Transport phenomena in active turbulence

Active or ‘self-propelled’ systems have emerged as a classical paradigm to investigate statistical mechanics of a wide variety of systems ranging from swimming bacteria, bird flocks, fish schools, to even Janus or ‘asymmetrically charged’ particles in a laboratory complex plasma. These systems are well known for their ability to form interesting phases that include swarming, jamming and even turbulence. Recently, it has been observed that changing the density of such suspensions can bring about a phase transition that is usually not amenable to equilibrium statistical mechanics. The turbulent phase in these active systems calls for an immediate attention due to its close resemblance to other physically distinct systems such as chemical reaction-diffusion processes. A few continuum models have already been constructed to describe this active turbulence and preliminary results are well supported by experiments. In this work, after presenting the phenomenology of active turbulence in two dimensions by discussing the available theoretical models and experiments, we focus on a recently proposed minimal continuum model that well describes (but not limited to) a dense suspension of bacteria. Using this model, important scaling laws are derived that may play a significant role in understanding transport phenomena in a generic active fluid that is usually far from equilibrium. We believe that these findings may be tested in any synthetic active matter system such as active complex plasmas, for example.