Polymers in Two-dimensional Bacterial Turbulence

In this talk, we experimentally investigate the effects of polymer additives on the collective dynamics of swarming Serratia marcescens in quasi-2D liquid films. We find that even minute amounts of polymers (< 20 ppm) can significantly enhance bacterial collective motion and promote large-scale coherent structures. Velocity statistics show that polymers reduce the likelihood of local velocity deviating from the mean swarming velocity, weakening the intermittency of fluctuations. Spatial and temporal correlations suggest that both the size and lifetime of flow structures are increased with polymers. In addition, we report an upscale transfer of enstrophy and energy using the recently developed filtering techniques. Unlike in classical 2D turbulence, both enstrophy and energy fluxes move primarily towards large scales in bacterial turbulence. The inverse enstrophy flux increases substantially with the addition of polymers, which is a potential mechanism for the increase in large-scale coherence.