Superhydrophobic drag reduction in turbulent flows: a critical review

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

A model for slip and drag in turbulent flows over superhydrophobic surfaces with surfactant

“…Average streamwise slip length and drag reduction In Fig. 3, we compare our laminar and turbulent model predictions (excluding surfactant effects, such that the average Marangoni shear rate γ Ma = 0) with available texture-resolving DNS (also exclusive of surfactant) for turbulent channel flows bounded by SHSs with long streamwise ridges that are periodic in the spanwise direction (Park et al, 2013;Türk et al, 2014;Rastegari and Akhavan, 2015;Egan et al, 2021;Park et al, 2021), as a function of the SHS texture period in wall units P + ∈ [0, 100] and gas fraction φ = 0.5 (blue symbols and lines), φ = 0.75 (red), φ = 0.88 (green) and φ = 0.94 (yellow). We investigate two quantities that are commonly used to characterise the local and global performance of SHSs compared to the no-slip flow: in Fig.…”

“…drag increase) to +90%, with five studies finding little (< 20%) or no drag reduction. A number of possible causes may explain these discrepancies, as discussed in detail in the review by Park et al (2021). For example, the liquid-gas interface at the SHS can deform due to pressure differences in the fluid and gas cavity, which has been shown to alter the drag reduction in laminar and turbulent flows over SHSs depending on the protrusion angle (Teo and Khoo, 2009;Rastegari and Akhavan, 2018).…”

One-Step Fabrication of Flexible Bioinspired Superomniphobic Surfaces

“…Superomniphobic surfaces, with unique micro/nanostructures to trap air, can prevent from getting wet by repelling liquids with large (>150°) contact angles (CAs) and low CA hysteresis. , The excellent liquid-repellent properties of the superomniphobic surfaces are attractive due to their potential applications in drag reduction, − antifouling surfaces, − anti-icing materials, , microfluidic devices, − flexible electronics, − and biological fields. , Extensive research on these functional surfaces was conducted after Choi et al. first proposed the concept of superomniphobic surface .…”

Two local slip modes at the liquid–liquid interface over liquid-infused surfaces

“…[1][2][3] Inspired by an unusual bounce behavior of water droplets on lotus leaves, superhydrophobic surfaces (SHSs) have been long explored to reduce flow drag. [4][5][6][7] Due to the presence of micro-and nanostructures and relatively low surface energy of SHSs, air can be trapped within these structures when SHSs are submerged under water. As water is prevented from a direct contact with a solid wall, a gas-liquid interface (GLI) is formed to form an apparent slip, which makes SHSs useful for applications that require drag reduction.…”