This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Soft materials such as colloidal suspensions, emulsions, foams and gels often display complex and unusual behaviors compared with ordinary solids or liquids. From a mechanical point of view, for instance, they can resist deformation like solids, but they can also start to flow like liquids when a sufficiently strong external drive is applied.
This transition from solid-like to liquid-like behavior, known as "yielding," is central to the physics of soft amorphous materials and to many industrial and biological processes. Some of these systems are also athermal: They are made of particles large enough that thermal motion plays no role in their dynamics. Their microscopic motion is therefore governed mainly by mechanical driving and interactions with neighboring particles, making their behavior even more unusual and intriguing.
A new study published in Communications Physics, establishes a connection between the mechanical yielding of soft athermal matter and a distinctive form of microscopic dynamics: Fickian yet non-Gaussian diffusion (FnGD). This phenomenon, also referred to as Brownian yet non-Gaussian diffusion, was first reported in 2009 and has since been identified in a variety of molecular systems, as well as in thermal and active soft matter, especially for particles moving in heterogeneous environments. The study was carried out at the University of Naples Federico II (Department of Chemical, Materials and Production Engineering), within the Italian national research project PRIN 2022 "Fickian non-Gaussian diffusion in static and dynamic environments," led by principal investigator Raffaele Pastore.
The team also includes Palak Patel, who joined Federico II as a postdoctoral researcher after completing her Ph.D. at the University of Pune (India), as well as Francesco Rusciano and Francesco Greco. Using numerical simulations, the team showed that FnGD can emerge even in athermal jammed matter, such as compressed emulsions, granular suspensions and foams, when particle motion is driven by imposed shear. The results connect this unusual microscopic transport regime to the onset of yielding rheology in soft jammed solids.
In recent years, FnGD has attracted considerable attention because it challenges the standard picture of microscopic diffusion and its usual dichotomous classification into standard Brownian motion and anomalous diffusion. Indeed, FnGD combines a mean-square displacement that increases linearly in time (Fickian), as in standard Brownian motion, with a non-Gaussian displacement distribution, as typically found in anomalous diffusion. So far, however, this hybrid form of diffusion has mostly been viewed as a signature of single-particle dynamics, with its relevance apparently confined to the microscopic realm.
Whether it is also connected to large-scale material responses has remained much less clear. "The central question is whether FnGD is merely a microscopic detail of particle dynamics, or whether it may have an impact on the macroscopic behavior of a material," says the project's principal investigator. "Soft jammed matter under shear, which combines a rich mechanical response at the macroscopic level with heterogeneous dynamics at the microscopic level, provides an ideal setting to address this question.
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