  Sedimentation in active fluids has come into focus due to the ubiquity of
swimming micro-organisms in natural and industrial processes. Here, we
investigate sedimentation dynamics of passive particles in a fluid as a
function of bacteria E. coli concentration. Results show that the presence of
swimming bacteria significantly reduces the speed of the sedimentation front
even in the dilute regime, in which the sedimentation speed is expected to be
independent of particle concentration. Furthermore, bacteria increase the
dispersion of the passive particles, which determines the width of the
sedimentation front. For short times, particle sedimentation speed has a linear
dependence on bacterial concentration. Mean square displacement data shows,
however, that bacterial activity decays over long experimental (sedimentation)
times. An advection-diffusion equation coupled to bacteria population dynamics
seems to capture concentration profiles relatively well. A single parameter,
the ratio of single particle speed to the bacteria flow speed can be used to
predict front sedimentation speed.
