Résumé:

Most metals and ceramics are aggregates of crystalline grains. Grain-boundaries (GBs) are two-dimensional lattice defects that separate the different grains and that control the bulk mechanical properties of polycrystalline materials. In particular, the sliding and the migration of GBs play important roles in the plastic deformation. Moreover the plastic behaviour depends crucially on the size of the crystallites. Colloidal polycrystals behave as most metals and ceramics at larger time and length scales, allowing to investigate their morphology and mechanical properties by direct 3-D visualization. In this talk I will show how the texture of a colloidal model polycrystalline system is tailored by adding controlled amounts of impurity and by changing the speed of crystallization. The morphology is analyzed quantitatively by confocal microscopy and for the first time the kinetics of growth of the crystallites, the distribution and the role of the impurities during solidification is observed proving theoretical predictions not detectable in molecular systems.

Ref HAL: hal-00712583_v1
DOI: 10.1093/acprof:oso/9780199691470.001.0001
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Concentrated colloidal suspensions are a well-tested model system which has a glass transition. Colloids are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. Because the particle size can be large (100 nm - 1000 nm), these samples can be studied with a variety of optical techniques including microscopy and dynamic light scattering. Here we review the phenomena associated with the colloidal glass transition, and in particular discuss observations of spatial and temporally heterogeneous dynamics within colloidal samples near the glass transition. Although this Chapter focuses primarily on results from hard-sphere-like colloidal particles, we also discuss other colloidal systems with attractive or soft repulsive interactions.

Résumé:

In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass and jamming transitions are approached. There is now ample evidence that structural relaxation in glassy systems can only occur through correlated rearrangements of particles, leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, the size of these rearranging domains grows in glass-formers, colloids, and driven granular materials alike, providing a possible explanation for kinetic slowing. However, little is known yet on the behavior of dynamical heterogeneity and relaxation time very close to dynamical arrest. In this talk, I'll present recent results from our group for a simple model system --colloidal hard spheres--, as well as for other glassy and jammed soft materials, including actin networks. We establish that the volume fraction dependence of relaxation time and dynamic heterogeneity in colloidal hard spheres follow mode coupling theory (MCT) predictions only in a restricted density range. Unlike previous studies, we provide equilibrium measurements above the MCT critical packing fraction, thereby proving unambiguously that in our samples the algebraic divergence is absent at the predicted packing fraction. The behavior of dynamical heterogeneity is even more surprising. While in supercooled hard spheres the size of domains undergoing cooperative rearrangements is limited to a few particles at most, for many systems approaching jamming the correlation length of the dynamics increases dramatically, approaching the system size. The role of elasticity and of its microscopic origin in shaping dynamical correlations will be discussed.

Ref HAL: hal-00524211_v1
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We use optical microscopy to investigate the yielding mechanism of concentrated emulsions submitted to an oscillatory strain. While rheological measurements suggest a smooth transition from linear elasticity to plastic behavior as the amplitude of the imposed strain is increased, direct imaging of the sheared emulsions reveals that at a microscopic level yielding occurs abruptly. Indeed, we find that a well-defined critical strain c separates a regime where no irreversible droplet rearrangements occur during oscillatory strain from a plastic regime where almost all droplets undergo irreversible displacements. Above the volume fraction at which droplets achieve random close packing, the amplitude of the critical strain c increases linearly with droplet volume fraction. Surprisingly, we find that yielding is associated with very small droplet displacements, with almost no structural rearrangement. These rearrangement events are very heterogeneous both in space and time, with bursts of plastic activity separating long quiescent periods. The typical size of the rearranged regions is modest at random close packing and increases with increasing droplet concentration

Ref HAL: hal-00522291_v1
PMID 21230079
Ref Arxiv: 1006.4422
DOI: 10.1103/PhysRevE.82.031503
WoS: 000282134100008
Ref. & Cit.: NASA ADS
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42 Citations
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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

Ref HAL: hal-00522272_v1
Ref Arxiv: 1009.5558
DOI: 10.1039/b926873a
WoS: 000279046300023
Ref. & Cit.: NASA ADS
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22 Citations
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Foams are ideal model systems to study stress-driven dynamics, as stress-imbalances within the system are continuously generated by the coarsening process, which unlike thermal fluctuations, can be conveniently quantified by optical means. However, the high turbidity of foams generally hinders the detailed study of the temporal and spatial distribution of rearrangement events, such that definite assessments regarding their contribution to the overall dynamics could not be made so far. In this paper, we use novel light scattering techniques to measure the frequency and position of events within a large sample volume. As recently reported (A. S. Gittings and D. J. Durian, Phys. Rev. E, 2008, 78, 066313), we find that the foam dynamics is determined by two distinct processes: intermittent bubble rearrangements of finite duration and a spatially homogeneous quasicontinuous process. Our experiments show that the convolution of these two processes determines the age-dependence of the mean dynamics, such that relations between intermittent rearrangements and coarsening process can not be established by considering means. By contrast the use of the recently introduced photon correlation imaging technique (A. Duri, D. A. Sessoms, V. Trappe, and L. Cipelletti, Phys. Rev. Lett., 2009, 102, 085702) enables us to assess that the event frequency is directly determined by the strain-rate imposed by the coarsening process. Surprisingly, we also find that, although the distribution of successive events in time is consistent with a random process, the spatial distribution of successive events is not random: rearrangements are more likely to occur within a recently rearranged zone. This implies that a topological rearrangement is likely to lead to an unstable configuration, such that a small amount of coarsening-induced strain is sufficient to trigger another event.

Commentaires: Journal: Soft Matter: 6, 3030-3037 (2010)

Ref HAL: hal-00512112_v1
DOI: 10.1103/PhysRevLett.104.169602
WoS: 000277007700063
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16 Citations
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