(155a) Active Matter Invasion of a Viscous Fluid and a No-Flow Theorem

We investigate the dynamics of hydrodynamically interacting motile and non-motile stress-generating swimmers or particles as they invade a surrounding viscous fluid. Colonies of aligned pusher particles are shown to elongate in the direction of particle orientation and undergo a cascade of transverse concentration instabilities. Colonies of aligned puller particles instead are found to elongate in the direction opposite the particle orientation and exhibit dramatic splay as the group moves into the bulk. A linear stability analysis of concentrated line distributions of particles is performed and growth rates are found, using an active slender-body approximation, to match the results of numerical simulations. Thin concentrated bands of aligned pusher particles are always unstable, while bands of aligned puller particles can either be stable (immotile particles) or unstable (motile particles) with a growth rate which is non monotonic in the force dipole strength. We also prove a surprising "no-flow theorem": a distribution initially isotropic in orientation loses isotropy immediately but in such a way that results in no fluid flow anywhere at any time.