# Collective Sedimentation of Squirmers under Gravity

## TL;DR

This study investigates how active spherical microswimmers called squirmers sediment under gravity, revealing layered structures, orientation patterns, and convection behaviors influenced by swimming speed and squirmer type.

## Contribution

It introduces a detailed simulation of large-scale squirmer sedimentation under gravity, highlighting the effects of swimming velocity and squirmer type on collective behavior.

## Key findings

- Layered sedimentation profiles with close-packed bottom layers.
- Exponential density distribution in the upper regions.
- Strong convection observed for swimming velocities exceeding sedimentation speed.

## 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.