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Transition vitreuse, hétérogénéité dynamique et vieillissement dans les systèmes a dynamique lente
(24) Production(s) de l'année 2021
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Relaxation dynamics of non-Brownian spheres below jamming
Auteur(s): Nishikawa Y., Ikeda A., Berthier L.
(Article) Publié:
Journal Of Statistical Physics, vol. p.37 (2021)
Texte intégral en Openaccess :
Ref HAL: hal-03138225_v1
Ref Arxiv: 2007.09418
DOI: 10.1007/s10955-021-02710-8
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé: We numerically study the relaxation dynamics and associated criticality of non-Brownian frictionless spheres below jamming in spatial dimensions $d=2$, $3$, $4$, and $8$, and in the mean-field Mari-Kurchan model. We discover non-trivial finite-size and volume fraction dependences of the relaxation time associated to the relaxation of unjammed packings. In particular, the relaxation time is shown to diverge logarithmically with system size at any density below jamming, and no critical exponent can characterise its behaviour approaching jamming. In mean-field, the relaxation time is instead well-defined: it diverges at jamming with a critical exponent that we determine numerically and differs from an earlier mean-field prediction. We rationalise the finite $d$ logarithmic divergence using an extreme-value statistics argument in which the relaxation time is dominated by the most connected region of the system. The same argument shows that the earlier proposition that relaxation dynamics and shear viscosity are directly related breaks down in large systems. The shear viscosity of non-Brownian packings is well-defined in all $d$ in the thermodynamic limit, but large finite-size effects plague its measurement close to jamming.
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Towards a better understanding of grass bed dynamics using remote sensing at high spatial and temporal resolutions
Auteur(s): Marion Menu, Guillaume Papuga, Frédéric Andrieu, Guilhem Debarros, Xavier Fortuny, Samuel Alleaume, Pitard E.
(Article) Publié:
Estuarine, Coastal And Shelf Science, vol. 251 p.107229 (2021)
Texte intégral en Openaccess :
Ref HAL: hal-03125706_v1
DOI: 10.1016/j.ecss.2021.107229
Exporter : BibTex | endNote
Résumé: Wetlands conservation and resilience capacities are key issues in many places over the globe. Understanding these issues will benefit from a precise knowledge of seagrass species occupancy and coverage over time and over space. Such information can be obtained from remote sensing images and their classification thanks to a vegetation index, to be used in a complementary manner to field work inventories. Sentinel-2 data, which are available with a frequent revisit time (<5 days) and a high spatial resolution (10m pixel size) can be used to map grassbeds at the surface or slightly below the surface of permanent lagoons, hence enabling the characterization of its seasonal dynamics, which was not possible with previous remote-sensing tools. We have proved the feasibility of such a method in the natural reserve of the Bagnas (Herault, France) where Stuckenia pectinata coverage can be tracked over a full year thanks to Sentinel-2 images and field work. Inter-annual dynamics (seasonal growth and senescence) can be mapped over time with 10m resolution and will be extended to pluriannual studies thanks to the long-term objective of the Sentinel-2 mission. This opens the way to a concerted management of natural reserves based on data analysis and field knowledge, a better understanding of seagrass coverage with fluctuating environmental conditions, and predictive mechanistic and/or stochastic models of future qualitative trends.
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Friction-Controlled Entropy-Stability Competition in Granular Systems
Auteur(s): Sun X., Kob W., Blumenfeld R., Tong H., Wang Y., Zhang J.
(Article) Publié:
Physical Review Letters, vol. 125 p.268005 (2021)
Texte intégral en Openaccess :
Ref HAL: hal-03117915_v1
Ref Arxiv: 2007.14145
DOI: 10.1103/PhysRevLett.125.268005
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé: Using cyclic shear to drive a two dimensional granular system, we determine the structural char-acteristics for different inter-particle friction coefficients. These characteristics are the result of acompetition between mechanical stability and entropy, with the latter’s effect increasing with fric-tion. We show that a parameter-free maximum-entropy argument alone predicts an exponential cell order distribution, with excellent agreement with the experimental observation. We show thatfriction only tunes the mean cell order and, consequently, the exponential decay rate and the pack-ing fraction. We further show that cells, which can be very large in such systems, are short-lived,implying that our systems are liquid-like rather than glassy..
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