Microswimmers interacting with deformable interfaces

Active microscopic swimmers at low Re in confined domains exhibit responses that differ from their passive counterparts.  For instance, while swimming in the vicinity of a plane boundary in a quiescent fluid, “pusher” type microswimmers, such as the bacteria E. coli, tend to preferentially migrate towards the boundary even in the dilute suspension limit, whereas, denser suspensions tend to exhibit more complex confinement-dependent spatiotemporal patterns. The dynamics of such systems depends crucially on the nature of the intervening boundary; namely free-slip versus no-slip, in case of plane boundaries, or non-deforming versus deforming interfaces. We analyze the latter case involving the coupled hydrodynamics of a dilute suspension of microswimmers in the vicinity of a deformable interface. We characterize the nature of the interface deformation due to the swimming motion and address how the swimming is itself modified by the interface. The analysis treats the role of the swimmer type (pusher or puller), the finite swimmer size and interface deformation due to both surface tension and resistance to bending. We highlight the non-trivial ways in which the local and non-local effects influence the coupled hydrodynamics.