In fluid-fluid multiphase flow problems, the ensemble averaged interfacial area concentration balance equation can have a source term coming from the interfacial interactions between the phases. This type of balance can develop its source term from an interfacial energy balance. The primary competing approach is to partition the dispersed phase into a collection of size classes and develop source terms from breakage and coalescence terms that depend primarily on the energy dissipation rate per unit mass and the size class. The advantage of this approach is that the result is both the distribution of bubble sizes and the ensemble averaged bubble size. The disadvantage is the large number of popuation balance equations and source terms needed to make the computation. The current approach gives only an ensemble averaged interfacial area concentration, but it only requires a single additional scalar balance equation in addition to the usual formaulation of the two fluid model. Frequently, the ensemble averaged interfacial area concentration distribution is sufficient for multiphase reaction and mixing.