Dimensionless numbers defining the onset of dispersed phase flow transitions in microfluidic flow-focusing devices

Microfluidic flow-focusing devices (FFD) play a crucial role in enabling precise control and production of emulsions, facilitating a wide spectrum of applications. Numerous studies have been conducted to investigate the flow behaviors in a flow-focusing device, such as squeezing, dripping, jetting, and plug flow. Despite this identification, the parameters governing the transitions between these behaviors are not fully known. In this study, we observed dispersed phase transitions experimentally using microfluidic flow-focusing devices (FFDs) by scanning 1800 flow rate conditions spanning two FFDs, two wettability conditions, 20 fluid combinations, four emulsifier systems, and dispersed-to-continuous phase viscosity ratios of 0.03–30. Four major patterns of droplet breakup were observed: squeezing, dripping, jetting, and plugging. We found that these dispersed phase transitions across the broad range can be described using two dimensionless numbers, Weber number based on the continuous phase, Wout, and Reynolds number based on the inner phase, Rin. The squeezing to jetting and dripping to jetting transition occurred at Wout ∼ 1. Similarly squeezing to plugging and dripping to plugging occurred at αRin ∼ 1, where α is a viscosity-dependent pre-factor. We compared our results to literature studies encompassing six FFD geometries, six surfactant systems, and a similar range of viscosity ratios, and found that the identified dimensionless numbers broadly capture the onset of dispersed phased transitions. Collectively, this study unifies a large body of datasets from FFD studies, simplifying the complex operating parameter space of emulsion generation into two dimensionless numbers that define the onset of disperse phase transitions.