Simulation of collective phenomena in microswimmer suspensions

Collective motion is ubiquitous in biological and synthetic systems across many
length- and timescales. On the macroscopic scale, examples include schools of fish, herds of sheep and flocks of birds. On the microscopic scale, bacteria, algae and synthetic self-propelled particles exhibit a range of collective phenomena. In suspensions of swimming bacteria, collective motion is often caused by hydrodynamic interactions between the swimmers, and is manifested as long-ranged chaotic flows, dubbed active turbulence. In this work, we study collective motion in simplified models of bacterial and algal suspensions with particle-resolved lattice Boltzmann simulations. Using an
extended force dipole as a minimal model for a microswimmer, we have been able to study large systems, containing up to 3 × 10^6 particles, and to capture information about large-scale collective behaviours. We have studied four separate aspects of collective motion in microswimmer suspensions. First, we performed unprecedentedly large simulations of 3-dimensional active suspensions to test predictions from kinetic theory about the transition to active turbulence and characterize the ensuing turbulent state. The focus was then turned to the effects of swimming velocity on the transition to active turbulence of pusher suspensions. In nature, front- and rearactuated microswimmers (so called pushers and pullers, respectively) coexist, which motivated us to study how the presence of pullers in the suspension changes the collective behaviour of pushers. Finally, motivated by the fact that most experiments are performed in 2-dimensional geometries, we also investigated and characterized the collective phenomena in a quasi-2-dimensional system, finding important qualitative differences compared to unbounded suspensions.