Shaping Self-Assembly and Mechanical Response in Dry and Polymer-Coated Granular Materials », Andrea Plati (Université Paris-Saclay).
Granular materials offer a playground to study emergent material properties at athermal scales. There, internal dissipation and external forcing intrinsically drive the system out of thermodynamic equilibrium. In this talk, I will connect two lines of work addressing how particle-scale interactions determine collective organization and mechanical response in granular matter.
The first part concerns self-assembly in binary mixtures of hard, dry grains vibrated on a substrate. Starting from a disordered state, and depending on the size ratio and relative proportions of the two species, we observe the spontaneous formation of quasi-crystalline structures and periodic square crystals [1,2]. Remarkably, these outcomes follow the equilibrium phase diagram of a hard-disk model originally devised for Brownian colloids, which highlights the key role of hard-core interactions and geometry in shaping collective organization. We also identify conditions for stable coexistence between the square crystal and an amorphous granular fluid, reminiscent of a first-order phase transition. Complementary numerical simulations reveal that this scenario is accompanied by a striking nonequilibrium effect: the coupling between local structure and energy transfer allows the two phases to coexist steadily at different effective temperatures.
In the second part, I will turn from purely repulsive grains to more complex contact mechanics engineered through polymer physical chemistry. I will give an overview of an ongoing experimental study on cohesive granular materials made of glass beads coated with polyborosiloxane (PBS), a crosslinked polymer with remarkable adhesive properties. The study aims to link the bulk rheology of PBS to grain-grain adhesion and, in turn, to the macroscopic mechanical response of the aggregates. In particular, I focus on how these properties depend on the physicochemical composition of the polymer coating, notably the crosslinker concentration.
[1] A. Plati, R. Maire, E. Fayen, F. Boulogne, F. Restagno, F. Smallenburg, and G. Foffi, Nat. Phys. 20, 465–471 (2024)
[2] A. Plati, R. Maire, F. Boulogne, F. Restagno, F. Smallenburg, and G. Foffi, J. Chem. Phys. 163, 054509 (2025)