Particle fluctuations in the sedimentation of active suspensions

Sedimentation of active organic matter occurs in various natural and industrial processes, including the ocean's biological pump and wastewater treatment. In passive systems, prior studies have shown that the correlation length of colloidal particles in the direction parallel to sedimentation decreases exponentially with distance. These correlations are influenced by factors such as particle concentration and size. However, the impact of biological activity arising from motile bacteria or other swimmers on the correlation lengths of settling passive particles remains a puzzling question. Here, we present experimental results on the effects of bacterial activity on the sedimentation process of dilute suspensions of spherical colloids. Through precise tracking of individual particles within an active suspension, our results reveal that the presence of swimming bacteria (E. coli) significantly enhances particle fluctuations. This enhancement reaches a plateau beyond a critical bacteria volume fraction, which is found to have bioconvective origins predicted earlier in simulations (Hillesdon & Pedley, J. Fluid Mech., 1996).