  Evidence suggests that the transport rate of a passive particle at long
timescales is enhanced due to interactions with the surrounding active ones in
a size- and composition-dependent manner. The collisions generate additional
friction on the particle while, on top of that, for nonequilibrium (active)
systems, the collisions can also lead to an exchange of energy between
different types of particles. Accordingly, the long-time diffusion coefficient
of a tagged particle is shaped by the interplay between the effective friction
and the energy transfer. Using a system of particles with different
temperatures, we probe these effects in dilute solutions and derive long-time
friction and self-diffusion coefficients as functions of volume fractions,
sizes and temperatures of particles. Remarkably, we show that both viscosity
and energy flux display a nonlinear dependence on size. The simplicity of our
formalism allows to discover various interesting scenarios that can be relevant
for biological systems.
