A lattice model of bacterial turbulence*

One of the most striking difference between active and passive systems is the appearance of collective motion in self-propelled particles suspended in a fluid. The phenomenology of this transition is well established: At low densities particles move in a seemingly uncorrelated fashion, while at higher densities they organise into jets and vortices comprising many individual microswimmers. Our recent work (Stenhammar et al, PRL 119, 028005 (2017)) suggests that this transition is caused by mutual reorientation of the microswimmers and is insensitive to their translational degrees of freedom.

Motivated by this obsrevation, we develop and study a lattice model of the collective phase. We confine model microswimmers to a regular lattice and fix their positions. The dynamics of each microswimmer then comprise re-orientations in the velocity fields created by other microswimmers and random orientational changes representing bacterial tumble event. We observe that in the absence of tumbling, all dynamics cease after some initial time, yielding a 'frozen' configuration. For sufficiently often tumbling, these configurations 'melt', and we discuss the implications of this scenario for bacterial turbulence.