Lubrication induced non-linear mobility triggers onset of bacterial turbulence

Knowledge on the propulsions of micro-swimmers is of key importance to understand the formation and structure of active matter collective motion. Establishing a link between propulsion of a single swimmer and the onset of collective motion in a dense suspension demands experiments where the activity of particles can be controlled. Another way is to monitor a vortex and its surrounding flow in active turbulence that exhibits large fluctuation. We mimic a 2D bacterial vortex with fixed mean chirality using an interfacial rotor. By mixing the rotor with 2D bacterial suspension, we study inter-vortex interaction between the artificial and free vortices, by looking at the rotation of the rotor and the nearby bacterial flow. We observe a lubrication-induced nonlinear increase of rotor speed that applies to free vortices and in turn triggers the onset of active turbulence. The result will inspire new active-matter based microfluidic devices with improved transport properties.