Unidirected motion and negative viscosity in microphase separationwith an active polar component

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 todevelop shear-thickening properties in contractile systems or induce in extensile suspensions a ”superfluid” regime under suitable condition.

We present results of LBM simulations for the behavior of a phase separating symmetric mixture of a passive isotropic fluid and an active polar gel under an applied shear flow. For small values of contractile activity(e.g., an actomyosin solution) and shear rates we observe the coexistence of phases characterized by passive and active droplets. In this case the constitutive curves show evidence of shear thickening behavior. For extensile activity (e.g., materials based on bacterial suspensions), instead, the constitutive curves indicates the presence of states characterized by negative viscosity. These states are marked by a negative slope of the velocity profile at the center of the channel, opposite to that of the applied shear.