Study on Developing Region and Regime Transitions of Confined High-Speed Coaxial Gas-Liquid Jets

Confined coaxial gas-liquid jets have been extensively investigated over the past two decades, primarily driven by advancements in droplet microfluidics. Near the nozzle, these jets typically exhibit significant contraction or expansion before reaching a stable downstream radius, a region referred to as the developing or entrance region. Historically, this region has received limited attention, as it was presumed to have negligible effects on the overall interfacial instability and subsequent droplet formation. In this study, experimental measurements were conducted on gas-liquid jets generated within a 3D flow-focusing microfluidic chip. The results demonstrated that the developing region extends considerably further downstream than previously recognized, dominating nearly the entire jet length before droplet breakup under most tested conditions. Further dimensional analysis revealed that the developing region comprises distinct sub-regions separately governed by inertial and viscous effects. The locations and transitions between these sub-regions were identified as critical factors influencing multiple flow regimes, including Dripping, Jetting, and Transition regimes. Through high-speed imaging combined with image processing techniques, the complex physical processes underpinning the transition regimes were elucidated, along with its intrinsic interactions with Dripping, Jetting, as well as with the developing region.