On EB科学技術大辞典
there's "falling film" for 流下薄膜.

On Google there are some instances of "falling thin film", so I think we can think that would be a direct translation of 流下薄膜 and that in the case of your question we can leave out the 薄 part.

Now there is only one instance on Google with "naturally falling film" (on the URL given below). So I would take it with a grain of salt, as all my answer altogether :)
Hope it helps anyway