Experimental Study on Millimeter-scale W1/O/W2 Compound Droplets Formation in a Co-flowing Device with Two-step Structure

Abstract W1/O/W2 compound droplets generation in a co-flowing microfluidic device with two-step structure was observed through experiments, and the formation process is quantitatively studied. Further the effects of the flow rates of all phases as well as the viscosity of W2 phase on the geometric size are extensively investigated. The results indicate that the outer diameter of compound droplets mainly depends on the flow rate and viscosity of external phase. The core water droplet diameter can be controlled by adjusting the flow rates of both inner and middle phases, and it is more sensitive to inner phase. Generally, the inner diameter of compound droplets is not only relative to the core droplet diameter, but also influenced by the encapsulated core water droplet number. Finally, a prediction correlation for dimensionless outer diameter with several dimensionless numbers is obtained. The results could provide an experimental basis for controllably preparing millimeter-scale monodisperse compound droplets.