Combined Effects of Formulation and Stirring on Emulsion Drop Size in the Vicinity of Three-Phase Behavior of Surfactant−Oil Water Systems

As surfactant−oil−water systems approach Winsor III phase behavior, at the so-called optimum formulation, the interfacial tension decreases, thus allowing the generation of smaller drops upon stirring. On the other hand, the coalescence rate increases, thus favoring the formation of larger drops. The two opposite effects do not alter the drop size in the same way and result in a minimum drop size. Such a minimum drop size is found on both sides (for O/W and W/O emulsions) of optimum formulation, whatever the variable used to produce the scan. The location of these minima, which correspond to the most efficient use of the stirring energy to make small droplets, is found to be slightly shifted by a change in stirring energy.