Interplay of flow and swimmer’s polar orientation in confined active suspensions*

We examine the dynamics of a continuum model of confined active bacterial suspensions where flows driven by active stresses are coupled to the tendency of microorganism to propel themselves along their orientation axis. Fluid flow is dissipated by both friction with the substrate and internal viscous forces. The relative importance of these two dissipation mechanisms, as quantified by the hydrodynamic screening length relative to the system size, modifies the nature of the spontaneous flow transition in the channel geometry. It also affects the nature of the crossover between laminar flows at low activity to periodic or oscillatory flow structures at high activity. In both disk and channel geometries, we also examine the role of the anchoring strength of the polar liquid crystals at the boundary.