Coffee Foam Effect

[LayC42]

To create the opacity map for the foam you can solve this with the material graph. A color gradient with radial mapping in the opacity channel would be my choice.
Maybe a shared scene will be improved by some users?

[Finema]

Here is my model, made in Autodesk Alias AutoStudio
The liquid is between the external surface of glass and the inner one
The last image is the render retouched in photoshop to hide the reflection on the edge of the cup

Andrea
Your glass model is not OK.
In attachment, i join a cup (.obj). See how it is cut out.

[Finema]

To create the opacity map for the foam you can solve this with the material graph. A color gradient with radial mapping in the opacity channel would be my choice.
Maybe a shared scene will be improved by some users?

To blur the edges of the foam, I've used Occlusion in Opacity map.