Creation of order by mechanical deformation in the dense active material

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Living or biological systems cannot be easily understood using standard laws of physics, such as thermodynamics, as scientists would for gases, liquids, or solids. Living systems are active, demonstrating fascinating properties such as adapting to their environment or repairing themselves. By exploring the questions posed by living systems using computer simulations, researchers at the University of Göttingen have now discovered a new type of order effect generated and sustained by simple mechanical deformation, in particular shear regular. The results were published in the PNAS.

Understanding living systems, such as the tissues formed by cells, poses a significant challenge due to their unique properties, such as adaptation, self-repair, and self-propulsion. Nevertheless, they can be studied with the help of models which treat them simply as an unusual and “active” form of physical matter. This can reveal extraordinary dynamic or mechanical properties. One of the puzzles is how active materials behave under shear (the strain produced by moving the top and bottom layers sideways in opposite directions, like the microscope cover plates sliding against each other) . Researchers at the Institute for Theoretical Physics at the University of Göttingen explored this question and discovered a new type of scheduling effect generated and maintained by constant shear strain. The researchers used a computer model of self-propelled particles where each particle is driven by a propulsive force that changes direction slowly and randomly. They discovered that although the flow of particles resembles that of ordinary liquids, there is a hidden order revealed by looking at the directions of the forces: these tend to point towards the nearest plate (upper or lower), while particles with lateral forces aggregate in the middle of the system.

“We were exploring the response of a model active material in steady state, where the system is sandwiched between two walls, one fixed and the other in motion to generate shear strain. What we have seen is that with a sufficiently strong driving force, an interesting order effect emerges, ”comments Dr Rituparno Mandal, Institute for Theoretical Physics, University of Göttingen. “We now also understand the control effect using a simple analytical theory, and the predictions of this theory match the simulation surprisingly well. “

Lead author Professor Peter Sollich, also from the Institute for Theoretical Physics at the University of Göttingen, explains: “Often an external force or a driving force destroys order. But here, the shear entrainment is essential to ensure the mobility of the particles that make up the active material, and they actually need that mobility to achieve the observed order. The results will open exciting possibilities for researchers studying the mechanical responses of living matter. “

Original publication: R Mandal, P Sollich “Shear fused orientational ordering in an active glass former”, Proceedings of the National Academy of Sciences (PNAS 2021). DoI: 10.1073 / pnas.2101964118

This research was made possible by funding from the European Union’s Horizon 2020 research and innovation program under a Marie Skłodowska-Curie grant.

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