  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. 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 in $d=2$ and 3 dimensions. Thus,
we model excluded-volume interactions for nonequilibrium systems but also
extend the scope to short-range soft potentials and compare our results to
Brownian-dynamics simulations. 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 and active colloids.
