Universal non-diffusive slow dynamics in aging soft matter
Luca Cipelletti, Laurence Ramos, S. Manley, E. Pitard, D. A. Weitz, Eugene E. Pashkovski, Marie Johansson
2002
Faraday discussions
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 À2 dependence of the characteristic relaxation time, where q is the scattering vector.
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... wever, 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) $ exp[À(t/t s ) p ], with p % 1.5 and t s $ q À1 , 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. I Introduction Disordered, solid-like materials are ubiquitous in soft condensed matter. They range from foams to polymer or particle gels, concentrated emulsions or colloidal suspensions. These materials, whose applications are countless, are typically obtained by a fluid-to-solid transition, which often quenches the system in a far-from-equilibrium configuration. Recent work has focused on the shared features of the solid-fluid transition in disordered materials, leading to the introduction of the concept of jamming. Liu and Nagel 1 have proposed a 3-dimensional jamming transition phase diagram, which unifies a wide range of fluid-solid transitions; Trappe et al. 2 have shown that a jamming phase diagram can indeed be established for a large variety of attractive colloidal systems. Other recent investigations have also pointed out the similarities between jammed systems such as gels and glasses. 3, 4 In jammed systems, the mobility of the constituents is extremely reduced, due to
doi:10.1039/b204495a
pmid:12638864
fatcat:tn74dsgomnfixpcneyl4fbbm2e