The interface position can be adjusted by controlling the pressure balance. When the interface is stable, the pressures on the rotor wall, which are imposed by the fluids above and below the heavy-phase channel, should be equal. The pressures of the fluids above and below the heavy-phase channel are represented by P1 and P2, respectively. The thicknesses of fluids flowing through the heavy-and light-phase weirs are negligible. P1 and