Active phase separation of biphasic polymer gels

Biological systems allow for the generation of active stresses, which can lead to mechanical and chemical instabilities and the formation of patterns. Many relevant systems - e.g. bone, developing tissue, cellular interiors - can be described as multiple immiscible phases of varying rheology, introducing a rich set of couplings between active stress generation and the corresponding passive mechanical responses. One area of particular interest is the generation of contractile forces within the cytoskeleton by the binding of myosin and kinesin motor proteins onto actin and microtubule networks, respectively. In this talk, we present a phenomenological model for active stresses induced by the fuel-dependent binding dynamics of molecular motors in biphasic, incompressible, transiently cross-linked polymer gels. We show that these stresses are analogous to a spatially and temporally varying dis-affinity between polymer and solvent which drives phase separation. The system's behavior on long time scales is investigated by time-integration of the complete non-linear system of gel equations, and the results are contrasted with the case of a demixing passive gel described by classical Flory-Huggins theory.