Active turbulence in suspensions of bacterial mimetics

Swimming bacteria exhibit self-organizing behavior, forming macro-scale patterns such as swarms and dynamic clusters. At high concentrations, bacterial suspensions display turbulent-like motion characterized by erratic flow and characteristic vortex size. In contrast to classical hydrodynamic turbulence, which occurs at high Reynolds numbers and is dominated by inertia with energy injected at the macroscale and cascading down to the microscales, bacterial turbulence occurs at very small Reynolds numbers where inertia is negligible. Instead, it is energized by the motion of microswimmers. The energy spectrum in classical turbulence follows a power-law dependence on the wavenumber, with E ∼ k^(−5/3). However, for bacterial turbulence, the energy dependence of E ∼ k^(−8/3) has been reported, although experimental results are limited and inconclusive. In this study, we present a model system to study the emergence of coherent structures and large-scale flows in active fluids that mimic bacterial suspensions under well-defined conditions. In this system, we utilize the Quincke instability to design motile colloids that execute a random walk. By experimentally investigating the collective dynamics of these Quincke random walkers at different levels of activity, density, and types of random walk, we aim to gain insights into the turbulent-like behaviors in low-Reynolds-number active flows.