Response Function of Coarsening Systems

The response function of domain growth processes, and in particular the violation of the fluctuation-dissipation theorem, are studied both analytically and numerically. In the asymptotic limit of large times, the fluctuation-dissipation ratio $X$, which quantifies this violation, tends to one if $C>m^2$ and to zero if $C<m^2$, corresponding to the fast (`bulk') and slow (`domain-wall') responses, respectively. In this paper, we focus on the pre-asymptotic behavior of the domain-wall response. This response is shown to scale with the typical domain length $L(t)$ as $1/L(t)$ for dimension $d>2$, and as $\ln (L(t))/L(t)$ for $d=2$. Numerical results confirming this analysis are presented

Superdiffusive, heterogeneous, and collective particle motion near the jamming transition in athermal disordered materials

We use computer simulations to study the microscopic dynamics of an athermal assembly of soft particles near the fluid-to-solid, jamming transition. Borrowing tools developed to study dynamic heterogeneity near glass transitions, we discover a number of original signatures of the jamming transition at the particle scale. We observe superdiffusive, spatially heterogeneous, and collective particle motion over a characteristic scale which displays a surprising non-monotonic behavior across the transition. In the solid phase, the dynamics is an intermittent succession of elastic deformations and plastic relaxations, which are both characterized by scale-free spatial correlations and system size dependent dynamic susceptibilities. Our results show that dynamic heterogeneities in dense athermal systems and glass-formers are very different, and shed light on recent experimental reports of `anomalous' dynamical behavior near the jamming transition of granular and colloidal assemblies

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

Can the glass transition be explained without a growing static length scale?

It was recently discovered that SWAP, a Monte Carlo algorithm that involves the exchange of pairs of particles of differing diameters, can dramatically accelerate the equilibration of simulated supercooled liquids in regimes where the normal dynamics is glassy. This spectacular effect was subsequently interpreted as direct evidence against a static, cooperative explanation of the glass transition such as the one offered by the random first-order transition (RFOT) theory. We review several empirical facts that support the opposite view, namely, that a local mechanism cannot explain the glass transition phenomenology. We explain the speedup induced by SWAP within the framework of the RFOT theory. We suggest that the efficiency of SWAP stems from a postponed onset of glassy dynamics, which allows the efficient exploration of configuration space even in the regime where the physical dynamics is dominated by activated events across free-energy barriers. We describe this effect in terms of `crumbling metastability' and use the example of nucleation to illustrate the possibility of circumventing free-energy barriers of thermodynamic origin by a change of the local dynamical rules.Comment: 15 pages, 3 figures; v2: improved discussions and clarification

Shearing a Glassy Material: Numerical Tests of Nonequilibrium Mode-Coupling Approaches and Experimental Proposals

The predictions of a nonequilibrium schematic mode-coupling theory developed to describe the nonlinear rheology of soft glassy materials have been numerically challenged in a sheared binary Lennard-Jones mixture. The theory gives an excellent description of the stress/temperature `jamming phase diagram' of the system. In the present paper, we focus on the issue of an effective temperature Teff for the slow modes of the fluid, as defined from a generalized fluctuation-dissipation theorem. As predicted theoretically, many different observables are found to lead to the same value of Teff, suggesting several experimental procedures to measure Teff. New, simple experimental protocols to access Teff from a generalized equipartition theorem are also proposed, and one such experiment is numerically performed. These results give strong support to the thermodynamic interpretation of Teff and make it experimentally accessible in a very direct way.Comment: Version accepted for publication - Physical Review Letter

Direct experimental evidence of a growing length scale accompanying the glass transition

Understanding glass formation is a challenge because the existence of a true glass state, distinct from liquid and solid, remains elusive: Glasses are liquids that have become too viscous to flow. An old idea, as yet unproven experimentally, is that the dynamics becomes sluggish as the glass transition approaches because increasingly larger regions of the material have to move simultaneously to allow flow. We introduce new multipoint dynamical susceptibilities to estimate quantitatively the size of these regions and provide direct experimental evidence that the glass formation of molecular liquids and colloidal suspensions is accompanied by growing dynamic correlation length scales.Comment: 5 pages, 2 figure

La motivation des usagers à participer sur Facebook : étude de la promotion numérique du festival Coup de coeur francophone

Ce mémoire porte sur la caractérisation des usages médiatiques à l'ère du Web 2.0. Plus spécifiquement, il traite de la participation des usagers à la page Facebook d'un festival de musique francophone québécois. Nous nous attachons ainsi à déterminer les motifs dont résulte la participation des individus aux festivals, mais aussi à mettre en avant les « nouveautés » liées à l'émergence du Web social et des médias socionumériques dans le paysage médiaticoculturel. De la problématisation de notre objet d'étude découle une question générale : au-delà des raisons qui poussent les publics à participer physiquement à un événement culturel annoncé comme un festival, quelles sont les motivations des internautes à participer à sa promotion sur les réseaux socionumériques comme Facebook? Afin d'y répondre, nous mobilisons le concept de prédominance des usages et des satisfactions (théorie des uses and gratifications) dont émane la consommation médiatique. Notre recherche consiste en une étude de cas de la participation des usagers sur la page d'une organisation culturelle : le festival québécois Coup de cœur francophone. Nous adoptons une stratégie de recherche mixte séquentielle explicative à prédominance quantitative reposant sur 1) une étude de contenu des commentaires laissés par les participants sur la page Facebook de ce festival et 2) une série d'entretiens individuels nous permettant de préciser et d'éclairer les résultats. Finalement, nous observons que les usagers participent sur la page Facebook du festival afin d'obtenir de l'information ou pour se divertir, mais qu'ils ne s'impliquent pas ou s'impliquent peu sur ces mêmes pages. Nous montrons ainsi que la créativité des usagers sur cette page est relativement limitée : il en ressort une participation principalement factuelle et peu créative. Enfin, nous proposons au lecteur des pistes de réflexion visant à approfondir la connaissance de l'objet d'étude.\ud ______________________________________________________________________________ \ud MOTS-CLÉS DE L’AUTEUR : festivals, réseaux socionumériques, participation, usages, gratification

Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and frictioninduced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature