2,941 research outputs found

### Density controls the kinetic stability of ultrastable glasses

We use a swap Monte Carlo algorithm to numerically prepare bulk glasses with
kinetic stability comparable to that of glass films produced experimentally by
physical vapor deposition. By melting these systems into the liquid state, we
show that some of our glasses retain their amorphous structures longer than
10^5 times the equilibrium structural relaxation time. This exceptional kinetic
stability cannot be achieved experimentally for bulk materials. We perform
simulations at both constant volume and constant pressure to demonstrate that
the density mismatch between the ultrastable glass and the equilibrium liquid
accounts for a major part of the observed kinetic stability.Comment: 7 Pages, 6 Figures. Figures 4b) and 5b) updated, revisions to text to
improve discussion, missing page numbers added to references, typos correcte

### Can the jamming transition be described using equilibrium statistical mechanics?

When materials such as foams or emulsions are compressed, they display solid
behaviour above the so-called `jamming' transition. Because compression is done
out-of-equilibrium in the absence of thermal fluctuations, jamming appears as a
new kind of a nonequilibrium phase transition. In this proceeding paper, we
suggest that tools from equilibrium statistical mechanics can in fact be used
to describe many specific features of the jamming transition. Our strategy is
to introduce thermal fluctuations and use statistical mechanics to describe the
complex phase behaviour of systems of soft repulsive particles, before sending
temperature to zero at the end of the calculation. We show that currently
available implementations of standard tools such as integral equations,
mode-coupling theory, or replica calculations all break down at low temperature
and large density, but we suggest that new analytical schemes can be developed
to provide a fully microscopic, quantitative description of the jamming
transition.Comment: 8 pages, 6 figs. Talk presented at Statphys24 (July 2010, Cairns,
Australia

### Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid

The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simple
shear flow is investigated by means of molecular dynamics simulations. The
range of temperature investigated covers both the liquid, supercooled and
glassy states, while the shear rate covers both the linear and nonlinear
regimes of rheology. The results can be interpreted in the context of a
nonequilibrium, schematic mode-coupling theory developed recently, which makes
the theory applicable to a wide range of soft glassy materials. The behavior of
the viscosity is first investigated. In the nonlinear regime, strong
shear-thinning is obtained. Scaling properties of the intermediate scattering
functions are studied. Standard `mode-coupling properties' of factorization and
time-superposition hold in this nonequilibrium situation. The
fluctuation-dissipation relation is violated in the shear flow in a way very
similar to that predicted theoretically, allowing for the definition of an
effective temperature Teff for the slow modes of the fluid. Temperature and
shear rate dependencies of Teff are studied using density fluctuations as an
observable. The observable dependence of Teff is also investigated. Many
different observables are found to lead to the same value of Teff, suggesting
several experimental procedures to access Teff. It is proposed that tracer
particle of large mass may play the role of an `effective thermometer'. When
the Einstein frequency of the tracers becomes smaller than the inverse
relaxation time of the fluid, a nonequilibrium equipartition theorem holds.
This last result gives strong support to the thermodynamic interpretation of
Teff and makes it experimentally accessible in a very direct way.Comment: Version accepted for publication in Journal of Chemical Physic

### A random walk description of the heterogeneous glassy dynamics of attracting colloids

We study the heterogeneous dynamics of attractive colloidal particles close
to the gel transition using confocal microscopy experiments combined with a
theoretical statistical analysis. We focus on single particle dynamics and show
that the self part of the van Hove distribution function is not the Gaussian
expected for a Fickian process, but that it reflects instead the existence, at
any given time, of colloids with widely different mobilities. Our confocal
microscopy measurements can be described well by a simple analytical model
based on a conventional continuous time random walk picture, as already found
in several other glassy materials. In particular, the theory successfully
accounts for the presence of broad tails in the van Hove distributions that
exhibit exponential, rather than Gaussian, decay at large distance.Comment: 13 pages, 5 figs. Submitted to special issue "Classical and Quantum
Glasses" of J. Phys.: Condens. Matter; v2: response to refere

### The role of attractive forces in viscous liquids

We present evidence from computer simulation that the slowdown of relaxation
of a standard Lennard-Jones glass-forming liquid and that of its reduction to a
model with truncated pair potentials without attractive tails is quantitatively
and qualitatively different in the viscous regime. The pair structure of the
two models is however very similar. This finding, which appears to contradict
the common view that the physics of dense liquids is dominated by the steep
repulsive forces between atoms, is characterized in detail, and its
consequences are explored. Beyond the role of attractive forces themselves, a
key aspect in explaining the differences in the dynamical behavior of the two
models is the truncation of the interaction potentials beyond a cutoff at
typical interatomic distance. This leads us to question the ability of the
jamming scenario to describe the physics of glass-forming liquids and polymers.Comment: 13 pages, 12 figure

### Identification of Known SSO in CU4 Object Processing

3 p.International audienceThe identification of known solar system objects (SSO) that will be observed by Gaia is a key point of the solar system object processing pipeline (CU4.SSO). It aims to associate the provisional tag assigned to observations of probably solar system objects to already known targets. At the time Gaia flies, it can be estimated that about 600,000 solar system objects (mainly small solar system objects) will be known and characterized by an orbit accurate enough to make their identification almost certain

### Shear localization in a model glass

Using molecular dynamics simulations, we show that a simple model of a glassy
material exhibits the shear localization phenomenon observed in many complex
fluids. At low shear rates, the system separates into a fluidized shear-band
and an unsheared part. The two bands are characterized by a very different
dynamics probed by a local intermediate scattering function. Furthermore, a
stick-slip motion is observed at very small shear rates. Our results, which
open the possibility of exploring complex rheological behavior using
simulations, are compared to recent experiments on various soft glasses.Comment: 4 pages, 4 figures (5 figure files

### A critical test of the mode-coupling theory of the glass transition

The mode-coupling theory of the glass transition predicts the time evolution
of the intermediate scattering functions in viscous liquids on the sole basis
of the structural information encoded in two-point density correlations. We
provide a critical test of this property and show that the theory fails to
describe the qualitatively distinct dynamical behavior obtained in two model
liquids characterized by very similar pair correlation functions. Because we
use `exact' static information provided by numerical simulations, our results
are a direct proof that some important information about the dynamics of
viscous liquids is not captured by pair correlations, and is thus not described
by the mode-coupling theory, even in the temperature regime where the theory is
usually applied.Comment: 7 pages, 5 figures

### Jamming transitions in amorphous packings of frictionless spheres occur over a continuous range of volume fractions

We numerically produce fully amorphous assemblies of frictionless spheres in
three dimensions and study the jamming transition these packings undergo at
large volume fractions. We specify four protocols yielding a critical value for
the jamming volume fraction which is sharply defined in the limit of large
system size, but is different for each protocol. Thus, we directly establish
the existence of a continuous range of volume fraction where nonequilibrium
jamming transitions occur. However, these jamming transitions share the same
critical behaviour. Our results suggest that, even in the absence of partial
crystalline ordering, a unique location of a random close packing does not
exist, and that volume fraction alone is not sufficient to describe the
properties of jammed states.Comment: 5 pages, 3 fig

### Front-mediated melting of ultrastable glasses

Ultrastable vapor-deposited glasses display uncommon material properties.
Most remarkably, upon heating they are believed to melt via a liquid front that
originates at the free surface and propagates over a mesoscopic crossover
length, before crossing over to bulk melting. We combine swap Monte Carlo with
molecular dynamics simulations to prepare and melt isotropic amorphous films of
unprecedendtly high kinetic stability. We are able to directly observe both
bulk and front melting, and the crossover between them. We measure the front
velocity over a broad range of conditions, and a crossover length scale that
grows to nearly $400$ particle diameters in the regime accessible to
simulations. Our results disentangle the relative roles of kinetic stability
and vapor deposition in the physical properties of stable glasses.Comment: 7 pages, 6 figures; accepted for publication in Phys. Rev. Let

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