Online Talk 

Zoom link: https://princeton.zoom.us/j/4745473988

Active fluids, which contain self-propelled particles, exhibit large scale flows arising from hydrodynamic interactions between swimmers. Here, we study the influence of confinement, in a channel or a sphere, on the collective pattern formation in suspensions of swimmers with a preferred orientation. The swimmers are modelled as singular solutions of Stokes equations. We run simulations of the system which are in turn explained through minimal models. In both case, we observe the emergence of collective motion even at relatively low concentration. Swimmers accumulating at a wall form plumes that drive convective flows in the channel. In a sphere, instabilities lead to the emergence of a large-scale vortex, with an orientation set by the chirality of the swimmers. Our model is validated against experiments on suspensions of magnetotactic bacteria, which align passively to external magnetic fields.