The physics of collective motion has been well studied over the past thirty years. So far, scientists have focused on researching “fluid” movements, such as flocks of birds or schools of fish. Now, researchers at the Gulliver Laboratory (ESPCI Paris-PSL / CNRS) have uncovered the possibility of collective motions in elastic solid structures using a sophisticated experimental device. Their work sheds light on the mechanism and parameters that control this so-called “collective actuation”. This work will be published in the journal natural physics.
Within the Gulliver laboratory, the team led by Olivier Dauchot, a CNRS researcher, has been studying collective movements for several years. The initial question was simple: How can the collective movements observed in nature, such as that of birds or fish, be reproduced in the laboratory? To do this, the team set up experiments using “active matter,” meaning matter whose elementary entities move autonomously: walking grains, floating drops, mini-robots — a veritable zoo of active (but non-living) systems that they were capable of to reproduce and study collective movements. More recently, her research has focused on the phenomena of congestion when the system becomes dense. From liquid, the system gradually becomes solid. Is collective motion possible within an active solid?
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“We chose Hexbugs as the active particles. These are small motorized robots that can be found in stores. As a solid, elastic material, we made a network of cylinders connected by springs. By placing a hexbug in each of the cylinders, we form the network, we form an active body,” explains Paul Baconnier, who is writing his doctoral thesis on the subject. Each hexbug deforms the network by trying to move while subject to the shifts caused by the efforts of its neighbors Remarkably, under certain conditions it is possible for a synchronized collective movement to emerge from this tug-of-war.
If the active solid is simply placed on the floor, the hexbugs spontaneously align and the entire solid begins to move through the lab. What if we hook the solid by its edges? A new kind of collective movement inside the solid is observed: all elements of the network oscillate periodically and synchronously around their position of equilibrium.
In order to explain this phenomenon of “collective actuation”, the researchers varied the parameters of the experiment, such as the stiffness of the springs or the shape of the network. They showed that the collective actuation results from the combination of the activity of the hexbugs and the elasticity of the network connections, allowing the structure to deform and each hexbug to orient itself in response to that deformation. The team modeled and numerically reproduced the observed behaviors, even in systems with several thousand active ingredients. This spontaneous collective activation is reminiscent of the observed movements in certain cellular dynamics, particularly in certain skin tissues, which could therefore be better understood in the light of this work.
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Paul Baconnier, Selective and Collective Actuation in Active Solids, natural physics (2022). DOI: 10.1038/s41567-022-01704-x. www.nature.com/articles/s41567-022-01704-x
Citation: Team investigates the collective actuation of an elastic network of “mini-robots” (2022, August 18), retrieved August 18, 2022 from https://phys.org/news/2022-08-team-actuation-elastic- network-mini-robots.html
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