148 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 ~ $N^{0.72 \pm 0.04}$, 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

### 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 ($\eta$)
variation with rms velocity ($u$). 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 $\eta \propto u^{-1.5}$ 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

### 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

### Unjamming due to local perturbations in granular packings with and without gravity

We investigate the unjamming response of disordered packings of frictional
hard disks with the help of computer simulations. First, we generate jammed
configurations of the disks and then force them to move again by local
perturbations. We study the spatial distribution of the stress and displacement
response and find long range effects of the perturbation in both cases. We
record the penetration depth of the displacements and the critical force that
is needed to make the system yield. These quantities are tested in two types of
systems: in ideal homogeneous packings in zero gravity and in packings settled
under gravity. The penetration depth and the critical force are sensitive to
the interparticle friction coefficient. Qualitatively, the same nonmonotonic
friction dependence is found both with and without gravity, however the
location of the extrema are at different friction values. We discuss the role
of the connectivity of the contact network and of the pressure gradient in the
unjamming response.Comment: 12 pages, 13 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 $\phi$ 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 $\phi=0.60$ for a range of speeds of the rod.Comment: 4 pages, 4 figures. The paper is significantly extended, including
new dat

### Universal and wide shear zones in granular bulk flow

We present experiments on slow granular flows in a modified (split-bottomed)
Couette geometry in which wide and tunable shear zones are created away from
the sidewalls. For increasing layer heights, the zones grow wider (apparently
without bound) and evolve towards the inner cylinder according to a simple,
particle-independent scaling law. After rescaling, the velocity profiles across
the zones fall onto a universal master curve given by an error function. We
study the shear zones also inside the material as function of both their local
height and the total layer height.Comment: Minor corrections, accepted for PRL (4 pages, 6 figures

### How Sandcastles Fall

Capillary forces significantly affect the stability of sandpiles. We analyze
the stability of sandpiles with such forces, and find that the critical angle
is unchanged in the limit of an infinitely large system; however, this angle is
increased for finite-sized systems. The failure occurs in the bulk of the
sandpile rather than at the surface. This is related to a standard result in
soil mechanics. The increase in the critical angle is determined by the surface
roughness of the particles, and exhibits three regimes as a function of the
added-fluid volume. Our theory is in qualitative agreement with the recent
experimental results of Hornbaker et al., although not with the interpretation
they make of these results.Comment: 4 pages, 2 figures, revte

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