Microswimmers in an axisymmetric vortex flow

TL;DR

This study explores how the shape and motility of ellipsoidal microswimmers influence their behavior in a vortex flow, revealing complex dynamics, trapping conditions, and the effects of noise on escape rates, advancing understanding of microswimmer-flow interactions.

Contribution

It provides a comprehensive dynamical systems analysis of microswimmer behavior in vortex flows, highlighting the impact of shape and motility on trapping and orientation, which was previously not fully understood.

Findings

Prolate microswimmers tend to align with the flow

Oblate microswimmers tend to stay perpendicular to the flow

Rotational noise increases escape rates near saddle points

Abstract

Microswimmers are encountered in a wide variety of biophysical settings. When interacting with flow fields, they show interesting dynamical features such as trapping, clustering, and preferential orientation. One important step towards the understanding of such features is to clarify the interplay of hydrodynamic flows with microswimmer motility and shape. Here, we study the dynamics of ellipsoidal microswimmers in a two-dimensional axisymmetric vortex flow. Despite this simple setting, we find surprisingly rich dynamics, which can be comprehensively characterized in the framework of dynamical systems theory. By classifying the fixed-point structure of the underlying phase space as a function of motility and microswimmer shape, we uncover the topology of the phase space and determine the conditions under which microswimmers are trapped in the vortex. For spherical microswimmers, we identify Hamiltonian dynamics, which are broken for microswimmers of a different shape. We find that prolate ellipsoidal microswimmers tend to align parallel to the velocity field, while oblate microswimmers tend to remain perpendicular to it. Additionally, we find that rotational noise allows microswimmers to escape the vortex with an enhanced escape rate close to the system's saddle point. Our results clarify the role of shape and motility on the occurrence of preferential concentration and clustering and provide a starting point to understand the dynamics in more complex flows.