Predicting longevity of submerged superhydrophobic surfaces with parallel grooves

Emami, Babak; Hemeda, Ahmed A.; Amrei, M.M.; Luzar, Alenka; Gad‐el‐Hak, Mohamed; Tafreshi, H. Vahedi

doi:10.1063/1.4811830

Cited by 64 publications

(80 citation statements)

References 43 publications

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“…The results also indicated that the theory of Poetes et al [11] was applicable for a much wider range of pressures. In a related theoretical study, Emami et al [110] modeled the longevity of parallel grooves superhydrophobic surfaces subjected to different pressures. It was found that for grooves with higher width-to-depth ratios, the critical pressure was higher due to stronger resistance to deflection of the air-water interface from the air trapped in such grooves.…”

Section: Longevity Of Superhydrophobic Coatingsmentioning

confidence: 99%

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: Longevity Of Superhydrophobic Coatingsmentioning

confidence: 99%

Polymeric Slippery Coatings: Nature and Applications

Samaha

Gad‐el‐Hak

2014

Polymers

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“…The sidewall of the trench has nanometer-scale roughness, as it was replicated from the "scalloped" surface [25] on the DRIE silicon mold. Let us first analyze how the trapped air is depleted from a hydrophobic trench after being submerged based on previous works [15,26,27]. Consider a simple trench with width w, length l, and depth h, as defined in Fig.…”

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confidence: 99%

“…Importantly for our purpose instead, a sample with single trench would allow clear images of one air-water meniscus; in comparison, multiple menisci in multiple trenches would overlap and blur the images. The transparent sample with a Let us first analyze how the trapped air is depleted from a hydrophobic trench after being submerged based on previous works [15,26,27]. Consider a simple trench with width w, length l, and depth h, as defined in Fig.…”

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Infinite Lifetime of Underwater Superhydrophobic States

2014

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Submerged superhydrophobic (SHPo) surfaces are well known to transition from the dewetted to wetted state over time. Here, a theoretical model is applied to describe the depletion of trapped air in a simple trench and re-arranged to prescribe the conditions for infinite lifetime. By fabricating a microscale trench in a transparent hydrophobic material, we directly observe the air depletion process and verify the model. The study leads to the demonstration of infinite lifetime (> 50 days) of air pockets on engineered microstructured surfaces under water for the first time.Environmental fluctuations are identified as the main factor behind the lack of a long-term underwater SHPo state to date.The stability of the air layer on superhydrophobic (SHPo [1]) surfaces fully submerged in water is of critical importance because their key anticipated applications, such as drag reduction [2][3][4][5] and anti-biofouling [6], are under water. Unfortunately, the air film on the underwater SHPo surface, often called a plastron, is fragile [2,7]. Over time, the air initially trapped on the SHPo surface diffuses away into the surrounding water, collapsing the air-water interface and causing a transition from the dewetted to wetted state [8]. Recent experimental studies showed that the lifetime of the underwater SHPo state is influenced by various environmental parameters [7,[9][10][11]. However, most of the studies only reported statistical information, such as average wetting time; more direct knowledge such as air-depletion dynamics or the effect of roughness geometries is needed to design the SHPo surface to be more robust against wetting. While some underwater insects boast a long-term or even indefinite (tested up to 120 days [12]) plastron, all artificial SHPo surfaces retained their plastron for much shorter length of time (mostly less than 2 an hour; rarely for days [7,9,[13][14][15] This paper aims to understand the air depletion process on submerged SHPo surfaces and to determine if an indefinite plastron is achievable. In order to systematically study the effect of geometric parameters of the surface structures for an intended goal, SHPo surfaces made of regular structures would be far more informative than those of random structures that produce only statistical data. Furthermore, for drag-reduction application in particular, SHPo surfaces with parallel trenches [3,4,17,18] have been found to outperform those with random structures [19,20], making SHPo surfaces with trenches a good candidate to study. Since multiple trenches are in parallel and isolated from each other, a single trench can represent the whole SHPo surface as far as the stability of the trapped air is concerned. The potential over-estimation of the depletion speed on single trench compared with the parallel trenches due to the edge effect [21,22] is considered minor to our goal. Importantly for our purpose instead, a sample with single trench would allow clear images of one air-water meniscus; in comparison, multiple menisci in multiple trench...

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“…The importance has been recognized only very recently, as the plastron loss became the main roadblock against testing SHPo surfaces in turbulent flows especially in large facilities. Samaha et al (2012a) have measured that the longevity of the plastron was shortened as the flow rate over the SHPo surface increased, and Emami et al (2013) have numerically investigated the unsteady behavior of the plastron interface by considering the diffusion of trapped air over time. They showed that the maximum hydrostatic pressure sustainable above the plastron decreases with increasing width of the water-air interface.…”

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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Predicting longevity of submerged superhydrophobic surfaces with parallel grooves

Cited by 64 publications

References 43 publications

Polymeric Slippery Coatings: Nature and Applications

Polymeric Slippery Coatings: Nature and Applications

Infinite Lifetime of Underwater Superhydrophobic States

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

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