I am working on the interdigitated flow channel, where mating surfaces (internal) are supposed to diffuse the liquids (water and ethanol) from one side to the other. I experimented with different boundary conditions: internal as internal, which led to merging the surfaces and making a single entity. Additionally, I made internal as wall, which prevented the liquids to cross and do diffusion is observed . I am intended to observe diffusion at mating surfaces. After consulting the experts, I have been suggested to deploy User defined function at the mating boundary. I am writing this mail seeking your suggestion on how to proceed with UDFs. I would be highly obliged if you could send me a few pertinent templates.