The dripping-to-jetting transition in a co-axial flow of aqueous two-phase systems with low interfacial tension

The dripping-to-jetting transition of co-axial flow with high interfacial tension has been extensively studied; however, little is known about this with low interfacial tension. We use an aqueous two-phase system as a model to study the transition at low interfacial tension (<1 mN m^-1). We identify a low critical jet velocity at which the transition occurs (V < 1 m s^-1) when compared with the velocity in systems with high interfacial tension (V > 1 m s^-1). The significant reduction in the critical jet velocity alters the relative importance of the inertial force (∝V²) and the viscous force (∝V). Counter-intuitively, even for systems with relatively low viscosities, the transition is no longer inertially-dominated. Therefore, we propose that all downstream forces, including inertial and viscous forces, contribute significantly in opposing the interfacial force. This comprehensive force balance accurately characterizes the dripping-to-jetting transition for a wide range of viscosities and low interfacial tension. The validity of this force balance is confirmed by the agreement in the interfacial tension values estimated by inputting the operating parameters of the flow into the force balance and that obtained by a commercial spinning drop tensiometer.