  The dynamics of wetting and dewetting is largely determined by the velocity
field near the contact lines. For water drops it has been observed that adding
surfactant decreases the dynamic receding contact angle even at a concentration
much lower than the critical micelle concentration (CMC). To better understand
why surfactants have such a drastic effect on drop dynamics, we constructed a
dedicated a setup on an inverted microscope, in which an aqueous drop is held
stationary while the transparent substrate is moved horizontally. Using
astigmatism particle tracking velocimetry, we track the 3D displacement of the
tracer particles in the flow. We study how surfactants alter the flow dynamics
near the receding contact line of a moving drop for capillary numbers in the
order of $10^{-6}$. Even for surfactant concentrations $c$ far below the
critical micelle concentration ($c \ll$ CMC) Marangoni stresses change the flow
drastically. We discuss our results first in a 2D model that considers
advective and diffusive surfactant transport and deduce estimates of the
magnitude and scaling of the Marangoni stress from this. Modeling and
experiment 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 the
Marangoni stress with the distance from the contact line suggests that the 2D
advection-diffusion model has to be extended to a full 3D model. The effect is
ubiquitous, since surfactant is present in many technical and natural dewetting
processes either deliberately or as contamination.
