Bacterial turbulence at fluid interfaces

Bacterial turbulence at fluid interfaces emerges as chaotic spatiotemporal flows at the boundary between two fluids or at the air-fluid interface within bulk fluids. The two-dimensional flows at these interface that are projected from the incompressible three-dimensional Stokesian fluid inevitably introduce the effect of fluid compressibility (analogous to pumping and leakage flow) and effective hydrodynamic interactions among active units, which are essential for the stability and ordering of active suspensions. However, fully unbounded bacterial layers at fluid interfaces embedded in a three-dimensional bulk fluid have yet to be experimentally realized. Here, we investigate interfacial bacterial turbulence using hydrophobic Serratia marcescens, forming a two-dimensional active monolayer at the water-air interface within a bulk fluid. We observe the onset of turbulence upon exceeding a critical bacterial concentration. The mean vortex size increases with the bulk fluid thickness, saturating at approximately 100 micrometers, independent of bacterial length and horizontal system size, suggesting the presence of an intrinsic correlation length unique to interfacial active turbulence. Our findings reveal that vertical flow and restricted quasi-2D motion reshape collective behaviors, resulting in a universal scaling exponent of -2 in the kinetic energy spectra and a new class of active turbulence. This work highlights fluid interfaces as a versatile platform for designing tunable turbulence.