title: Anomalous dynamical light scattering in soft glassy gels.
abstract: We compute the dynamical structure factor S( q,tau) of an elastic medium where force dipoles appear at random in space and in time, due to "micro-collapses" of the structure. Various regimes are found, depending on the wave vector q and the collapse time theta. In an early time regime, the logarithm of the structure factor behaves as (q tau)(3/2), as predicted in (L. Cipelletti et al., Phys. Rev Lett. 84, 2275 (2000)) using heuristic arguments. However, in an intermediate-time regime we rather obtain a (q tau)(5/4) behaviour. Finally, the asymptotic long-time regime is found to behave as q(3/2)tau. We also give a plausible scenario for aging, in terms of a strain-dependent energy barrier for micro-collapses. The relaxation time is found to grow with the age t(w), quasi-exponentially at first, and then as t(w)(4/5) with logarithmic corrections.
authors.affiliation: Service de Physique de l'Etat Condensé, Centre d'études de Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France. bouchau@spec.saclay.cea.fr
authors.forename: J-P
authors.initials: JP
authors.lastname: Bouchaud
authors.forename: E
authors.initials: E
authors.lastname: Pitard