Measurement of hydrodynamic frictional drag on superhydrophobic flat plates in high Reynolds number flows

In this paper, we report the characterization of large-scale superhydrophobic surfaces for hydrodynamic drag reduction in boundary layer flows using a high-speed towing tank system. For making superhydrophobic surfaces, flat aluminum plates (4 ft ×?2 ft ×?3/8 in, with sharpened leading/trailing edges) were prepared and coated with nano-structured hydrophobic particles. The static and dynamic contact angle measurements indicate that the coated surfaces correspond to a de-wetting (Cassie) state with air retained on the nano-structured surfaces. Hydrodynamic drag of the large-area superhydrophobic plates was measured to cover turbulent flows (water flow speeds up to 30 ft/s, Reynolds number in the range of 10 ⁵-10⁷) and compared with that of an uncoated bare aluminum control plate. Results show that an acceptable drag reduction was obtained up to ∼30% in the early stage of the turbulent regime which is due to reduced shear forces on the plates because of the lubricating air layer on the surface. However, in a fully developed turbulent flow regime, an increase in drag was measured which is mainly attributed to the amplified surface roughness due to the protrusions of air bubbles formed on the surface. Meanwhile, a qualitative observation suggests that the air bubbles are prone to be depleted during several runs of the high shear-rate flows, as revealed by streak lines of depleted air bubbles. This suggests that the superhydrophobic coating is unstable in maintaining the de-wetted state under dynamic flow conditions and that the increased drag results from the inherent surface roughness of the coating layer where the de-wetted state collapses to a wetted (Wenzel) state due to the depletion of air bubbles. However, it was also observed that the air bubbles would reform on the surface, with the same properties as a dry surface immersed in water, while the plate was kept statically immersed in water for 12 hours, suggesting that the superhydrophobic coating retains static stability for a de-wetted state. The experimental results illustrate that drag reduction is not solely dependent on the superhydrophobicity of a surface (e.g., contact angle and air fraction), but the morphology and stability of the surface air layer are also critical for the design and use of superhydrophobic surfaces for large-scale hydrodynamic drag reduction, especially in turbulent flow regimes.