What do you want to achieve with the UDF? That is the question which I have.

@DrAmine, I'm sorry I wasn't clearer earlier. Please refer to the picture in my original post above. I'm adding a source term to model the surface tension which is exactly the same as the CSF model by Brackbill et al. already implemented in Fluent, but there is one small change which is indicated in bold at the bottom of the image (I want to smoothen the interface by changing the definition of the cell phase fraction, and area-averaging it).nn