Domaines de Recherche:  Physique/Matière Condensée/Mécanique statistique
 Physique/Matière Condensée/Matière Molle

Productions scientifiques :


Catching up with experiments: Equilibrium simulations of supercooled liquids beyond laboratory time scales
Auteur(s): Coslovich D., Berthier L., Ninarello A. S., Ozawa M.
Conference: 10th Liquid Matter Conference (Ljubljana, SI, 20170717)
Ref HAL: hal01576120_v1
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Résumé: Computer simulations give precious insight into the microscopic behavior of disordered and amorphous materials, but their typical time scales are orders of magnitude shorter than the experimentally relevant ones. In particular, simulations of supercooled liquids cover at most 45 decades of viscous slowing down, which falls far short of the 13 decades commonly accessible in experimental studies. We close this enormous gap for a class of realistic models of liquids, which we successfully equilibrate beyond laboratory time scales by means of the swap Monte Carlo algorithm. We show that combined optimization of selected features of the interaction potential, such as particle softness, polydispersity and nonadditivity, leads to computer models with excellent glassforming ability. For such models, we achieve over 10 orders of magnitude speedup in equilibration time scale. This numerical advance allows us to address some outstanding questions concerning glass formation, such as the role of local structure and the relevance of an entropy crisis, in a dynamical range that remains inaccessible in experiments. Our results support the view that nontrivial static correlations continue to build up steadily in supercooled liquids even below the laboratory glass temperature.



Models and algorithms for the next generation of glass transition studies
Auteur(s): Ninarello A. S., Berthier L., Coslovich D.
(Article) Publié:
Physical Review X, vol. 7 p.021039 (2017)
Ref HAL: hal01539636_v1
Ref Arxiv: 1704.08864
DOI: 10.1103/PhysRevX.7.021039
Ref. & Cit.: NASA ADS
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1 citation
Résumé: Successful computer studies of glassforming materials need to overcome both the natural tendency to structural ordering and the dramatic increase of relaxation times at low temperatures. We present a comprehensive analysis of eleven glassforming models to demonstrate that both challenges can be efficiently tackled using carefully designed models of size polydisperse supercooled liquids together with an efficient Monte Carlo algorithm where translational particle displacements are complemented by swaps of particle pairs. We study a broad range of size polydispersities, using both discrete and continuous mixtures, and we systematically investigate the role of particle softness, attractivity and nonadditivity of the interactions. Each system is characterized by its robustness against structural ordering and by the efficiency of the swap Monte Carlo algorithm. We show that the combined optimisation of the potential's softness, polydispersity and nonadditivity leads to novel computer models with excellent glassforming ability. For such models, we achieve over ten orders of magnitude gain in the equilibration timescale using the swap Monte Carlo algorithm, thus paving the way to computational studies of static and thermodynamic properties under experimental conditions. In addition, we provide microscopic insights into the performance of the swap algorithm which should help optimizing models and algorithms even further.



Equilibrium sampling of hard spheres up to the jamming density and beyond
Auteur(s): Berthier L., Coslovich D., Ninarello A. S., Ozawa M.
(Article) Publié:
Physical Review Letters, vol. 116 p.238002 (2016)
Ref HAL: hal01331594_v1
Ref Arxiv: 1511.06182
DOI: 10.1103/PhysRevLett.116.238002
Ref. & Cit.: NASA ADS
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10 citations
Résumé: We implement and optimize a particleswap MonteCarlo algorithm that allows us to thermalize a polydisperse system of hard spheres up to unprecedentedlylarge volume fractions, where \revise{previous} algorithms and experiments fail to equilibrate. We show that no glass singularity intervenes before the jamming density, which we independently determine through two distinct nonequilibrium protocols. We demonstrate that equilibrium fluid and nonequilibrium jammed states can have the same density, showing that the jamming transition cannot be the endpoint of the fluid branch.
Commentaires: 5 pages, 3 figs; To be published in Phys. Rev. Lett. Réf Journal: Phys. Rev. Lett. 116, 238002 (2016)



Structure and dynamics of coupled viscous liquids
Auteur(s): Ninarello A. S., Berthier L., Coslovich D.
(Article) Publié:
Molecular Physics, vol. 113 p.2707 (2015)
Ref HAL: hal01218876_v1
Ref Arxiv: 1504.06221
DOI: 10.1080/00268976.2015.1039089
Ref. & Cit.: NASA ADS
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3 citations
Résumé: We perform MonteCarlo simulations to analyse the structure and microscopic dynamics of a viscous LennardJones liquid coupled to a quenched reference configuration of the same liquid. The coupling between the two replicas is introduced via a field epsilon conjugate to the overlap Q between the two particle configurations. This allows us to study the evolution of various static and dynamic correlation functions across the (epsilon, T) equilibrium phase diagram. As the temperature is decreased, we identify increasingly marked precursors of a firstorder phase transition between a lowQ and a highQ phase induced by the field epsilon. We show in particular that both static and dynamic susceptibilities have a maximum at a temperaturedependent value of the coupling field, which defines a `Widom line'. We also show that, in the highoverlap regime, diffusion and structural relaxation are strongly decoupled because single particle motion mostly occurs via discrete hopping on the sites defined by the reference configuration. These results, obtained using conventional numerical tools, provide encouraging signs that an equilibrium phase transition exists in coupled viscous liquids, but also demonstrate that important numerical challenges must be overcome to obtain more conclusive numerical evidence.
Commentaires: 14 pages, 8 figures. Accepted for publication in Molecular Physics (Special Issue in honour of J.P. Hansen). Réf Journal: Mol. Physics 113, 2707 (2015)
