Rheology of active polar emulsions: from linear to unidirectional and unviscid flow, and intermittent viscosity

Active fluids are systems where active components present in the fluid (microtubules with molecular motors such as kinesin or actomyosin bundles) display interesting collective ordering properties. Active fluids also exhibit peculiar rheological properties. Depending on the characteristic of the active stress, activity is capable to heighten viscosity, enough to develop shear-thickening properties in contractile systems or induce in extensile suspensions a superfluid regime under suitable condition. We study, by lattice Boltzmann methods, the rheological behavior of an emulsion made of an active polar component and an isotropic passive fluid. Different flow regimes are found by varying the values of shear rate and extensile activity (occurring, e.g., in microtubule-motor suspensions). By increasing activity, a first transition occurs from linear flow regime to spontaneous persistent unidirectional macro-scale flow, followed by another transition either to (low shear) intermittent flow regime with coexistence of states with positive, negative, and vanishing apparent viscosity, or to (high shear) symmetric shear thinning regime.