Dimensionless Analyses of Liquid–Liquid–Liquid Three-Phase Flow Patterns in a Confined Microchannel

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.