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Ref Arxiv: 1805.06017
DOI: 10.1063/1.5040975
WoS: 000448216600025
Ref. & Cit.: NASA ADS
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11 Citations
Résumé:

We propose a computational method to measure the configurational entropy in generic polydisperse glass-formers. In particular, our method resolves issues related to the diverging mixing entropy term due to a continuous polydispersity. The configurational entropy is measured as the difference between the well-defined fluid entropy and a more problematic glass entropy. We show that the glass entropy can be computed by a simple generalisation of the Frenkel-Ladd thermodynamic integration method, which includes permutations of the particle diameters. This approach automatically provides a physically meaningful mixing entropy, and includes contributions that are not purely vibrational. The proposed configurational entropy is thus devoid of conceptual and technical difficulties due to continuous polydispersity, while being conceptually closer and technically simpler than alternative free energy approaches.

Commentaires: 11 pages, 4 figures. Réf Journal: J. Chem. Phys. 149, 154501 (2018)

PMID 29758608
DOI: 10.1103/PhysRevE.97.040601
WoS: WOS:000429636700001
30 Citations
Résumé:

We explore the glassy dynamics of soft colloids using microgels and charged particles interacting by steric and screened Coulomb interactions, respectively. In the supercooled regime, the structural relaxation time τα of both systems grows steeply with volume fraction, reminiscent of the behavior of colloidal hard spheres. Computer simulations confirm that the growth of τα on approaching the glass transition is independent of particle softness. By contrast, softness becomes relevant at very large packing fractions when the system falls out of equilibrium. In this nonequilibrium regime, τα depends surprisingly weakly on packing fraction, and time correlation functions exhibit a compressed exponential decay consistent with stress-driven relaxation. The transition to this novel regime coincides with the onset of an anomalous decrease in local order with increasing density typical of ultrasoft systems. We propose that these peculiar dynamics results from the combination of the nonequilibrium aging dynamics expected in the glassy state and the tendency of colloids interacting through soft potentials to refluidize at high packing fractions.

Ref HAL: hal-01877609_v1
Ref Arxiv: 1804.06800
DOI: 10.1103/PhysRevE.98.012609
WoS: WOS:000440141300011
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10 Citations
Résumé:

Discontinuous shear-thickening in dense suspensions naturally emerges from the activation of frictional forces by shear flow in non-Brownian systems close to jamming. Yet, this physical picture is incomplete as most experiments study soft colloidal particles subject to thermal fluctuations. To characterise discontinuous shear-thickening in colloidal suspensions we use computer simulations to provide a complete description of the competition between athermal jamming, frictional forces, thermal motion, particle softness, and shear flow. We intentionally neglect hydrodynamics, electrostatics, lubrication, and inertia, but can nevertheless achieve quantitative agreement with experimental findings. In particular, shear-thickening corresponds to a crossover between frictionless and frictional jamming regimes which is controlled by thermal fluctuations and particle softness and occurs at a softness dependent P\'eclet number. We also explore the consequences of our findings for constant pressure experiments, and critically discuss the reported emergence of `S-shaped' flow curves. Our work provides the minimal ingredients to quantitatively interpret a large body of experimental work on discontinuous shear-thickening in colloidal suspensions.

Commentaires: 17 pages, 9 figures. Accepted for publication in Phys. Rev. E. Réf Journal: Phys. Rev. E 98, 012609 (2018)

Ref HAL: hal-01876145_v1
DOI: 10.1038/s41467-018-05329-8
WoS: WOS:000439687600009
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8 Citations
Résumé:

Upon mechanical loading, granular materials yield and undergo plastic deformation. The nature of plastic deformation is essential for the development of the macroscopic constitutive models and the understanding of shear band formation. However, we still do not fully understand the microscopic nature of plastic deformation in disordered granular materials. Here we used synchrotron X-ray tomography technique to track the structural evolutions of three-dimensional granular materials under shear. We establish that highly distorted coplanar tetrahedra are the structural defects responsible for microscopic plasticity in disordered granular packings. The elementary plastic events occur through flip events which correspond to a neighbor switching process among these coplanar tetrahedra (or equivalently as the rotation motion of 4-ring disclinations). These events are discrete in space and possess specific orientations with the principal stress direction.

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