Molecular dynamics simulations of active Brownian particles in dilute suspension: diffusion in free space and distribution in confinement

TL;DR

This paper introduces a new molecular dynamics simulation method for active Brownian particles, analyzing their diffusion in free space and distribution under confinement, revealing how persistence time influences stationary distributions and diffusion properties.

Contribution

A novel MD simulation approach for ABPs that captures hydrodynamics and active forces, providing insights into diffusion and distribution behaviors in different environments.

Findings

Effective diffusion coefficient in free space agrees with analytical models.

Stationary distribution transitions from Boltzmann to non-Boltzmann with increased persistence.

Active diffusion in confinement aligns semi-quantitatively with free space results.

Abstract

In this work, we report a new method to simulate active Brownian particles (ABPs) in molecular dynamics (MD) simulations. Immersed in a fluid, each ABP consists of a head particle and a spherical phantom region of fluid where the flagellum of a microswimmer takes effect. The orientation of the active particle is governed by a stochastic dynamics, with the orientational persistence time determined by the rotational diffusivity. To hydrodynamically drive the active particle as a pusher, a pair of active forces are exerted on the head particle and the phantom fluid region respectively. The active velocity measured along the particle orientation is proportional to the magnitude of the active force. The effective diffusion coefficient of the active particle is first measured in free space, showing semi-quantitative agreement with the analytical result predicted by a minimal model for ABPs. We then turn to the probability distribution of the active particle in confinement potential. We find that the stationary particle distribution undergoes an evolution from the Boltzmann-type to non-Boltzmann distribution as the orientational persistence time is increased relative to the relaxation time in the potential well. From the stationary distribution in confinement potential, the active part of the diffusion coefficient is measured and compared to that obtained in free space, showing a good semi-quantitative agreement while the orientational persistence time varies greatly relative to the relaxation time.