Scaled-up droplet generation in parallelised 3D flow focusing junctions

Monodispersed organic phase droplets with an average diameter from 20 to 200 µm were produced at the rate of up to 20,000 droplets per second in a glass microfluidic chip composed of 7 parallel 3D flow focusing junctions with 100 μm-deep channels. The continuous phase was 2 wt% polyvinyl alcohol solution, while the dispersed phase was dichloromethane, n-dodecane, and polydimethylsiloxane 10 cSt fluid corresponding to the dispersed-to-continuous-phase viscosity ratio of 0.2, 0.8 and 6.1, respectively. Four different droplet generation regimes were observed, dripping, squeezing, jetting, and threading. The regions where each of these regimes was stable were mapped using Weber number of the dispersed phase and capillary number of the continuous phase. The transitions between the droplet formation regimes were governed by the Weber number of the dispersed phase, indicating that inertial forces in the dispersed phase were more relevant than viscous forces in controlling the transition. Stable droplet generation in dripping regime in each junction was maintained for at least 6 hours. The coefficient of variation of droplet sizes from individual junctions and all junctions combined was <3% under optimal conditions. The droplet size variations between different junctions were greater than those within each junction. Liquid plugs were produced in the squeezing regime at the dispersed-to-continuous phase flow rate ratio greater than one. This study is the first investigation of droplet generation in multiple 3D flow-focusing junctions with potential applications for the production of drug microcarriers using emulsification solvent evaporation, especially for the encapsulation and controlled delivery of lipophilic drugs.