Emulsions consist of two liquid phases: the disperse and continuous phases. The phase in the form of finely divided droplets is the disperse or internal phase; the phase forming the matrix in which these droplets are suspended is the continuous or external phase. Based on the liquid phases, a typical water-oil emulsion can exist either as an oil-in-water (oil is the disperse phase) or water-in-oil emulsion. These types are abbreviated as o/w and w/o, respectively.
When an assembly of spheres of equal radii are in a position of densest packing, they occupy 74% of the total volume; the remaining 26% is empty space. Theoretically, this relationship means that for a given system, o/w and w/o emulsions are both possible between the phase volume concentrations 0.26 and 0.74. Below 0.26 and above 0.74, only one form can exist. In the intermediate range of volume concentrations, no one form of emulsion is favored. These systems are described as multiple or dual emulsions with the disperse phase containing globules of the other phase.
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