Sedimentation of active fluids: Bacteria hinders passive particle sedimentation rates

Numerous natural systems showcase the sedimentation of particles in presence of microorganisms. Here, we experimentally investigate the effects of bacteria activity on the sedimentation process of dilute suspensions of (passive) particles. Presence of swimming bacteria (E. coli) significantly reduces the speed of the sedimentation front of Brownian passive particles; but passive particles did not seem to affect the sedimentation rates of bacteria. Two fronts appear during the sedimentation process, a spherical particle front and a bacteria front. The develop an active hindering settling function which shows the typical decrease in particle sedimentation as bacteria volume fraction increases, followed by a regime in which particle sedimentation speed is independent of bacterial volume fraction.  Results show that a timescale of the second front (E. coli) captures the behavior of both regimes and sedimentation rates of spherical colloids can be tuned via a-priori quantification of bacteria-particle interactions.