Résumé:

Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition.

Résumé:

We use conventional and multispeckle dynamic light scattering to investigate the dynamics of a wide variety of jammed soft materials, including colloidal gels, concentrated emulsions, and concentrated surfactant phases. For all systems, the dynamic structure factor f (q, t) exhibits a two-step decay. The initial decay is due to the thermally activated diffusive motion of the scatterers, as indicated by the q(-2) dependence of the characteristic relaxation time, where q is the scattering vector. However, due to the constrained motion of the scatterers in jammed systems, the dynamics are arrested and the initial decay terminates in a plateau. Surprisingly, we find that a final, ultraslow decay leads to the complete relaxation of f (q, t), indicative of rearrangements on length scales as large as several microns or tens of microns. Remarkably, for all systems the same very peculiar form is found for the final relaxation of the dynamic structure factor: f (q, t) similar to exp [ (t /tau(s))(p)], with p approximate to 1.5 and t(s) similar to q(-1), thus suggesting the generality of this behavior. Additionally, for all samples the final relaxation slows down with age, although the aging behavior is found to be sample dependent. We propose that the unusual ultraslow dynamics are due to the relaxation of internal stresses, built into the sample at the jamming transition, and present simple scaling arguments that support this hypothesis.

Ref HAL: hal-00004280_v1
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Résumé:

We introduce a new scheme to investigate temporally heterogeneous dynamics, which is termed Time Resolved Correlation (TRC). TRC is applied to data obtained by Diffusing Wave Spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

Commentaires: proceedings of the 5th Liquid Matter Conference (Konstanz 2002)

Résumé:

We investigate the slow dynamics of strongly aggregated particle gels by diffusing wave spectroscopy (DWS) using a charge coupled device (CCD) camera as a multispeckle detector. The evolution of the slow dynamics after the gel-point is followed using two different techniques, the multispeckle method and a novel correlation scheme, time resolved correlation (TRC), which allows us to obtain time resolved information on the dynamics of the gel. We find that the intensity autocorrelation function measured by multispeckle DWS exhibits an ultraslow relaxation, whose characteristic time strongly depends on the age of the gel. An analysis of the TRC data reveals that the slow dynamics is intermittent. We propose that the intermittency is due to rare disruptions or formations of bonds affecting the elastic backbone of the network, which lead to sudden long range rearrangements of the particle position and thus to a sudden change of the speckle pattern.