Active turbulence in microswimmer suspensions -- the role of active hydrodynamic stress and volume exclusion

TL;DR

This study investigates active turbulence in microswimmer suspensions, highlighting how active hydrodynamic stress and volume exclusion influence collective behaviors like clustering, phase separation, and turbulence, using mesoscale simulations of squirmers.

Contribution

It introduces a mesoscale MPC simulation model including active stress and rotlet dipoles, revealing the emergence of turbulence and phase behaviors in bacterial microswimmer suspensions.

Findings

Emergent clustering and phase separation depending on density and active stress.

Observation of Kolmogorov-Kraichnan-type turbulence at high concentrations.

Hydrodynamic flow fields are crucial for swarming and turbulence phenomena.

Abstract

Microswimmers exhibit an intriguing, highly-dynamic collective motion with large-scale swirling and streaming patterns, denoted as active turbulence -- reminiscent of classical high-Reynolds-number hydrodynamic turbulence. Various experimental, numerical, and theoretical approaches have been applied to elucidate similarities and differences to inertial hydrodynamic and active turbulence. These studies reveal a wide spectrum of possible structural and dynamical behaviors of active mesoscale systems, not necessarily consistent with the predictions of the Kolmogorov-Kraichnan theory of turbulence. We use squirmers embedded in a mesoscale fluid, modeled by the multiparticle collision dynamics (MPC) approach, to explore the collective behavior of bacteria-type microswimmers. Our model includes the active hydrodynamic stress generated by propulsion, and a rotlet dipole characteristic for flagellated bacteria. We find emergent clusters, activity-induced phase separation, and swarming, depending on density, active stress, and the rotlet dipole strength. The analysis of the squirmer dynamics in the swarming phase yields Kolomogorov-Kraichnan-type hydrodynamic turbulence and energy spectra for sufficiently high concentrations and strong rotlet dipoles. This emphasizes the paramount importance of the hydrodynamic flow field for swarming and bacterial turbulence.