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

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.