Diffusion of species concentration in Castro

[RemiLehe]

My understanding is that Castro currently implements diffusion of temperature (i.e. a fluid with areas of hot and cold temperatures will evolve towards a uniform temperature eventually), but not yet diffusion of species concentration (i.e. a fluid with areas of high concentration in species 1 and 2 respectively will evolve towards a uniform concentration eventually).

I was wondering: are there plans to implement diffusion of species concentration in Castro?
If not, would you have pointers as to how to implement this in Castro, if I had to do it myself?

[zingale]

in general species diffusion is not important in astrophysics, which is our traditional problem space. So we have not implemented it. If it could be handled explicitly, then it should be doable by mirroring how thermal diffusion is done. But it is not in our plans at the moment.