Dynamics of Active Brownian Particles in 2D: macro and microphase separation, cluster diffusion, particle geometry effects

Active matter systems are non-equilibrium systems in which individual particles continually consume internal energy at a local scale and in a sustained way. Their non-equilibrium nature allows for a variety of intriguing phenomena as for instance motility induced phase separation (MIPS). Here, we illustrate peculiar features of MIPS in a system of Active Brownian Particles (ABPs) in 2D. We explain the growth exponent z ≈ 1/3 in the L∼t^z law, L being the typical size of clusters of the dense phase and t the time, in terms of an aggregation-condensation mechanism, also taking into account the fractal geometry of aggregates and the diffusion properties of single clusters. We find anomalous dependence of the cluster diffusion coefficient, which decreases as the inverse of the square root of mass, yet increasing with the square of the Peclet number as in the case of single particles. Moreover, we found evidence of another ordering mechanism, i.e., the micro-phase separation of the dense phase into hexatic domains and vapour bubbles. The growth rate of hexatic domains differs from that of the whole clusters and behaves as L_H≈t^0.2. The size of bubbles at steady-state are controlled by the propulsion force and are also quantitatively described. Finally, we show that changing the geometry of active particles from disks to dumbbells has enormous effects on the dynamics of MIPS, both affecting the value of the growth exponent (z ≈ 3/5) and the motion of single clusters that acquire evident ballistic behaviour.