Superhydrophobic drag reduction in turbulent flows: a critical review

Hyungmin Park,Chang-Jin Kim,Chang-Hwan Choi

doi:10.1007/s00348-021-03322-4

Abstract

Superhydrophobic (SHPo) surfaces have been investigated vigorously since around 2000 due in large part to their unique potential for hydrodynamic frictional drag reduction without any energy or material input. The mechanisms and key factors affecting SHPo drag reduction have become relatively well understood for laminar flows by around 2010, as has been reviewed before [Lee et al. Exp Fluids 57:176 (2016)], but the progress for turbulent flows has been rather tortuous. While improved flow tests made positive SHPo drag reduction in fully turbulent flows more regular since around 2010, such a success in a natural, open water environment was reported only in 2020 [Xu et al. Phys Rev Appl 13:034056 (2020b)]. In this article, we review studies from the literature about turbulent flows over SHPo surfaces, with a focus on experimental studies. We summarize the key knowledge obtained, including the drag-reduction mechanism in the turbulent regime, the effect of the surface roughness morphology, and the fate and role of the plastron. This review is aimed to help guide the design and application of SHPo surfaces for drag reduction in the large-scale turbulent flows of field conditions.Graphic abstract

Highlights

A superhydrophobic (SHPo) surface, which typically consists of a micro- and/or nanoscale roughness treated chemically to be hydrophobic, is well known to trap pockets or a layer of air [called plastron (Brocher 1912)] between the roughness elements when submerged under water
Park et al (2014) used longitudinally grooved SHPo surfaces monolithically fabricated on floating elements and flexure beams, following the silicon microlithographic process developed by Sun et al (2015), to directly measure the skin-friction drag in TBL flows in a water tunnel, and achieved a drag reduction as high as 75% (or ~ 65% after compensating for the effect of small sample size, following Park (2015)) with gas fraction (GF) = 0.97 at Re = 250, which corresponds to w+ ≈ 0.9
Hydrodynamic drag reduction using SHPo surface has attained a great attention for its potential for economic and environmental benefits of global scale, its field demonstration has not materialized until very recently

Summary

Introduction

A superhydrophobic (SHPo) surface, which typically consists of a micro- and/or nanoscale roughness treated chemically to be hydrophobic, is well known to trap pockets or a layer of air [called plastron (Brocher 1912)] between the roughness elements when submerged under water. Some reported a consistent drag reduction (Daniello et al 2009; Park et al 2014; Li et al 2020a, b), while others showed little drag reduction or even drag increase (Zhao et al 2007; Peguero and Breuer 2009; Aljallis et al 2013; Gose et al 2018). This discrepancy might have been partially caused by the measurement techniques, we believe the main culprit has been a deteriorated or depleted plastron, looking back with the updated knowledge we have today (Lee et al 2016). The plastron is lost by the increased shear stress (Wexler et al 2015a), inertia, and

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