Focusing of Active Rods in a Converging Flow*

We consider active rodlike particles swimming in a convergent fluid flow in a trapezoid nozzle by using mathematical modeling to analyze trajectories of these particles inside the nozzle and numerical simulations to show that trajectories are strongly affected by the background fluid flow and geometry of the nozzle leading to wall accumulation and rheotaxis. We describe the non-trivial focusing of active rods depending on physical as well as geometrical parameters. It is also established that the convergent component of the background flow leads to stability of both downstream and upstream swimming at the centerline. The stability of downstream swimming enhances focusing, and the stability of upstream swimming enables rheotaxis in the bulk.