Droplet microfluidic method for measurement of ultralow interfacial tension in ternary fluid systems

We experimentally investigate droplet pattern formation in coaxial microchannels using ternary mixtures of two immiscible fluids and a miscible solvent. The influence of solvent concentration is examined through periodic pattern analysis of droplet flow and functional relationships are developed to determine the initial interfacial tension of dispersions made of aqueous mixtures of solvent and oil at short timescales, i.e., when solvent diffusion into the continuous phase has a negligible effect on flow morphologies. We examine a wide range of flow rates and delineate vast flow maps of droplet regimes, including dripping and jetting flows, to clarify the hydrodynamic behavior of conjugate fluid mixtures in square microcapillaries. A method based on analysis of droplet size and spacing is implemented to predict the role of the miscible fluid additive concentration in microfluidic multiphase flows of water-isopropanol and ethanol-isopropanol blends in viscous silicone oil. This approach enables measurement of extremely small values of interfacial tension at large solvent concentrations. This work shows a technique for exploring and characterizing numerous ternary flow systems of interest with a variety of organic solvents and oils.