Dynamics of Active Brownian Particles in two dimensions: macro and microphase separation, cluster diffusion, particle geometry effects.

by Prof. Giuseppe Gonnella

Wednesday 12 Feb 2025, 16:00 → 17:00 Europe/Rome

2G26

Active matter systems are non-equilibrium systems in which individual particles, biological or artificial, continually consume internal energy to self-propel, breaking time-reversal symmetry at a local scale and in a sustained way. Their non-equilibrium nature allows a variety of intriguing phenomena to appear, for example, the phase separation in a dense and dilute phase in the complete absence of attractive interactions, called motility induced phase separation (MIPS). We would like to illustrate peculiar features of MIPS in a paradigmatic model of active matter, the Active Brownian Particles (ABPs) in two spatial dimensions. Even though MIPS preserves many aspects of an equilibrium phase separation, the physics at play is more complex, leading to new phenomena. We explain the growth exponent z =1/3 in terms of an aggregation-condensation mechanism, also taking into account the fractal geometry of aggregates and the diffusion properties of single clusters. We found anomalous dependence of the cluster diffusion coefficient on the cluster mass, since it decreases as the inverse of the square root of mass but still increasing with the square of the Peclet number as in the case of single particles. Moreover, on top of this ordinary equilibrium-like coarsening, we found evidence of another ordering mechanism, i.e., the micro-phase separation of the dense phase into hexatic domains and vapour bubbles. 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 and the motion of single clusters that acquire evident ballistic behavior.