Abstract
Intermittent dynamics driven by internal stress imbalances in disordered systems is a fascinating yet poorly understood phenomenon. Here, we study it for a coarsening foam. By exploiting differential dynamic microscopy and particle tracking we determine the dynamical characteristics of the foam at different ages in reciprocal and direct space, respectively. At all wavevectors $q$ investigated, the intermediate scattering function exhibits a compressed exponential decay. However, the access to unprecedentedly small values of $q$ highlights the existence of two distinct regimes for the $q$-dependence of the foam relaxation rate $\Gamma(q)$. At high $q$, $\Gamma(q) \sim q$ consistent with directionally-persistent and intermittent bubble displacements. At low $q$, we find the surprising scaling $\Gamma(q) \sim q^{\delta}$, with $\delta$ = 1.6 $\pm$ 0.2. The analysis of the bubble displacement distribution in real space reveals the existence of a displacement cut-off of the order of the bubble diameter. Introducing such cut-off length in an existing model, describing stress-driven dynamics in disordered systems, fully accounts for the observed behavior in direct and reciprocal space.
Roberto Cerbino
Professor of Experimental Soft Matter Physics
My research interests include Soft matter physics, living matter, cell biophysics and quantitative microscopy.