  The surface tension of living cells and tissues originates from the
generation of nonequilibrium active stresses within the cell cytoskeleton.
Here, using laser ablation, we generate gradients in the surface tension of
cellular aggregates as models of simple tissues. These gradients of active
surface stress drive large-scale and rapid toroidal motion. Subsequently, the
motions spontaneously reverse as stresses reaccumulate and cells return to
their original positions. Both forward and reverse motions resemble Marangoni
flows in viscous fluids. However, the motions are faster than the timescales of
viscoelastic relaxation, and the surface tension gradient is proportional to
mechanical strain at the surface. Further, due to active stress, both the
surface tension gradient and surface strain are dependent upon the volume of
the aggregate. These results indicate that surface tension can induce rapid and
highly correlated elastic deformations in the maintenance of tissue shape and
configuration.
