Study of Active Brownian Particle Diffusion in Polymer Solutions

TL;DR

This study investigates how active Brownian particles diffuse in polymer solutions, revealing non-monotonic size dependence and the influence of active forces on effective viscosity and diffusion behavior.

Contribution

It introduces a phenomenological effective viscosity for active particles and uncovers non-trivial size and force dependencies in diffusion within complex fluids.

Findings

Long-time diffusion coefficient shows non-monotonic dependence on particle size at high active forces.

Effective viscosity experienced by active particles is larger and depends on size and force.

Diffusion scaling with propelling force exhibits non-monotonic behavior with particle size.

Abstract

The diffusion behavior of an active Brownian particle (ABP) in polymer solutions is studied using Langevin dynamics simulations. We find that the long time diffusion coefficient $D$ can show a non-monotonic dependence on the particle size $R$ if the active force $F_{a}$ is large enough, wherein a bigger particle would diffuse faster than a smaller one which is quite counterintuitive. By analyzing the short time dynamics in comparison to the passive one, we find that such non-trivial dependence results from the competition between persistence motion of the ABP and the length-scale dependent effective viscosity that the particle experienced in the polymer solution. \textcolor{black}{We have also introduced an effective viscosity $\eta_{\text{eff}}$ experienced by the ABP phenomenologically. Such an active $\eta_{\text{eff}}$ is found to be larger than a passive one and strongly depends on $R$ and $F_{a}$}\textcolor{magenta}{.} In addition, we find that the dependence of $D$ on propelling force $F_{a}$ presents a well scaling form at a fixed $R$ and the scaling factor changes non-monotonically with $R$. Such results demonstrate that active issue plays rather subtle roles on the diffusion of nano-particle in complex solutions.