Whats the blurry layer between two immiscible liquids called?

Question

maybe i'm crazy but i thought there was a term for that blurred effect between layers. similar to the mixing of different air densities, (heck it may be the same thing) when heat rises from asphalt in a distance. the word i'm not looking for is mirage. for some reason i thought it started with an s. its driving me crazy!!!

Answer 1

The proper term is phase boundary.