Velocity fluctuations in the sedimentation of active suspensions

Sedimentation of active matter is found in many natural and industrial processes, such as ocean's biological pump and wastewater treatment. Sedimentation processes are usually characterized by studying the concentration profiles, hindered settling functions, and velocity fluctuations. In passive systems, previous results show that the correlation length of colloidal particles in the direction parallel to sedimentation decreases exponentially with distance. These correlations depend on the concentration and size of the dispersed particles. However, biological activity, due to the presence of motile bacteria or other swimmers, affect the correlation lengths of settling passive particles is a question that remains unanswered. To address it, we experimentally investigate the effects of bacteria activity on the sedimentation process of dilute suspensions of spherical colloids. By tracking individual particles in an active suspension, results show that the presence of swimming bacteria (E. coli) disturbs the sedimentation particles trajectories. We find an enhancement particle mean-square displacement in the lateral direction once activity is introduced in the system. This leads to a significant difference in the correlation lengths between passive and active systems.