Ref HAL: hal-00512108_v1
PMID 19933125
Ref Arxiv: 0907.2329
DOI: 10.1098/rsta.2009.0178
WoS: 000271780000003
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65 Citations
Résumé:

We investigate dynamical heterogeneities in the collective relaxation of a concentrated microgel system, for which the packing fraction can be conveniently varied by changing the temperature. The packing fraction dependent mechanical properties are characterised by a fluid-solid transition, where the system properties switch from a viscous to an elastic low-frequency behaviour. Approaching this transition from below, we find that the range of spatial correlations in the dynamics increases. Beyond this transition, the spatial correlation range reaches a maximum, extending over the entire observable system size of approximately 5 mm. Increasing the packing fraction even further leads to a second transition, which is characterised by the development of large zones of lower and higher dynamical activity that are well separated from each other; the range of correlation decreases at this point. This striking non-monotonic dependence of the spatial correlation length on volume fraction is reminiscent of the behaviour recently observed at the jamming/rigidity transition in granular systems (Lechenault et al. 2008). We identify this second transition as the transition to 'squeezed' states, where the constituents of the system start to exert direct contact forces on each other, such that the dynamics becomes increasingly determined by imbalanced stresses. Evidence of this transition is also found in the frequency dependence of the storage and loss moduli, which become increasingly coupled as direct friction between the particles starts to contribute to the dissipative losses within the system. To our knowledge, our data provide the first observation of a qualitative change in dynamical heterogeneity as the dynamics switch from purely thermally-driven to stress-driven.

Commentaires: published in Phil. Trans. R. Soc. A

Ref HAL: hal-00512107_v1
Ref Arxiv: 0903.1933
DOI: 10.1088/1742-5468/2009/07/P07015
WoS: 000269353300015
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42 Citations
Résumé:

We use dynamic light scattering and numerical simulations to study the approach to equilibrium and the equilibrium dynamics of systems of colloidal hard spheres over a broad range of density, from dilute systems up to very concentrated suspensions undergoing glassy dynamics. We discuss several experimental issues (sedimentation, thermal control, non-equilibrium aging effects, dynamic heterogeneity) arising when very large relaxation times are measured. When analyzed over more than seven decades in time, we find that the equilibrium relaxation time, tau_alpha, of our system is described by the algebraic divergence predicted by mode-coupling theory over a window of about three decades. At higher density, tau_alpha increases exponentially with distance to a critical volume fraction phi_0 which is much larger than the mode-coupling singularity. This is reminiscent of the behavior of molecular glass-formers in the activated regime. We compare these results to previous work, carefully discussing crystallization and size polydispersity effects. Our results suggest the absence of a genuine algebraic divergence of tau_alpha in colloidal hard spheres.

Commentaires: 16 pages; 12 figures

Ref HAL: hal-00512105_v1
PMID 19257033
Ref Arxiv: 0812.3847
DOI: 10.1103/PhysRevE.79.011405
WoS: 000262976600046
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22 Citations
Résumé:

We use x-ray photon correlation spectroscopy to investigate the dynamics of a high volume fraction emulsion creaming under gravity. The dodecane-in-water emulsion has interfaces stabilized solely by colloidal particles (silica). The samples were observed soon after mixing: as the emulsion becomes compact we discern two regimes of ageing with a cross-over between them. The young emulsion has faster dynamics associated with creaming in a crowded environment accompanied by local rearrangements. The dynamics slow down for the older emulsion although our studies show that motion is associated with large intermittent events. The relaxation rate, as seen from the intensity autocorrelation function, depends linearly on the wave vector at all times; however, the exponent associated with the line shape changes from 1.5 for young samples to less than 1 as the emulsion ages. The combination of ballisticlike dynamics, an exponent that drops below 1 and large intermittent fluctuations has not been reported before.

Commentaires: 8 pages, 5 figures

Ref HAL: hal-00330408_v3
PMID 19257754
Ref Arxiv: 0810.2431
DOI: 10.1103/PhysRevLett.102.085702
WoS: 000263816200033
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74 Citations
Résumé:

We introduce a new dynamic light scattering method, termed photon correlation imaging, which enables us to resolve the dynamics of soft matter in space and time. We demonstrate photon correlation imaging by investigating the slow dynamics of a quasi two-dimensional coarsening foam made of highly packed, deformable bubbles and a rigid gel network formed by dilute, attractive colloidal particles. We find the dynamics of both systems to be determined by intermittent rearrangement events. For the foam, the rearrangements extend over a few bubbles, but a small dynamical correlation is observed up to macroscopic length scales. For the gel, dynamical correlations extend up to the system size. These results indicate that dynamical correlations can be extremely long-ranged in jammed systems and point to the key role of mechanical properties in determining their nature.

Commentaires: Published version (Phys. Rev. Lett. 102, 085702 (2009)) The Dynamical Activity Mapsprovided as Supplementary Online Material are also available on w3.lcvn.univ-montp2.fr/~lucacip/dam/movies.htm