Title: Collective Sedimentation of Squirmers under Gravity,
Abstract: Active particles, which interact hydrodynamically, display a remarkable
variety of emergent collective phenomena. We use squirmers to model spherical
microswimmers and explore the collective behavior of thousands of them under
the influence of strong gravity using the method of multi-particle collision
dynamics for simulating fluid flow. The sedimentation profile depends on the
ratio of swimming to sedimentation velocity as well as on the squirmer type. It
shows close packed squirmer layers at the bottom and a highly dynamic region
with exponential density dependence towards the top. The mean vertical
orientation of the squirmers strongly depends on height. For swimming
velocities larger than the sedimentation velocity, squirmers show strong
convection in the exponential region. We quantify the strength of convection
and the extent of convection cells by the vertical current density and its
current dipole, which are large for neutral squirmers as well as for weak
pushers and pullers.