Interacting passive advective scalars in an active medium

Recent experimental studies, both in vivo and in vitro, have revealed that membrane components that bind to the cortical actomyosin meshwork are driven by active fluctuations, whereas membrane components that do not bind to cortical actin are not. Here we study the statistics of density fluc- tuations and dynamics of particles advected in an active quasi-two dimensional medium comprising self-propelled filaments with no net orientational order, using a combination of agent-based Brow- nian dynamics simulations and analytical calculations. The particles interact with each other and with the self-propelled active filaments via steric interactions. We find that the particles show a tendency to cluster and their density fluctuations reflect their binding to and driving by the active filaments. The late-time dynamics of tagged particles is diffusive, with an active diffusion coefficient that is independent of (or at most weakly-dependent on) temperature at low temperatures. Our results are in qualitative agreement with the experiments mentioned above. In addition, we make predictions that can be tested in future experiments.