Effect of device geometry on droplet size in co-axial flow-focusing microfluidic droplet generation devices

The Co-axial Flow Focusing (CFF) technique is an established microfluidic method for the generation of microdroplets. It consists of a nozzle co-axially aligned in a (circular cross-section) channel with a downstream orifice. The continuous-phase (c-phase) fluid in the orifice is used to shear off droplets of the dispersed-phase (d-phase) fluid ejected from the nozzle. Circular cross-section CFF devices are preferred due to their simpler fluid hydrodynamics, but not studied extensively as compared to rectangular channels. Understanding the effect of various parameters such as the channel design and fluid properties in CFF requires the development of reliable computational models. In this paper, the effects of multiple local geometries on droplet formation in a microfluidic CFF device were investigated numerically with experimental validations. The geometries included the d-phase nozzle inner diameter, nozzle tip distance from the c-phase channel orifice, and the inner diameter and length of the flow-focusing orifice. Our model is able to successfully predict the effects of these geometrical parameters on the droplet size for a wide range of c-phase flow rates spanning both the dripping and the jetting flow regimes, with an error range of 3-6%. Based on the simulation results, the droplet size is strongly dependent on the c-phase flow rate and the sizes of both the dispensing nozzle and the flow-focusing orifice. The effects of nozzle-to-orifice distance and the length of the orifice on the droplet size were not as pronounced in the range of the simulated c-phase flow rates. Our model can be used for the design and fabrication of CFF devices which yield predetermined droplet sizes with high precision.