Foundation and challenges in modelling Dilute Active Suspensions

TL;DR

This paper rigorously derives continuum models for dilute active suspensions from microscopic dynamics, clarifying assumptions, limitations, and challenges in modeling self-propelling particle systems like bacteria and algae.

Contribution

It provides a detailed derivation of continuum models from microscopic equations, explicitly states assumptions, and discusses limitations and boundary condition challenges.

Findings

Derivation of the mean-field model from microscopic dynamics

Explicit assumptions for continuum model derivation

Discussion of boundary condition and singularity issues

Abstract

Active suspensions, which consist of suspended self-propelling particles such as swimming microorganisms, often exhibit non-trivial transport properties. Continuum models are frequently employed to elucidate phenomena in active suspensions, such as shear trapping of bacteria, bacterial turbulence, and bioconvection patterns in suspensions of algae. Yet, these models are often empirically derived and may not always agree with the individual-based description of active particles. Here we establish a more rigorous foundation to fully develop a continuum model based on the respective microscopic dynamics through coarse-graining. All the assumptions needed to reach popular continuum models from a multi-particle Fokker-Planck equation, which governs the probability of the full configuration space, are explicitly presented. In the dilute limit, this approach leads to the mean-field model (a.k.a. Doi-Saintillan-Shelley model), which can be further reduced to a continuum equation for particle density. Moreover, we review the limitations and highlight the challenges related to continuum descriptions, including significant issues in implementing physical boundary conditions and the possible emergence of singular solutions.