69 research outputs found
Dynamical Heterogeneity close to the Jamming Transition in a Sheared Granular Material
The dynamics of a bi-dimensional dense granular packing under cyclic shear is
experimentally investigated close to the jamming transition. Measurement of
multi-point correlation functions are produced. The self-intermediate
scattering function, displaying slower than exponential relaxation, suggests
dynamic heterogeneity. Further analysis of four point correlation functions
reveal that the grain relaxations are strongly correlated and spatially
heterogeneous, especially at the time scale of the collective rearrangements.
Finally, a dynamical correlation length is extracted from spatio-temporal
pattern of mobility. Our experimental results open the way to a systematic
study of dynamic correlation functions in granular materials.Comment: 4 pages, final version accepted for publication in Phys. Rev. Let
Jamming transition of a granular pile below the angle of repose
We study experimentally the relaxation towards mechanical equilibrium of a
granular pile which has just experienced an avalanche and discuss it in the
more general context of the granular jamming transition. Two coexisting
dynamics are observed in the surface layer: a short time exponential decay
consisting in rapid and independent moves of grains and intermittent bursts
consisting in spatially correlated moves lasting for longer time. The
competition of both dynamics results in long-lived intermittent transients, the
total duration of which can late more than a thousand of seconds. We measure a
two-time relaxation function, and relate it via a simple statistical model to a
more usual two-time correlation function which exhibits strong similarities
with auto-correlation functions found in aging systems. Localized perturbation
experiments also allow us to test the pile surface layer receptivity.Comment: 9 pages, 10 figure
Super-diffusion around the rigidity transition: Levy and the Lilliputians
By analyzing the displacement statistics of an assembly of horizontally
vibrated bidisperse frictional grains in the vicinity of the jamming transition
experimentally studied before, we establish that their superdiffusive motion is
a genuine Levy flight, but with `jump' size very small compared to the diameter
of the grains. The vibration induces a broad distribution of jumps that are
random in time, but correlated in space, and that can be interpreted as
micro-crack events at all scales. As the volume fraction departs from the
critical jamming density, this distribution is truncated at a smaller and
smaller jump size, inducing a crossover towards standard diffusive motion at
long times. This interpretation contrasts with the idea of temporally
persistent, spatially correlated currents and raises new issues regarding the
analysis of the dynamics in terms of vibrational modes.Comment: 7 pages, 6 figure
Relevance of visco-plastic theory in a multi-directional inhomogeneous granular flow
We confront a recent visco-plastic description of dense granular flows [P.
Jop et al, Nature, {\bf 441} (2006) 727] with multi-directional inhomogeneous
steady flows observed in non-smooth contact dynamics simulations of 2D
half-filled rotating drums. Special attention is paid to check separately the
two underlying fundamental statements into which the considered theory can be
recast, namely (i) a single relation between the invariants of stress and
strain rate tensors and (ii) the alignment between these tensors.
Interestingly, the first prediction is fairly well verified over more than four
decades of small strain rate, from the surface rapid flow to the quasi-static
creep phase, where it is usually believed to fail because of jamming. On the
other hand, the alignment between stress and strain rate tensors is shown to
fail over the whole flow, what yields an apparent violation of the
visco-plastic rheology when applied without care. In the quasi-static phase,
the particularly large misalignment is conjectured to be related to transient
dilatancy effects
Flow rule, self-channelization and levees in unconfined granular flows
Unconfined granular flows along an inclined plane are investigated
experimentally. During a long transient, the flow gets confined by quasistatic
banks but still spreads laterally towards a well-defined asymptotic state
following a nontrivial process. Far enough from the banks a scaling for the
depth averaged velocity is obtained, which extends the one obtained for
homogeneous steady flows. Close to jamming it exhibits a crossover towards a
nonlocal rheology. We show that the levees, commonly observed along the sides
of the deposit upon interruption of the flow, disappear for long flow
durations. We demonstrate that the morphology of the deposit builds up during
the flow, in the form of an underlying static layer, which can be deduced from
surface velocity profiles, by imposing the same flow rule everywhere in the
flow.Comment: 4 pages, 5 figure
The building blocks of dynamical heterogeneities in dense granular media
We investigate experimentally the connection between short time dynamics and
long time dynamical heterogeneities within a dense granular media under cyclic
shear. We show that dynamical heterogeneities result from a two timescales
process. Short time but already collective events consisting in clustered cage
jumps concentrate most of the non affine displacements. On larger timescales
such clusters appear aggregated both temporally and spatially in avalanches
which eventually build the large scales dynamical heterogeneities. Our results
indicate that facilitation plays an important role in the relaxation process
although it does not appear to be conserved as proposed in many models studied
in the literature.Comment: 4 pages, 4 figure
Avalanches and Dynamical Correlations in supercooled liquids
We identify the pattern of microscopic dynamical relaxation for a two
dimensional glass forming liquid. On short timescales, bursts of irreversible
particle motion, called cage jumps, aggregate into clusters. On larger time
scales, clusters aggregate both spatially and temporally into avalanches. This
propagation of mobility, or dynamic facilitation, takes place along the soft
regions of the systems, which have been identified by computing
isoconfigurational Debye-Waller maps. Our results characterize the way in which
dynamical heterogeneity evolves in moderately supercooled liquids and reveal
that it is astonishingly similar to the one found for dense glassy granular
media.Comment: 4 pages, 3 figure
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