Alignment of non-spherical active particles in chaotic flows

TL;DR

This paper investigates how non-spherical microswimmers, like rods and disks, align with chaotic flows, revealing a robust alignment mechanism driven by flow correlations and particle shape, with implications for ecology.

Contribution

It introduces a statistical model explaining the alignment of spheroidal active particles in turbulent flows, highlighting the role of flow-particle interactions and shape effects.

Findings

Rod-like particles preferentially align with flow velocity.

Alignment is caused by flow velocity-gradient correlations and shape asymmetry.

The alignment effect is robust and flow-independent.

Abstract

We study the orientation statistics of spheroidal, axisymmetric microswimmers, with shapes ranging from disks to rods, swimming in chaotic, moderately turbulent flows. Numerical simulations show that rod-like active particles preferentially align with the flow velocity. To explain the underlying mechanism we solve a statistical model via perturbation theory. We show that such alignment is caused by correlations of fluid velocity and its gradients along particle paths combined with fore-aft symmetry breaking due to both swimming and particle nonsphericity. Remarkably, the discovered alignment is found to be a robust kinematical effect, independent of the underlying flow evolution. We discuss its possible relevance for aquatic ecology.