2,326 research outputs found
Heterogeneities in amorphous systems under shear
The last decade has seen major progresses in studies of elementary mechanisms
of deformation in amorphous materials. Here, we start with a review of
physically-based theories of plasticity, going back to the identification of
"shear-transformations" as early as the 70's. We show how constructive
criticism of the theoretical models permits to formulate questions concerning
the role of structural disorder, mechanical noise, and long-ranged elastic
interactions. These questions provide the necessary context to understand what
has motivated recent numerical studies. We then summarize their results, show
why they had to focus on athermal systems, and point out the outstanding
questions.Comment: Chapter of "Dynamical Heterogeneities in glasses, colloids and
granular materials", Eds.: L. Berthier, G. Biroli, J-P Bouchaud, L.
Cipelletti and W. van Saarloos (Oxford University Press, to appear), more
info at http://w3.lcvn.univ-montp2.fr/~lucacip/DH_book.ht
Local dynamics and primitive path analysis for a model polymer melt near a surface
By applying local primitive path and Rouse modes analysis we study the chains
conformations, local dynamics and viscosity of a model polymer melt in a
polymer-wall interface. We establish that the presence of a repulsive wall
leads to acceleration of the dynamics both for unentangled and weakly entangled
melts and to a depletion in the entanglement density in the wall vicinity. When
the surface bears some grafted chains, we show that the melt chains are
accelerated in the unentangled regime and slowed down in the entangled regime.
By analyzing the primitive paths we attribute the observed slowdown to an
increase in the entanglement density in the interfacial layer. The presence of
a relatively small density of grafting sites thus leads to improved mechanical
properties (reinforcement) and decreases locally the entanglement length even
if the surface is repulsive
On the definition of temperature in dense granular media
In this Letter we report the measurement of a pseudo-temperature for
compacting granular media on the basis of the Fluctuation-Dissipation relations
in the aging dynamics of a model system. From the violation of the
Fluctuation-Dissipation Theorem an effective temperature emerges (a dynamical
temperature T_{dyn}) whose ratio with the equilibrium temperature T_d^{eq}
depends on the particle density. We compare the results for the
Fluctuation-Dissipation Ratio (FDR) T_{dyn}/T_d^{eq} at several densities with
the outcomes of Edwards' approach at the corresponding densities. It turns out
that the FDR and the so-called Edwards' ratio coincide at several densities
(very different ages of the system), opening in this way the door to
experimental checks as well as theoretical constructions.Comment: RevTex4 4 pages, 4 eps figure
Modeling transient absorption and thermal conductivity in a simple nanofluid
Molecular dynamics simulations are used to simulate the thermal properties of
a model fluid containing nanoparticles (nanofluid). By modelling transient
absorption experiments, we show that they provide a reliable determination of
interfacial resistance between the particle and the fluid. The flexibility of
molecular simulation allows us to consider separately the effect of
confinement, particle mass and Brownian motion on the thermal transfer between
fluid and particle. Finally, we show that in the absence of collective effects,
the heat conductivity of the nanofluid is well described by the classical
Maxwell Garnet equation model
Nucleation in hydrophobic cylindrical pores : a lattice model
We consider the nucleation process associated with capillary condensation of
a vapor in a hydrophobic cylindrical pore (capillary evaporation). The
liquid-vapor transition is described within the framework of a simple lattice
model. The phase properties are characterized both at the mean-field level and
using Monte-Carlo simulations. The nucleation process for the liquid to vapor
transition is then specifically considered. Using umbrella sampling techniques,
we show that nucleation occurs through the condensation of an asymmetric vapor
bubble at the pore surface. Even for highly confined systems, good agreement is
found with macroscopic considerations based on classical nucleation theory. The
results are discussed in the context of recent experimental work on the
extrusion of water in hydrophobic pores
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
Continuum limit of amorphous elastic bodies (III): Three dimensional systems
Extending recent numerical studies on two dimensional amorphous bodies, we
characterize the approach of elastic continuum limit in three dimensional
(weakly polydisperse) Lennard-Jones systems. While performing a systematic
finite-size analysis (for two different quench protocols) we investigate the
non-affine displacement field under external strain, the linear response to an
external delta force and the low-frequency harmonic eigenmodes and their
density distribution. Qualitatively similar behavior is found as in two
dimensions. We demonstrate that the classical elasticity description breaks
down below an intermediate length scale , which in our system is
approximately 23 molecular sizes. This length characterizes the correlations of
the non-affine displacement field, the self-averaging of external noise with
distance from the source and gives the lower wave length bound for the
applicability of the classical eigenfrequency calculations. We trace back the
"Boson-peak" of the density of eigenfrequencies (obtained from the velocity
auto-correlation function) to the inhomogeneities on wave lengths smaller than
.Comment: 27 pages, 11 figures, submitted to Phys. Rev.
Basins of attraction of metastable states of the spherical -spin model
We study the basins of attraction of metastable states in the spherical
-spin spin glass model, starting the relaxation dynamics at a given distance
from a thermalized condition. Weighting the initial condition with the
Boltzmann distribution we find a finite size for the basins. On the contrary, a
white weighting of the initial condition implies vanishing basins of
attraction. We make the corresponding of our results with the ones of a
recently constructed effective potential.Comment: LaTeX, 7 pages, 7 eps figure
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