Abstract

Bacteria, like E. coli, when placed in confined environments are known to be attracted to solid surfaces and swim in contact with them for long times. In this talk, several aspects related to the motion of bacteria near surfaces are considered. First, a simple swimmer model will be introduced. It reproduces some features of bacteria-surface interaction including the observed circular motion. It will be shown that a purely hydrodynamic model is sufficient to take account of the attraction and alignment parallel to the walls but and extra, non-hydrodynamic interaction, is needed to obtain the persistent motion. Second, the induced diffusion of tracers in presence of a bacterial bath is studied. A kinetic theory combined with a hydrodynamic approach allow us to compute the resulting diffusivity. It is shown that close to surfaces, the induced diffusivity is enhanced as a result of the smaller swimming efficiency of bacteria. Finally, the case of swimmers placed in an oscillatory shear flow is analyzed. As a result of flow fluctuations, the coarse grained displacement of a swimmer in the vorticity direction is maximized for a finite fluctuation intensity. This phenomenon corresponds to an example of stochastic resonance.