  Living systems at the sub-cellular, cellular, and multi-cellular level are
often crowded systems that contain active particles. The active motion of these
particles can also propel passive particles, which typically results in
enhanced effective diffusion of the passive particles. Here we study the
diffusion of a passive tracer particle in such a dense system of active
crowders using a minimal lattice model incorporating particles pushing each
other. We show that the model exhibits several regimes of motility and quantify
the enhanced diffusion as a function of density and activity of the active
crowders. Moreover, we demonstrate an interplay of tracer diffusion and
clustering of active particles, which suppresses the enhanced diffusion.
Simulations of mixtures of passive and active crowders show that a rather small
fraction of active particles is sufficient for the observation of enhanced
diffusion.
