Significant and stable drag reduction with air rings confined by alternated superhydrophobic and hydrophilic strips

Hu, Haibao; Wen, Jun; Bao, Luyao; Jia, Laibing; Song, Dong; Song, Baowei; Pan, Guang; Scaraggi, Michele; Dini, Daniele; Xue, Qunji; Zhou, Feng

doi:10.1126/sciadv.1603288

Cited by 142 publications

(94 citation statements)

References 55 publications

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“…A series of measurements have been performed to explore the details of the boundary layer near the superhydrophobic surfaces and the slip velocity at the micro air pockets within the surface asperities has been successfully measured using particle image velocimetry (micro-PIV). Significant slip velocity has been directly observed at the superhydrophobic surfaces and the slippage will reduce the shear stress, turbulence production, or strength and patterns of turbulent vortex [1,[11][12][13][14][15][16][17][18].…”

Section: Introductionmentioning

confidence: 99%

Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Song

et al. 2018

Phys. Rev. Fluids

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Superhydrophobic surfaces have been shown to produce significant drag reduction in both laminar and turbulent flows by introducing an apparent slip velocity along an air-water interface trapped within the surface roughness. In the experiments presented within this study, we demonstrate the existence of a surface tension gradient associated with the resultant Marangoni flow along an air-water interface that causes the slip velocity and slip length to be significantly reduced. In this study, the slip velocity along a millimeter-sized air-water interface was investigated experimentally. This large-scale air-water interface facilitated detailed investigation of the interfacial velocity profiles as the flow rate, interfacial curvature and interface geometry were varied. For the air-water interfaces supported above continuous grooves (concentric rings within a torsional shear flow) where no surface tension gradient exists, slip velocity as high as 30% of the bulk velocity was observed. However, for the air-water interfaces supported above discontinuous grooves (rectangular channels in a Poiseuille flow), the presence of a surface tension gradient reduced the slip velocity and in some cases resulted in an interfacial velocity that was opposite to the main flow direction. The curvature of the air-water interface in the spanwise direction was found to dictate the details of the interfacial flow profile with reverse flow in the center of the interface for concave surfaces and along the outside of the interface for convex surfaces. The deflection of the air-water interface was also found to greatly affect the magnitude of the slip. Numerical simulations imposed with a relatively small surface tension gradient along air-water interface were able to predict both the reduced slip velocity and back flow along the air-water interface.

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Section: Introductionmentioning

confidence: 99%

Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Song

et al. 2018

Phys. Rev. Fluids

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“…Interfaces 2019, 6,1901033 temperature with 60% relative humidity, were deposited at a volume rate of 100 µL min −1 . Mater.…”

Section: Wwwadvmatinterfacesdementioning

confidence: 99%

“…Interfaces 2019,6, 1901033 We hypothesize that the CAs along the solid lines are the same at stage I and stage II ( rs I θ = rs II θ ) based on the experimental observation.…”

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confidence: 92%

“…Droplet manipulation is a ubiquitous phenomenon in various natural processes and biological processes such as selfcleaning, [1][2][3][4] drag reduction, [3,[5][6][7][8] and microfluidics. [9][10][11][12][13] As one of the key aspects for droplet manipulation, [7,[14][15][16][17][18] the directional transportation of liquid droplets has aroused great interest due to its importance in applications including water collecting, [19][20][21] anti-icing [16,22,23] and materials transportation.…”

Section: Introductionmentioning

confidence: 99%

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Steerable Droplet Bouncing for Precise Materials Transportation

Zhao

et al. 2019

Adv Materials Inter

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the spider silk and the peristome surface of Nepenthes alata are used for directional water collection, which are induced by the surface energy gradient and the differences in Laplace pressure. [27,28] Learning from nature, directional water collection system is found in cactus spine structure, which is the result of the Laplace pressure gradients and the multi-functions integration. [29] By introducing anisotropic microstructures, guided self-propelled leaping of droplet is realized on the superhydrophobic surface. [22] The dominating factor for the droplet directional movement is to induce the asymmetric forces to the droplet. [5,22,[30][31][32] However, due to the complexity in the interaction between the droplet and the solid, [33][34][35][36][37][38] the precise control of the lateral droplet movement, both in the direction and the quantity, remains a great challenge.Recently, the development of the surface wettability provides an efficient way to control the liquid behaviors. [19,39,40] Through rational design of the surface wettability pattern, we report a facile method to realize the directional transportation of rebounding droplets with precise controllability. The lateral movement of the droplet originates from the asymmetric adhesion force that accumulates during the droplet receding process at the interface. We reveal that the droplet lateral momentum shows positive correlation to the area of a geometrical region that depends on the coupling location of the droplet impacting center and the wettability pattern. The findings help us get further understanding of basic droplet impact process at the interface and offer a promising strategy to accurately control the droplet behaviors, which shows great potential in functional materials transportation, microfluidics, heat-transfer, etc. Figure 1a shows the schematic illustration of the steerable droplet transportation by impacting on the wettability patterned surface, which consists of a superhydrophilic stripe on the superhydrophobic surface (see Figure S1, Supporting Information). The droplet impact center is set as coordinate Directional transportation of liquid droplets plays a significant role in various processes including anti-fogging, anti-icing, and materials transportation. Diverse strategies have been developed to achieve lateral bouncing of impacting droplets. However, due to the complexity of the interactions on the interface between a droplet and the solid, quantitatively manipulating the directional movement of the droplet still remains challenging. Here, it is proposed that the directional transportation of a droplet with precise controllability can be achieved by impacting it on a heterogenously wettable surface. It is found that the droplet lateral momentum correlates with the surface area of a geometric region that depends on the position-coupling between the droplet maximum spreading and the wettability pattern. The well-controlled droplet directional movement has generality for different Weber numbers and diverse superhydrophilic pattern...

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“…will be transformed into elastic potential energy of the microsized structure. In this work, 3D profile measurement reveals that the average length of the individual microsized structure is ≈32 µm which is much higher than that of the normal hierarchical rough structure for constructing superhydrophobic coatings It is also noting that the coral‐reef‐like structure could greatly improve the flexibility of the microsized structure. Each of the tentacle on the coral‐reef‐like structure could act as an individual cantilever and consequently, every individual coral‐reef‐like structure could be compared to a laminated cantilever beam such as the leaf spring used on vehicles .…”

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confidence: 99%

Bioinspired Mechanically Robust Metal‐Based Water Repellent Surface Enabled by Scalable Construction of a Flexible Coral‐Reef‐Like Architecture

Luo

2019

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Mechanical robustness is a central concern for moving artificial superhydrophobic surfaces to application practices. It is believed that bulk hydrophilic materials cannot be use to construct micro/nanoarchitectures for superhydrophobicity since abrasion‐induced exposure of hydrophilic surfaces leads to remarkable degradation of water repellency. To address this challenge, the robust mechanical durability of a superhydrophobic surface with metal (hydrophilic) textures, through scalable construction of a flexible coral‐reef‐like hierarchical architecture on various substrates including metals, glasses, and ceramics, is demonstrated. Discontinuous coral‐reef‐like Cu architecture is built by solid‐state spraying commercial electrolytic Cu particles (15–65 µm) at supersonic particle velocities. Subsequent flame oxidation is applied to introduce a porous hard surface oxide layer. Owing to the unique combination of the flexible coral‐reef‐like architecture and self‐similar manner of the fluorinated hard oxide surface layer, the coating surface retains its water repellency with an extremely low roll‐off angle (<2°) after cyclic sand‐paper abrasion, mechanical bending, sand‐grit erosion, knife‐scratching, and heavy loading of simulated acid rain droplets. Strong adhesion to glass, ceramics, and metals up to 34 MPa can be achieved without using adhesive. The results show that the present superhydrophobic coating can have wide outdoor applications for self‐cleaning and corrosion protection of metal parts.

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Significant and stable drag reduction with air rings confined by alternated superhydrophobic and hydrophilic strips

Abstract: Drag reduction of up to 77.2% is achieved with air rings confined by alternating superhydrophobic and hydrophilic strips.

Cited by 142 publications

References 55 publications

Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Effect of a surface tension gradient on the slip flow along a superhydrophobic air-water interface

Steerable Droplet Bouncing for Precise Materials Transportation

Bioinspired Mechanically Robust Metal‐Based Water Repellent Surface Enabled by Scalable Construction of a Flexible Coral‐Reef‐Like Architecture

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