Gyrotactic cluster formation of bottom-heavy squirmers

TL;DR

This study uses hydrodynamic simulations to explore how bottom-heavy squirmers form clusters under gravity and flow interactions, revealing the influence of gravitational torque, squirmer type, and boundary effects on clustering behavior.

Contribution

It introduces a detailed simulation analysis of gyrotactic clustering in squirmers, highlighting the roles of gravitational torque and flow field types in cluster formation.

Findings

Clusters form more readily in neutral squirmers.

Pullers require larger gravitational torques to cluster.

No clustering observed for pusher squirmers.

Abstract

Squirmers that are bottom-heavy experience a torque that aligns them along the vertical so that they swim upwards. In a suspension of many squirmers, they also interact hydrodynamically via flow fields that are initiated by their swimming motion and by gravity. Swimming under the combined action of flow field vorticity and gravitational torque is called gyrotaxis. Using the method of multi-particle collision dynamics, we perform hydrodynamic simulations of a many-squirmer system floating above the bottom surface. Due to gyrotaxis they exhibit pronounced cluster formation with increasing gravitational torque. The clusters are more volatile at low values but compactify to smaller clusters at larger torques. The mean distance between clusters is mainly controlled by the gravitational torque and not the global density. Furthermore, we observe that neutral squirmers form clusters more easily, whereas pullers require larger gravitational torques due to their additional force-dipole flow fields. We do not observe clustering for pusher squirmers. Adding a rotlet dipole to the squirmer flow field induces swirling clusters. At high gravitational strengths, the hydrodynamic interactions with the no-slip boundary create an additional vertical alignment for neutral squirmers, which also supports cluster formation.