Hot spots in an athermal system

We study experimentally the dynamical heterogeneities occuring at slow shear, in a model amorphous glassy material i.e. a 3D granular packing. The deformation field is resolved spatially using a Diffusive Wave Spectroscopy technique. The heterogeneities show up as localized regions of strong deformations spanning a mesoscopic size of about $10$ grains and called the 'hot spots'. The spatial clustering of hot-spots is linked to the subsequent emergence of shear bands. Quantitively, their apparition is associated with the macroscopic plastic deformation and their rate of occurence gives a physical meaning to the concept of ``fluidity,'' recently used to describe the local and non-local rheology of soft glassy materials.