Controlling Waves in Polar Active Fluids

In polar active fluids, macroscopic flows are generated by self-propelled constituent particles, such as swimming bacteria or colloidal Quincke rollers. These constituents typically experience aligning interactions that allow the emergence of ordered flocking flows, which spontaneously break rotational symmetry. This symmetry breaking and the lack of energy conservation have profound consequences for the dynamics of wave-like excitations about a flocking state. Such dynamics naturally underpin the propagation of long-range signals, but how can we control them? Combining numerical simulations and theory, we characterize the excitation spectrum of waves in the Toner-Tu model, and quantify its dependence on parameters, domain geometry, and inhomogeneity of the base state (e.g. a steady vortex). As a minimal control strategy, we leverage periodic driving of the self-propulsion speed to parametrically excite waves in the fluid, demonstrating a simple route for spatial control of patterns through time-modulated activity.