Dispersion in two dimensional channels - the Fick-Jacobs approximation revisited.

We examine the dispersion of Brownian particles in a symmetric two dimensional channel, this classical problem has been widely studied in the literature using the so called Fick-Jacobs' approximation and its various improvements. Most studies rely on the reduction to an effective one dimensional diffusion equation, here we drive an explicit formula for the diffusion constant which avoids this reduction. Using this formula the effective diffusion constant can be evaluated numerically without resorting to Brownian simulations. In addition a perturbation theory can be developed in $\varepsilon = h_0/L$ where $h_0$ is the characteristic channel height and $L$ the period. This perturbation theory confirms the results of Kalinay and Percus (Phys. Rev. E 74, 041203 (2006)), based on the reduction, to one dimensional diffusion are exact at least to ${\cal O}(\varepsilon^6)$. Furthermore, we show how the Kalinay and Percus pseudo-linear approximation can be straightforwardly recovered. The approach proposed here can also be exploited to yield exact results an appropriate limit $\varepsilon \to \infty$, we show that here the diffusion constant remains finite and show how the result can be obtained with a simple physical argument. Moreover we show that the correction to the effective diffusion constant is of order $1/\varepsilon$ and remarkably has a some universal characteristics. Numerically we compare the analytical results obtained with exact numerical calculations for a number of interesting channel geometries.