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Review Article

Underwater Superhydrophobicity: Stability, Design and Regulation, and Applications

[+] Author and Article Information
Yahui Xue, Pengyu Lv

State Key Laboratory for Turbulence
and Complex Systems,
Mechanics and Engineering Science,
College of Engineering,
Peking University,
Beijing 100871, China

Hao Lin

Mechanical and Aerospace Engineering,
Rutgers, The State University of New Jersey,
Piscataway, NJ 08854

Huiling Duan

State Key Laboratory for Turbulence
and Complex Systems,
Mechanics and Engineering Science,
College of Engineering,
Peking University,
Beijing 100871, China;
CAPT, HEDPS, and IFSA Collaborative
Innovation Center of MoE,
BIC-ESAT,
Peking University,
Beijing 100871, China
e-mail: hlduan@pku.edu.cn

1Corresponding author.

Manuscript received December 30, 2015; final manuscript received May 16, 2016; published online June 21, 2016. Assoc. Editor: Xiaodong Li.

Appl. Mech. Rev 68(3), 030803 (Jun 21, 2016) (38 pages) Paper No: AMR-15-1145; doi: 10.1115/1.4033706 History: Received December 30, 2015; Revised May 16, 2016

Bioinspired superhydrophobic surfaces have attracted great interest from fundamental research to engineering applications. The stability, design, and regulation of superhydrophobicity, especially in a submerged environment, have been one of the main focuses of recent efforts. This review is dedicated to illustrating the fundamental characteristics of underwater superhydrophobicity, introducing novel and effective strategies for robust design and regulation, and to providing an overview of the state-of-the-art engineering applications in drag reduction and cavitation/boiling control. First, the underlying mechanisms of wetting transition on superhydrophobic surfaces submerged underwater induced by physical phenomena including pressurization, air diffusion, fluid flow, and condensation are reviewed. The influence of the closed/open state of entrapped air cavities is differentiated. Landmark experiments demonstrating wetting transition mechanisms are surveyed. Then, novel strategies for designing robust superhydrophobic surfaces are summarized, including hierarchical, reentrant, lubricant-infused, and mechanically durable structures. Moreover, strategies for superhydrophobicity regulation are introduced, which are classified into two types: self-healing and dewetting, based on the failure regime (surface damage or meniscus collapse). The current state-of-the-art engineering applications in drag reduction and cavitation/boiling control are comprehensively reviewed. Last but not least, remaining challenges for future research are given at the conclusion.

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References

Figures

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Fig. 1

(a)–(c) Lotus plant (Nelumbo nucifera): contamination (a) and self-cleaning (b) of a lotus leaf, and SEM image of the leaf microstructure with papillose cells and epicuticular wax tubules (c). (Reprinted with permission from Koch et al. [9]. Copyright 2008 by Elsevier Ltd.) (d) and (e) Water strider (Gerris remigis): a resting water strider on water (d), and SEM image of the strider's leg with microsetae structures, inset showing the nanogrooves on one seta (e). (d) (Reprinted with permission from Feng and Jiang [10]. Copyright 2006 by Wiley-VCH Verlag GmbH & Co. KGaA). (e) (Reprinted with permission from Gao and Jiang [11]. Copyright 2004 by Nature Publishing Group.) (f)–(h) Springtail (Orthonychiurus stachianus): the entire animal immersed in water (f) and olive oil (g) showing surrounding plastron, and SEM image of nanoscopic surface topography on the springtail cuticle with comblike reentrant structures (h). (Reprinted with permission from Hensel et al. [12]. Copyright 2013 by American Chemical Society.) (i) and (j) Salvinia molesta: the leaf surface densely covered with hairs, rendering it superhydrophobic (i), and SEM image of the complex hairy structures with four multicellular hairs connected at the terminal end (j). (Reprinted with permission from Barthlott et al. [13]. Copyright 2010 by Wiley-VCH Verlag GmbH & Co. KGaA.) (k) and (l) Pitcher plant (Nepenthes bicalcarata): photograph of the pitcher with the position of peristome indicated (k), and SEM image of the peristome surface with ridge structures, arrow showing the inside direction of the pitcher (l). (Reprinted with permission from Bohn and Federle [14]. Copyright 2004 by National Academy of Sciences).

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Fig. 2

Schematics of different wetting states on structured hydrophobic surfaces underwater. (a)–(c) are the CB, depinned metastable, and Wenzel states, respectively. Depinning of the contact line happens when the contact angle, θ, reaches an advancing one, θa. Here, h denotes the sagging depth.

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Fig. 3

(a) A series of photographs showing a droplet in millimeter scale compressed between two identical superhydrophobic surfaces with increasing pressure. (Reprinted with permission from Lafuma and Quéré [19]. Copyright 2003 by Nature Publishing Group.) (b) Two sequences of snapshots with a 15 ms time interval showing water droplets impacting on a microtextured hydrophobic surface with relatively lower (top panel) and higher (bottom panel) speeds, respectively. (Reprinted with permission from Bartolo et al. [20]. Copyright 2006 by EDP Sciences.) (c) Successive photographs of 5 s time interval showing an evaporating droplet sitting on a micropillar-patterned hydrophobic surface. (Reprinted with permission from Reyssat et al. [22]. Copyright 2008 by EPLA).

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Fig. 4

Wetting transition on structured hydrophobic surfaces with open cavities. (a) and (b) illustrate the depinning and sag transition mechanisms, respectively. (c) The experiment results of the critical impalement pressure as a function of the pillar height. Circles, squares, and triangles denote evaporation, compression, and droplet impact experiments, respectively. Solid and dashed lines are corresponding best fits. (Reprinted with permission from Bartolo et al. [20]. Copyright 2006 by EDP Sciences).

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Fig. 5

(a) Observation of wetting transition of a CB water droplet on a textured PDMS surface using RICM: the progressive profiles of the water interface with time. (Reprinted with permission from Moulinet and Bartolo [63]. Copyright 2007 by EDP Sciences, Societ Italiana di Fisica and Springer-Verlag.) (b) and (c) LSCM observation of CB-to-Wenzel transition: 3D images of a water droplet dyed with fluorescence on a pillar-patterned surface before (d) and after (e) the collapse of the meniscus. Artificial pillars are shown for better visualization. (Reprinted with permission from Papadopoulos et al. [65]. Copyright 2013 by National Academy of Sciences.) (d)–(g) A series of images from a bottom view showing the stepwise propagation of the liquid front during the transition to the Wenzel state (d), and the resultant square (f) and circularly (g) shaped wetted areas. (Reprinted with permission from Sbragaglia et al. [67]. Copyright 2007 by American Physical Society).

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Fig. 6

Comparison of nondimensionalized thermodynamic potential energies, Ω̃, as a function of liquid pressure, pL, normalized by the atmosphere pressure, p0, among the CB, metastable, and Wenzel states. Inset shows the surface patterned with pores. P1, the pressure at which the energies of the Wenzel and CB states become equal; P2 (or C), the critical depinning pressure. (Reprinted with permission from Xue et al. [28]. Copyright 2012 by American Chemical Society).

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Fig. 7

Pressure-induced wetting transition on structured hydrophobic surfaces submerged underwater. (a)–(c) Line-scanning LSCM images of the liquid–air interfaces 5 min after immersion under 4 kPa (a), 50 kPa (b), and 15 min after immersion under 50 kPa (c), respectively. (d) and (e) Plots of contact angle, θ, (d) and sagging depth, h, (e) as functions of applied overpressure Δp. Dots: experiments; solid and dashed lines: theoretical predictions with and without considering air diffusion. Here, 5 min is the time needed to settle the system before data acquisition. (Reprinted with permission from Lv et al. [88]. Copyright 2014 by American Physical Society).

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Fig. 8

Variation of filling fraction, ϕV, versus hydrostatic overpressure, ΔP, for the nanocavities with vertical (circles) and inclined (triangles) sidewalls. Solid and open symbols: intrusion and extrusion curves, respectively. Dashed lines: theoretical predictions. Inset: nanocavities with inclined sidewalls. (Reprinted with permission from Checco et al. [106]. Copyright 2014 by American Physical Society).

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Fig. 9

(a)–(c) Representative configurations of CB, transition, and Wenzel states extracted from molecular simulations, respectively; (d) comparison of grand potential and free energy profiles as a function of the filling fraction obtained by the continuum theory and the molecular simulations, respectively. (Reprinted with permission from Giacomello et al. [108]. Copyright 2012 by American Physical Society).

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Fig. 10

LSCM images of symmetric (a) and asymmetric (b) collapse of menisci inside hydrophobic micropores at the last stage of wetting transition. (Reprinted with permission from Lv et al. [109]. Copyright 2014 by American Chemical Society).

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Fig. 11

(a) A water boatman (Notonecta glauca) resting right beneath water. The silvery shine indicates the air film surrounding the insect body, and (b) SEM image of micro and nanoscale setae on the elytra of the water boatman: standing and bent microsetae over densely patterned nanosetae. (Reprinted with permission from Balmert et al. [115]. Copyright 2011 by Wiley-LISS, Inc.) (c) and (d) Schematics showing the typically bent hairs on the body surface of an aquatic insect (c) and the two-dimensional menisci supported on these parallel hairs (d). (Reprinted with permission from Flynn and and Bush [114]. Copyright 2008 by Cambridge University Press).

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Fig. 12

(a) Photographs showing the decay of entrapped air cavities with time on a micropore-patterned hydrophobic surface submerged underwater. (Reprinted with permission from Bobji et al. [25]. Copyright 2009 by American Chemical Society.) (b) The image series shows the reflection of immersed superhydrophobic samples at a depth of 126 cm for 30 min, 35 min, and 60 min, implying the decay of underwater superhydrophobicity with time, and (c) the cross section series obtained by confocal microscopy shows the plastron on a sample at fresh immersion, after immersion for 1 hr, and after extended laser scanning, respectively. The plastron decays from an air layer to individual bubbles. (Reprinted with permission from Poetes et al. [26]. Copyright 2010 by American Physical Society).

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Fig. 13

(a) Schematics of the gas-diffusion model, and (b) evolution of the depinned metastable state: normalized sagging depth, (h−h0)/H, as a function of normalized time, t/tD, in the metastable states. (Reprinted with permission from Lv et al. [88]. Copyright 2014 by American Physical Society).

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Fig. 14

Evolution of the pinned CB state: model prediction for the evolution of the contact angle, θ, with respect to time, t, under different pressures driven by air diffusion. The dotted line is the average advancing contact angle, θa, inside the cavity (∼120 deg). The close and open symbols are experimental data for 0 and 12 kPa, respectively. (Reprinted with permission from Lv et al. [88]. Copyright 2014 by American Physical Society).

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Fig. 15

Variation of the meniscus position at a hydrophobic trench over time. Under different immersion depth, H, two distinctive stability conditions are revealed. The meniscus position is defined as the distance from the top edge of the trench to the lowest position of meniscus. The dotted–dashed line denotes the maximum deflection of the meniscus while the meniscus remains pinned at the top edge. (Reprinted with permission from Xu et al. [129]. Copyright 2014 by American Physical Society).

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Fig. 16

(a) Oxygen concentration distribution over protruding bubbles of 43 deg in the flow channel, as represented by the lifetime field (in ns) resolved by FD-FLIM. Lg and Ls denote the widths of the groove and the grate, respectively. (b) Oxygen flux, JO2, as functions of axial x positions and Reynolds numbers, Re. Dots: experiments; Dashed and solid lines: statistical rate theory-based and equilibrium condition-based numerical results, respectively. (Reprinted with from Karatay et al. [138]. Copyright 2013 by The Royal Society of Chemistry).

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Fig. 17

ESEM image series showing the growth and merging of condensed microdroplets on structured hydrophobic surfaces. (Reprinted with permission from Jung and Bhushan [48]. Copyright 2007 by John Wiley and Sons).

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Fig. 18

(a) Side-view images of self-propelled jumping motion of coalesced droplets. The merged droplet propels itself upward into the air with a vertical velocity of 0.14 m/s. (Reprinted with permission from Boreyko and Chen [144]. Copyright 2009 by American Physical Society.) (b) Field emission SEM image of a CuO surface with side view, and observation of condensed droplets jumping away from a superhydrophobic nanostructured CuO tube (Inset: magnified view, scale bar: 500 μm). (Reprinted with permission from Miljkovic et al. [149]. Copyright 2012 by American Chemical Society.) (c) ESEM images of condensed droplet growth and departure on surfaces patterned with hierarchical nanograssed micropyramids. These circular condensed droplets frequently detach from the substrates, exposing fresh space for new drop formation (as indicated by the dashed circles). (Reprinted with permission from Chen et al. [150]. Copyright 2011 by Wiley-VCH Verlag GmbH & Co. KGaA).

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Fig. 19

(a) Variation of contact and rolling angles on superhydrophobic surfaces versus immersion time underwater. (Reprinted with permission from Boinovich et al. [156]. Copyright 2010 by American Chemical Society.) (b) Formation and morphology of microdroplets growing from the bottom of entrapped air cavities underwater, observed by confocal microscopy. Top panel: top view of microdroplets at initial stages (indicated by 1); Bottom panel: line-scanning images across the diameter of the micropore indicated by 2 in the top row. The growth of the condensed droplet (indicated by 3) eventually leads to the collapse of the suspended liquid–air interface by coalescence. (Reprinted with permission from Lv et al. [109]. Copyright 2014 by American Chemical Society).

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Fig. 20

(a) Typical example of a nano–micro hierarchical surface with microscale pillars fabricated through deep reactive etching and nanoscale roughness etched by XeF2 gas. (Reprinted with permission from Kwon et al. [161]. Copyright 2009 by American Chemical Society.) (b) Schematics showing how an N-level hierarchical structure to resist high liquid pressure. (Reprinted with permission from Su et al. [169]. Copyright 2010 by American Chemical Society.) (c) Schematics illustrating the pinning enhancement of the contact line by sublevel structures on the sidewalls with increased advancing contact angle. (Reprinted with permission from Xue et al. [28]. Copyright 2012 by American Chemical Society.) (d) Successive positions of meniscus pinned at the fins of length l and thickness d on the sidewalls. (Reprinted with permission from Hemeda et al. [174]. Copyright 2014 by AIP Publishing LLC).

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Fig. 21

Reentrant structures and wetting behaviors. (a) Schematics of a reentrant structure sustaining a heterogeneous wetting sate, with top surface wetted and lower surface nonwetted, and (b) SEM image of a surface with reentrant topology, i.e., microhoodoos with circular flat caps. (Reprinted with permission from Tuteja et al. [178]. Copyright 2007 by American Association for the Advancement of Science.) (c) SEM image of a nanostructured surface consisting of nanonails, and (d) pearl-like droplets of water and ethanol on the nanonail surface. (Reprinted with permission from Ahuja et al. [180]. Copyright 2008 by American Chemical Society.) (e) SEM images of a surface with doubly reentrant topology and a magnified bottom view of the nanooverhang structure on one post, and (f) apparent advancing (θA) and receding (θR) contact angles of different liquids measured on three liquid-repellent surfaces: doubly reentrant surface, (singly) reentrant surface, and cylindrical postpatterned surface of the same nominal solid fraction (∼5%). Their data are denoted by circles, triangles, and squares, respectively. (Reprinted with permission from Liu and Kim [183]. Copyright 2014 by American Association for the Advancement of Science).

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Fig. 22

(a) Schematics of the process to prepare a SLIPS: infusing a nano/microporous substrate with a highly wetted lubricant to form a stable and smooth liquid film, and (b) plot of the sliding angle of a decane droplet versus the nitrogen gas pressure in a chamber, showing negligible dependence on the gas pressure up to 676 atm. (Reprinted with permission from Wong et al. [159]. Copyright 2011 by Nature Publishing Group).

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Fig. 23

SEM (a) and TEM (b) images of the constituent nanoparticles in the paint. Sizes varied from 60 to 200 nm for the TiO2 nanoparticles. (c) and (d) One cycle of the sandpaper abrasion test. (Reprinted with permission from Lu et al. [217]. Copyright 2015 by American Association for the Advancement of Science).

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Fig. 24

(a) Schematics showing the self-healing mechanism of superhydrophobic surfaces: sufficient healing agents (e.g., fluoroalkylsilane) are stored within the porous surface, and the release of the preserved fluoroalkylsilane to the top surface where the fluoroalkylsilane layer has decomposed enables the restoration of the superhydrophobicity. (Reprinted with permission from Li et al. [227]. Copyright 2010 by Wiley-VCH Verlag GmbH & Co. KGaA.) (b) Schematics of a hierarchical alumina substrate providing reservoirs for healing agents (i.e., hydrophobic perfluorooctyl acid) to realize its self-healing capability by gradually releasing the agent. (Reprinted with permission from Wang et al. [234]. Copyright 2010 by Royal Society of Chemistry).

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Fig. 25

(a) A sequence of snapshots showing the droplet trampolining motion on a pillared superhydrophobic surface (SEM image, see inset), and (b) plot of droplet vertical position (y) versus time (t) during trampolining with the position of y = 0 indicated by the dotted lines in (a). (Reprinted with permission from Schutzius et al. [240]. Copyright 2015 by Nature Publishing Group).

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Fig. 26

(a)–(c) Photograph series showing the reversible wetting transitions on a nanostructured hydrophobic surface: droplets in an original superhydrophobic state (a), in a wetted state induced by applying a voltage of 35 V (b), and in a restored superhydrophobic state (c). (Reprinted with permission from Krupenkin et al. [241]. Copyright 2007 by American Chemical Society.) (d) and (e) Wettability dependence on the temperature of an intrinsically hydrophilic surface texture: at room temperature, the surface is superhydrophilic, and deposited droplets spread out immediately (d), and when the temperature is elevated to 160 °C, the droplet retains a superhydrophobic state, inset showing the composite droplet-substrate interface (e), and (f) schematics showing the principle of evaporation-induced superhydrophobicity: the substrate is heated from the bottom to induce continuous liquid evaporation to support the droplet with an overpressure. (Reprinted with permission from Adera et al. [242]. Copyright 2013 by Nature Publishing Group).

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Fig. 27

(a) SEM images of a microstructured hydrophobic surface with nanostructured bottom. Au electrode is coated and patterned on the bottom for electrolysis, and (b) a series of photographs showing the restoration of underwater superhydrophobicity by gas generation on the proposed surface shown in (a). (Reprinted with permission from Lee and Kim [243]. Copyright 2011 by American Physical Society.) (c) Schematics showing the reversible wetting transitions on a hierarchical superhydrophobic surface enabled by negative liquid pressure, and (d) SEM image of the hierarchical surface with microposts and nanofilament coating. (Reprinted with permission from Verho et al. [244]. Copyright 2012 by National Academy of Sciences).

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Fig. 28

Schematics of no-slip and slip boundary conditions at a solid–liquid interface

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Fig. 29

Dependence of liquid slippage on surface wettability. (a) Plot of slip length (b) versus static contact angle (θc) on different surfaces. (b) Plot of slip length (b) versus liquid depletion length (δ), inset illustrating the liquid depletion at a solid surface. Dots: simulation results; dashed line: scaling fit. (Reprinted with permission from Huang et al. [277]. Copyright 2008 by American Physical Society).

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Fig. 30

Schematics showing the principle of the regulation of superhydrophobic surfaces by temperature: (a) liquid adhesion is reduced with collapsed polymer chains when the temperature is above the LCST of pNIPAM, and (b) liquid adhesion is enhanced with stretched polymer chains when the temperature is below the LCST. (c) Variation of slip length with adhesive force on surfaces with different initiator/PFOTS ratios at two temperatures below and above the LCST (e.g., 20 °C and 50 °C). (Reprinted with permission from Wu et al. [284]. Copyright 2014 by American Chemical Society).

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Fig. 31

(a) Schematics showing the microflow channel with microridged hydrophobic bottom and smooth top, and (b) plot of flow velocity profiles in the microchannel obtained by micro-PIV measurements (dots) and numerical simulations (lines). Triangles indicate the profile over the ridge surface and squares indicate the profiles along the liquid–air interfaces. (Reprinted with permission from Ou and Rothstein [301]. Copyright 2005 by AIP Publishing LLC).

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Fig. 32

(a) SEM images of carbon nanotube forests with two different lateral roughness length scales (L), and (b) variation of slip length (b) versus L. Dots: experiments; lines, theoretical predictions. Circles indicate a superhydrophobic state. Squares indicate a wetted state. The triangle shows a reference value on a bare silicon surface. (Reprinted with permission from Joseph et al. [263]. Copyright 2006 by American Physical Society).

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Fig. 33

(a) and (b) SEM images of microstructured surfaces with posts and grates of constant pitch and different gas fractions as indicated, respectively, and (c) variation of slip length measured by a rheometer system versus gas fraction on the two kinds of surfaces. Dots: experiments; lines: theoretical predictions by Eq. (16). (Reprinted with permission from Lee et al. [265]. Copyright 2008 by American Physical Society).

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Fig. 34

(a)–(c) Schematics showing undeformed (a), deformed (b), and cylindrical (c) bubbles entrapped on structured surfaces within a unit cell. (Reprinted with permission from Hyväluoma and Harting [327]. Copyright 2008 by American Physical Society.) (d) Variation of slip length (B) versus meniscus protruding angle (θ). Dots: numerical results for CB states; dashed line, referenced value for a Wenzel state; inset: schematics of a protruding bubble. (Reprinted with permission from Steinberger et al. [316]. Copyright 2007 by Nature Publishing Group.) (e) and (f) Velocity measurements by micro-PIV: velocity field in a flow channel with entrapped bubbles of θ = 43 deg at the sidewall (e), and average velocity profiles for bubble protruding angles of θ = 43 deg and 21 deg, respectively (f). (Reprinted with permission from Karatay et al. [323]. Copyright 2013 by National Academy of Sciences).

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Fig. 35

Schematics showing the mechanisms of drag reduction with a streamwise slip boundary condition (a) and drag enhancement with a spanwise slip boundary condition. (Reprinted with permission from Min and Kim [329]. Copyright 2004 by AIP Publishing LLC).

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Fig. 36

Comparison of the population and strength of vortical structures on regular channels (a), (b) and superhydrophobic channels with microgrates (c), (d) at relatively lower (a), (c) and higher (b), (d) Reynolds numbers. (Reprinted with permission from Lee et al. [340]. Copyright 2015 by Springer Science+Business Media Dordrecht).

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Fig. 37

Variation of drag reduction rate versus Reynolds number for superhydrophobic channels with streamwise grates of fraction 50% and relatively small (open circles) and large (solid squares) spanwise dimensions. (Reprinted with permission from Daniello et al. [357]. Copyright 2009 by AIP Publishing LLC).

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Fig. 38

(a) Photograph of a combined sample with a partial superhydrophobic surface and a partial smooth surface for simultaneous comparison. The plastron state on superhydrophobic surface is easily detectable from the silvery shine, and (b) variation of normalized drag versus gas fraction on superhydrophobic surfaces with two different micrograte spacings. The triangle and the diamond are data from Refs. [347] and [357] for turbulent channel flows, respectively. (Reprinted with permission from Park et al. [352]. Copyright 2014 by Cambridge University Press).

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Fig. 39

(a) Schematics showing the turbulent Taylor–Couette flow setup with superhydrophobic coating (SEM image shown in (d), scale bar: 50 μm) on the inner rotor. The unconnected and connected configurations of the plastron on the coating are shown in (b) and (c), respectively. (e) Variation of the dimensionless slip length, b+ =  b/δν, as a function of a square root of the Reynolds number, Re. Dashed lines: linear fits for the last six data points. (Reprinted with permission from Srinivasan et al. [46]. Copyright 2015 by American Physical Society).

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Fig. 40

Instantaneous streamlines and vortex swirling strength (λci) contours of smooth (a) and superhydrophobic (b) surfaces at a friction Reynolds number of 990. (Reprinted with permission from Zhang et al. [348]. Copyright 2015 by Springer-Verlag).

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Fig. 41

Variation of threshold cavitation pressure (pm) versus the cavity radius (rc). Crosses: cavitation; circles: no cavitation; inset: zoom view. (Reprinted with permission from Borkent et al. [374]. Copyright 2009 by AIP Publishing LLC).

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Fig. 42

Confocal microscopic images of cavity morphology in different phases on a pore-patterned hydrophobic surface (top, scale bars: 10 μm), and quantitative comparison of experiment results (dots) with theoretical predictions (line segments) of the normalized cavity volume, V/VH, versus the negative liquid pressure, (pV−pL)/p0 (bottom). Phases (I), (II), (III), (IV), and (V) are observed at applied pressures elative to the atmospheric one of 35, 1, −9, −30, and −32 kPa, respectively. (Reprinted with permission from Xue et al. [379]. Copyright 2015 by AIP Publishing LLC).

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Fig. 43

Photographs showing the cooling process of heated steel spheres (200 °C) with rough superhydrophobic (a), (b) and smooth hydrophobic (c), (d) surfaces in a water pool with temperature of 100 °C. (Reprinted with permission from Vakarelski et al. [394]. Copyright 2012 by Nature Publishing Group).

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Fig. 44

Schematics of experiment setup (a) and photograph sequence showing the active control of pool boiling (b)–(j). The de-ionized water is dissolved with positively charged DTAB surfactant. The potentials of separate gold electrodes are switched between 2.0 V and 0.1 V to realize the on/off control of bubble formation. Scale bar: 1 cm. (Reprinted from Cho et al. [398]. Copyright 2015 by Nature Publishing Group).

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