Active flows in dense suspensions of motile colloids*

Dense bacterial suspensions  generate a distinct turbulent-like motion at low Reynolds numbers, energized by the microswimmers’ activity. The energy spectrum of bacterial turbulence is predicted to scale with the wavenumber k as  E ∼ k^(−8/3), which contrasts  the E ∼ k^(−5/3) observed in classical inertial turbulence, and reflects an energy flow from micro to macro scales.

In our study, we introduce a model system to investigate the formation of coherent structures and large-scale flows in active fluids that mimic bacterial suspensions. We employ motile colloids driven by the Quincke instability that replicate the individual bacterial locomotion. Through experimental investigation, we examine the flow in a dense suspension of Quincke-driven random walkers at various levels of field strength, particle number density, and types of random walk. Our findings reveal a universal trend where the combined effect of field strength and density, termed activity, results  in energy spectrum's cascading slope close to  -4 .