4.7 Article

Emergence of active turbulence in microswimmer suspensions due to active hydrodynamic stress and volume exclusion

Journal

COMMUNICATIONS PHYSICS
Volume 5, Issue 1, Pages -

Publisher

NATURE PORTFOLIO
DOI: 10.1038/s42005-022-00820-7

Keywords

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Funding

  1. Projekt DEAL

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This study presents computational simulations of spheroidal model-microswimmers confined between two walls, revealing two collective dynamic states, depending on density and hydrodynamic interactions. Microswimmers exhibit an highly-dynamic collective motion with large-scale swirling patterns denoted as active turbulence. Various approaches have been applied to elucidate similarities and differences of inertial hydrodynamic and active turbulence.
Microswimmers are ubiquitous in nature and present highly-dynamic collective motion. This study presents computational simulations of spheroidal model-microswimmers confined between two walls, revealing two collective dynamic states, giant motile clusters and active turbulence, depending on density and hydrodynamic interactions. 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 of inertial hydrodynamic and active 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 behavior, 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 a strong rotlet dipole. This emphasizes the paramount importance of the hydrodynamic flow field for swarming motility and bacterial turbulence.

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