Phase separation in a binary mixture of self-propelled particles with variable speed

Phase separation in a binary mixture of self-propelled particles is an active area of research. Many factors are responsible for the phase separation among different types of self-propelled particles. In this paper, we consider a binary mixture of self-propelled particles moving with variable speed. The variable speed model is introduced, where the propulsion speed of each particle varies with their neighbours orientation through a power γ. Different types of particles can be characterised by different γ. Hence we ask the question: what happens if we mix two types of particles (γ1,γ2)? We find that the transition from the disordered-to-ordered state remains invariant for any power γ. In the ordered state, we find phase separation when the difference in the two γ’s is large. Phase separation decreases by decreasing the difference of |γ1−γ2|. Which is also confirmed by linearised hydrodynamic equations of motion in the ordered state. In the disordered state when the difference in the two γ’s is large, one type of particles with large γ form static clusters which act as a nucleation site for another type particles with smaller γ. We argue that the mechanism proposed here is one of the reasons for the formation of aggregates in the binary mixture of self-propelled particles.