  Superhydrophobic surfaces have been shown to produce significant drag
reduction in both laminar and turbulent flows by introducing an apparent slip
velocity along an air-water interface trapped within the surface roughness. In
the experiments presented within this study, we demonstrate the existence of a
surface tension gradient associated with the resultant Marangoni flow along an
air-water interface that causes the slip velocity and slip length to be
significantly reduced. In this study, the slip velocity along a
millimeter-sized air-water interface was investigated experimentally. This
large-scale air-water interface facilitated a detailed investigation of the
interfacial velocity profiles as the flow rate, interfacial curvature and
interface geometry were varied. For the air-water interfaces supported above
continuous grooves (concentric rings within a torsional shear flow) where no
surface tension gradient exists, a slip velocity as high as 30% of the bulk
velocity was observed. However, for the air-water interfaces supported above
discontinuous grooves (rectangular channels in a Poiseuille flow), the presence
of a surface tension gradient reduced the slip velocity and in some cases
resulted in an interfacial velocity that was opposite to the main flow
direction. The curvature of the air-water interface in the spanwise direction
was found to dictate the details of the interfacial flow profile with reverse
flow in the center of the interface for concave surfaces and along the outside
of the interface for convex surfaces. The deflection of the air-water interface
was also found to greatly affect the magnitude of the slip. Numerical
simulations imposed with a relatively small surface tension gradient along the
air-water interface were able to predict both the reduced slip velocity and
back flow along the air-water interface.
