred region, suggesting a
non-local rheology: the relation between applied stress and observed strain
rate in one location depends on the strain rate in another location
Solid-fluid transition in a granular shear flow
The rheology of a granular shear flow is studied in a quasi-2d rotating
cylinder. Measurements are carried out near the midpoint along the length of
the surface flowing layer where the flow is steady and non-accelerating.
Streakline photography and image analysis are used to obtain particle
velocities and positions. Different particle sizes and rotational speeds are
considered. We find a sharp transition in the apparent viscosity ()
variation with rms velocity (). In the fluid-like region above the depth
corresponding to the transition point (higher rms velocities) there is a rapid
increase in viscosity with decreasing rms velocity. Below the transition depth
we find for all the different cases studied and the
material approaches an amorphous solid-like state deep in the layer. The
velocity distribution is Maxwellian above the transition point and a Poisson
velocity distribution is obtained deep in the layer. The observed transition
appears to be analogous to a glass transition.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Finite width of quasi-static shear bands
I study the average deformation rate of an amorphous material submitted to an
external uniform shear strain rate, in the geometry known as the split-bottom
configuration. The material is described using a stochastic model of plasticity
at a mesoscopic scale. A shear band is observed to start at the split point at
the bottom, and widen progressively towards the surface. In a two-dimensional
geometry the average statistical properties of the shear band look similar to
those of the directed polymer model. In particular, the surface width of the
shear band is found to scale with the system height H as H^q with q=0.68 +/-
0.02. In more realistic three dimensional simulations the exponent changes to
q=0.60 +/- 0.02 and the bulk profile of the width of the shear band is closer
to a quarter of circle, as it was observed to be the case in recent simulations
of granular materials.Comment: 10 pages, 10 figure
Force Chains, Microelasticity and Macroelasticity
It has been claimed that quasistatic granular materials, as well as nanoscale
materials, exhibit departures from elasticity even at small loadings. It is
demonstrated, using 2D and 3D models with interparticle harmonic interactions,
that such departures are expected at small scales [below O(100) particle
diameters], at which continuum elasticity is invalid, and vanish at large
scales. The models exhibit force chains on small scales, and force and stress
distributions which agree with experimental findings. Effects of anisotropy,
disorder and boundary conditions are discussed as well.Comment: 4 pages, 11 figures, RevTeX 4, revised and resubmitted to Phys. Rev.
Let
Continuum approach to wide shear zones in quasi-static granular matter
Slow and dense granular flows often exhibit narrow shear bands, making them
ill-suited for a continuum description. However, smooth granular flows have
been shown to occur in specific geometries such as linear shear in the absence
of gravity, slow inclined plane flows and, recently, flows in split-bottom
Couette geometries. The wide shear regions in these systems should be amenable
to a continuum description, and the theoretical challenge lies in finding
constitutive relations between the internal stresses and the flow field. We
propose a set of testable constitutive assumptions, including
rate-independence, and investigate the additional restrictions on the
constitutive relations imposed by the flow geometries. The wide shear layers in
the highly symmetric linear shear and inclined plane flows are consistent with
the simple constitutive assumption that, in analogy with solid friction, the
effective-friction coefficient (ratio between shear and normal stresses) is a
constant. However, this standard picture of granular flows is shown to be
inconsistent with flows in the less symmetric split-bottom geometry - here the
effective friction coefficient must vary throughout the shear zone, or else the
shear zone localizes. We suggest that a subtle dependence of the
effective-friction coefficient on the orientation of the sliding layers with
respect to the bulk force is crucial for the understanding of slow granular
flows.Comment: 11 pages, 7 figure
Diffusion and mixing in gravity-driven dense granular flows
We study the transport properties of particles draining from a silo using
imaging and direct particle tracking. The particle displacements show a
universal transition from super-diffusion to normal diffusion, as a function of
the distance fallen, independent of the flow speed. In the super-diffusive (but
sub-ballistic) regime, which occurs before a particle falls through its
diameter, the displacements have fat-tailed and anisotropic distributions. In
the diffusive regime, we observe very slow cage breaking and Peclet numbers of
order 100, contrary to the only previous microscopic model (based on diffusing
voids). Overall, our experiments show that diffusion and mixing are dominated
by geometry, consistent with fluctuating contact networks but not thermal
collisions, as in normal fluids
Phase transition in a static granular system
We find that a column of glass beads exhibits a well-defined transition
between two phases that differ in their resistance to shear. Pulses of
fluidization are used to prepare static states with well-defined particle
volume fractions in the range 0.57-0.63. The resistance to shear is
determined by slowly inserting a rod into the column of beads. The transition
occurs at for a range of speeds of the rod.Comment: 4 pages, 4 figures. The paper is significantly extended, including
new dat
Two scenarios for avalanche dynamics in inclined granular layers
We report experimental measurements of avalanche behavior of thin granular
layers on an inclined plane for low volume flow rate. The dynamical properties
of avalanches were quantitatively and qualitatively different for smooth glass
beads compared to irregular granular materials such as sand. Two scenarios for
granular avalanches on an incline are identified and a theoretical explanation
for these different scenarios is developed based on a depth-averaged approach
that takes into account the differing rheologies of the granular materials.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
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