Dynamics of droplet formation in rotary flow focusing

Rotary flow focusing (RFF) is distinguished from conventional microfluidic platforms through its capacity to accommodate wide viscosity ranges in both continuous and dispersed phases during droplet formation. The dynamic mechanisms during droplet formation and the parametric dependencies within RFF systems are examined systematically. Four distinct flow modes, including squeezing, dripping, jetting and tip-streaming, are achieved by varying the rotational velocity and the dispersed-phase flow rate, and the corresponding transition boundaries are identified. In the squeezing and dripping modes, scaling laws are derived to predict droplet size based on interfacial dynamics during the breakup of the dispersed phase. In the jetting mode, functional relationships describing how jet diameter, droplet size and jet length depend on flow parameters are established through external flow field analysis. The tip-streaming mode facilitates the production of droplets at very small scale, with the effects of flow control parameters on droplet size quantitatively evaluated. Additionally, the effects of geometric parameters and fluid physical properties on RFF performance are investigated, enabling the successful production of high-viscosity fluid droplets ranging from micrometre to millimetre scales. © The Author(s), 2025. Published by Cambridge University Press.