Disentangling the mechanisms of surfactant-driven contact line motion: From transient Marangoni flow to adsorption-dominated dynamics

The contact line motion in liquid–liquid–solid systems involves a complex interplay of surfactants and interfacial forces, which is a critical issue not yet fully resolved. While Marangoni flow, interfacial tension reduction, and surfactant adsorption are all implicated, their respective roles across different time scales remain contentious. To elucidate these dynamics, we investigated the coalescence between a surfactant-laden pendant drop and a pure water sessile drop on a substrate in decane. Using a high-speed camera and drop shape analyzer (DSA 100), we systematically analyzed the process across milliseconds to minutes. Experiments were conducted by employing a series of surfactants: anionic sodium dodecylbenzenesulfonate, cationic cetyltrimethylammonium bromide, zwitterionic lauramidopropyl hydroxysulfobetaine, and nonionic polyoxyethylene (20) sorbitan monolaurate (TW 20), on both hydrophilic and hydrophobic substrates. The results reveal a timescale-dependent transition in the governing mechanisms of contact line motion. The millisecond-scale dynamics are initiated by the coalescence-induced capillary waves and Marangoni flow. Subsequently, contact line motion transitions to a long-term stage (seconds to minutes) driven by surfactant adsorption at the solid–liquid interface. The overall behavior of the contact line results from synergistic effects of surfactant properties, concentration, and substrate wettability. Our work clarifies the apparent complexity by establishing a time-resolved framework: the sustained contact line motion is predominantly controlled by surfactant adsorption on the solid substrate, with Marangoni effects playing negligible roles in the long-term stage.