Initially observed for colloidal tracers in complex fluids [1], Fickian yet non-Gaussian diffusion (FnGD) has since emerged in a wide range of thermal and active systems, revealing its rather ubiquitous nature when particles move in heterogeneous environments.
In this talk, I will start by briefly recalling what we recently learned by investigating equilibrium supercooled liquids, where FnGD is closely related to dynamic heterogeneity [2] and to rare but very large cage-jumps that become especially relevant below the mode-coupling temperature [3].
I will then focus on out-of-equilibrium glassy materials, showing that FnGD occurs also in athermal jammed systems under steady shear [4]. On varying the shear rate γ̇ and the volume fraction φ in a model of soft non-Brownian suspensions, we find that the statistics of non-affine single-particle displacements obey robust master curves, with dependence on γ̇ and φ controlled only by three shifting factors related to characteristic time- and length-scales of microscopic dynamics. Shear-induced FnGD appears close to the onset of the apparent decoupling between shear stress and shear rate characterizing the yielding rheology of soft jammed solids, and extends over increasingly broad time windows on decreasing γ̇. A minimal stochastic model, which we term heterogeneous persistent random walk, captures the emerging dynamical features in terms of domain-dependent persistence times and finite renewal time of the local environment.
Overall, our results reveal the existence of FnGD in non-thermal systems and contribute to a better understanding of the rheology of soft jammed matter from a single-particle perspective [4].
References
[1] B. Wang, S. M. Anthony, S. C. Bae, and S. Granick, Proceedings of the National Academy of Sciences 106, 15160–15164 (2009).
[2] F. Rusciano, R. Pastore and F. Greco, Physical Review Letters 128, 168001 (2022).
[3] F. Rusciano, R. Pastore, F. Greco, and W. Kob, Physical Review X 16, 021061 (2026)
[4] P. Patel, F. Rusciano, F. Greco, and R. Pastore, to appear.