Active particles, which interact hydrodynamically, display a remarkablevariety of emergent collective phenomena. We use squirmers to model sphericalmicroswimmers and explore the collective behavior of thousands of them underthe influence of strong gravity using the method of multi-particle collisiondynamics for simulating fluid flow. The sedimentation profile depends on theratio of swimming to sedimentation velocity as well as on the squirmer type. Itshows close packed squirmer layers at the bottom and a highly dynamic regionwith exponential density dependence towards the top. The mean verticalorientation of the squirmers strongly depends on height. For swimmingvelocities larger than the sedimentation velocity, squirmers show strongconvection in the exponential region. We quantify the strength of convectionand the extent of convection cells by the vertical current density and itscurrent dipole, which are large for neutral squirmers as well as for weakpushers and pullers.
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