Crossing the granular fluid-glass transition : soft modes, acoustics and rheology – JamVibe

.Granular systems are known to display jamming, i.e. a sharp transition between fluid-like and solid-like states. The aim of this project, through the scope of granular matter under vibration, will address explicitly the relation between internal relaxation processes (soft-modes, granular reorganization) and external driving (vibration and/or shear). It will mix experimental, numerical and theoretical efforts and will consider the mechanical status of the jammed region both viewed from the solid and the fluid phases, by developing the following four points. (i) Elastic behavior and fragility anomalies in the vicinity of the jamming transition will be probed experimentally with an acoustic surface wave resonance method. The experiments will be complemented by the numerical study of an inclined layer using the stress response technique and a theoretical investigation on the interaction between soft-mode and elastic waves. (ii) Under a weak level of vibration, the ability for a packing to unjamm and reorganize will be investigated. We will address specifically the relation between soft-mode properties and the existence cooperative processes leading to a slow relaxation and eventually to a blockade dynamics. A statistical technique based on the particle position covariance matrix will be used to address the question of irreversible processes leading to a diffusive dynamics. The study will be also performed in model packing with the objective to understand the impact of mechanisms such as local dissipation due to friction. The robustness of the soft-mode vision to understand granular matter irreversible processes will be especially under scope. (iii) We wish to establish a deeper vision of transport properties (rheology) of granular matter in the near-jammed phase. This will clarify the role of collective reorganizations in the dynamics of the system as well as the elusive notions of stress threshold, intermittent dynamics and effective friction. A key task for this point is the study of several rheometric devices in the presence of vibration to express in a tunable way, the relation between the fragile solid matrix and the fluid phase. Experimentally, we will use two rheometric devices based on the motion of an intruder and granular flows, both systems will have the ability to be weakly vibrated. Numerically, we will simulate a 2D shear cell, also submitted to a vibration of given amplitude and frequency. In all cases, the transient existence of solid and liquid domains intermixed in various proportions will be addressed by the development of novel statistical physics indicators. The impact of this phase mixing on the rheology will be evaluated and possibly will lead to a practicable phenomenological model. (iv) We will address experimentally the validity of a new scenario describing a positive feed-back coupling between shear and sound waves (internal vibrations). This will shed light on the metastable character of the jamming transition as well as the mechanism of shear band formation in granular flows and the possibility to synchronized internal degrees of freedom with collective granular motion. We see that, transversally to all these four points, comes the recurrent question of the relation between internal degrees of freedom, collective reorganizations of the packing and mechanical and dynamical properties of the system. The presence and distribution of (quasi) soft modes as the system approaches mobility onset, has been evidenced in many context besides granular matter but we ignore so far its real impact of the macroscopic. This conceptually open question, will serve as the leading thread for the proposal. A second objective on the theoretical side will be to take the outcome of the experimental/numerical studies in order to gain new insights on glassy dynamics. Hence we expect to gain a new vision, based on an original approach, leading possibly to a reformulation of the statistical physics of the glassy transition.