Effects of topological changes in microchannel geometries on the hydrodynamic formation and breakup of all-aqueous droplets

Microfluidic gravity-driven droplet generation in aqueous two-phase systems (ATPS) has recently emerged as an effective technique used to passively create all-biocompatible environments. The hydrodynamic formation and breakup of all-aqueous viscous threads in flow-focusing microfluidic devices were experimentally studied by applying different intersection angles (θ). Four typical flow regimes are observed over a range of 0.2–2 kPa, namely, transitional, jetting, threading, and tubing. The flow rate ratio based on the specifications of the entire device was calculated, and the flow regimes were mapped based on the capillary number of the flows. Scaling laws that describe the flow behavior are then put forth based on physical arguments. In the jetting regime, droplet size and jet width are measured and correlated with flow rate ratio. The critical thread length before droplet pinch-off is determined by flow rate ratio and the capillary number of any of the phases. In the transitional regime, droplet length is affected by flow rate ratio and the capillary number of the continuous phase. In addition, through variations in θ, the influence of altering the geometry of the device on the flow regimes was considered. The findings presented in this article provide insight into the intricate process of droplet generation in passive ATPS microfluidics.