transient mass diffusion
 

I'm having trouble modeling the following: A vertical pipe is separted with a valve in the middle. Above the valve is fluid-1, below is fluid-2. I want to model the diffusion/mixing; both conc gradient driven, and gravity driven. How would I do this? When I use the species model I can't figure out to specify the zone composition just at the beginning. Right now I specify one zone as a fixed value of pure fluid and see how it diffuses into the other zone. Is there a way to just specify at time=0, and model what happens? Thanks