Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of "active matter" in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano-and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveling, predicting, and controlling the behavior of active matter is a truly interdisciplinary endeavor at the interface of biology, chemistry, ecology, engineering, mathematics, and physics.
Most spindle-shaped cells (smooth muscles, sarcomas etc…) routinely organize in vivo in wellaligned "nematic" domains 1-3 . This organization implies intrinsic topological defects that may be used to probe the behavior of these active nematic systems. Active non-cellular nematics have been shown to be dominated by activity, yielding complex chaotic flows 4,5 . However, the regime in which live spindle-shaped cells operate, and in particular the importance of cell-substrate friction, remains largely unexplored. By designing model in vitro experiments, we show that these active cellular nematics operate in a regime where activity is effectively damped by friction and therefore that the interaction between defects is controlled by the system's elastic nematic energy. Due to the activity of the cells, these defects behave as self-propelled particles and pairwise annihilate until all displacements freeze as cell crowding increases 6,7 . When confined in mesoscopic circular domains, the system evolves toward two identical +1/2 disclinations facing each other. The most likely reduced positions of these defects are found to be independent of the size of the disk, the cells' activity or even the cell type. These results are well described by equilibrium Liquid Crystal theory.Therefore, these cell-based systems operate in a regime more stable than other active nematics, which is likely to be required for their biological function.
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