Abstract
This study presents a three-dimensional numerical investigation of droplet breakup in asymmetric Y-junction microchannels using a coupled volume of fluid-level set method. The motivation arises from the limited understanding of how geometric asymmetry and viscosity contrast jointly influence droplet splitting, which has been extensively explored only in symmetric T-junctions. The effects of the viscosity ratio \({{\lambda }}\) and the outlet width ratio \({{w}_{2}}{\text{/}}{{w}_{1}}\) on the critical capillary number \({\text{Ca}}\) and the droplet length ratio l/w governing the transition between the breakup and non-breakup regimes were systematically analyzed. The results reveal that the higher viscosity ratios promote breakup by enhancing the viscous stresses with respect to interfacial tension, while the larger outlet width ratios favor non-breakup as droplets tend to move into the wider branch. A modified predictive model, developed by extending the existing T-junction framework, successfully captures the regime transition behavior in asymmetric Y-junctions. Furthermore, the daughter droplet length ratio after breakup deviates increasingly from ideal geometric scaling with greater outlet asymmetry. These findings provide new insight into the coupled influence of the viscosity ratio and geometric asymmetry on droplet dynamics and offer a predictive approach for designing microfluidic systems with controlled droplet splitting.
