# Gravity induced formation of spinners and polar order of spherical microswimmers on a surface

## TL;DR

This study numerically investigates how hydrodynamic interactions induce the formation of chiral spinners and polar order among spherical microswimmers on a surface, revealing stable structures and collective behaviors influenced by concentration and confinement.

## Contribution

It introduces a detailed numerical analysis of hydrodynamic effects leading to spinner formation and polar order in spherical microswimmers on surfaces, highlighting the role of sedimentation and confinement.

## Key findings

- Formation of stable chiral spinners at low coverage
- Transition to polar order at higher concentrations
- Emergence of particle vortex in confined geometries

## Abstract

We study numerically the hydrodynamics of a self-propelled particle system, consisting of spherical squirmers sedimented on a flat surface. We observe the emergence of dynamic structures, due to the interplay of particle-particle and particle-wall hydrodynamic interactions. At low coverages, our results demonstrate the formation of small chiral spinners: two or three particles are bound together via near-field hydrodynamic interactions and form a rotating dimer or trimer respectively. The stability of the self-organised spinners can be tuned by the strength of the sedimentation. Increasing the particle concentration leads more interactions between particles and the spinners become unstable. At higher area fractions we find that pusher particles can align their swimming directions leading to a stable polar order and enhanced motility. Further, we test the stability of the polar order in the presence of a solid boundary. We observe the emergence of a particle vortex in a cylindrical confinement.