Two-dimensional bacterial turbulence at a liquid-air interface

Many ecological issues at a liquid-air interface are complicated due to the non-equilibrium and multi-scale nature of the interface. Active matter such as bacterial suspension at an interface drives the system even further from equilibrium. In this talk, we show our recent results on active turbulence at a 2D interface. In experiments, 2D bacterial turbulence is normally formed in a thin layer of liquid above solid, or between two liquid phases. It is difficult to quantify how hydrodynamic interactions influence the structure of active turbulence, since the 2D suspension is either too far or too close to a no-slip boundary that is parallel to the bacterial motion. Here, we use concentrated Serratia marcescens bacteria that swim at a liquid-air interface to form 2D active turbulence. By cell culture, we obtain bacteria with lengths from 1μm to 10μm to tune the hydrodynamic dipole lengths. By using a microbubble assay, the thickness of the liquid layer can be adjusted from 1μm to several mm. The mean vortex size of bacterial turbulence scales with hydrodynamic dipole lengths. More interestingly, the vortex size decreases exponentially with the distance to the solid wall, which we believe is a unique feature of the 2d bacterial suspension at a water-air interface.