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The dynamics of wetting and dewetting is largely determined by the velocityfield near the contact lines. For water drops it has been observed that addingsurfactant decreases the dynamic receding contact angle even at a concentrationmuch lower than the critical micelle concentration (CMC). To better understandwhy surfactants have such a drastic effect on drop dynamics, we constructed adedicated a setup on an inverted microscope, in which an aqueous drop is heldstationary while the transparent substrate is moved horizontally. Usingastigmatism particle tracking velocimetry, we track the 3D displacement of thetracer particles in the flow. We study how surfactants alter the flow dynamicsnear the receding contact line of a moving drop for capillary numbers in theorder of $10^{-6}$. Even for surfactant concentrations $c$ far below thecritical micelle concentration ($c \ll$ CMC) Marangoni stresses change the flowdrastically. We discuss our results first in a 2D model that considersadvective and diffusive surfactant transport and deduce estimates of themagnitude and scaling of the Marangoni stress from this. Modeling andexperiment agree that a tiny gradient in surface tension of a few $\mu N \,m^{-1}$ is enough to alter the flow profile significantly. The variation of theMarangoni stress with the distance from the contact line suggests that the 2Dadvection-diffusion model has to be extended to a full 3D model. The effect isubiquitous, since surfactant is present in many technical and natural dewettingprocesses either deliberately or as contamination.

Flow profiles near receding three--phase contact lines: Influence of surfactants