Anisotropic run-and-tumble-turn dynamics

Abstract: Run-and-tumble processes successfully model several living systems. While studies have typically focused on particles with isotropic tumbles, recent examples exhibit "tumble-turns", in which particles undergo 90{\deg} tumbles and so possess explicitly anisotropic dynamics. We study the consequences of such tumble-turn anisotropicity at both short and long-time scales. We model run-and-tumble-turn particles as self-propelled particles subjected to an angular potential. Using agent-based simulations, we study the interplay of noise and potential on the particles' trajectories, demonstrating that the long-time effect is to alter the tumble-turn time, which governs the long-time dynamics. In particular, when normalized by this timescale, trajectories become independent of the underlying details of the potential. As such, we develop a simplified continuum theory, which quantitatively agrees with agent simulations. The hydrodynamic limit reveals that the transition to diffusive dynamics precedes the transition to isotropic dynamics as the hydrodynamic limit, while purely diffusive, can drive anisotropicity at intermediate times.