Modeling microbubble production rates from expanding nozzle flow-focusing microfluidic devices

Flow-focusing microfluidic devices (FFMDs) have been reported to produce monodisperse microbubbles with tunable diameters determined by device geometry and input flow/pressure conditions. However, no model for microbubble production rate has yet been reported. In this paper, we develop a model describing the production rate of monodisperse spherical microbubbles from planar two-dimensional PDMS-based expanding-nozzle FFMDs as a function of gas pressure and liquid flow rate. High speed optical microscopy was used to determine the diameter and production rate. Microbubble generation was observed to occur in five discrete regimes. Within the spherical monodisperse regime, the generation cycle was characterized by two phases: bubbling and waiting. The generation cycle was found to be inversely proportional to liquid flow rate for a fixed gas pressure. Excellent agreement was found between predicted and measured microbubble production rate (R² = 0.91).