Robust surfaces capable of reducing flow drag, controlling heat and mass transfer, and resisting fouling in fluid flows are important for various applications. In this context, textured surfaces impregnated with a liquid lubricant show promise due to their ability to sustain a liquid–liquid interface that induces slippage. However, theoretical and numerical studies suggest that the slippage can be compromised by surfactants in the overlying fluid, which contaminate the liquid–liquid interface and generate Marangoni stresses. In this study, we use Doppler-optical coherence tomography, an interferometric imaging technique, combined with numerical simulations to investigate how surfactants influence the slip length of lubricant-infused surfaces with longitudinal grooves in a laminar flow. Surfactants are endogenously present in the contrast agent (milk) which is added to the working fluid (water). Local measurements of slip length at the liquid–liquid interface are significantly smaller than theoretical predictions for clean interfaces (Schönecker & Hardt 2013). In contrast, measurements are in good agreement with numerical simulations of fully immobilized interfaces, indicating that milk surfactants adsorbed at the interface are responsible for the reduction in slippage. This work provides the first experimental evidence that liquid–liquid interfaces within textured surfaces can become immobilised in the presence of surfactants and flow.

Robust surfaces capable of reducing flow drag, controlling heat and mass transfer,and resisting fouling in fluid flows are important for various applications. In thiscontext, textured surfaces impregnated with a liquid lubricant show promise dueto their ability to sustain a liquid-liquid layer that induces slippage. However,theoretical and numerical studies suggest that the slippage can be compromisedby surfactants in the overlying fluid, which contaminate the liquid-liquid interfaceand generate Marangoni stresses. In this study, we use Doppler-optical coherencetomography, an interferometric imaging technique, combined with numericalsimulations to investigate how surfactants influence the slip length of lubricant-infused surfaces with longitudinal grooves in a laminar flow. We introducesurfactants by adding tracer particles (milk) to the working fluid (water). Localmeasurements of slip length at the liquid-liquid interface are significantly smallerthan theoretical predictions for clean interfaces (Schönecker & Hardt 2013).In contrast, measurements are in good agreement with numerical simulationsof fully immobilized interfaces, indicating that milk particles adsorbed at theinterface are responsible for the reduction in slippage. This work provides thefirst experimental evidence that liquid-liquid interfaces within textured surfacescan become immobilized in the presence of surfactants and flow.