710 research outputs found
Disordered jammed packings of frictionless spheres
At low volume fraction, disordered arrangements of frictionless spheres are
found in un--jammed states unable to support applied stresses, while at high
volume fraction they are found in jammed states with mechanical strength. Here
we show, focusing on the hard sphere zero pressure limit, that the transition
between un-jammed and jammed states does not occur at a single value of the
volume fraction, but in a whole volume fraction range. This result is obtained
via the direct numerical construction of disordered jammed states with a volume
fraction varying between two limits, and . We identify these
limits with the random loose packing volume fraction \rl and the random close
packing volume fraction \rc of frictionless spheres, respectively
Spatial correlations of elementary relaxation events in glass-forming liquids
The dynamical facilitation scenario, by which localized relaxation events
promote nearby relaxation events in an avalanching process, has been suggested
as the key mechanism connecting the microscopic and the macroscopic dynamics of
structural glasses. Here we investigate the statistical features of this
process via the numerical simulation of a model structural glass. First we show
that the relaxation dynamics of the system occurs through particle jumps that
are irreversible, and that cannot be decomposed in smaller irreversible events.
Then we show that each jump does actually trigger an avalanche. The
characteristic of this avalanche change on cooling, suggesting that the
relaxation dynamics crossovers from a noise dominated regime where jumps do not
trigger other relaxation events, to a regime dominated by the facilitation
process, where a jump trigger more relaxation events.Comment: 8 pages, 6 figure
Absence of `fragility' and mechanical response of jammed granular materials
We perform molecular dynamic (MD) simulations of frictional non-thermal
particles driven by an externally applied shear stress. After the system jams
following a transient flow, we probe its mechanical response in order to
clarify whether the resulting solid is 'fragile'. We find the system to respond
elastically and isotropically to small perturbations of the shear stress,
suggesting absence of fragility. These results are interpreted in terms of the
energy landscape of dissipative systems. For the same values of the control
parameters, we check the behaviour of the system during a stress cycle.
Increasing the maximum stress value, a crossover from a visco-elastic to a
plastic regime is observed.Comment: 6 pages, 9 figures, accepted in Granular Matter on 01-02-201
Pacman Percolation and the Glass Transition
We investigate via Monte Carlo simulations the kinetically constrained
Kob-Andersen lattice glass model showing that, contrary to current
expectations, the relaxation process and the dynamical heterogeneities seems to
be characterized by different time scales. Indeed, we found that the relaxation
time is related to a reverse percolation transition, whereas the time of
maximum heterogeneity is related to the spatial correlation between particles.
This investigation leads to a geometrical interpretation of the relaxation
processes and of the different observed time scales.Comment: 12 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1109.428
Thermodynamics and Statistical Mechanics of dense granular media
By detailed Molecular Dynamics and Monte Carlo simulations %of a realistic
model we show that granular materials at rest can be described as
thermodynamics systems. First we show that granular packs can be characterized
by few parameters, as much as fluids or solids. Then, in a second independent
step, we demonstrate that these states can be described in terms of equilibrium
distributions which coincide with the Statistical Mechanics of powders first
proposed by Edwards. We also derive the system equation of state as a function
of the ``configurational temperature'', its new intensive thermodynamic
parameter.Comment: Supplementary Informations adde
Shear instabilities in granular mixtures
Dynamical instabilities in fluid mechanics are responsible of a variety of
important common phenomena, such as waves on the sea surface or Taylor vorteces
in Couette flow. In granular media dynamical instabilities has just begun to be
discovered. Here we show by means of molecular dynamics simulation the
existence of a new dynamical instability of a granular mixture under
oscillating horizontal shear, which leads to the formation of a striped pattern
where the components are segregated. We investigate the properties of such a
Kelvin-Helmholtz like instability and show how it is connected to pattern
formation in granular flow and segregation.Comment: Phys. Rev. Lett. 94, 18800
Jamming at zero temperature, zero friction, and finite applied shear stress
Via molecular dynamics simulations, we unveil the hysteretic nature of the
jamming transition of soft repulsive frictionless spheres, as it occurs varying
the volume fraction or the shear stress. In a given range of control parameters
the system may be found both in a flowing and in an jammed state, depending on
the preparation protocol. The hysteresis is due to an underlying energy
landscape with many minima, as explained by a simple model, and disappears in
the presence of strong viscous forces and in the small limit. In this
limit, structural quantities are continuous at the transition, while the
asymptotic values of two time quantities such as the self-intermediate
scattering function are discontinuous, giving to the jamming transition a mixed
first-order second-order character close to that found at the glass transition
of thermal systems
Particle jumps in structural glasses
Particles in structural glasses rattle around temporary equilibriumpositions,
that seldom change through a process which is much faster than the relaxation
time, known as particle jump. Since the relaxation of the system is due to the
accumulation of many such jumps, it could be possible to connect the single
particle short time motion to the macroscopic relaxation by understanding the
features of the jump dynamics. Here we review recent results in this research
direction, clarifying the features of particles jumps that have been understood
and those that are still under investigation, and examining the role of
particle jumps in different theories of the glass transition.Comment: 10 pages, 4 figures, Review articl
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