Physical phases of matter consist of vapor, liquid, and solid of various crystalline forms. In a flow system with a mixture of phases, multiple interactions occur at the interfaces of various configurations or geometries. Within a given physical phase in the mixture, each element of similar dynamic response constitutes a dynamic phase in the flowfield and the potential field of the system. Multiphase flow arises from a multiplicity of elements of different dynamic responses. An example is a gaseous suspension of solid particles of identical material; particles of different sizes constitute different phases because of their different inertial responses to a changing flowfield. This multiphase concept is also applicable to the case of a suspension of monodispersed (Identical sized) particles with different electrostatic charges; in a given electric field, particles of different charges constitute different phases because of different forces acting on them [Soo, 1965]. Similar examples can be drawn from the cases of bubbles, droplets, and particles suspended in a fluid, be it a gas or liquid, as long as the phases are immiscible. In general, these phases can be separated by a variety of configurations of interfaces.