Abstract
In this study, we present experiments on the generation and control of droplets with a wide range of size in a microfluidic device composed of flow-focusing (cross-junction) geometry with a converging-diverging nozzle-shaped section. The microfluidic structure was made of poly(dimethylsiloxane) (PDMS) for the generation of water-in-oil droplets. It is found that adding the converging-diverging section to the cross-junction significantly reduces the droplet size without greatly increasing the overall flow resistance and still achieves good uniformity. The throat of the converging-diverging section determines the breakup location of the droplet emulsion. The effects of the throat width on the scaling for the size of droplets are most pronounced in the higher range of the continuous-phase to dispersed-phase flow rate ratio, where the shear-driven mechanism dominates the breakup. In this regime, the shearing effects are largely enhanced by the strong acceleration and deceleration created in the converging-diverging section. These effects enable us to generate droplets with a wide range of diameter, including those as small as 10 µm or less, in the microfluidic devices having channel dimensions on the order of 100 µm.