Study of compound drop formation in axisymmetric microfluidic devices with different geometries

Via comparing drop formation regimes and the properties of the produced compound drops, the compound drop formation in axisymmetric co-flowing and flow-focusing microfluidic devices is numerically studied to clarify the performances of these two devices on fabricating the compound drops. The hydrodynamics underlying the drop formation as well as the effects of flow conditions and local geometries of the devices on the fabricating performances are also elucidated. The results indicate that, differing from co-flowing device, flow-focusing device induces strong hydrodynamic focusing effect and high viscous action through its orifice, resulting in distinct compound drops formation behaviors from those in the co-flowing device. In the flow-focusing device, the breakup of the necks is accelerated under dripping regime and the formation of the jet is facilitated under jetting regime. Compared with the co-flowing device, transition from dripping to jetting happens under smaller Capillary number in the flow-focusing device and the consequent drop size is smaller with thinner shells and higher generation rate. The orifice radius plays a significant role in compound drop formation while the orifice length has little effect. Additionally, a regime diagram is organized to provide the quantitative description for the flow regimes of compound drop formation in the axisymmetric microfluidic devices, depending on the dimensionless orifice radius and Capillary number of the matrix fluid.