Triangular nozzle structure control of droplet generation hydrodynamics in microfluidic step emulsification devices

The nozzle structure is identified as a critical factor in controlling monodisperse droplet generation in microfluidic step emulsification devices. Two groups of meticulously designed microfluidic step emulsification devices with distinct triangular nozzles are introduced in this study to systematically investigate the hydrodynamic control of droplet generation by the triangular nozzle structure. Dripping and jetting regimes were characterized through regime diagrams, with the dripping regime identified as optimal for producing highly monodisperse droplets that are insensitive to flow rate variations. An opening angle of 10° was determined as the optimal triangular nozzle design, significantly enhancing the stability of the dripping regime. In this regime, the duration of confined-growing stage exhibits a positive correlation with nozzle volume and ultimately determines the resulting droplet size. Furthermore, the morphological evolution of the dispersed phase within the triangular nozzle was demonstrated to govern the interfacial dynamics throughout the entire droplet generation cycle, closely associated with the nozzle structure. Based on these insights, a predictive model incorporating the triangular nozzle structure was proposed to estimate droplet size in step emulsification devices under the dripping regime, achieving reliable predictions with a maximum relative error of 30%. These findings provide a theoretical and practical framework for customizing droplet size in microfluidic applications, contributing to enhanced precision and efficiency in monodisperse droplet generation.