  Trace amounts of surfactants have been shown to critically prevent the drag
reduction of superhydrophobic surfaces (SHSs), yet predictive models including
their effects in realistic geometries are still lacking. We derive theoretical
predictions for the velocity and resulting slip of a laminar fluid flow over
three-dimensional SHS gratings contaminated with surfactant, which allow for
the first direct comparison with experiments. The results are in good agreement
with our numerical simulations and with measurements of the slip in
microfluidic channels lined with SHSs, which we obtain via confocal microscopy
and micro-particle image velocimetry. Our model enables the estimation of a
priori unknown parameters of surfactants naturally present in applications,
highlighting its relevance for microfluidic technologies.
