66 research outputs found
Correlation between Voronoi volumes in disc packings
We measure the two-point correlation of free Voronoi volumes in binary disc
packings, where the packing fraction ranges from 0.8175 to
0.8380. We observe short-ranged correlations over the whole range of and anti-correlations for . The spatial extent of
the anti-correlation increases with while the position of the
maximum of the anti-correlation and the extent of the positive correlation
shrink with . We conjecture that the onset of anti-correlation
corresponds to dilatancy onset in this system
"Barchan" dunes in the lab
We demonstrate the feasibility of studying dunes in a laboratory experiment.
It is shown that an initial sand pile, under a wind flow carrying sand,
flattens and gets a shape recalling barchan dunes. An evolution law is proposed
for the profile and the summit of the dune. The dune dynamics is shown to be
shape invariant. The invariant shape, the ``dune function'' is isolated.Comment: 3 pages, 4 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
Evidence of Deep Water Penetration in Silica during Stress Corrosion Fracture
We measure the thickness of the heavy water layer trapped under the stress corrosion fracture surface of silica using neutron reflectivity experiments. We show that the penetration depth is 65–85 Å, suggesting the presence of a damaged zone of ~100 Å extending ahead of the crack tip during its propagation. This estimate of the size of the damaged zone is compatible with other recent results
Kinetic Heterogeneities at Dynamical Crossovers
We perform molecular dynamics simulations of a model glass-forming liquid to
measure the size of kinetic heterogeneities, using a dynamic susceptibility
that quantifies the number of particles whose dynamics
are correlated on the length scale and time scale . By measuring
as a function of both and , we locate local maxima
at distances and times . Near the dynamical
glass transition, we find two types of maxima, both correlated with crossovers
in the dynamical behavior: a smaller maximum corresponding to the crossover
from ballistic to sub-diffusive motion, and a larger maximum corresponding to
the crossover from sub-diffusive to diffusive motion. Our results indicate that
kinetic heterogeneities are not necessarily signatures of an impending glass or
jamming transition.Comment: 6 pages, 4 figure
A deductive statistical mechanics approach for granular matter
We introduce a deductive statistical mechanics approach for granular
materials which is formally built from few realistic physical assumptions. The
main finding is an universal behavior for the distribution of the density
fluctuations. Such a distribution is the equivalent of the Maxwell-Boltzmann's
distribution in the kinetic theory of gasses. The comparison with a very
extensive set of experimental and simulation data for packings of monosized
spherical grains, reveals a remarkably good quantitative agreement with the
theoretical predictions for the density fluctuations both at the grain level
and at the global system level. Such agreement is robust over a broad range of
packing fractions and it is observed in several distinct systems prepared by
using different methods. The equilibrium distributions are characterized by
only one parameter () which is a quantity very sensitive to changes in the
structural organization. The thermodynamical equivalent of and its relation
with the `granular temperature' are also discussed.Comment: 15 pages, 6 figure
Rheology and dynamical heterogeneity in frictionless beads at jamming density
We investigate the rheological properties of an assembly of inelastic (but
frictionless) particles close to the jamming density using numerical
simulation, in which uniform steady states with a constant shear rate
is realized. The system behaves as a power-law fluid and the
relevant exponents are estimated; e.g., the shear stress is proportional to
, where . It is also found that
the relaxation time and the correlation length of the velocity
increase obeying power laws: and
, where and
Critical scaling and heterogeneous superdiffusion across the jamming/rigidity transition of a granular glass
The dynamical properties of a dense horizontally vibrated bidisperse granular
monolayer are experimentally investigated. The quench protocol produces states
with a frozen structure of the assembly, but the remaining degrees of freedom
associated with contact dynamics control the appearance of macroscopic
rigidity. We provide decisive experimental evidence that this transition is a
critical phenomenon, with increasingly collective and heterogeneous
rearrangements occurring at length scales much smaller than the grains'
diameter, presumably reflecting the contact force network fluctuations.
Dynamical correlation time and length scales soar on both sides of the
transition, as the volume fraction varies over a remarkably tiny range (). We characterize the motion of individual grains,
which becomes super-diffusive at the jamming transition , signaling
long-ranged temporal correlations. Correspondingly, the system exhibits
long-ranged four-point dynamical correlations in space that obey critical
scaling at the transition density.Comment: 4 pages, 8 figure
Jaming and Geometry of Two-Dimensional Foams
We experimentally probe the vicinity of the jamming point J, located at a
density corresponding to random close packing (), in
two dimensional, bidisperse packings of foam bubbles. We vary the density of
the foam layer and extract geometrical measures by image analysis. We confirm
the predicted scaling of the average contact number Z with and compare
the distribution of local contact numbers to a simple model. We further
establish that the distribution of areas strongly depends on .
Finally, we find that the distribution of contact forces systematically
varies with density.Comment: 6 pages, 5 figures, submitte
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