Superhydrophobic surfaces: From the lotus leaf to the submarine

Samaha, Mohamed A.; Tafreshi, H. Vahedi; Gad‐el‐Hak, Mohamed

doi:10.1016/j.crme.2011.11.002

Cited by 174 publications

(81 citation statements)

References 56 publications

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“…Especially in underwater applications, the longevity of a superhydrophobic surface-how long the surface could maintain the entrapped air-is critical [8,9,11,20]. Recent overviews of the phenomenon are available [12,13,21]. The last reference in particular emphasizes the dominant role of surface tension forces at the small scale.…”

Section: Introductionmentioning

confidence: 99%

“…The presence of the air-water interface is responsible for the slip effect, characterized by an effective "slip length" [19]. This, in turn, results in a significant reduction in the skin-friction drag exerted on a moving, submerged surface [6,10,13]. As long as air pockets are entrapped in the surface pores, the surface remains superhydrophobic.…”

Section: Introductionmentioning

confidence: 99%

“…Superhydrophobic surfaces (i.e., water-repellent surfaces) have been developed by well designed chemistry and roughness [1][2][3][4][5][6][7][8][9][10][11][12][13]. Superhydrophobicity could be achieved by a combination of low surface energy and micro-or nanoscale surface structure.…”

Section: Introductionmentioning

confidence: 99%

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Polymeric Slippery Coatings: Nature and Applications

Samaha

Gad‐el‐Hak

2014

Polymers

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Abstract:We review recent developments in nature-inspired superhydrophobic and omniphobic surfaces. Water droplets beading on a surface at significantly high static contact angles and low contact-angle hystereses characterize superhydrophobicity. Microscopically, rough hydrophobic surfaces could entrap air in their pores resulting in a portion of a submerged surface with air-water interface, which is responsible for the slip effect. Suberhydrophobicity enhances the mobility of droplets on lotus leaves for self-cleaning purposes, so-called lotus effect. Amongst other applications, superhydrophobicity could be used to design slippery surfaces with minimal skin-friction drag for energy conservation. Another kind of slippery coatings is the recently invented slippery liquid-infused porous surfaces (SLIPS), which are one type of omniphobic surfaces. Certain plants such as the carnivorous Nepenthes pitcher inspired SLIPS. Their interior surfaces have microstructural roughness, which can lock in place an infused lubricating liquid. The lubricant is then utilized as a repellent surface for other liquids such as water, blood, crude oil, and alcohol. In this review, we discuss the concepts of both lotus effect and Nepenthes slippery mechanism. We then present a review of recent advances in manufacturing polymeric and non-polymeric slippery surfaces with ordered and disordered micro/nanostructures. Furthermore, we discuss the performance and longevity of such surfaces. Techniques used to characterize the surfaces are also detailed. We conclude the article with an overview of the latest advances in characterizing and using slippery surfaces for different applications.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Polymeric Slippery Coatings: Nature and Applications

Samaha

Gad‐el‐Hak

2014

Polymers

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“…[1][2][3]. Because a large portion of the interface between the solid wall and the liquid is replaced by an air-liquid interface, which can be considered almost as a shear-free surface (known as a "plastron"), SHSs could be used to obtain significant drag reduction in fluid flow applications (4,5).…”

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confidence: 99%

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Peaudecerf

Landel

Goldstein

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

162

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Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air-water interface (the "plastron") for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of streamwise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.superhydrophobic surface | drag reduction | surfactant | Marangoni stress | plastron S uperhydrophobic surfaces (SHSs) combine hydrophobic surface chemistry and micro-or nanoscale patterning to retain a network of air pockets when exposed to a liquid (e.g., reviews in refs. 1-3). Because a large portion of the interface between the solid wall and the liquid is replaced by an air-liquid interface, which can be considered almost as a shear-free surface (known as a "plastron"), SHSs could be used to obtain significant drag reduction in fluid flow applications (4, 5). Microchannel tests have recorded drag reductions of over 20% (e.g., refs. 6-11) and rheometer tests reported slip lengths of up to 185 µm (12). Turbulent flow experiments have reduced drag by up to 75% (13-16). However, a wide range of experiments have provided inconsistent results, with several studies reporting little or no drag reduction (16)(17)(18)(19)(20)(21)(22)(23)(24)(25).A key step toward solving this puzzle has come with the realization that surfactants could induce Marangoni stresses that impair drag reduction. This was first hypothesized to account for experiments that rev...

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“…Recent establishment in the slips and drag reductions obtained on engineered SHPo surfaces in laminar flows (Ou et al 2004; have heightened the anticipation that someday an appreciable reduction can be reliably obtained in TBL flows as well (Rothstein 2010;Samaha et al 2012b). While most SHPo surfaces in the literature were characterized simply by liquid droplets on them and not Authors' final manuscript for: H. Park, G. Sun, and C.--J. Kim, "Superhydrophobic turbulent drag reduction as a function of surface grating parameters," Journal of Fluid Mechanics, Vol.…”

Section: Introductionmentioning

confidence: 99%

Development of a Miniature Shear Sensor for Direct Comparison of Skin-Friction Drags

Sun

Park

Kim

2015

J. Microelectromech. Syst.

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Despite the confirmation of slip flows and successful drag reduction in small-scaled laminar flows, the full impact of superhydrophobic (SHPo) drag reduction remained questionable because of the sporadic and inconsistent experimental results in turbulent flows. Here we report a systematic set of bias-free reduction data obtained by measuring the skin-friction drags on a SHPo surface and a smooth surface at the same time and location in a turbulent boundary layer flow. Each monolithic sample consists of a SHPo surface and a smooth surface suspended by flexure springs, all carved out from a 2.7 × 2.7 mm 2 silicon chip by photolithographic microfabrication. The flow tests allow continuous monitoring of the plastron on the SHPo surfaces, so that the drag reduction data are genuine and consistent. A family ofSHPo samples with precise profiles reveals the effects of grating parameters on turbulent drag reduction, which was measured to be as much as ~ 75%.

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Superhydrophobic surfaces: From the lotus leaf to the submarine

Cited by 174 publications

References 56 publications

Polymeric Slippery Coatings: Nature and Applications

Polymeric Slippery Coatings: Nature and Applications

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Development of a Miniature Shear Sensor for Direct Comparison of Skin-Friction Drags

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