Competing effect of activity and non-inert crowding on the dynamics of self-propelled tracer particles.

Transport of single molecules via passive or active processes is crucial for biological functioning. In this study, we investigate the dynamics of point-like self-propelled tracer particles in a non-inert, ordered crowded environment using underdamped Langevin dynamics. The crowders are modeled by creating a biologically relevant setting with high damping and attractive centers. We analyze the diffusion of passive and self-propelled tracers as a function of crowder stickiness and packing fraction. Passive tracers exhibit subdiffusive behavior, while self-propelled tracers show superdiffusion despite obstruction by crowders. At intermediate packing fractions, diffusion is suppressed due to perfectly separated attractive centers, an effect overcome by active propulsion, which enhances tracer mobility. In extreme crowding situations, the attractive potential valleys start to overlap, contributing to enhanced diffusion. The displacement probability distribution of passive tracers is non-Gaussian at intermediate packing fractions, displaying a double-Gaussian structure. This non-Gaussianity is reduced for active tracers, whose distributions exhibit soft multi-step features arising from repeated hopping and trapping events. Furthermore, propulsion restores ergodicity, which is otherwise broken due to the presence of sticky crowders.