Transverse Self-Propulsion Enhances the Aggregation of Active Dumbbells

TL;DR

This study uses simulations to show that active dumbbells propelled transversely tend to form larger, more stable clusters with spontaneous rotation, differing from axially propelled dumbbells in phase behavior and cluster dynamics.

Contribution

It introduces a novel model of transverse self-propulsion in active dumbbells and analyzes its effects on phase separation, cluster stability, and rotation, highlighting differences from axial propulsion.

Findings

Transverse propulsion enlarges the phase-separated region at all activities.

Clusters exhibit spontaneous rotation with angular velocity scaling as ω∼r_g^{-2}.

Transversely propelled dumbbells form more stable clusters with interior orientation.

Abstract

We investigate a two-dimensional system of active Brownian dumbbells using molecular dynamics simulations. In this model, each dumbbell is driven by an active force oriented perpendicular to the axis connecting its two constituent beads. We characterize the resulting phase behavior and find that, across all values of activity, the system undergoes phase separation between dilute and dense phases. The dense phase exhibits hexatic order, and for large enough activity, we observe a marked increase in local polarization, with dumbbells predominantly oriented towards the interior of the clusters. Compared to the case of axially self-propelled dumbbells, we find that the binodal region is enlarged towards lower densities at all activities. This shift arises because dumbbells with transverse propulsion can more easily form stable cluster cores, serving as nucleation seeds, and show a highly suppressed escaping rate from the cluster boundary. Finally, we observe that clusters exhibit spontaneous rotation, with the modulus of the angular velocity scaling as $\omega\sim r_g^{-2}$, where $r_g$ is the cluster's radius of gyration. This contrasts with axially propelled dumbbells, where the scaling follows $\omega\sim r_g^{-1}$. We develop a simplified analytical model to rationalize this scaling behavior.