Microfluidic phase inversion for nanoemulsification via surfactant-affinity modulation

Phase inversion emulsification (PIE) is an energy-efficient route to nanoemulsions, but its dynamics are difficult to resolve in conventional stirred systems because formulation, interfacial transport, and hydrodynamic effects are strongly coupled. Here, we present a continuous-flow microfluidic platform for PIE using an aqueous phase containing Tween 80 and an oil phase containing Span 20. The relative flow rates of the two phases were varied to adjust the aqueous volume fraction and the nominal blended hydrophilic-lipophilic balance of the formulation. In a 1 m-long hydrophobic microchannel, W/O droplets generated upstream evolved along the channel. At an aqueous volume fraction of 0.9, water-continuous structures coexisted with W/O/W double-emulsion-like intermediates near the outlet, coinciding with a marked decrease in the aqueous-phase inlet pressure. Outlet conductivity measurements and dynamic light scattering indicated that the collected samples contained O/W nanoemulsions with mean hydrodynamic diameters of 100-250 nm. Dynamic pendant-drop measurements further showed that the interfacial tension of the two-surfactant formulation decreased to approximately 0.2 mN m^-1 within 7 s before drop detachment. These observations are consistent with surfactant-mediated interfacial reorganization during transport, although the local surfactant-transfer pathway and the independent effects of flow rate and residence time were not resolved. The platform provides an optically accessible continuous-flow system for studying phase inversion and low-energy nanoemulsion formation.