Motility-induced clustering and meso-scale turbulence in active polar fluids

Abstract Meso-scale turbulence was originally observed experimentally in various suspensions of swimming bacteria, as well the collective motion active colloids. The corresponding large scale dynamical patterns were reproduced a simple model polar fluid, assuming constant density particles. Recent, more detailed studies variety experimental realizations fluids revealed additional interesting aspects, such anomalous velocity statistics and clustering phenomena. Those phenomena cannot be explained by currently available models for fluids. Herein, we extend continuum suggested Dunkel et al to include variations local feedback between self-propulsion speed If decreases strong enough with density, linear stability analysis resulting shows that, addition short-wavelength instability original model, long-wavelength occurs. This is typically high densities particles analogous well-known phenomenon motility-induced phase separation (MIPS) scalar matter. We determine diagram indicating instabilities perform systematic numerical simulations regions parameter space. interplay understood short-range (leading meso-scale turbulence) long-range (associated MIPS) leads dynamics novel concerning nucleation coarsening processes. Our simulation results display rich patterns, including into domains dynamically changing irregularly shaped boundaries. Anomalous are all phases where system segregates low densities. offers explanation their occurrence recent experiments bacterial suspensions.