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PMID 21230079
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DOI: 10.1103/PhysRevE.82.031503
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We study the nonequilibrium aging dynamics in a system of quasi-hard spheres at large density by means of computer simulations. We find that, after a sudden quench to large density, the relaxation time initially increases exponentially with the age of the system. After a surprisingly large crossover time, the system enters the asymptotic aging regime characterized by a linear increase of the relaxation time with age. In this aging regime, single particle motion is strongly non-Fickian, with a mean-squared displacement increasing subdiffusively, associated to broad, non-Gaussian tails in the distribution of particle displacements. We find that the system ages through temporally intermittent relaxation events, and a detailed finite size analysis of these collective dynamic fluctuations reveals that these events are not spanning the entire system, but remain spatially localized.

Commentaires: 11 pages; 10 figs

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DOI: 10.1103/PhysRevLett.104.169602
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Brambilla et al. Reply:

Commentaires: Reply to a comment by van Megen and Williams

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PMID 21517290
Ref Arxiv: 1008.2899
DOI: 10.1103/PhysRevLett.106.120601
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70 Citations
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We analyze the large scale structure and fluctuations of jammed packings of polydisperse spheres produced both numerically and in a granular experiment. While the structure factors of the packings reveal no unusual behavior for small wavevectors, the compressibility displays an anomalous linear dependence at low wavectors and vanishes when q -> 0. Our results apply to arbitrary particle size distributions. For continuous distributions, we derive simple perturbative approximations for the compressibility that are accurate for polydispersity up to about 30%. We show that the compressibility vanishes because jammed packings of polydisperse spheres have no bulk fluctuations of the volume fraction and are thus hyperuniform, a property not observed experimentally before.

Commentaires: 4 pages; 2 figures

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PMID 21230636
Ref Arxiv: 1009.4531
DOI: 10.1103/PhysRevE.82.060501
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We use theory and simulations to investigate the existence of amorphous glassy states in ultrasoft colloids. We combine the hyper-netted chain approximation with mode-coupling theory to study the dynamic phase diagram of soft repulsive spheres interacting with a Hertzian potential, focusing on low temperatures and large densities. At constant temperature, we find that an amorphous glassy state is entered upon compression, as in colloidal hard spheres, but the glass unexpectedly melts when density increases further. We attribute this re-entrant fluid-glass transition to particle softness, and correlate this behaviour to previously reported anomalies in soft systems, thus emphasizing its generality. The predicted fluid-glass-fluid sequence is confirmed numerically.

Commentaires: 4 pages, 3 figs Journal: Phys. Rev. E 82, 060501(R) (2010)

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PMID 21303135
Ref Arxiv: 1010.5607
DOI: 10.1063/1.3541248
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We use replica exchange Monte-Carlo simulations to measure the equilibrium equation of state of the disordered fluid state for a binary hard sphere mixture up to very large densities where standard Monte-Carlo simulations do not easily reach thermal equilibrium. For the moderate system sizes we use (up to N=100), we find no sign of a pressure discontinuity near the location of dynamic glass singularities extrapolated using either algebraic or simple exponential divergences, suggesting they do not correspond to genuine thermodynamic glass transitions. Several scenarios are proposed for the fate of the fluid state in the thermodynamic limit.

Commentaires: 10 pages, 8 figs

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Ref Arxiv: 1001.0914
DOI: 10.1209/0295-5075/90/20005
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59 Citations
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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.

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Ref Arxiv: 1011.5637
DOI: 10.1088/1742-5468/2011/01/P01004
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When materials such as foams or emulsions are compressed, they display solid behaviour above the so-called 'jamming' transition. Because compression is done out-of-equilibrium in the absence of thermal fluctuations, jamming appears as a new kind of a nonequilibrium phase transition. In this proceeding paper, we suggest that tools from equilibrium statistical mechanics can in fact be used to describe many specific features of the jamming transition. Our strategy is to introduce thermal fluctuations and use statistical mechanics to describe the complex phase behaviour of systems of soft repulsive particles, before sending temperature to zero at the end of the calculation. We show that currently available implementations of standard tools such as integral equations, mode-coupling theory, or replica calculations all break down at low temperature and large density, but we suggest that new analytical schemes can be developed to provide a fully microscopic, quantitative description of the jamming transition.

Commentaires: 8 pages, 6 figs. Talk presented at Statphys24 (July 2010, Cairns, Australia)