  We computationally studied the phase behavior and dynamics of binary mixtures
of active particles, where each 'species' had distinct activities leading to
distinct velocities, fast and slow. We obtained phase diagrams demonstrating
motility-induced phase separation (MIPS) upon varying the activity and
concentration of each species, and extended current kinetic theory of
active/passive mixtures to active/active mixtures. We discovered two regimes of
behavior quantified through the participation of each species in the dense
phase compared to their monodisperse counterparts. In regime I (active/passive
and active/weakly-active), we found that the dense phase was segregated by
particle type into domains of fast and slow particles. Moreover, fast particles
were suppressed from entering the dense phase while slow particles were
enhanced entering the dense phase, compared to monodisperse systems of all-fast
or all-slow particles. These effects decayed asymptotically as the activity of
the slow species increased, approaching the activity of the fast species until
they were negligible (regime II). In regime II, the dense phase was
homogeneously mixed and each species participated in the dense phase as if it
were it a monodisperse system; each species behaved as if it weren't mixed at
all. Finally, we collapsed our data defining two quantities that measure the
total activity of the system. We thus found expressions that predict, a priori,
the percentage of each particle type that participates in the dense phase
through MIPS.
