185 research outputs found
Fluctuations, response and aging dynamics in a simple glass-forming liquid out of equilibrium
By means of molecular dynamics computer simulations we investigate the out of
equilibrium relaxation dynamics of a simple glass former, a binary
Lennard-Jones system, after a quench to low temperatures. We study both one
time quantities and two-times correlation functions. Two-times correlation
functions show a strong time and waiting time dependence. For large
and times corresponding to the early -relaxation regime the correlators
approach the Edwards-Anderson value by means of a power-law in time. at long
times the correlation functions can be expressed as and compute the function . This function is
found to show a -dependence which is a bit stronger than a logarithm and to
depend on the observable considered.
Finally we discuss our measurements of the time dependent response function.
We find that at long times the correlation functions and the response are not
related by the usual fluctuation dissipation theorem but that this relation is
similar to the one found for spin glasses with one step replica symmetry
breaking.Comment: 23 pages, 14 figure
Spatial cooperativity in microchannel flows of soft jammed materials: A mesoscopic approach
The flow of amorphous solids results from a combination of elastic
deformation and local structural rearrangements, which induce non-local elastic
deformations. These elements are incorporated into a mechanically-consistent
mesoscopic model of interacting elastoplastic blocks. We investigate the
specific case of channel flow with numerical simulations, paying particular
attention to situations of strong confinement. We find that the simple picture
of plastic events embedded in an elastic matrix successfully accounts for
manifestations of spatial cooperativity. Shear rate fluctuations are observed
in seemingly quiescent regions, and the velocity profiles in confined flows at
high applied pressure deviate from those expected in the absence of non-local
effects, in agreement with experimental data. However, we suggest a different
physical origin for the large deviations observed when walls have rough
surfaces, associated with "bumps" of the particles against the asperities of
the walls.Comment: 5 figure
A mesoscopic model for the rheology of soft amorphous solids, with application to mi- crochannel flows
We study a mesoscopic model for the flow of amorphous solids. The model is
based on the key features identified at the microscopic level, namely peri- ods
of elastic deformation interspersed with localised rearrangements of parti-
cles that induce long-range elastic deformation. These long-range deformations
are derived following a continuum mechanics approach, in the presence of solid
boundaries, and are included in full in the model. Indeed, they mediate spatial
cooperativity in the flow, whereby a localised rearrangement may lead a distant
region to yield. In particular, we simulate a channel flow and find
manifestations of spatial cooperativity that are consistent with published
experimental obser- vations for concentrated emulsions in microchannels. Two
categories of effects are distinguished. On the one hand, the coupling of
regions subject to different shear rates, for instance,leads to finite shear
rate fluctuations in the seemingly un- sheared "plug" in the centre of the
channel. On the other hand, there is convinc- ing experimental evidence of a
specific rheology near rough walls. We discuss diverse possible physical
origins for this effect, and we suggest that it may be associated with the
bumps of particles into surface asperities as they slide along the wall
Correlation and shear bands in a plastically deformed granular medium
Recent experiments (Le Bouil et al., Phys. Rev. Lett., 2014, 112, 246001)
have analyzed the statistics of local deformation in a granular solid
undergoing plastic deformation. Experiments report strongly anisotropic
correlation between events, with a characteristic angle that was interpreted
using elasticity theory and the concept of Eshelby transformations with
dilation; interestingly, the shear bands that characterize macroscopic failure
occur at an angle that is different from the one observed in microscopic
correlations. Here, we interpret this behavior using a mesoscale elastoplastic
model of solid flow that incorporates a local Mohr-Coulomb failure criterion.
We show that the angle observed in the microscopic correlations can be
understood by combining the elastic interactions associated with Eshelby
transformation with the local failure criterion. At large strains, we also
induce permanent shear bands at an angle that is different from the one
observed in the correlation pattern. We interpret this angle as the one that
leads to the maximal instability of slip lines
The jamming transition as probed by quasistatic shear flow
We study the rheology of amorphous packings of soft, frictionless particles
close to jamming. Implementing a quasistatic simulation method we generate a
well defined ensemble of states that directly samples the system at its
yield-stress. A continuous jamming transition from a freely-flowing state to a
yield stress situation takes place at a well defined packing fraction, where
the scaling laws characteristic of isostatic solids are observed. We propose
that long-range correlations observed below the transition are dominated by
this isostatic point, while those that are observed above the transition are
characteristic of dense, disordered elastic media.Comment: 4 pages, 6 figures, revised versio
Acoustic excitations and elastic heterogeneities in disordered solids
In the recent years, much attention has been devoted to the inhomogeneous
nature of the mechanical response at the nano-scale in disordered solids.
Clearly, the elastic heterogeneities that have been characterized in this
context are expected to strongly impact the nature of the sound waves which, in
contrast to the case of perfect crystals, cannot be completely rationalized in
terms of phonons. Building on previous work on a toy model showing an
amorphisation transition [Mizuno H, Mossa S, Barrat JL (2013) EPL {\bf
104}:56001], we investigate the relationship between sound waves and elastic
heterogeneities in a unified framework, by continuously interpolating from the
perfect crystal, through increasingly defective phases, to fully developed
glasses. We provide strong evidence of a direct correlation between sound waves
features and the extent of the heterogeneous mechanical response at the
nano-scale
Effects of inertia on the steady-shear rheology of disordered solids
We study the finite-shear-rate rheology of disordered solids by means of
molecular dynamics simulations in two dimensions. By systematically varying the
damping magnitude in the low-temperature limit, we identify two well
defined flow regimes, separated by a thin (temperature-dependent) crossover
region. In the overdamped regime, the athermal rheology is governed by the
competition between elastic forces and viscous forces, whose ratio gives the
Weissenberg number (up to elastic parameters); the
macroscopic stress follows the frequently encountered Herschel-Bulkley
law , with yield stress
\Sigma\_0\textgreater{}0. In the underdamped (inertial) regime, dramatic
changes in the rheology are observed for low damping: the flow curve becomes
non-monotonic. This change is not caused by longer-lived correlations in the
particle dynamics at lower damping; instead, for weak dissipation, the sample
heats up considerably due to, and in proportion to, the driving. By suitably
thermostatting more or less underdamped systems, we show that their rheology
only depends on their kinetic temperature and the shear rate, rescaled with
Einstein's vibration frequency.Comment: Accepted for publication in Phys. Rev. Let
Influence of Wetting Properties on Diffusion in a Confined Fluid
We briefly discuss how the wetting properties of a fluid/solid interface can
indirectly influence the diffusion properties of fluid confined between two
solid walls. This influence is related to the variability of the hydrodynamic
boundary conditions at the interface, which correlates to the wetting
properties.Comment: Paper presented at the ILL workshop "Dynamics in confinment",
Grenoble, January 2000 (http://www.ill.fr
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