461 research outputs found
Cage rattling does not correlate with the local geometry in molecular liquids
Molecular-dynamics simulations of a liquid of short linear molecules have
been performed to investigate the correlation between the particle dynamics in
the cage of the neighbors and the local geometry. The latter is characterized
in terms of the size and the asphericity of the Voronoi polyhedra. The
correlation is found to be poor. In particular, in spite of the different
Voronoi volume around the end and the inner monomers of a molecule, all the
monomers exhibit coinciding displacement distribution when they are caged (as
well as at longer times during the structural relaxation). It is concluded that
the fast dynamics during the cage trapping is a non-local collective process
involving monomers beyond the nearest neighbours.Comment: 15 pages, 6 figure
Elastic consequences of a single plastic event: towards a realistic account of structural disorder and shear wave propagation in models of flowing amorphous solids
Shear transformations (i.e., localised rearrangements of particles resulting
in the shear deformation of a small region of the sample) are the building
blocks of mesoscale models for the flow of disordered solids. In order to
compute the time-dependent response of the solid material to such a shear
transformation, with a proper account of elastic heterogeneity and shear wave
propagation, we propose and implement a very simple Finite-Element (FE) -based
method. Molecular Dynamics (MD) simulations of a binary Lennard-Jones glass are
used as a benchmark for comparison, and information about the microscopic
viscosity and the local elastic constants is directly extracted from the MD
system and used as input in FE. We find very good agreement between FE and MD
regarding the temporal evolution of the disorder-averaged displacement field
induced by a shear transformation, which turns out to coincide with the
response of a uniform elastic medium. However, fluctuations are relatively
large, and their magnitude is satisfactorily captured by the FE simulations of
an elastically heterogeneous system. Besides, accounting for elastic anisotropy
on the mesoscale is not crucial in this respect. The proposed method thus paves
the way for models of the rheology of amorphous solids which are both
computationally efficient and realistic, in that structural disorder and
inertial effects are accounted for.Comment: Submitted to the Journal of Mechanics and Physics of Solid
Correlazioni spazio-temporali tra dinamica veloce e rilassamento in liquidi sottoraffreddati e polimeri
L'esistenza di una correlazione tra dinamica vibrazionale e rilassamento strutturale in liquidi sottoraffreddati e polimeri è stata confermata di recente. Scopo del presente lavoro di tesi è iniziare uno studio dell'origine microscopica di queasta correlazione. L'approccio seguito è quello delle simulazioni numeriche con tecnica Molecular Dynamics per un sistema di polimero lineare in fase liquida. Nel corso del presente lavoro, la dinamica del sistema è studiata in dettaglio tramite analisi di differente tipo
Direct calculation of the critical Casimir force in a binary fluid
We show that critical Casimir effects can be accessed through direct
simulation of a model binary fluid passing through the demixing transition. We
work in the semi grand canonical ensemble, in slab geometry, in which the
Casimir force appears as the excess of the generalized pressure,
. The excesses of the perpendicular pressure, , and of
, are individually of much larger amplitude. A critical pressure
anisotropy is observed between forces parallel and perpendicular to the
confinement direction, which collapses onto a universal scaling function
closely related to that of the critical Casimir force
Driving rate dependence of avalanche statistics and shapes at the yielding transition
We study stress time series caused by plastic avalanches in athermally
sheared disordered materials. Using particle-based simulations and a mesoscopic
elasto-plastic model, we analyze size and shear-rate dependence of the
stress-drop durations and size distributions together with their average
temporal shape. We find critical exponents different from mean-field
predictions, and a clear asymmetry for individual avalanches. We probe scaling
relations for the rate dependency of the dynamics and we report a crossover
towards mean-field results for strong driving.Comment: 5 pages, 3 figures, 1 table, supplementary material to be found at
http://www-liphy.ujf-grenoble.fr/pagesperso/martens/documents/liu2015-sm.pd
Thermodynamic scaling of relaxation: Insights from anharmonic elasticity
Using molecular dynamics simulations of a molecular liquid, we investigate the thermodynamic scaling (TS) of the structural relaxation time Tα in terms of the quantity Tp-γts, where T and p are the temperature and density, respectively. The liquid does not exhibit strong virial-energy correlations. We propose a method for evaluating both the characteristic exponent γts and the TS master curve that uses experimentally accessible quantities that characterise the anharmonic elasticity and does not use details about the microscopic interactions. In particular, we express the TS characteristic exponent γts in terms of the lattice Grneisen parameter δL and the isochoric anharmonicity δL. An analytic expression of the TS master curve of Tα with δL as the key adjustable parameter is found. The comparison with the experimental TS master curves and the isochoric fragilities of 34 glassformers is satisfying. In a few cases, where thermodynamic data are available, we test (i) the predicted characteristic exponent γts and (ii) the isochoric anharmonicity δL, as drawn by the best fit of the TS of the structural relaxation, against the available thermodynamic data. A linear relation between the isochoric fragility and the isochoric anharmonicity δL is found and compared favourably with the results of experiments with no adjustable parameters. A relation between the increase of the isochoric vibrational heat capacity due to anharmonicity and the isochoric fragility is derived
Probing relevant ingredients in mean-field approaches for the athermal rheology of yield stress materials
International audienceAlthough the notion of mechanical noise is expected to play a key role in the non-linear rheology of athermally sheared amorphous systems, its characterization has so far remained elusive. Here, we show using molecular dynamic simulations that in spite of the presence of strong spatio-temporal correlations in the system, the local stress exhibits normal diffusion under the effect of the mechanical noise in the finite driving regime. The diffusion constant appears to be proportional to the mean plastic activity. Our data suggests that the corresponding proportionality constant is density independent, and can be directly related to the specific form of the rheological flow curve, pointing the way to a generic way of modeling mechanical noise in mean-field equations
Relaxation, short time dynamics and elastic properties in glass-forming liquids
When they are cooled or compressed, several systems such as liquids, mixtures, polymers, biomaterials, metals, and molten salts may avoid the crystallization, resulting in a metastable supercooled phase. A full understanding of the extremely complex phenomenology in supercooled liquids is still missing. First there is the issue of how crystallization can be prevented and how deeply the liquid can be supercooled. However by far the most interesting feature of supercooled liquids is the glass transition (GT): following a huge increase in the viscosity as the temperature decreases, the liquid freezes into a glass, a microscopically disordered solid-like state. Understanding the
extraordinary viscous slow-down that accompanies glass formation is one of the major open challenges in condensed matter physics.
During my Ph.D. period (January 2009 - December 2011), I worked on several projects, all connected with the aim of understanding from microscopic basis the relaxation processes in glass-forming liquids. In the light of recent works, particular attention has been addressed to the connection between fast vibrational dynamics on picosecond time scales and the slow relaxation. The first part of my work has been devote to deepen some interesting aspects of this result and to discuss its implications on other aspects of the supercooled liquid phenomenology such as the diffusion and the violation of the Stokes-Einstein relation. Then I focused on the issue of the repulsive interactions controlling the static and dynamics in viscous liquids, and the related topic of the density-temperature scaling. In the last part of my research activity, investigated the elastic models of the GT, which relate the huge slowing down of glass-forming systems with the increasing solidity.
The study of supercooled liquids is approached here from a numerical point of view. Due to these huge potentialities, in the last years, computer experiments played an increasingly important role in glass transition studies. By performing Molecular Dynamics (MD) simulations, we were able to study the dynamics on the microscopic level and to collect informations on every observable of interest with quite a high level of precision, while the same process in experiments would require much more effort. MD simulations allow us to test the validity of theoretical models, as in the case of the elastic models, in a fully controlled environment. During all the study, we have maintained a close connection with the "real world", by comparing, whenever possible, MD results with experimental ones.
To study the complex glassy phenomenology, the chosen prototype of viscous liquid is the simple beads and springs model for polymeric chains. Polymers play a central role in several studies on the GT because of their natural inclination to disorder: in most cases a polymer liquid, rather than crystallize in a regular lattice, reaches the amorphous glassy state
Competition of the connectivity with the local and the global order in polymer melts and crystals
The competition between the connectivity and the local or global order in
model fully-flexible chain molecules is investigated by molecular-dynamics
simulations. States with both missing (melts) and high (crystal) global order
are considered. Local order is characterized within the first coordination
shell (FCS) of a tagged monomer and found to be lower than in atomic systems in
both melt and crystal. The role played by the bonds linking the tagged monomer
to FCS monomers (radial bonds), and the bonds linking two FCS monomers (shell
bonds) is investigated. The detailed analysis in terms of Steinhardt's
orientation order parameters Q_l (l = 2 - 10) reveals that increasing the
number of shell bonds decreases the FCS order in both melt and crystal.
Differently, the FCS arrangements organize the radial bonds. Even if the
molecular chains are fully flexible, the distribution of the angle formed by
adjacent radial bonds exhibits sharp contributions at the characteristic angles
{\theta} = 70{\deg}, 122{\deg}, 180{\deg}. The fractions of adjacent radial
bonds with {\theta} = 122{\deg}, 180{\deg} are enhanced by the global order of
the crystal, whereas the fraction with 70{\deg} < {\theta} < 110{\deg} is
nearly unaffected by the crystallization. Kink defects, i.e. large lateral
displacements of the chains, are evidenced in the crystalline state.Comment: J. Chem. Phys. in pres
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