Dynamic Mode Structure of Bacterial Turbulence

Dense suspensions of swimming bacteria exhibit chaotic flow patterns that promote mixing and transport of resources and signaling chemicals within cell colonies. Whilst the importance of this so-called “bacterial turbulence” is widely recognized, the structure of the resulting flow is not well understood. Here, we extend the use of modal decomposition to study the dynamical flow structure of this model active matter system. Particle image velocimetry (PIV) quantifies the two-dimensional velocity field of dense bacterial suspensions of the rod-shaped Bacillus subtilis. The dominant spatial structures of the velocity and vorticity fields are extracted using proper orthogonal decomposition (POD) and ranked in order of energy and enstrophy magnitude, respectively. The time-dependent amplitudes of the spatial structures reveal velocity fluctuation frequencies within the turbulent system. We also examine the spatial mode structure and fluctuation frequency as a function of mean cell swimming speed to characterize changes in the kinetic energy distribution with cell activity. These results contribute to the fundamental understanding of active matter system dynamics and chaotic flow structures responsible for mixing.