We investigate the effect of thin air layers naturally sustained on superhydrophobic surfaces on the terminal velocity and drag force of metallic spheres free falling in water. The surface of 20 mm to 60 mm steel or tungsten-carbide spheres is rendered superhydrophobic by a simple coating process that uses commercially available hydrophobic agent. By comparing the free fall of unmodified spheres and superhydrophobic spheres in a 2.5 meters tall water tank, It is demonstrated that even a very thin air layer (~ 1 – 2 μm) that covers the freshly dipped superhydrophobic sphere, can reduce the drag force on the spheres by up to 80 %, at Reynolds numbers 105 - 3×105 , owing to an early drag crisis transition. This study complements prior investigations on the drag reduction efficiency of model gas layers sustained on heated metal spheres falling in liquid by the Leidenfrost effect. The drag reduction effects are expected to have significant implication for the development of sustainable air-layer-based energy saving technologies.
Jetly A, Vakarelski IU, Thoroddsen ST (2018) Drag crisis moderation by thin air layers sustained on superhydrophobic spheres falling in water. Soft Matter. Available: http://dx.doi.org/10.1039/c7sm01904a.
This work was supported by the King Abdullah University of Science and Technology (KAUST). We acknowledge Mr. Ziqiang Yang for the assistance in the some of the experiments. The AFM imaging was performed in the KAUST Microfluidics Thrust Area Labs.