Ref HAL: hal-01904079_v1
Ref Arxiv: 1804.01810
DOI: 10.1073/pnas.1717403115
WoS: 000429012500051
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
19 Citations
Résumé:

Material failure is ubiquitous, with implications from geology to everyday life and material science. It often involves sudden, unpredictable events, with little or no macroscopically detectable precursors. A deeper understanding of the microscopic mechanisms eventually leading to failure is clearly required, but experiments remain scarce. Here, we show that the microscopic dynamics of a colloidal gel, a model network-forming system, exhibit dramatic changes that precede its macroscopic failure by thousands of seconds. Using an original setup coupling light scattering and rheology, we simultaneously measure the macroscopic deformation and the microscopic dynamics of the gel, while applying a constant shear stress. We show that the network failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements.

Commentaires: . Réf Journal: PNAS 115, 3587 (2018)

Ref HAL: hal-01896690_v1
DOI: 10.1039/c8sm00911b
WoS: WOS:000442269000021
Exporter : BibTex | endNote
1 Citation
Résumé:

We use space-resolved dynamic light scattering in the highly multiple scattering regime (Photon Correlation Imaging Diffusing Wave Spectroscopy, PCI-DWS) to investigate temperature-induced phase transitions in polymorphic materials. We study paraffin wax as a simple model system and chocolate, a prototypical example of fat-based products exhibiting complex, history-dependent phase transitions. We find that microscopic dynamics measured using PCI-DWS show remarkable, non-monotonic behavior upon heating: they transiently accelerate when crossing phase transition and slow down above the transition temperature. Sub-micron resolution measurements of the local drift of the sample surface reveal that the speed-up of the dynamics is due to the strain field induced by the change in density at transition temperature. The transition temperatures obtained from PCI-DWS are found to be in excellent agreement with those inferred from complementary differential scanning calorimetry and X-ray scattering experiments, thereby validating PCI-DWS as a new, powerful tool for the characterization of phase transitions in complex soft matter. Finally, we demonstrate the unique possibilities afforded by space-resolved DWS by investigating the spatially heterogeneous response of poorly manufactured or composite chocolate samples.

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-01889803_v1
Ref Arxiv: 1802.03820
DOI: 10.1122/1.5025622
WoS: 000449684700010
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
10 Citations
Résumé:

Power law rheology is of widespread occurrence in complex materials that are characterized by the presence of a very broad range of microstructural length and time scales. Although phenomenological models able to reproduce the observed rheological features exist, in general a well-established connection with the microscopic origin of this mechanical behavior is still missing. As a model system, this work focuses on a fractal colloidal gel. We thoroughly characterize the linear power law rheology of the sample and its age dependence. We show that at all sample ages and for a variety of rheological tests the gel linear viscoelasticity is very accurately described by a Fractional Maxwell (FM) model, characterized by a power law behavior. Thanks to a unique set-up that couples small-angle static and dynamic light scattering to rheological measurements, we demonstrate that the power law rheology observed in the linear regime originates from reversible non-affine rearrangements and discuss the possible relationship between the FM model and the microscopic structure of the gel.

Commentaires: . Réf Journal: Journal of Rheology, 62, 1429-1441 (2018)

Ref HAL: hal-01877609_v1
Ref Arxiv: 1804.06800
DOI: 10.1103/PhysRevE.98.012609
WoS: WOS:000440141300011
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
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
Exporter : BibTex | endNote
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.

Ref HAL: hal-01870809_v1
Exporter : BibTex | endNote