Fluidization and anomalous density fluctuations in 2D Voronoi cell tissues with pulsating activity.

Cells not only can be motile by crawling but are also capable of nonmotility active motions like periodic contraction or pulsation. In this work, based on a Voronoi cell model, we show how this nonmotility activity affects the structure, dynamic, and density fluctuations of cellular monolayers. Our model shows that random cell pulsation fluidizes solid epithelial tissues into a hyperuniform fluid state, while pulsation synchronization inhibits the fluidity and causes a reverse solidification. Our results indicate this solidification is a Berezinskii-Kosterlitz-Thouless-type transition, characterized by strong density/dynamic heterogeneity arising from the annihilation of topological defects in the pulsating phase space. The magnitude and length scale of density heterogeneity diverge with the pulsating period, resulting in an opposite giant density fluctuation or anti-hyperuniformity. We propose a fluctuating hydrodynamic theory that can unify the two opposite anomalous fluctuation phenomena. Our findings can help to understand recent experimental observations in Madin-Darby canine kidney monolayers.