I have no idea about Mechanical - whilst I can use the software due to it being the same interface as Ansys Meshing I don't know any more about the buttons. As a Chemical Engineer we were taught that Mechanical Engineers did have a use... ;)
Typically a membrane is used to separate two regions, osmosis type systems being a common application. So, the membrane would be the surface boundary between two fluid regions. That may make the set up easier.
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