328 research outputs found
An experimental investigation of the force network ensemble
We present an experiment in which a horizontal quasi-2D granular system with
a fixed neighbor network is cyclically compressed and decompressed over 1000
cycles. We remove basal friction by floating the particles on a thin air
cushion, so that particles only interact in-plane. As expected for a granular
system, the applied load is not distributed uniformly, but is instead
concentrated in force chains which form a network throughout the system. To
visualize the structure of these networks, we use particles made from
photoelastic material. The experimental setup and a new data-processing
pipeline allow us to map out the evolution subject to the cyclic compressions.
We characterize several statistical properties of the packing, including the
probability density function of the contact force, and compare them with
theoretical and numerical predictions from the force network ensemble theory.Comment: accepted for publication in the conference proceedings of Powders and
Grains 201
Statistics of defect motion in spatiotemporal chaos in inclined layer convection
We report experiments on defect-tracking in the state of undulation chaos
observed in thermal convection of an inclined fluid layer. We characterize the
ensemble of defect trajectories according to their velocities, relative
positions, diffusion, and gain and loss rates. In particular, the defects
exhibit incidents of rapid transverse motion which result in power law
distributions for a number of quantitative measures. We examine connections
between this behavior and L\'evy flights and anomalous diffusion. In addition,
we describe time-reversal and system size invariance for defect creation and
annihilation rates.Comment: (21 pages, 17 figures
Sound propagation and force chains in granular materials
Granular materials are inherently heterogeneous, leading to challenges in
formulating accurate models of sound propagation. In order to quantify acoustic
responses in space and time, we perform experiments in a photoelastic granular
material in which the internal stress pattern (in the form of force chains) is
visible. We utilize two complementary methods, high-speed imaging and
piezoelectric transduction, to provide particle-scale measurements of both the
amplitude and speed of an acoustic wave in the near-field regime. We observe
that the wave amplitude is on average largest within particles experiencing the
largest forces, particularly in those chains radiating away from the source,
with the force-dependence of this amplitude in qualitative agreement with a
simple Hertzian-like model of particle contact area. In addition, we are able
to directly observe rare transient force chains formed by the opening and
closing of contacts during propagation. The speed of the leading edge of the
pulse is in quantitative agreement with predictions for one-dimensional chains,
while the slower speed of the peak response suggests that it contains waves
which have travelled over multiple paths even within just this near-field
region. These effects highlight the importance of particle-scale behaviors in
determining the acoustical properties of granular materials
Defect turbulence and generalized statistical mechanics
We present experimental evidence that the motion of point defects in thermal
convection patterns in an inclined fluid layer is well-described by Tsallis
statistics with an entropic index . The dynamical properties of
the defects (anomalous diffusion, shape of velocity distributions, power law
decay of correlations) are in good agreement with typical predictions of
nonextensive models, over a range of driving parameters
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