Effective diffusion along the backbone of combs with finite-span 1D and 2D fingers

TL;DR

This paper derives a general analytical expression for effective diffusion in comb-like structures with finite 1D and 2D fingers, revealing conditions under which diffusion is enhanced and connecting it to reversible adsorption and potential traps.

Contribution

It introduces a unified analytical framework for effective diffusion in comb structures with finite-length fingers, applicable to various trapping scenarios and energy landscapes.

Findings

Existence of a critical width of 2D fingers for faster diffusion

Derived a universal formula for effective diffusion coefficient

Connected diffusion in combs to reversible adsorption and potential traps

Abstract

Diffusion in complex heterogeneous media such as biological tissues or porous materials typically involves constrained displacements in tortuous structures and {\em sticky} environments. Therefore, diffusing particles experience both entropic (excluded-volume) forces and the presence of complex energy landscapes. In this situation, one may describe transport through an effective diffusion coefficient. In this paper, we examine comb structures with finite-length 1D and finite-area 2D fingers, which act as purely diffusive traps. We find that there exists a critical width of 2D fingers above which the effective diffusion along the backbone is faster than for an equivalent arrangement of 1D fingers. Moreover, we show that the effective diffusion coefficient is described by a general analytical form for both 1D and 2D fingers, provided the correct scaling variable is identified as a function of the structural parameters. Interestingly, this formula corresponds to the well-known general situation of diffusion in a medium with fast reversible adsorption. Finally, we show that the same formula describes diffusion in the presence of dilute potential energy traps, e.g. through a landscape of square wells. While diffusion is ultimately always the results of microscopic interactions (with particles in the fluid, other solutes and the environment), effective representations are often of great practical use. The results reported in this paper help clarify the microscopic origins and the applicability of global, integrated descriptions of diffusion in complex media.