# Nonsteady-state diffusion in two-dimensional periodic channels

## TL;DR

This paper develops a non-steady-state approach to derive an effective one-dimensional diffusion coefficient in two-dimensional periodic channels, providing a series expansion that aligns with previous models and is highly accurate for long wavelength channels.

## Contribution

It introduces a non-steady-state method to derive the diffusion coefficient, offering a series expansion that improves understanding of diffusion in periodic channels beyond stationary flow assumptions.

## Key findings

- The leading term recovers Zwanzig's expression for D(x).
- The expansion converges rapidly for long wavelength channels.
- The approach aligns with previous models and clarifies validity limits.

## Abstract

The dynamics of a freely diffusing particle in a two-dimensional channel with cross sectional area $A(x)$, can be effectively described by a one-dimensional diffusion equation under the action of a potential of mean force $U(x)=-k_BT\ln [A(x)]$ (where $k_BT$ is the thermal energy) in a system with a spatially-dependent diffusion coefficient $D(x)$. Several attempts to derive expressions relating $D(x)$ to $A(x)$ and its derivatives have been made, which were based on considering stationary flows in periodic channels. Here, we take an alternative approach and consider non-steady state single particle diffusion in an open periodic channel. The approach allows us to express $D(x)$ as a series of terms of increasing powers of $\epsilon$ - a parameter associated with the aspect ratio of the channel. When the expansion is truncated at the leading term, we recover the expression suggested by Zwanzig [J. Phys. Chem. {\bf 96}, 3926 (1992)] for $D(x)$. Furthermore, comparison of the first few terms in our expansion for $D(x)$ with the one proposed by Kalinay and Percus [Phys. Rev. E {\bf 74}, 041203 (2006)] shows that they are consistent with each other. In the limit of long wavelength channels ($\epsilon\ll 1$), the expansion converges rapidly and the leading approximation provides a very accurate description of the two-dimensional dynamics. For short wavelength channels, the expansion does not converge and the validity of the effective one-dimensional description is questionable.