A. Maitra, M. Lenz (LPTMS)
Living systems often organize themselves as « flocks » of quasi-identical objects, from the synchronized swimming of a school of fish to the coordinated migration of cells during wound healing. These large scale movements fundamentally differ from those of « dead » objects as each constituent of a flock moves on its own. Theoretical considerations suggest that this intrinsic motion makes flocks fundamentally unstable, implying that they are doomed to be disorganized when they exceed a certain size. This prediction is however contradicted by some experimental observations.
To account for this unexpected stability, two researchers as the Laboratoire de Physique Théorique et Modèles Statistiques have theoretically studied the behaviour of a special type of self-propelled object. They focused on objects that are able to autonomously rotate like spinning tops, a characteristic shared by some of the bacteria present in our intestine, as well as the cells that line the inside of our organs. In the presence of small imperfections in their initial arrangement, such particles are always subjected to a combination of the two perturbations illustrated below, and previous theories predict that either one of the two always grows catastrophically, eventually destroying the order of the flock. In their article, published in the journal Nature Communications, the researchers however show that the rapid rotation of their particles allow them to escape this fate by periodically transforming the unstable configurations into stable ones, which stunts their growth. This new mechanism may contribute to a better understanding of our cells, as well as lead to the design principles for artificial active systems able to manipulate objects at the micrometer scale.
Reference : A. Maitra, M. Lenz, Spontaneous rotation can stabilise ordered chiral active fluids, Nature Communications 10, 920, (2019)
Results obtained in the framework of the project Disordered assemblies of biofilaments: from aggregation to contractility (BioFib) funded by topic 2 of PALM and coordinated by Martin Lenz (LPTMS).