Diffusion of two molecular species in a crowded environment: theory and experiments

TL;DR

This paper develops a theoretical model for the diffusion of two molecular species in crowded environments, validated by experiments showing crowding-induced barriers that hinder mixing, extending standard diffusion models to account for steric effects.

Contribution

The paper introduces a mesoscopic model with finite capacity that captures non-linear cross diffusion and crowding effects, supported by experimental validation.

Findings

Crowding creates a dynamical barrier preventing mixing.

The model accurately predicts the evolution of dense ink drops.

Standard diffusion models are extended to multispecies crowded environments.

Abstract

Diffusion of a two component fluid is studied in the framework of differential equations, but where these equations are systematically derived from a well-defined microscopic model. The model has a finite carrying capacity imposed upon it at the mesoscopic level and this is shown to lead to non-linear cross diffusion terms that modify the conventional Fickean picture. After reviewing the derivation of the model, the experiments carried out to test the model are described. It is found that it can adequately explain the dynamics of two dense ink drops simultaneously evolving in a container filled with water. The experiment shows that molecular crowding results in the formation of a dynamical barrier that prevents the mixing of the drops. This phenomenon is successfully captured by the model. This suggests that the proposed model can be justifiably viewed as a generalization of standard diffusion to a multispecies setting, where crowding and steric interferences are taken into account.