Viscoelasticity Encodes Longevity of Transient Dynamical States in Active Fluids

The out-of-equilibrium active reorganization of cytoskeletal networks by molecular motors is necessary for fundamental life processes, such as cell division, cell motility, and environmental sensing. While the passive structure and mechanics of such materials have been well documented, the effects of their steady-state out-of-equilibrium reorganization is a site of current research. In this work, we introduce an active cytoskeletal composite material whose viscoelasticity is controlled by the actin filament concentration. At the lowest actin concentrations, the system exhibits a single steady-state of extensile active flow. Increasing the actin concentration triggers a contractile state in coexistence with the active fluid. The resultant filament-rich structures are transient and their lifetimes increase with actin concentration. These self-organized structures are also captured by the dynamical behavior of the active composite. Phase portraits reveal two separate dynamical states which underlie the observed spatiotemporal organization. This work captures the rich, multi-stable phenomena of far-from-equilibrium active materials within a well-controlled model experimental system.