175 research outputs found
Comment on ``Granular Entropy: Explicit Calculations for Planar Assemblies''
A Comment on the Letter by Raphael Blumenfeld and Sam F. Edwards, [Phys. Rev.
Lett. 90, 114303 (2003)]
High--order jamming crossovers and density anomalies
We demonstrate the existence of high--order jamming crossovers in systems of
particles with repulsive contact interactions, which originate from the
collapse of successive coordination shells. At zero temperature, these
crossovers induce an anomalous behavior of the bulk modulus, which varies
non--monotonically with the density, while at finite temperature they induce
density anomalies consisting in an increased diffusivity upon isothermal
compression and in a negative thermal expansion coefficient. We rationalize the
dependence of these crossovers on the softness of the interaction potential,
and relate the jamming crossovers and the anomalous diffusivity through the
investigation of the vibrational spectrum
Dynamics and instantaneous normal modes in a liquid with density anomalies
We investigate the relation between the dynamical features of a supercooled
liquid and those of its potential energy landscape, focusing on a model liquid
with density anomalies. We consider, at fixed temperature, pairs of state
points with different density but the same diffusion constant, and find that
surprisingly they have identical dynamical features at all length and time
scales. This is shown by the collapse of their mean square displacements and of
their self--intermediate scattering functions at different wavevectors. We then
investigate how the features of the energy landscape change with density, and
establish that state points with equal diffusion constant have different
landscapes. In particular, we find a correlation between the fraction of
instantaneous normal modes connecting different energy minima and the diffusion
constant, but unlike in other systems these two quantities are not in
one--to--one correspondence with each other, showing that additional landscape
features must be relevant in determining the diffusion constant
Density anomalies and high-order jamming crossovers
Jamming crossovers occur at zero temperature in assemblies of particles
interacting via finite range repulsive potentials, when on increasing the
density particles make contacts with those of subsequent coordination shells.
Density anomalies, including an increased diffusivity upon isothermal
compression and a negative thermal expansion coefficient, are the finite
temperature signatures of the jamming crossovers. In this manuscript we show
that the jamming crossovers are correlated to an increase of the non affine
response of the system to density changes, and clarify that jammed systems
evolve upon compression through subsequent Eshlby-like plastic instabilities
Attraction tames two-dimensional melting: from continuous to discontinuous transitions
Two-dimensional systems may admit a hexatic phase and hexatic-liquid
transitions of different natures. The determination of their phase diagrams
proved challenging, and indeed those of hard-disks, hard regular polygons, and
inverse power-law potentials, have been only recently clarified. In this
context, the role of attractive forces is currently speculative, despite their
prevalence at both the molecular and colloidal scale. Here we demonstrate, via
numerical simulations, that attraction promotes a discontinuous melting
scenario with no hexatic phase. At high-temperature, Lennard-Jones particles
and attractive polygons follow the shape-dominated melting scenario observed in
hard-disks and hard polygons, respectively. Conversely, all systems melt via a
first-order transition with no hexatic phase at low temperature, where
attractive forces dominate. The intermediate temperature melting scenario is
shape-dependent. Our results suggest that, in colloidal experiments, the
tunability of the strength of the attractive forces allows for the observation
of different melting scenario in the same system.Comment: SI include
Role of cell deformability in the two-dimensional melting of biological tissues
The size and shape of a large variety of polymeric particles, including
biological cells, star polymers, dendrimes, and microgels, depend on the
applied stresses as the particles are extremely soft. In high-density
suspensions these particles deform as stressed by their neighbors, which
implies that the interparticle interaction becomes of many-body type.
Investigating a two-dimensional model of cell tissue, where the single particle
shear modulus is related to the cell adhesion strength, here we show that the
particle deformability affects the melting scenario. On increasing the
temperature, stiff particles undergo a first-order solid/liquid transition,
while soft ones undergo a continuous solid/hexatic transition followed by a
discontinuous hexatic/liquid transition. At zero temperature the melting
transition driven by the decrease of the adhesion strength occurs through two
continuous transitions as in the Kosterlitz, Thouless, Halperin, Nelson, and
Young scenario. Thus, there is a range of adhesion strength values where the
hexatic phase is stable at zero temperature, which suggests that the
intermediate phase of the epithelial-to-mesenchymal transition could be hexatic
type
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
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
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