Stability loss in granular media – STABINGRAM

The ANR project STABINGRAM aims to deepen our understanding of the jamming transition in dry and liquid immersed granular materials. Even though our knowledge of these materials has greatly improved, the modelization of this transition remains a challenge, because theoretical advances are hindered by fundamental problems such as memory effects and the heterogeneous character of the flows. To make progress, experimentalists need access to the grain-scale dynamics (micro-displacements and rearrangements) that leads to solid-liquid transition in granular packings. This is possible today with recently developed and validated techniques such as X-ray tomography, the diffusion of light, and nonlinear acoustics. Another useful tool for studying grain-scale dynamics is numerical simulation, and two main applications will be developed here: the simulation of acoustic propagation by the technique of molecular dynamics, and the simulation of liquid immersed grains in the dense regime by a technique of contact dynamics completed by lubrication. The confrontation between experiment and simulation is indispensable for improving modelization.

The project will be lead by researchers from different communities within complementary partners: physicists (partner 2: physics), engineers in mechanics (partner 1: FAST) and acoustics (partner 3: LAUM) and geophysicists (partner 4: Geosciences Rennes), associated with specialists in numerical methods (mathematicians from Orsay university (LMO) and Ecole Polytechnique (CMAP)) and theoreticians (partner 1: LPTMS). This choice of partners is motivated by the complexity of the phenomena considered, and the large range of numerical and experimental methods that will be used as well as the applications to Earth Sciences.

The experiments and numerical simulations proposed in the project STABINGRAM are motivated by several goals related to open questions in the physics of granular materials:
- We wish to characterize and understand the microrearrangements or "precursors" responsable for destabilization of packings and flows.
- We will seek to understand the connection between precursors and the resulting instability
We will also seek to distinguish effects due to the external forcing and those that are intrinsic to granular materials.

The scientific program consists of several small-scale experiments leading to the destabilization of a granular packing combined with the development and adaptation of experimental and numerical tools for studying packings on the threshold of stability. The project is organized into three interlocking tasks. Task 1 is devoted to the characterization of packings at the threshold of instability, whereas task 2 is concerned with the structural dynamics of the flow. Each one of these tasks has three parts that treat different instability situations: (i) avalanches due to the slow inclination of granular packing, (ii) compression followed by penetration, and (iii) local fluidization or erosion by a upwards or downwards normal jet flow. Task 3 unites the first two by developing experimental tools (acoustics, light scattering, X-ray tomography) and numerical tools (molecular and contact dynamics).