Geometric confinement reveals scale-free velocity correlations in epithelial cell monolayer.

Collective cell flows are a hallmark of tissue dynamics in development, wound healing, and various diseases. Here, we investigate how the size, shape, topology and rigidity of patterned substrate influence the organization of flows and mechanical fields in an epithelial MDCK cell monolayer at several time and space scales. Using micropatterned substrates with and without free front (a strip and a closed racetrack), we show that confinement and obstacles induce spatial heterogeneities in velocity and force fields, leading to the emergence of domains, waves, and long-range correlations. We show that spatial velocity correlations are scale-free, following a power law whose exponent evolves as the monolayer matures. This challenges the notion of a single intrinsic correlation length. We also show that in absence of free front, spontaneous collective motions are stronger on soft than on hard substrate. Our findings provide new insights into the rheology of epithelial tissues and the interplay between mechanics and collective migration.