When the soft material begins to flow, the diffusion of the particles takes an unexpected turn

Soft materials, such as colloidal suspensions, emulsions, foams, and gels, often exhibit complex and unusual behavior compared to ordinary solids or liquids. From a mechanical standpoint, for example, they can resist deformation like solids but also begin to flow like liquids when a sufficiently intense external stress is applied. This transition from solid to liquid behavior, known as yielding, is central to the physics of amorphous soft materials and to many processes of industrial and biological interest. Some of these systems are also athermic: they consist of particles large enough not to be affected by thermal motion. Their microscopic motion is therefore governed primarily by external mechanical stress and interactions with neighboring particles, making their behavior even richer and more intriguing.

A new study published in *Communications Physics*, a journal in the Nature Portfolio, establishes a connection between mechanical yielding in athermic soft matter and a peculiar form of microscopic dynamics known as Fickian yet non-Gaussian diffusion (FnGD). This phenomenon was first observed in 2009 and has since been identified in various molecular systems, as well as in thermal and active soft materials, particularly when particles move in heterogeneous environments. The study was conducted at the Department of Chemical, Materials, and Industrial Production Engineering at Federico II, as part of the PRIN 2022 project “Fickian non-Gaussian diffusion in static and dynamic environments, ” led by Principal Investigator Raffaele Pastore. The team also includes Palak Patel, a postdoctoral researcher at Federico II following her Ph.D. at the University of Pune in India, along with Francesco Rusciano and Francesco Greco. Through numerical simulations, the group has shown that FnGD can also emerge in athermal jammed systems, such as compressed emulsions, dense granular suspensions, and foams, when particle motion is induced by a shear flow. The results link this unusual regime of microscopic transport to the onset of rheological yielding in soft jammed solids.

From Microscopic Motion to Mechanical Response. In recent years, FnGD has attracted significant attention because it challenges the standard framework of microscopic diffusion, with its dichotomous distinction between Brownian motion and anomalous diffusion. Until now, however, this hybrid form of diffusion had been interpreted primarily as a property of single-particle dynamics, seemingly confined to the microscopic world. It was much less clear whether it could also be linked to the material’s large-scale response.

Thus, a central question is whether FnGD is merely a microscopic detail of particle dynamics, or whether it can have an impact on the macroscopic behavior of the material. Jammed soft matter, which combines a rich mechanical response at the macroscopic level with heterogeneous dynamics at the microscopic level, offers an ideal context for addressing this question. The study shows that, in these materials, FnGD is not merely a microscopic feature of particle dynamics but is closely linked to the mechanical response near yielding.

A key finding of the study is that the microscopic time scales characterizing FnGD are closely linked to the macroscopic rheology of the material. In soft, jammed materials near yielding, an apparent decoupling between shear rate and shear stress is observed: the shear stress tends to saturate toward a plateau value—the so-called yield stress—becoming nearly independent of the shear rate. The authors have shown that a similar decoupling also emerges in the microscopic dynamics: the characteristic time marking the entry into the Fickian regime tends to saturate, analogous to shear stress, while the time required to recover a Gaussian distribution of displacements continues to grow as the inverse of the shear rate. This separation between time scales gives rise to FnGD and suggests that it can be considered a microscopic counterpart to mechanical yielding .

Bibliographic Reference
Palak Patel, Francesco Rusciano, Francesco Greco, and Raffaele Pastore,“Shear-induced Fickian yet non-Gaussian diffusion in athermal jammed matter,” Communications Physics 9, 284 (2026).

Article link: https://doi.org/10.1038/s42005-026-02817-y