  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.
