Interfacial instability and transition of jetting and dripping modes in a co-flow focusing process

Abstract

The breakup dynamics of coaxial liquid interfaces into compound droplets in a co-flow focusing process is studied systematically. In experiments, the jetting and dripping modes downstream the focusing orifice are identified within the parametric regime where a coaxial liquid cone can be established steadily, and the phase diagram is plotted under different flow rates of inner, outer, and driving liquids. The force balance for the jet interface is analyzed numerically to explore the critical conditions for the jetting-dripping transition. It is found that the instability of the inner interface is much easier to trigger the modes transition, and the transition criterion is decided by the balance of inertia force, shear stress, and interfacial tension at the local inner jet. The linear spatiotemporal instability analysis is further carried out to study the convective and absolute instabilities of the coaxial jets. The effects of main process parameters on jet instability are accessed, and the boundary between the absolute/convective instabilities is further compared with the experimental and numerical results, which achieves good agreement. Finally, the energy budget analysis of the instability of coaxial liquid jets is performed to provide more understanding of physical mechanisms for the mode transition.