We study fluid flow in the vicinity of textured and superhydrophobically coated surfaces with characteristic texture sizes on the order of 10μm. Both for droplets moving down an inclined surface and for an external flow near the surface (hydrofoil), there is evidence of appreciable drag reduction in the presence of surface texture combined with superhydrophobic coating. On textured inclined surfaces, the drops roll faster than on a coated untextured surface at the same angle. The highest drop velocities are achieved on surfaces with irregular textures with characteristic feature size ∼8μm. Application of the same texture and coating to the surface of a hydrofoil in a water tunnel results in drag reduction on the order of 10% or higher. This behavior is explained by the reduction of the contact area between the surface and the fluid, which can be interpreted in terms of changing the macroscopic boundary condition to allow nonzero slip velocity.

We present an experimental investigation of water flow around a hydrofoil with a superhydrophobic patterned surface. The experimental setup uses a water tunnel to measure the drag over the hydrofoil and acquire velocity field measurements using particle image velocimetry (PIV). Drag reduction on the order of 10% or higher was observed on hydrofoils with irregular surface textures combined with super-hydrophobic coating, leading to effective Navier slip on the surface. However, we report that other macroscopic flow characteristics, including the stall angle, are also changed by application of the coating.