Mapping flow-focusing microfluidic droplet formation to determine high-throughput droplet generation configurations

Abstract

Droplet microfluidics enables the generation of monodisperse droplets of desired size using immiscible multiphase flows. These droplets serve as individual reactors in bio (chemical) analysis. In addition to monodispersity, high throughput is also necessary for many applications, especially in synthesizing nano- and microscale materials. Therefore, high-frequency droplet formation in the stable regimes (dripping and squeezing) is of great importance. In this work, the flow-focusing geometry is numerically investigated to determine the geometrical dimensions and flow rate ratio by which high-frequency, high mass flow rate, and monodispersed droplet formation are achievable. 3D numerical simulation has been performed with the commercial software ANSYS Fluent using the VOF method. The lowest and highest error values were 1.5% and 7% in the validation results, respectively, confirming the solver's reliability. The numerical model determined droplets' diameter and mass flow rate, and their generation frequency for a simple geometry that had been experimentally studied. The geometry, then, was changed both in dimension and junction, seeking the best performing configuration. Our results demonstrated that the highest mass flow rate and generation frequency are obtained with the flow rate ratio of 5 between the continuous and dispersed phases. Furthermore, by introducing a modified geometry using a rib, it was revealed that reducing the channel width in the simple geometry leads to much more improvement in the generation frequency and droplet diameter than using the modified geometry. Subsequently, the aspect ratio study showed that the aspect ratio 1 creates the highest production frequency in the simple geometry. Finally, taking into account all the previous findings, a relation is introduced which determines the required channel dimensions when a specific droplet diameter is of interest.