Parallelizing monodisperse microbubble generators within microfl uidic devices off ers a promising strategy for signifi -

cantly increasing production throughput. However, gases diff er fundamentally from liquids in key properties—such as 

low viscosity and density, high interfacial tension with liquids, and high compressibility—complicating the uniform generation

of bubbles across multiple generators. These gas-phase characteristics introduce coupling eff ects between parallel 

units and result in hydrodynamic feedback from the collection channels, making scalable operation more challenging. In 

microfl uidic systems, gas and liquid enter the bubble-generation channels as single-phase fl ows. After bubble formation, 

two-phase fl ow resistance dominates in the collection channels. Therefore, this study aims to investigate the uniformity of 

bubbles produced by eight parallel fl ow-focusing generators under two-phase fl ow conditions. The infl uence of gas pressure,

capillary number, and viscosity ratio is examined experimentally. We fi nd that achieving uniform fl uid distribution 

for monodisperse bubble generation requires consideration of additional two-phase hydrodynamic resistance, determined 

by both the capillary number and viscosity ratio. This study addresses a critical gap in understanding two-phase fl ow 

resistance and provides design guidelines for the scalable production of uniform bubbles.