Percolation transitions in a binary mixture of active Brownian particles with different softness.

Homogeneous active Brownian particle (ABP) systems with purely repulsive interactions are considered to exhibit a 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 the 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, we observed a novel feature: the formation of a space-filling structure made of particles with higher stiffness, within the dense cluster of MIPS containing both types of particles. Our systematic analysis shows that this structure formation occurs only if the difference in softness of both types of particles is sufficiently large. Furthermore, the presence of a non-linear scaling for different compositions of binary 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 lead to complex phase behavior.