From Exotic grain shapes to smart metagranular structures: the road to Geometric Cohesion origins – Exo2GeCo

In classical constructions, building granular structure is only possible when the grains are cemented together (as concrete where gravel is made cohesive from cement paste). Adhesive forces transform the forces network into a self-stressed network of tensile/compressive forces. These adhesive forces permit to get a cohesive system but are energy consuming and make the system irreversibly cohesive. Self-supported granular structure can also emerge from non-convex grains without any external energy consuming process. In this case, self- stressed force chains result from grain entanglement. Geometric cohesion (i.e., the existence of cohesion induced by geometry) is a new paradigm, very little studied and poorly understood. This lack of knowledge can be attributed to three major facts: 1) this field of investigation is very young, 2) it requires large experimental & financial resources to design a representative quantity of grains and explore as much shapes as possible, 3) the most common Discrete Element Method (DEM) used for studying granular systems is not suitable. In general, contact laws are chosen proportional to grain overlapping (Hertz law) inducing ad'hoc damping parameters to be calibrated. This is clearly problematic for exotic grains, of evolving shapes, with a potential huge number of contacts. Also, in experiments, mainly spherical or at least convex grains are used. Because of the difficulty to image and to measure force in 3D, granular systems are mostly studied in 2D. All these scientific & technical challenges explain the small number of numerical, experimental, theoretical studies on systems made of exotic grains, from their static to flow properties. Systematical approaches are urgently needed to 1) Model assemblies of exotic grains properties to build the database of grains inducing geometric cohesion, to 2) Identify the micro-mechanisms at play when geometric cohesion appear, and to 3) Link geometric cohesion to structure strength.