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The flow pattern transition of a liquid−liquid− liquid three-phase flow in a confined microchannel has been ascertained by systematic numerical simulations with a volume of fluid-continuum surface force (VOF-CSF) model. Three typical flow patterns of the liquid−liquid−liquid three-phase flow can be distinguished by the flow field structures, namely, plug, slug, and blocked-slug. Further, the effects of the key dimensionless numbers on the quantitative transition laws of flow patterns are intensively investigated. It is found that the transition from plug to slug is only determined by the capillary number of the outer phase. However, as for the transition from slug to blocked-slug, it can be significantly regulated by the coupling of all involved dimensionless numbers. In addition, to reveal the underlying mechanisms of flow pattern transition, the force field characteristics have been thoroughly discussed. The results indicate that, in plug flow, the Laplace pressure force acting on the outer interface is dominant. Once the pressure differential force and shear force together acting on the outer interface overcome the Laplace pressure force, the flow pattern of the slug emerges. As the pressure differential force and the shear force acting on the inner interface increase, the flow pattern falls into the blocked-slug because of an enhanced hindering effect of the inner interface on the deformation of the outer interface. The results offer a theoretical guide for precisely controlling the liquid−liquid−liquid three-phase flow behaviors to fulfill the process intensification of microfluids in a confined microchannel.

Droplet step emulsification has been proven to possess the unique advantage of decoupling the flow parameters, which obviously contributes to the realization of the mass production of monodisperse droplets. However, a complete understanding of the dynamic characteristics underlying droplet spontaneous formation in step emulsification has not been fully revealed and remains a challenge because the channel confinement effect always results in the complexity in interface spatiotemporal evolution under various conditions. In this work, the spontaneous formation mechanisms of droplets in step emulsification are numerically investigated via a VOF–CSF model. The physics behind the two distinct flow patterns regarding dripping and jetting are deeply revealed based on the local flow field structures and pressure distribution, and it was found that in dripping, before final pinch-off of the neck, a finite time singularity usually exists, thus leading to infinite velocity and pressure inside the neck. However, in jetting, as the droplet steadily expands, the velocity and pressure inside the neck finally reach an equilibrium state. Besides, by taking multiple variables with a wide range into account, the flow pattern diagram and the prediction correlation of droplet size are established with several dimensionless numbers, exhibiting excellent universality. In particular, the force field characteristics previously undocumented for droplet step emulsification are also quantitatively clarified from a new perspective of momentum conservation. The results obtained in this study reveal the spontaneous formation mechanisms of droplets in step emulsification, thus providing theoretical guidance for precisely regulating the emulsification process.