I would like to show diffusion between fluids using level-set method. I've tried some of tricks to couple diffusion equation and level-set method but I couldn't manage it.
Is there a particular reason you wish to use the level-set method? I know that it is quite computationally intensive.
There is a good tutorial to show diffusion between two laminar fluid flows in a micromixer that uses the single-phase flow (spf) and transport of diluted species (tds) modules: