Two-phase displacement in capillary tubes: wetting transition and interface evolution

The displacement of one fluid by another immiscible fluid is ubiquitous to many natural and industrial settings from geological CO₂ sequestration to microfluidics. One fundamental aspect of fluid-fluid displacement in the presence of a solid is the dynamic wetting transition: the displacement rate above which liquid films are formed on the confining surfaces. Despite its importance and fundamental nature, the wetting transition in confined geometries is far from well understood, and recent studies have reported conflicting wetting-transition scenarios. In one scenario, the fluid-fluid interface deforms outwards (in the direction of flow), and the invading fluid becomes completely nonwetting upon the transition, above which the defending fluid is left as a trailing thin film behind the meniscus (Zhao et al., PRL 2018). In the other scenario, the fluid-fluid interface deforms inwards, and the invading fluid becomes completely wetting and forms a leading film ahead of the meniscus (Levache & Bartolo, PRL 2014). In this study, we establish a unified description of the dynamic wetting transition in a capillary tube at arbitrary equilibrium contact angle θ, viscosity ratio Μ and capillary number Ca. To do so, we combine phase-field modeling and laboratory experiments to investigate the fluid-fluid displacement configurations, which are delineated in the form of a phase diagram in θ-Μ-Ca space. The crossovers of the different displacement regimes are then rationalized by the hydrodynamic theory.