Spontaneous phase transition in bacterial polar fluids

Numerous efforts have focused on understanding and controlling active turbulence, yet a spontaneous phase transition from an initially aligned polar state to active turbulence has not been demonstrated in bacterial systems. Here we report such a transition, which proceeds through a previously unrecognized intermediate phase of counter-rotating concentric rings in bacterial fluids. Using an open microfluidic device, we generate large-scale bacterial polar fluids by imposing shear flow. Intrinsic bend instabilities transform these polar fluids into counter-rotating concentric rings that persist before breaking into turbulence. We find that the emergence of the ring pattern is independent of both confinement and activity, due to stabilizing steric interactions among converging bacteria. The ring wavelength and vortex size converge to a single, confinement-tunable length scale, whereas the ring lifetime, vortex duration, and the self-shear time collapse onto a common time scale independent of length. These results show that universal scales are selected at the onset of instability but arise from distinct mechanisms. Together, our findings reveal a unique mechanism of bacterial turbulence, distinct from microtubule–kinesin systems and consistent with numerical simulations, providing a framework for spatiotemporal control in active fluids and establishing new principles governing non-equilibrium phase transitions.