We consider two systems of active swimmers moving close to a solid surface, one being a living population of wild-type E. coli and the other being an assembly of self-propelled Au-Pt rods. In both situations, we have identiﬁed two diﬀerent types of motion at the surface and evaluated the fraction of the population that displayed ballistic trajectories (active swimmers) with respect to those showing diﬀusive-like behavior. We studied the eﬀect of this complex swimming activity on the diﬀusivity of passive tracers also present at the surface. We found that the tracer diﬀusivity is enhanced with respect to standard Brownian motion and increases linearly with the activity of the ﬂuid, deﬁned as the product of the fraction of active swimmers and their mean velocity. This result can be understood in terms of series of elementary encounters between the active swimmers and the tracers
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.