Diffusion of multiple species with excluded-volume effects

TL;DR

This paper develops a continuum model for multiple interacting species with excluded-volume effects, linking stochastic particle dynamics to PDEs, and demonstrates how inter-species competition influences diffusion behaviors.

Contribution

It systematically derives a nonlinear cross-diffusion PDE system for multiple species from stochastic particle models using matched asymptotic expansions.

Findings

The model captures enhancement and diminishment of diffusion due to intra- and inter-species interactions.

Simulations agree well with the analytical continuum model.

The approach clarifies the distinction between collective and self-diffusion coefficients.

Abstract

Stochastic models of diffusion with excluded-volume effects are used to model many biological and physical systems at a discrete level. The average properties of the population may be described by a continuum model based on partial differential equations. In this paper we consider multiple interacting subpopulations/species and study how the inter-species competition emerges at the population level. Each individual is described as a finite-size hard core interacting particle undergoing Brownian motion. The link between the discrete stochastic equations of motion and the continuum model is considered systematically using the method of matched asymptotic expansions. The system for two species leads to a nonlinear cross-diffusion system for each subpopulation, which captures the enhancement of the effective diffusion rate due to excluded-volume interactions between particles of the same species, and the diminishment due to particles of the other species. This model can explain two alternative notions of the diffusion coefficient that are often confounded, namely collective diffusion and self-diffusion. Simulations of the discrete system show good agreement with the analytic results.