center of mass

Hi all

In my simulation system there are two immisible liquid phases, one of them is in the right side of the simulation box and the other is in the left side in Z direction. In X and Y direction center of mass velocity is 10^-5 in read units and in Z direction the velocity is about 10^-4. Visually I see displacement of phase 1 by phase 2 periodically. I used periodic boundary conditions but I think after equilibration I should not see the displacement. Would you please help me about this?

Thanks alot for your time an d attention.

Best Regards