Dynamics of a homogeneous active dumbbell system

TL;DR

This study investigates the dynamics of active dumbbells in two dimensions, revealing complex dependencies of diffusion, response, and effective temperature on activity, density, and temperature, with implications for understanding active matter behavior.

Contribution

It provides a detailed analysis of how activity and density influence the dynamical properties and effective temperature in a homogeneous active dumbbell system, highlighting non-monotonic behaviors.

Findings

Diffusion constant increases with activity, decreases with packing fraction.

Effective temperature is always higher than ambient temperature and varies non-monotonically with activity.

Finite-size clustering affects the effective temperature and dynamical responses.

Abstract

We analyse the dynamics of a two dimensional system of interacting active dumbbells. We characterise the mean-square displacement, linear response function and deviation from the equilibrium fluctuation-dissipation theorem as a function of activity strength, packing fraction and temperature for parameters such that the system is in its homogeneous phase. While the diffusion constant in the last diffusive regime naturally increases with activity and decreases with packing fraction, we exhibit an intriguing non-monotonic dependence on the activity of the ratio between the finite density and the single particle diffusion constants. At fixed packing fraction, the time-integrated linear response function depends non-monotonically on activity strength. The effective temperature extracted from the ratio between the integrated linear response and the mean-square displacement in the last diffusive regime is always higher than the ambient temperature, increases with increasing activity and, for small active force it monotonically increases with density while for sufficiently high activity it first increases to next decrease with the packing fraction. We ascribe this peculiar effect to the existence of finite-size clusters for sufficiently high activity and density at the fixed (low) temperatures at which we worked. The crossover occurs at lower activity or density the lower the external temperature. The finite density effective temperature is higher (lower) than the single dumbbell one below (above) a cross-over value of the Peclet number.