Gradients in Surface Tension Drive Global Flows in Cell Aggregates

The surface tension of living cells and tissues originates from active stresses within the cytoskeleton. We perturb surface tension in model tissues using laser ablation. Upon sudden release of surface tension at a point, cells move rapidly throughout the entire sample. Subsequently, cells reverse and slowly return to their original positions as tensional homeostasis is re-established. These movements resemble viscous flow, even for flow durations that are significantly smaller than the viscoelastic timescales. Analysis of deformation fields allows us to evaluate the magnitude of the surface tension perturbation, which we find to be proportional to the aggregate size and correlated to the pre-strain of cells at the surface. Together these analyses suggest that the existence of a size-dependent tension and requirement of maintaining tensional homeostasis determines the magnitude of global flow velocities and their temporal orientation, respectively.