# Collective Dynamics in a Monolayer of Squirmers Confined to a Boundary by Gravity

## TL;DR

This study explores the complex collective behaviors of microswimmers confined to a boundary under gravity, revealing various dynamic states influenced by swimmer type, density, and interactions, including fluid-like phases, clustering, and swarming.

## Contribution

It provides a comprehensive hydrodynamic simulation analysis of monolayer microswimmers under gravity, highlighting new collective states and behaviors based on swimmer type and density.

## Key findings

- Formation of hydrodynamic Wigner fluid at low densities
- Emergence of fluctuating chains, trimers, and clusters at higher densities
- Swarming and chaotic behaviors in pushers at high densities

## Abstract

We present a hydrodynamic study of a monolayer of squirmer model microswimmers confined to a boundary by strong gravity using the simulation method of multi-particle collision dynamics. The squirmers interact with each other via their self-generated hydrodynamic flow fields and thereby form a variety of fascinating dynamic states when density and squirmer type are varied. Weak pushers, neutral squirmers, and pullers have an upright orientation. With their flow fields they push neighbors away and thereby form a hydrodynamic Wigner fluid at lower densities. Furthermore, states of fluctuating chains and trimers, of kissing, and at large densities a global cluster exist. Finally, pushers at all densities can tilt against the wall normal and their in-plane velocities align to show swarming. It turns into chaotic swarming for strong pushers at high densities. We characterize all these states quantitatively.