Investigation of In-Air Droplet Generation in Confined PDMS Microchannels Operating in the Jetting Regime

Liquid-in-air generation of monodisperse, microscale droplets is an alternative to conventional liquid-in-liquid methods. Previous work has validated the use of a highly inertial gaseous continuous phase in the production of monodisperse droplets in the dripping regime using planar, flow-focusing, PDMS microchannels. The jetting flow regime, characteristic of small droplet size and high generation rates, is studied here in novel microfluidic geometries. The region associated with the jetting regime is characterized using the liquid Weber number (We_l) and the gas Reynolds number (Re_g). We explore the effects of microchannel confinement on the development and subsequent breakup of the liquid jet as well as the physical interactions between the jet and continuous gaseous flow. Droplet breakup in the jetting regime is also studied numerically and the influence of different geometrical parameters is investigated. Numerical simulations of the jetting regime include axisymmetric cases where the jet diameter and length are studied. This work represents a vital investigation into the physics of droplet breakup in the jetting regime subject to a confined gaseous co-flow. By understanding the effects that different flow and geometry conditions have on the generation of droplets, the use of this system can be optimized for specific high-demand applications in the aerospace, material, and biological industries.