Active time scales for passive particles in bacterial baths
The interactions of swimming bacteria with passive particles have been shown to give rise to a super diffusive component in the displacement of the particles. The imparted active motion on the particles can lead to effective attractive interactions and may facilitate out-of-equilibrium transport effects. In this work, to elucidate the origin of active-passive interactions, we study the effects of a bath of swimming E. coli bacteria on dilute colloidal particles of similar size, specifically of radii between 1 and 4 microns, which have sedimented on a glass substrate. We focus on time ranges between a few milliseconds to a few seconds and by looking at the particle displacements through particle tracking, we experimentally observe two components in the active diffusivity, characterized by a short (~20 ms) and a larger correlation time (~100 ms). The larger active time scale is found to be linked to close range interactions between particles and bacteria, while the shorter time scale is discussed in terms of longer ranged hydrodynamic interactions, through experiments which separately track bacterial and particle motions and correlate them as a function of distance.
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