Francois N. Frenkiel Award Talk: Swimming bacteria at complex interfaces

Flagellated bacteria exploiting helical propulsion are known to swim along circular trajectories near surfaces. Past fluid dynamic studies predict this circular motion to be clockwise above a rigid surface (when viewed from inside the fluid) and counter- clockwise below a free surface. Recent experimental investigations showed that complex physicochemical processes at the nearby surface could lead to a change in the direction of rotation, both at solid surfaces absorbing slip-inducing polymers and interfaces covered with surfactants. Motivated by these results, we use a far-field hydrodynamic model to predict the kinematics of swimming near three types of interfaces: clean fluid-fluid interface, slipping rigid wall, and a fluid interface covered by incompressible surfactants. Representing the helical swimmer by a superposition of hydrodynamic singularities, we first show that in all cases the surfaces reorient the swimmer parallel to the surface and attract it, both of which are a consequence of the Stokes dipole component of the swimmer flow field. We then show that circular motion is induced by a higher-order singularity, namely, a rotlet dipole, and that its rotation direction is strongly affected by the boundary conditions at the interface and the bacteria shape. \\[4pt] In collaboration with Eric Lauga, DAMTP, University of Cambridge.