Direct numerical simulation of contact-line-driven dewetting and pinch-off dynamics in surfactant-laden flows

We investigate the role of surfactants in the immiscible displacement of a more viscous fluid by a less viscous one within a cylindrical capillary tube, under conditions of partial wetting. Our analysis is carried out through high-fidelity, three-dimensional direct numerical simulations (DNS), which enable a fully resolved treatment of fluid–fluid interfaces and interfacial forces. The regime of interest is one in which capillary and viscous forces dominate over inertial effects. In this regime, above a critical displacement rate, the motion of the contact line is governed exclusively by the wettability of the capillary wall. In the absence of surfactants, the interface forms a central finger-like structure, leaving behind a thin film of the displaced fluid on the tube walls. This film becomes unstable due to the Rayleigh–Plateau instability, leading to bubble pinch-off. Building upon this baseline understanding, we introduce surfactants, amphiphilic monomers that adsorb at fluid–fluid interfaces and modulate local surface tension, into the system. Although surfactants are known to significantly affect interfacial flows, their impact on displacement dynamics under partial wetting conditions remains underexplored. Our study aims to fill this gap by systematically varying key surfactant properties, including interfacial elasticity, solubility in the bulk phases, and adsorption/desorption kinetics at the interface. We find that increased interfacial elasticity, due to surfactant-induced Marangoni stresses, can delay bubble pinch-off. Moreover, surfactant solubility introduces a dynamic exchange between the interface and the bulk, resulting in more complex pinch-off behaviour. Adsorption and desorption kinetics can give rise to non-uniform surfactant distributions along the interface, which in turn induce relatively strong Marangoni flows that feed back into the bulk hydrodynamics. Overall, our results highlight the critical importance of surfactant properties in modulating displacement dynamics in confined geometries.