Résumé
We use Photon Correlation Imaging, a recently introduced space-resolved
dynamic light scattering method, to investigate the spatial correlation of the
dynamics of a variety of jammed and glassy soft materials. Strikingly, we find
that in deeply jammed soft materials spatial correlations of the dynamics are
quite generally ultra-long ranged, extending up to the system size, orders of
magnitude larger than any relevant structural length scale, such as the
particle size, or the mesh size for colloidal gel systems. This has to be
contrasted with the case of molecular, colloidal and granular ``supercooled''
fluids, where spatial correlations of the dynamics extend over a few particles
at most. Our findings suggest that ultra long range spatial correlations in the
dynamics of a system are directly related to the origin of elasticity. While
solid-like systems with entropic elasticity exhibit very moderate correlations,
systems with enthalpic elasticity exhibit ultra-long range correlations due to
the effective transmission of strains throughout the contact network.