Clogging and Depinning of Ballistic Active Matter Systems in Disordered Media

TL;DR

This study uses simulations to explore how ballistic active disks behave in disordered media, revealing various phases including clogging, depinning, and phase separation, with activity level influencing flow and clogging phenomena.

Contribution

It introduces a detailed phase diagram of active disks in disordered media, highlighting the effects of activity and obstacle density on depinning and clogging behaviors.

Findings

Active disks clog at lower obstacle densities than passive disks.

Multiple distinct phases including pinned, jammed, and gel states are identified.

An optimal activity level maximizes particle flux through the medium.

Abstract

We numerically examine ballistic active disks driven through a random obstacle array. Formation of a pinned or clogged state occurs at much lower obstacle densities for the active disks than for passive disks. As a function of obstacle density we identify several distinct phases including a depinned fluctuating cluster state, a pinned single cluster or jammed state, a pinned multicluster state, a pinned gel state, and a pinned disordered state. At lower active disk densities, a drifting uniform liquid forms in the absence of obstacles, but when even a small number of obstacles are introduced, the disks organize into a pinned phase-separated cluster state in which clusters nucleate around the obstacles, similar to a wetting phenomenon. We examine how the depinning threshold changes as a function of disk or obstacle density, and find a crossover from a collectively pinned cluster state to a disordered plastic depinning transition as a function of increasing obstacle density. We compare this to the behavior of nonballistic active particles and show that as we vary the activity from completely passive to completely ballistic, a clogged phase-separated state appears in both the active and passive limits, while for intermediate activity, a readily flowing liquid state appears and there is an optimal activity level that maximizes the flux through the sample.