Effect of polydispersity on the dynamics of active Brownian particles

TL;DR

This study numerically investigates how size diversity affects the dynamics and phase behavior of active Brownian particles, revealing four distinct phases and enhanced diffusion with increased polydispersity.

Contribution

It introduces a comprehensive analysis of polydispersity effects on active particle phases, identifying new phase distinctions and diffusion behaviors not previously characterized.

Findings

Four distinct phases identified: jammed, liquid, solid-jammed, and liquid jammed.

Enhanced diffusion observed with increasing polydispersity.

System behavior consistent across different packing densities.

Abstract

We numerically study the dynamics and the phases of self-propelled disk-shaped particles of different sizes with soft repulsive potential in two dimensions. Size diversity is introduced by the polydispersity index (PDI) $\epsilon$, which is the width of the uniform distribution of the particle's radius. The self-propulsion speed of the particles controls the activity $v$. We observe enhanced dynamics for large size diversity among the particles. We calculate the effective diffusion coefficient $D_{eff}$ in the steady-state. The system exhibits four distinct phases, jammed phase with small $D_{eff}$ for small activity and liquid phase with enhanced $D_{eff}$ for large activity. The number fluctuation is larger and smaller than the equilibrium limit in the liquid and jammed phase, respectively. Further, the jammed phase is of two types: solid-jammed and liquid jammed for small and large PDI. Whereas the liquid phase is called motility induced phase separation (MIPS)-liquid for small PDI and for large PDI, we find enhanced diffusivity and call it the {\em pure liquid} phase. The system is studied for three packing densities $\phi$, and the response of the system for polydispersity is the same for all $\phi$'s. Our study can help understand the behavior of cells of various sizes in a tissue, artificial self-driven granular particles, or living organisms of different sizes in a dense environment.