Entrainment and confinement amplify transport by schooling micro-swimmers

Active agents serve an important function in the transport and distribution of drugs or chemical signals. While by intuition one might think of transport in the agent's interior, it can also be mediated by entrainment of the surrounding bulk liquid. Here, we investigate the fluid dynamics of entrainment by squirmers in confined geometries, represented by an active droplet, constituting a weak pusher, in a quasi-2D microfluidic environment. We compare experimental measurements of flow fields and the displacement of tracer colloids to a theory model of a swimmer in a Brinkman medium, which accounts for the quasi 2D-confinement. The model compares favourably with an unconfined Stokes description. We further extend this model by simulations of a school of squirmers collectively entraining particles in the bulk medium. We show that the entrainment is enhanced both by the collective action of the school and by the strong confinement in the presence of boundaries, giving the school a significant cargo capacity.