Abstract: A mixer especially useful for making mechanical foams is disclosed. The mixer includes a stator and a rotor mounted for rotation with respect to the stator, the stator and rotor defining a fluid flow path between them. The fluid flow path includes two shearing zones spaced axially along the rotor. A surface of the rotor extending from the first shearing zone to the second shearing zone defines one boundary of the fluid flow path. The cross-sectional area of the flow path through the mixer is kept substantially constant, varying by no more than about 25 percent, so that zones of stalled flow and low pressure are avoided, thereby maintaining the velocity of the mixture in the flow path relatively high at all times and maintaining a substantial back pressure on the mixture. Portions of the stator and rotor are easily assemblable or disassemblable so that the mixer can be readily converted from a multi-head form to a single-head form.