Velocity fluctuations in sedimentation of particles stirred by bacterial suspensions

Interaction of passive particles with active matter is ubiquitous in environmental and biological processes. However, the effects of bacterial activity on the settling dynamics of passive particles remain elusive. To understand these effects, we built a custom-made experimental setup to explore the sedimentation dynamics of polystyrene particles in dilute E. coli suspensions. We showed that time-dependent concentration profiles at the macroscale (Singh et al., Soft Matter 2021) and mesoscale (Maldonando et al., Phys. Fluids 2022) are functions of bacterial activity. Probing the system at the microscale, we demonstrated that the sedimentation dynamics of passive colloids in an active bacterial bath exhibit a diffusion-to-bioconvection transition set by a critical bacterial concentration (Maldonado et al., J. Fluid Mech 2024). This transition from Stokes-Einstein diffusion dynamics to activity-driven convection is accompanied by fluctuations in sedimentation velocities. We characterize the statistics of these fluctuations,  in the components of sedimentation velocities parallel and orthogonal to the direction of the gravitational force, stoboscopically at the colloidal particle-scale and model them using a Hinch-blob type-model that accouts the effect of bacterial activity. This work provides a new perspective on underlying mechanism associated with the sedimentation dynamics of passive matter in an active bath.