There are many important flow situations involving a dispersed liquid or solid material in a continuous gas or liquid. A few everyday examples are fuel injection in a automobile engine,
rain, dust storms and all sorts of atomizers for painting, cleaning and applying medicine. In most cases there are significant interactions between the continuous and dispersed materials
that arise because of the drag experienced by the dispersed particles as they move through the continuous fluid. A secondary interaction effect is the displacement of fluid volume by particle
Of the two interaction effects, i.e., volume exclusion and momentum exchange, the most important is momentum exchange because this can be significant even when the volume of
particles is small. To see why this is so, consider a typical two-phase system with properties similar to that of water and air. The density ratio between water and air is about 1000. This
means that a liquid fraction of only 10E-4 translates into 10% of the mixture mass. Thus, even when the water volume is a negligible fraction of the whole, it still may account for a significant
portion of the mixture momentum.