Microfluidic droplet flow regimes of ternary mixtures with solvent at ultralow interfacial tension

We experimentally investigate droplet pattern formation in coaxial microchannels using ternary mixtures of two immiscible fluids and a miscible solvent. The role of solvent concentration is examined through analysis of periodic flow patterns and scaling relationships are developed to calculate a wide range of interfacial tensions at short time-scales. In particular, low-viscosity aqueous and ethylic mixtures of isopropanol are injected into a viscous silicone oil using square microcapillaries. The flow maps obtained display dispersed flows, such as dripping and jetting regimes, at low solvent concentration and separated flows, including core-annular flows, at large concentration. A method based on analysis of droplet size and spacing in the stable dripping regime is implemented to measure extremely small values of interfacial tension in ternary fluid systems.