Investigation on Millimeter-scale W1/O/W2 Compound Droplets Generation in a Co-flowing Device with One-step Structure

Abstract Millimeter-scale W1/O/W2 compound droplets generation in a co-flowing microfluidic device with one-step structure was symmetrically observed through experiments, and the formation process is distinguished according to the variation law of thread tip velocity. Specially, under a wide range of operation conditions, W1/O/W2 compound droplets form accompanying with the generation of oil phase droplet. The formation ratio of compound droplets to oil phase droplet always depends on the outer phase Capillary number, Caw2, and the flow rate ratio, R, of middle to inner phases, and thus the flow pattern diagram is obtained. Further the influences of all phases flow rates, outer phase viscosity as well as the interfacial tension on the formation period and geometric sizes are extensively studied. Finally, a prediction correlation for dimensionless inner and outer diameters with several dimensionless numbers is also obtained. The results could provide an experimental basis for controllably preparing monodisperse compound droplets with millimeter-scale.