  In this work, we focus on the behavior of a single passive Brownian particle
in a suspension of passive particles with short-range repulsive interactions
and a larger self-diffusion coefficient. While the forces affecting the
single-particle are thermal-like fluctuations and repulsion, due to other
particles in the suspension, our numerical simulations show that on
intermediate time scales directed motion on a single-particle level emerges.
This emergent directional motion leads to a breakdown of the Einstein relation
and non-monotonic augmentation of the measured diffusion coefficient.
Directional tendency increases with the density of the suspension and leads to
growth of the diffusivity with the density of the suspension, a phenomenon
recently observed for a system of hard spheres by Ilker, Castellana, and
Joanny. Counter-intuitively, the directional flow originates from the tendency
of different particles to push each other out of their way. Due to such
strictly repulsive interactions, nearby particles form into temporally
correlated pairs and move cooperatively, thus creating a preferred direction of
motion on intermediate time scales. We show that directional motion emerges
when the ratio of the self-diffusion coefficients of the tracked particle and
suspension constituents is below a critical value.
