# Diffusivity

The PorousFlowDispersiveFlux Kernel requires a diffusion coefficient for each component in each phase, as well as a tortuosity, .

These are provided as material properties in PorousFlow. Two formulations are available:

## Constant diffusivity

PorousFlowDiffusivityConst

A simple model where the diffusion constants and tortuosity are constant.

## Millington-Quirk diffusivity

PorousFlowDiffusivityMillingtonQuirk

A saturation dependent model proposed by Millington and Quirk (1961), where the diffusion coefficients are constant but the tortuosity is (1)

## References

1. R. J. Millington and J. P. Quirk. Permeability of porous solids. Trans. Faraday Soc., 57:1200–1207, 1961.[BibTeX]