Hydrodynamic alignment of microswimmers in pipes

TL;DR

This study uses 3D numerical simulations to explore how microswimmers align within cylindrical pipes, revealing phase transitions and the influence of confinement on collective behavior.

Contribution

It demonstrates that microswimmers exhibit alignment and phase transitions under confinement, with insights into how geometry and size ratios affect their collective dynamics.

Findings

Microswimmers show alignment in pipes similar to bulk behavior.

An order/disorder phase transition occurs when pipe size matches swimmer size.

Dynamics are independent of the specific confinement geometry.

Abstract

We investigate the dynamics of model microswimmers under confinement, in cylindrical geometries, by means of three dimensional direct numerical calculations with fully resolved hydrodynamics. Such swimmers are known to show collective alignment in bulk, and we confirmed that similar alignment can be observed even in pipes, although the volume fraction dependency shows qualitative differences. By comparing the structural information, we investigated the cause of such differences. We found an order/disorder phase transition for a specific type of swimmers, as the size of the pipe becomes comparable to the size of the swimmers. Such dynamics are found not to depend on the geometry of confinement.