  Homogeneous active Brownian particle (ABP) systems with purely repulsive
interactions are considered to have simple phase behavior, but various physical
attributes of active entities can lead to variation in the collective dynamics.
Recent studies have shown that even homogeneous ABPs exhibit complex behavior
due to an interplay between particle softness and motility. However, the
heterogeneity in composition of ABPs has not been explored yet. In this paper,
we study the structural properties of a binary mixture of ABPs with different
particle softness, by varying the relative softness and composition. We found
that upon varying the motility parameter, the system underwent a
motility-induced phase separation (MIPS) followed by a percolation transition
similar to the homogeneous systems; however, there is complex structure
formation within the dense phase of the MIPS which depends on the relative
softness of the binary system. Furthermore, the presence of a non-linear
scaling for different compositions of heterogeneous ABPs suggests that there is
a complex relationship between the composition and the structural properties.
Our study demonstrates that the composition heterogeneity of ABPs can also lead
to complex phase behavior.
