Phase separation in mixed systems of active particles with low and high inertia

Active matter refers to a class of substances capable of autonomously moving by harnessing energy from their surrounding environment. These substances exhibit unique non-equilibrium phenomena, and hence have attracted great attention in the scientific community. Many active matters, such as bacteria, cells, micro-swimmers, and self-propelled colloidal particles, operate in viscous environment and their motions are usually described using overdamped models. Examples include overdamped active Brownian particle (ABP) model for self-propelled colloidal particles in solution and run-and-tumble (RTP) model for swimming bacteria. In recent years, increasing research studies have focused on the impact of inertia on the behavior of active matter. Vibrating robots, runners, flying insects, and micro-fliers are example active systems in the underdamped condition. The motion of these active matters can be modelled by underdamped Langevin equation, known as the active inertial particle (AIP) model. Previous studies have demonstrated that, similar to ABP systems, motility-induced phase separation (MIPS) phenomena also happen in AIP systems under certain density conditions. However, due to the strong collision-and-rebound effect, aggregation of AIP particles and hence the MIPS are impeded. In complex living/application environments, mixture of different active agents is often seen. Some studies on mixed systems of active matter show that the composition is an important quantity, influencing the phase separation phenomena. In this paper, we study the phase separation phenomena in mixed systems composed of low- and high-inertia active particles by underdamped Langevin dynamics simulations. We find that, compared to single-component system, the mixed systems are unexpectedly more favorable for the occurrence of phase separation at moderate overall concentration and certain range of component fraction, while more unfavorable for phase separation at high overall concentration. The underlying mechanism is that the presence of a small amount of the high-inertia particles could accelerate the motion of the low-inertia particles, and hence facilitate their aggregation and promote the phase separation. However, when the fraction of the high-inertia particles is large, frequent elastic collisions would disturb the aggregation of the low-inertia particles and suppress the occurrence of phase separation. Our results provide new sights into the collective behavior of active materials and also a reference for their design and applications.