Séminaire Théo Lenavetier

Surfactant dynamics at liquid-liquid interfaces under shear: experimental study of the kinematic condition

Lubricant-infused surfaces (LIS) are engineered materials that incorporate a stable liquid-liquid interface between a trapped lubricant and a working fluid. This configuration is expected to promote remarkable interfacial slip, enabling applications from anti-biofouling to thrombosis prevention. However, unlike controlled lab settings, real environments expose LIS to surface-active chemical contaminants. Even traces amounts of these surfactants can accumulate at the sheared water/oil interface, inducing Marangoni stresses that oppose the flow and reduce interfacial slip.

This raises a key question: what kinematic condition should be imposed at the interface to predict the averaged behavior of the flow, especially in engineering contexts? In the presence of surfactants, accurately determining the stopping point positions and the resulting Marangoni stress poses a significant challenge due to the numerous hidden physico-chemical variables. Consequently, many numerical and analytical models fail to predict the experimental velocity field at the liquid-liquid interface, leading to inaccurate predictions of the flow within the working fluid.

We propose using Doppler Optical Coherence Tomography (D-OCT) to characterize local flows near the liquid-liquid interface of a LIS under controlled surfactant conditions. This technique is relatively new in the soft-matter hydrodynamics community, and enables live in-situ velocimetry with velocities up to v ~ 10cm/s and with a spanwise/depthwise resolution of d ~ 2µm.

By resolving the 3D velocity field near the interface, we aim to track interfacial motion and compression to infer surfactant flux and kinematic constraints. This allows us to quantify and map Marangoni stresses, revealing surfactant transport routes and clarifying the physical origins of interfacial immobilization in LIS. Ultimately, this approach offers a path to bridge the gap between real chemical conditions and analytical models