Microswimmers in an axisymmetric vortex flow

José-Agustín Arguedas-Leiva, Michael Wilczek

Published 2019-11-04Version 1

Microswimmers are encountered in a wide variety of biophysical settings. When interacting with flow fields, they show interesting dynamical features such as hydrodynamical trapping, clustering, and preferential orientation. One important step towards the understanding of such features is to clarify the interplay of hydrodynamic flow 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 swimmer shape, we uncover the topology of the phase space and determine the conditions under which swimmers are trapped in the vortex. For spherical swimmers, we identify Hamiltonian dynamics, which are broken for swimmers of a different shape. We find that prolate ellipsoidal shaped 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.