Characterization of MIPS in a suspension of repulsive Active Brownian Particles through dynamical features

TL;DR

This study investigates how the softness of interaction potentials influences Motility Induced Phase Separation in Active Brownian Particles, highlighting the importance of dynamical features over structural ones for phase boundary detection.

Contribution

It demonstrates that potential softness and dynamical measures significantly affect phase behavior in ABP systems, proposing dynamical features as more effective indicators of MIPS boundary.

Findings

Harder potentials reduce overlapping effects.

Dynamical features outperform structural ones in detecting MIPS.

Diffusion decay follows Vogel-Fulcher law, akin to fragile glasses.

Abstract

The two-dimensional Active Brownian Particles system is meant to be composed of hard disks, that show excluded volume interactions, usually simulated via molecular dynamics using pure repulsive potentials. We show that the softness of the chosen potential plays a role in the result of the simulation, focusing on the case of the emergence of Motility Induced Phase Separation. In a pure hard-sphere system with no traslational diffusion,the phase diagram should be completely determined by their density and P\'eclet number. However, we have found two additional effects that affect the phase diagram in the ABP model we simulate: the relative strength of the traslational diffusion compared to the propulsion term and the overlapping of the particles. As we show, the second effect can be strongly mitigated if we use, instead of the standard Weeks-Chandler- Andersen potential, a harder one, the pseudo-hard spheres potential. Moreover, in determining the boundary of our phase space, we have tried different approaches to detect MIPS and concluded that observing dynamical features, via the non-Gaussian parameter, is more efficient than observing structural ones, via the local density distribution function. We also demonstrate that the Vogel-Fulcher equation successfully reproduces the decay of the diffusion as a function of density, except for very high density cases. Thus, the ABP system behaves similarly to a fragile glass in this regard.