A diffusive mass flux link between two pathways has two "sides" (one for each of the linked pathways). Mass diffuses from one pathway, through the "diffusive layer" on one side of the link, through the "diffusive layer" on the other side of the link, and into the other pathway.
Note that the two fluids on either side of the link do not have to be identical. For example, you could connect two cells via a diffusive link with Water on one side of the link and Air on the other. In fact, transfer across the air-water interface (e.g., of a lake) is often mathematically represented in this way.
In such a situation, when a species diffuses through one side of the link, it must partition from one fluid into the other before it can diffuse through the other side of the link. GoldSim handles this partitioning automatically, based on the specified partition coefficient(s) for the species to the two fluids.
Warning: Particulate Solids
will not diffuse in a link in which the two fluids on either side of the link do
not have the same name (since typically it would be inappropriate to assume that
a particulate solid
would "partition" between fluids.) If you wish to transport Solids across
a fluid interface, you can do so by defining an advective mass flux link
between the pathways involving the Solid.
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