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, ranging from microbial colonies to epithelial tissues. Despite the potential of manipulating such topological defects to control diverse self-organized structures and collective dynamics, controlling the position of defects in active matter remains a challenging area of research. In this study, we investigated the geometry-guided 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 a paired and ordered distribution of half-integer topological defects that remained stable even when the size of the spatial constraint was altered using geometric parameters. These defects direct the inward flow of cells, induced by the curved boundary shape, towards the geometric center of the confined space. This inward flow contributes to an increase in a local cell density, and furthermore the geometry-induced nematic order provides mechanical stimulation to confined cells, as indicated by the elongated cell nucleus. Our geometry-based approach sets the foundation for controlling defect pairing and provides insights into the interplay among geometry, topology, and collective dynamics.