Unraveling Oil-Content-Regulated Marangoni Flow Dynamics at Oil-Water Interfaces by In Situ Oblique-Incidence...

A critical challenge in understanding the Marangoni effect at oil-water interfaces lies in the lack of techniques that simultaneously provide high sensitivity, in situ real-time monitoring, and spatiotemporally synchronous characterization, which hinders the elucidation of the dynamic mechanisms and regulation principles of interfacial flows. To address this gap, we employed oblique-incidence reflectivity difference (OIRD) as a label-free optical method to investigate, in real time and in situ, the Marangoni flow dynamics in oil-water systems (oil phase: tridecane) with W/O ratios of 100:1, 100:2, and 100:3 over 0-500 s. By extracting oscillation features of the OIRD imaginary-part signal Im{Δp-Δs}, and conducting peak-valley analysis, amplitude calculation, and spreading velocity fitting together with spatial imaging, the regulatory role of oil content on interfacial Marangoni behavior was systematically revealed. The periodic oscillations of the OIRD signal sensitively capture fluid disturbances driven by interfacial tension gradients, and the flow intensity exhibits a significant negative correlation with oil content: lower oil content yields a larger interfacial tension gradient, resulting in more pronounced oscillation amplitude, faster initial spreading velocity, and a prolonged relaxation process. For the W/O = 100:1 system, OIRD spatial images at t = 50 s display regular concentric alternating bright and dark rings, and the transverse and longitudinal cross-sectional signals both exhibit orientation-independent periodic oscillations, suggesting that the interfacial fluctuation forms an isotropic periodic ripple structure that exhibits a suggestive spatiotemporal correspondence with the time-domain signals at this representative time point. This work establishes OIRD as a powerful tool for resolving the in situ dynamics of Marangoni flows and provides mechanistic insights into how oil content governs interfacial flow intensity and pattern formation.