Résumé Court

The glass transition is the dramatic dynamical slowdown one observes when cooling down a liquid or densifying a set of particles into an amorphous state. In this thesis we explore two distinct situations related to this phenomenology, mostly in the case of granular materials. In a first part we study the strongly non-linear response of an assembly of frictional grains under vibration when they are very densely packed. To this aim we pull a probe particle – an “intruder” – through the media and follow the dynamics of the particles around. We evidence the presence of two transitions : the first one is analogous to a fluidisation transition, as shown by the evolution from a continuous to a highly intermittent motion and by the appearance of a yield stress in the response; the second one is identified as the jamming transition that occurs in several systems. We show that the reorganizations induced by the intruder display critical signatures at the transition, as previously shown by the study of the same system without drag, leading to interesting connections with recent simulations in athermal and frictionless particles systems. In a second part, we study the relations between the very short term dynamics – essentially made of vibrations of the particles around their equilibrium positions – and the long time scale dynamics where dynamical heterogeneities are responsible for the slowdown of the structural relaxations on approach to the glass transition. The dynamics of the metastable states is analyzed in two experimental granular systems (grains under cyclic shear or in a fluidized bed) as well as in simulations of repulsive particles. We show the dominant role of quasi-instantaneous cooperative displacements to build the large intermittent decorrelation patterns that one observes on longer time scales through a facilitation mechanism. Facilitation becomes less and less efficient when the packing fraction increases, leading to increasingly separated and concentrated dynamical events in space and time. The vibrational dynamics leading to the cooperative motion are also investigated, and we show that the latter are essentially driven by the structure, coarse grained at a mesoscopic scale.