Geometric confinement guides topological defect pairings and emergent flow in nematic cell populations

Topological defects in nematically aligned cell populations play a critical role in modulating collective motion, from microbial colonies to epithelial tissues. Despite the potential of manipulating such topological defects to control diverse self-organized structures and collective dynamics, defect manipulation in active matter remains an challenging area of research. In this study, we investigated the geometric control of defect positioning and alignment in a nematic cell population by imposing spatial constraints consisting of two or three overlapping circular boundaries. The confined cell population exhibited an ordered pairing of half-integer topological defects that remained stable even when the size of the spatial constraint was altered using geometric parameters. These defects also elicited robust contractile flow that induced a negative divergence in the velocity field of collective motion. Such net contractile flow can contribute to mechanical stimulation on confined cells, as evidenced by the stretched cell nucleus. Our geometry-based approach paves the way for controlling defect pairing, providing a deeper understanding of the interplay among geometry, topology, and collective dynamics.