Viscoelasticity enables self-organization of bacterial active matter*

Simultaneous control of spatial and temporal organization of active matter is challenging and generally requires complex interactions, such as reaction-diffusion hierarchies or genetically engineered cellular circuits. Here we found that tuning the rheological properties of bacterial active fluids enables large-scale spatial and temporal self-organization. As the viscoelasticity of the suspending fluid is varied, a confined bacterial active fluid first self-organizes in space into a millimeter-scale rotating vortex; then displays temporal organization as the giant vortex switches its global chirality periodically with tunable frequency, reminiscent of a self-driven torsional pendulum. The phenomenon can be explained in terms of the interplay between active forcing and viscoelastic stress relaxation. Our findings advance the understanding of bacterial behavior in complex fluids and demonstrate experimentally that rheological properties can be harnessed to control active matter flows.