Internal fluid-fluid boundary issue

Hello all,
Inexperienced comsol user here. So I have a 3-d model of a cylinder. In the lower half of the cylinder there is water while in the upper we have air. Air is rotating around the cylinder due to an inlet and an outlet. My problem is that I want to have an interior boundary on the air-water surface that is as follows: ux_air=ux_water and uy_air=uy_water and w_z=0 so that there is no mixing between air and water.
Do you have any suggestions on how this could be applied?

Thanks in advance,
Lefteris