Rotational and translational diffusion in an interacting active dumbbell system

TL;DR

This study investigates the diffusive behavior of interacting active dumbbells in two dimensions, revealing how activity, density, and phase transitions influence translational and rotational diffusion properties.

Contribution

It provides a detailed analysis of diffusion regimes and their dependence on Péclet number and density in an interacting active dumbbell system, highlighting new dynamical behaviors.

Findings

Four regimes of translational mean square displacement persist at finite density.

Diffusion constants depend on Péclet number and density, with specific ratios remaining constant.

Complex rotational behavior emerges at finite density, with non-monotonic dependence on density at high Péclet numbers.

Abstract

We study the dynamical properties of a two-dimensional ensemble of self-propelled dumbbells with only repulsive interactions. This model undergoes a phase transition between a homogeneous and a segregated phase and we focus on the former. We analyse the translational and rotational mean square displacements in terms of the P\'eclet number, describing the relative role of active forces and thermal fluctuations, and of particle density. We find that the four distinct regimes of the translational mean square displacement of the single active dumbbell survive at finite density for parameters that lead to a separation of time-scales. We establish the P\'eclet number and density dependence of the diffusion constant in the last diffusive regime. We prove that the ratio between the diffusion constant and its value for the single dumbbell depends on temperature and active force only through the P\'eclet number at all densities explored. We also study the rotational mean square displacement proving the existence of a rich behavior with intermediate regimes only appearing at finite density. The ratio of the rotational late-time diffusion constant and its vanishing density limit depends on the P\'eclet number and density only. At low P\'eclet number it is a monotonically decreasing function of density. At high P\'eclet number it first increases to reach a maximum and next decreases as a function of density. We interpret the latter result advocating the presence of large-scale fluctuations close to the transition, at large enough density, that favour coherent rotation inhibiting, however, rotational motion for even larger packing fractions.