Collective Motion of Filamentous Bacteria

This chapter describes our experimental study on collective motion of long, filamentous, non-tumbling bacteria swimming in a thin fluid layer. The confinement in the quasi-two-dimensional plane and the high aspect ratio of the cells induce weak nematic alignment upon collision due to the weak excluded volume interactions, which, for sufficiently high density of cells, gives rise to global nematic order. This homogeneous but highly-fluctuating phase, observed on the largest experimentally-accessible scale of millimeters, exhibits the properties predicted by the standard flocking models, especially the Vicsek-style self-propelled rods: true long-range nematic order and non-trivial giant number fluctuations. Thus, our experimental system is recognized as the first unambiguous example that falls into the Vicsek universality class. Our results suggest necessary conditions for the Vicsek universality class in comparison with other experimental studies. This chapter is based on our publication [Nishiguchi et al., Physical Review E 95, 020601(R) (2017)] but also includes further detailed analysis of correlations and collisions.