Drag crisis moderation by thin air layers sustained on superhydrophobic spheres falling in water

Abstract

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 a commercially available hydrophobic agent. By comparing the free fall of unmodified spheres and superhydrophobic spheres in a 2.5 meter 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 from 10 to 3 × 10, 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 implications for the development of sustainable air-layer-based energy saving technologies.

Stable-streamlined cavities following the impact of non-superhydrophobic spheres on water

“…However this is perhaps not surprising, having in mind that the air layer between the front of the sphere and the cavity before the cavity separation from the sphere is very thin and provides only partial effective slip. [30][31][32] The direct contact between the sphere and the liquid could explain the residual drag on the formation.…”

Giant drag reduction on Leidenfrost spheres evaluated from extended free-fall trajectories

“…In the case of Fig. 4b we used data collected in our previous study [6] for sphere falling in a water-filled tank of 2.5 meter height and 40×40 cm cross-section, which results in less sideway velocity fluctuation due to the tank wall effects.…”

“…The introduction of a gas layer on a solid moving in liquid is also an intriguing fluid dynamics phenomenon, as even a thin gas layer can modify the no-slip boundary condition at the solid surface and alter the flow patterns around the body and the related drag forces. Methods to induce gas layers at the solid body interface include bubbles injection, supercavitation, assisted cavitation [1], cavity entrapment during impact [2], and the use of superhydrophobic surfaces that can naturally sustain a thin air layer under water [3][4][5][6].…”

The air entrapment under a drop impacting on a nano-rough surface

“…The coating of the surface follows our earlier protocol used to superhydrophobize steel spheres by Vakarelski et al 23,24 A glass microscope slide (Fisher Scientific) was coated with Glaco Mirror Coat Zero (Soft 99 Ltd, Japan), which is an alcohol suspension containing nano-particles (d B 40 nm) and a superhydrophobizing agent. The coating was cured in an oven at 160 1C for 10 minutes.…”

“…The overall effects of surface roughness particularly as it relates to superhydrophobic surfaces have been intensively studied due to the potential for drag reduction, 23,24 self-cleaning surfaces 25 and prevention of icing of aircraft wings, 26 among others. Typically, superhydrophobic surfaces are created by a natural or tailored texture or pattern, such as an array of micro-pillars or ribs, imprinted on the surface and then coating the surface with a hydrophobizing agent, if necessary.…”