We studied aqueous phase addition rate and stirring speed effects on emulsion characteristics, particularly on water mass fraction value (Fw) at the inversion point. The studied system is composed of silicone oils (viscosities between 1 and 12 Pa.s), aqueous phase (1 % NaCl) and a mixture of nonylphenol polyethoxylate as surfactant. The emulsification process is followed-up, in situ, by simultaneous viscosity and conductivity measurements. The aqueous phase is added to the oily one, under continuous agitation until the inversion point, detected by an abrupt change in viscosity and conductivity measurements, is reached.
Water mass fraction (at the inversion point) depends on multiple emulsion formation, and is also affected by the oil phase viscosity. There is a total or partial inversion depending on the viscosity phase ratio. According to microscopic observations of local morphologies obtained during the inversion process, for systems with low viscosity phase ratio, the inversion is triggered through the following morphology evolution: W/O → w/o/W/O → w/O/W → O/W. For more viscous oils, no morphology formation of type w/o/W/O has been observed before inversion point. Final emulsions are highly concentrated emulsions either because Fw is about 5 to 9% in high oil viscosity systems, or because of the persistence of multiple emulsions (w/O/W) in low oil viscosity systems.