Collective dynamics of densely confined active polar disks with self- and mutual alignment

TL;DR

This study investigates how dense groups of self-propelled polar disks with off-centered rotation exhibit various collective behaviors, including oscillations and milling, under different conditions, revealing robust and observable phenomena.

Contribution

It introduces a mechanical model combining self- and mutual alignment mechanisms, demonstrating diverse collective states in confined active polar disks.

Findings

Emergence of high-frequency oscillations and low-frequency milling states.

Robustness of collective behaviors across different parameters.

Potential observability in natural or artificial systems.

Abstract

We study the emerging collective states in a simple mechanical model of a dense group of self-propelled polar disks with off-centered rotation, confined within a circular arena. Each disk presents self-alignment towards the sum of contact forces acting on it, resulting from disk-substrate interactions, while also displaying mutual alignment with neighbors due to having its center of rotation located a distance R behind its centroid so that central contact forces can also introduce torques. The effect of both alignment mechanisms produces a variety of collective states that combine high-frequency localized circular oscillations with low-frequency milling around the center of the arena, in fluid or solid regimes. We consider cases with small/large R values, isotropic/anisotropic disk-substrate damping, smooth/rough arena boundaries, different densities, and multiple systems sizes, showing that the emergent collective states that we identify are robust, generic, and potentially observable in real-world natural or artificial systems.