Dynamic behaviors of double emulsion formation in a flow-focusing device

Double emulsion drop formation in flow-focusing microfluidic device is investigated numerically by computational fluid dynamics simulation using VOF method. The simulation produces two typical drop formation modes, including dripping and jetting, and provides the detailed hydrodynamic information underlying these two modes. In addition, the effects of viscosity ratio, flow rate ratio and interfacial tension ratio on the double emulsion formation by the flow-focusing system are clarified quantitatively. It is indicated that the cooperation of the interfacial tension and viscous drag of the outer fluid induce the detaching of the double emulsion droplet under both dripping and jetting modes, while the large viscous drag from the outer fluid under jetting mode generates a long stretched neck which leads to the different formation behaviors. The drop formation mode transits from dripping to jetting abruptly when the middle fluid is more viscous than other fluids. And the size of the generated drops increases with the rise in viscosity ratio of the middle fluid to the inner one. The flow rate of the outer fluid determines the drop formation mode as well as the generated drop size, while the flow rate of the middle fluid contributes only to the drop size but have no obvious effect on drop formation regime in a wide range of flow rate ratio. In addition, the interfacial tension ratio shows little influence on drop sizes but large effect on the interface shapes. (C) 2014 Elsevier Ltd. All rights reserved.