Reorganization of a dense granular assembly:: The unjamming response function -: art. no. 031306

We investigate the mechanical properties of a static dense granular assembly in response to a local forcing. To this end, a small cyclic displacement is applied on a grain in the bulk of a two-dimensional disordered packing under gravity and the displacement fields are monitored. We evidence a dominant long range radial response in the upper half part above the solicitation and after a large number of cycles the response is quasireversible with a remanent dissipation field exhibiting long range streams and vortexlike symmetry.