  While superhydrophobic surfaces (SHSs) show promise for drag reduction
applications, their performance can be compromised by traces of surfactant,
which generate Marangoni stresses that increase drag. This question is
addressed for soluble surfactant in a three-dimensional laminar channel flow,
with periodic SHSs on both walls. We assume that diffusion is sufficiently
strong for cross-channel concentration gradients to be small. Exploiting a
long-wave theory that accounts for a rapid transverse Marangoni-driven flow, we
derive a one-dimensional model for surfactant evolution, which allows us to
predict the drag reduction across the parameter space. The system exhibits
multiple regimes, involving competition between Marangoni effects, bulk and
interfacial diffusion, advection and shear dispersion. We map out asymptotic
regions in the high-dimensional parameter space, deriving approximations of the
drag reduction in each region and comparing them to numerical simulations. Our
atlas of maps provides a comprehensive analytical guide for designing
surfactant-contaminated channels with SHSs, to maximise the drag reduction in
applications.
