Plastron morphology and drag of a superhydrophobic surface in turbulent regime*

The relationship between the state of the plastron, slip velocity, and drag of a superhydrophobic surface in a turbulent boundary layer was investigated. The experiments were carried out using a body-of-revolution and tested in a water tunnel at Re from 5.0×10⁵ to 1.5×10⁶ (based on the length of the model). Visualization of the plastron and particle tracking velocimetry (PTV) of the near-wall boundary layer was carried. The load measurement showed a 36.4% reduction in drag at the lowest Re of 5.0×10⁵, which decreased to 5.6% at the highest Re of 1.5×10⁶. The microscopic PTV showed an increase in the slip velocity from 0.131 to 0.602 m/s, and a relatively constant slip-length (85 to 66 μm) over the superhydrophobic surface as Re increased from 5.0×10⁵ to 1.5×10⁶. The visualizations of the surface showed frequent appearance of a full air plastron with average thickness of ~10 μm at the two lowest Re. This full plastron was essential for obtaining a considerable drag reduction (>16%). At the three higher Re with smaller drag reduction (<8%), the plastron demonstrated isolated menisci of air, pinned between the tip and the valley of large roughness protrusions.