373 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)]
Random walk, cluster growth, and the morphology of urban conglomerations
We propose a new model of cluster growth according to which the probability
that a new unit is placed in a point at a distance from the city center is
a Gaussian with mean equal to the cluster radius and variance proportional to
the mean, modulated by the local density . The model is analytically
solvable in dimensions, where the density profile varies as a
complementary error function. The model reproduces experimental observations
relative to the morphology of cities, determined via an original analysis of
digital maps with a very high spatial resolution, and helps understanding the
emergence of vehicular traffic.Comment: Physica A. To appea
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
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
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
Collecting livestock data: What, who, who pays?
Joint brief of the World Bank, FAO, ILRI, AU-IBAR with support from the Gates Foundatio
"New Science Theater". Intervista a Martin Reckhaus
The paper introduces the play "New Science" â directed by Martin Reckhaus, book by Jessica Slote, performed in New York in 2006 â with the aim to understand its declared inspiration from G. Vico. Moving from the main question of the play: "What does it mean to be human?", the interview with Martin Reckhaus connects Vicoâs ideas about the creation of history, the importance of collectivity, the strength of sensuality, the trust in the meaningfulness of the changement and in the power of imagination, to the existential experience pursued by a theatre practice deeply rooted in the tradition of the Living Theatre and of its revolutionary critic of reality
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
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