Hydrodynamics of flagellated microswimmers near free-slip interfaces

The hydrodynamics of a flagellated microorganism is investigated when swimming close to a planar free-slip surface by means of numerical solutions of the Stokes equations obtained via a Boundary Element Method. Depending on the initial condition, the swimmer can either escape from the free-slip surface or collide with the boundary. Interestingly, the microorganism does not exhibit a stable orbit. Independently of escape or attraction to the interface, close to a free-slip surface, the swimmer follows a counter-clockwise trajectory, in agreement with experimental findings, [15]. The hydrodynamics is indeed modified by the free-surface. In fact, when the same swimmer moves close to a no-slip wall, a set of initial conditions exists which result in stable orbits. Moreover when moving close to a free-slip or a no-slip boundary the swimmer assumes a different orientation with respect to its trajectory. Taken together, these results contribute to shed light on the hydrodynamical behaviour of microorganisms close to liquid-air interfaces which are relevant for the formation of interfacial biofilms of aerobic bacteria.