Andrea Plati

At rest, granular materials are trapped in metastable configurations, but when subjected to sufficiently strong mechanical vibrations, they begin to flow and rearrange. Such vibrofluidized systems can reach a nonequilibrium steady state, in which the statistical properties of the dynamics remain constant but time-reversal symmetry is broken by forcing and dissipative mechanisms. The main focus of my talk will be to show how vibrofluidized granular materials exhibit phenomena in which equilibrium-like and nonequilibrium features are deeply intertwined.

First, I will present our experimental study of the self-assembly processes of granular binary mixtures vibrated on a substrate. Starting from a disordered configuration and depending on the size ratio and proportions of the two grain species, we observed the spontaneous formation of quasi-crystalline structures [1] and periodic square crystals [2]. Remarkably, this follows predictions from the equilibrium phase diagram of a hard disk model with conservative dynamics. We have also obtained experimental conditions that realize stable coexistence between the square crystal phase and a granular amorphous fluid. Through numerical simulations, we demonstrated that the observed phenomena resemble those of a first-order equilibrium phase transition. However, the two phases coexist at different effective temperatures, which is a striking nonequilibrium effect. Unexpectedly, the denser crystalline phase is hotter than the less dense liquid phase. Through qualitative arguments supported by kinetic theory, we elucidate the role of coupling between local structure and energy transfer mechanisms in maintaining kinetic temperature gradients across the fluid-crystalline interface.

In the final part of my talk, I will discuss how our setup can also be used as a playground for the study of absorbing phase transitions and hyperuniform fluids. I will summarize the relevant theoretical and numerical results [3] and present some recent experimental findings.

[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)

[3]: R. Maire, A. Plati, M. Stockinger, E. Trizac, F. Smallenburg, G. Foffi, Phys. Rev. Lett. 132, 238202 (2024)