Controllable fabrication of millimeter-scale double droplets in co-flowing devices

Abstract

The fabrication of millimeter-scale water-in-oil-in-water (W1/O/W2) double droplets in five self-manufacturing co-flowing devices is investigated. Double droplets with a diameter range of 1000–4000 µm are successfully fabricated. Three typical flow patterns are observed in a wide range of flow rates, namely squeezing, dripping-like, and jetting-like. A flow pattern diagram is obtained to form double droplets without a single oil droplet and rupture. Furthermore, during the transition from the squeezing to jetting-like patterns, the frequency, filament length and satellite droplet size increase, while the double droplet size and formation period decrease. In addition, due to the increase in middle O and outer W2 phase tube sizes, the double droplet size and its distribution increase, but the frequency decreases. The effect of the flow rate of different phases and the sizes of the tubes on the formation process of millimeter-scale double droplets is also systematically investigated. These results provide more in-depth insight into the formation mechanism of millimeter-scale double droplets and enhance the fabrication controllability of double droplets.