Dynamics of Moving Contact Line Influenced by Rotational Forcing

The present study explores the influence of surface wettability and viscosity ratio on capillary filling, wetting, and interface evolution of an immiscible binary system in a rotational microfluidic system. A thermodynamically consistent phase-field model is used to capture the spatiotemporal evolution of the interface, influenced by the complex interplay among the rotational force, surface tension, and viscous resistance. A comprehensive regime map, characterized by the local Weber number, is developed to classify the distinct interfacial transitions. Unlike classical pressure-driven systems that yield uniform centerline viscous fingering, we demonstrate that rotational forcing introduces velocity-dependent transverse Coriolis momentum. This Coriolis force acts as an active symmetry-breaking mechanism, laterally shifting the advancing fluid and driving a skewed morphological distortion. Under hydrophilic conditions, the interface transitions from a concave shape through weakly and strongly centrifugal force-dominated regimes as the rotational Reynolds number increases. Conversely, hydrophobic conditions maintain a convex meniscus, accelerating these regime transitions. Moreover, increasing the viscosity ratio significantly delays regime transitions because of increased viscous resistance from the displaced fluid. For hydrophobic substrates, overcoming this resistive capillary force requires a critical rotational Reynolds number to initiate flow, which scales with both the contact angle and the fluid properties. We believe that the findings of this study will advance the fundamental understanding of interfacial dynamics in rotational microfluidics and provide a foundation for the rational design of next-generation centrifugal lab-on-chip devices, typically used in diagnostics, sample preparation, and biochemical assays.