Flow regime analysis and prediction for compound droplet generation in a microfluidic device coupling flow-focusing and step emulsification

In this work, a tandem microfluidic device coupling flow-focusing and step emulsification was fabricated to generate compound droplets. Through systematic visualization experiments, four distinct flow regimes were identified, namely two-step compound droplet generation (TSCG), one-step compound droplet generation (OSCG), intermittent generation of multiple single droplets and one compound droplet (IGS&C), and two-step multi-core compound droplet generation (TSMCG). Furthermore, the hydrodynamic mechanisms underlying these flow regimes were elucidated. Both TSCG and OSCG exhibit robust compound droplet generation performance since the generation of the outer droplets is insensitive to flow rate variations, yielding highly monodisperse compound droplets with coefficient of variation ( C.V. ) below 5%. The OSCG dominated by interfacial tension is more favorable for producing the compound droplets with a larger core/shell ratio under higher frequency. Moreover, it produced compound droplets with higher monodispersity than other flow regimes. The influences of the Weber numbers ( We ) of the inner and the middle phases, as well as the distance between the flow-focusing junction and step emulsification nozzle on the compound droplet generation performance were examined and clarified. Regime maps were also provided to quantitatively identify the above four flow regimes depending on these influencing factors. Furthermore, a flow regime prediction method was proposed to predict the flow regime transitions, achieving a prediction accuracy of up to 82.01%.