Exploring dense active matter systems and their forces

Everywhere around us, there are dense groups of self-propelled particles that can form solid-like states we call active glasses. These dense active matter systems range from cytoplasm to cell tissues and from bacterial biofilms to traffic jams. Scientists have recently been studying the dynamical and mechanical properties of such active glasses, but these systems are so structurally disordered and out of equilibrium that understanding them has proven challenging.

A way to study the properties of active glasses is to treat them as unusual, active forms of physical matter. Researchers supported by the EU-funded RMAG project sought to gain insight into these systems and particularly on how they behave under shear – the stress produced by pressure in a substance’s structure when its layers are shifted laterally in opposite directions. Their findings were published in the journal ‘Proceedings of the National Academy of Sciences of the United States of America’.

The research team simulated a model active glassy system under steady shear. In this system, each self-propelling particle was driven by a propulsion force whose direction undergoes slow and random changes. “We were exploring the response of a model active material under steady driving, where the system is sandwiched between two walls, one stationary and the other moving to generate shear deformation,” states study lead author Dr Rituparno Mandal of RMAG project coordinator University of Göttingen in a ‘EurekAlert!’ news item.The team found that although particle flow resembles that of ordinary liquids, force directions reveal a hidden order: they tend to point towards the top or bottom plate – whichever is nearest – while the particles with lateral forces cluster in the middle of the glassy system. “What we saw was that at a sufficiently strong driving force, an interesting ordering effect emerges,” Dr Mandal goes on to explain. “We now also understand the ordering effect using a simple analytical theory and the predictions from this theory match surprisingly well with the simulation.”

Study senior author Prof. Peter Sollich, also from the University of Göttingen, explains: “Often an external force or driving force destroys ordering. But here the driving by shear flow is key in providing mobility to the particles that make up the active material, and they actually need this mobility to achieve the observed order.”

According to Prof. Sollich, the results of this study are expected to “open up exciting possibilities for researchers investigating the mechanical responses of living matter.” The RMAG (Rheology and Mechanics of Active Glasses) project aims to offer scientists new insight into cell biology and materials science and pave the way for the design of new active materials with remarkable abilities. The 2-year project ends in October 2022.

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RMAG project