Active Matter Shepherding and Clustering in Inhomogeneous Environments

TL;DR

This paper investigates how active and passive disks behave in an inhomogeneous environment with pinning, revealing various dynamical phases including shepherding, phase separation, and clustering, depending on motor force and pinning density.

Contribution

It introduces a detailed phase diagram of active-passive disk mixtures in inhomogeneous environments, highlighting novel shepherding and phase separation phenomena.

Findings

Active disks shepherd passive disks into unpinned regions.

High pinning and motor force lead to rapid accumulation and phase separation.

Different dynamical phases are mapped in a comprehensive phase diagram.

Abstract

We consider a mixture of active and passive run-and-tumble disks in an inhomogeneous environment where only half of the sample contains quenched disorder or pinning. The disks are initialized in a fully mixed state of uniform density. We identify several distinct dynamical phases as a function of motor force and pinning density. At high pinning densities and high motor forces, there is a two step process initiated by a rapid accumulation of both active and passive disks in the pinned region, which produces a large density gradient in the system. This is followed by a slower species phase separation process where the inactive disks are shepherded by the active disks into the pin-free region, forming a non-clustered fluid and producing a more uniform density with species phase separation. For higher pinning densities and low motor forces, the dynamics becomes very slow and the system maintains a strong density gradient. For weaker pinning and large motor forces, a floating clustered state appears and the time averaged density of the system is uniform. We illustrate the appearance of these phases in a dynamic phase diagram.