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

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

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

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, $P_{\bot}-n\mu$. The excesses of the perpendicular pressure, $P_{\bot}$, and of $n\mu$, 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

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

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

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

Plastic ridge formation in a compressed thin amorphous film

We demonstrate that surface morphogenesis in compressed thin films may result from spatially correlated plastic activity. A soft glassy film strongly adhering to a smooth and rigid substrate and subjected to uniaxial compression, indeed, does not undergo any global elastic pattern-forming instability, but responds plastically via localized burst events that self-organize, leading to the emergence of a series of parallel ridges transverse to the compression axis. This phenomenon has been completely overlooked, but results from common features of the plastic response of glasses, hence should be highly generic for compressed glassy thin films