I have attached a hip where demonstrated the problem, one source where each hemisphere has different density and viscosity 1000/0 and 100/2. When viscosity for both is 0 everything works great, less dens fluid floats but when I am adding even small viscosity you can see what happens on the video or the attached hip file. The viscous object starts running with out viscosity, then almost stops and then running and again later stops while the water fluid keep going (when is not trapped under the viscous fluid).

Why?

flipbook:
https://docs.google.com/file/d/0Bw4OEXwfSYFsZ1QyRkVCam1PaE0/edit?usp=sharing [docs.google.com]