Noise-induced transitions from contractile to extensile active stress in isotropic fluids

Tissues of living cells are a prime example of active fluids. There is experimental evidence that tissues generate extensile active stress even though their constituting cells are contractile. Fluctuating forces that could result from cell-substrate interactions have been proposed to be able to induce a transition from contractile to extensile active stress. We define the notion of contractile and extensile active stress in isotropic and anisotropic active matter. Through analytic calculations and numerical computations, we then show that in isotropic active fluids, nonlinearities and coupling between fluctuating forces and fluid density are necessary for such a transition to occur. Here, both transitions from extensile to contractile and vice versa are possible.