Structured Superhydrophobic Surfaces Research Articles

Trifolium repens L.-Like Periodic Micronano Structured Superhydrophobic Surface with Ultralow Ice Adhesion for Efficient Anti-Icing/Deicing.

Wind turbine blades are often covered with ice and snow, which inevitably reduces their power generation efficiency and lifetime. Recently, a superhydrophobic surface has attracted widespread attention due to its potential values in anti-icing/deicing. However, the superhydrophobic surface can easily transition from Cassie-Baxter to Wenzel at low temperature, limiting its wide applications. Herein, inspired by the excellent water resistance and cold tolerance of Trifolium repens L. endowed by its micronano structure and low surface energy, a fresh structure was prepared by combining femtosecond laser processing technology and a boiling water treatment method. The prepared icephobic surface aluminum alloy (ISAl) mainly consists of a periodic microcrater array, nonuniform microclusters, and irregular nanosheets. This three-scale structure greatly promotes the stability of the Cassie-Baxter state. The critical Laplace pressure of ISAl is up to 1437 Pa, and the apparent water contact angle (CA) is higher than 150° at 0 °C. Those two factors contribute to its excellent anti-icing and deicing performances. The results show that the static icing delay time reaches 2577 s, and the ice adhesion strength is only 1.60 kPa. Furthermore, the anti-icing and deicing abilities of the proposed ISAl were examined under the environment of low temperature and high relative humidity to demonstrate its effectiveness. The dynamic anti-icing time of ISAl in extreme environments is up to 5 h, and ice can quickly fall with a speed of 34 r/min when it is in a horizontal rotational motion. Finally, ISAl has excellent reusability and mechanical durability, with the ice adhesion strength still being less than 6 kPa and the CA greater than 150° after 15 cycles of icing-deicing tests. The proposed structure would offer a promising strategy for the efficient anti-icing and deicing of wind turbine blades.

Multiscale Microflower Structured Superhydrophobic Surface via Electrostatic Air Spray

Multiscale Microflower Structured Superhydrophobic Surface via Electrostatic Air Spray

A pine needle-like superhydrophobic Zn/ZnO coating with excellent mechanochemical robustness and corrosion resistance

The superhydrophobic Zn-based coating with hierarchical structure has good prospects in metal anticorrosion. However, the practical application of superhydrophobic coating is still limited by the poor mechanical stability of micro/nano hierarchical structure. Herein, a robust superhydrophobic Zn/ZnO coating with pine needle-like structure and superior corrosion resistance was design and constructed. The micro-scale polyhedral Zn was fabricated on steel followed by the deposition of radially aligned ZnO nanorods, forming a novel pine needle-like hierarchical structure, which was finally modified by stearic acid. Notably, the pine needle-like structured Zn/ZnO (Zn/ZnO-3) coating exhibits excellent superhydrophobicity with contact angle of 166.8 ± 1.4° and sliding angle of 2.6 ± 0.5°. What's more, Zn/ZnO-3 coating maintains superhydrophobicity after suffering from strong acid/alkali, tape peeling, water drop impact and sandpaper abrasion tests. Compared with carbon steel and hydrophobic Zn coating, the corrosion current density of superhydrophobic Zn/ZnO-3 coating is decreased by approximately 4 and 3 orders of magnitude, respectively. The superior mechanochemical stability and anti-corrosion performance of superhydrophobic Zn/ZnO-3 coating is ascribed to its pine needle-like structure. Overall, this work provides a novel strategy to design hierarchical structured superhydrophobic surfaces with excellent mechanochemical stability and corrosion resistance, holding great prospects in metallic corrosion protection.

Design of dual-scale composite structured superhydrophobic surfaces for atmospheric corrosion prevention based on coalescence-induced droplet jumping

BackgroundThe coalescence-induced droplet self-jumping behavior on superhydrophobic surfaces (SHS) provides a new idea for atmospheric corrosion prevention. Although the influence on droplet self-jumping on different scales in single structure has been well understood, the mechanism of composite structures on droplet self-jumping, especially the process of atmospheric corrosion prevention is still unclear. MethodsIn this work, SHS that composite structured on micron-scale and nano-scale were designed on copper substrate at given salt solution concentration using hydrothermal method; and the difference of their droplet self-jumping behavior on surface was explored in energy perspective. Significant findingsResults showed that the nano-scale composite structured SHS was more conducive to droplet self-jumping for having smaller top layer structures and interfacial adhesion (Eint). Furthermore, the nano-scale composite structured SHS exhibited superior corrosion protection performance due to the wetting transition by droplet self-jumping. This study provides theoretical guidance for the development of corrosion prevention with composite structured SHS based on coalescence-induced droplet self-jumping behavior.

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

Design of Dual-Scale Composite Structured Superhydrophobic Surfaces for Atmospheric Corrosion Prevention Based on Coalescence-Induced Droplet Jumping

The effect of drop volume on the apparent contact angle of hierarchical structured superhydrophobic surfaces

If the apparent contact angle (ACA) of superhydrophobic materials could be significantly affected by drop volume, it would not be reliable to use ACA as an important index to evaluate the wettability of superhydrophobic materials. In order to explore the effect of drop volume on the ACA of hierarchical structured superhydrophobic surfaces, two kinds of hierarchical structured superhydrophobic surfaces, natural lotus leaf surface and superhydrophobic coating, were used for experimental analysis. In addition, a physical model of ACA considering drop volume was established. And the relationship curve between drop volume and ACA was calculated. The experiment and simulation results show that for each kind of drop, the volume has no significant effect on the ACA of hierarchical structured superhydrophobic surfaces. And the ACA can reflect the wettability of the superhydrophobic material effectively.

Droplet Directional Movement on the Homogeneously Structured Superhydrophobic Surface with the Gradient Non-Wettability.

The surface with the gradient non-wettability intensely appeals to researchers because of its academic significance and applications for directional droplet movement. Herein, we developed a homogeneous structure superhydrophobic surface with the gradient non-wettability by a combination strategy of chemical etching and vapor diffusion modification. As a consequence, the as-prepared surface exhibits a remarkable gradient characteristic of water repellency, and the water contact angle is mainly located within the range of 162 ± 0.5 to 149 ± 0.4°. Meanwhile, the sliding angle also exhibits a corresponding change from 3 to 11°. On this basis, the gradient characteristic of non-wettability induces the distinguishing droplet adhesion on the surface, that is, from 19 μN for the most hydrophobic end to 57 μN for the opposite one. Because of the difference of the water adhesion force, droplets on the as-prepared surface can well roll alongside a specific direction (i.e., gradient direction of non-wettability). In terms of dynamic impact droplets, they can rapidly rebound off the sample surface with the short contact time of 12.8 ms, and the finally fallen droplets mainly deviate toward weaker regions because of water repellency. To analyze this phenomenon, it is found that the asymmetric mechanic behavior is mainly caused by the unbalanced retraction force between the both ends of the impact droplet. This work provides a novel strategy to construct the homogeneous structure superhydrophobic surface with the gradient non-wettability for the applications in the droplet movement control or transport.

Research on sliding angles of water droplets on the hierarchical structured superhydrophobic surfaces

Certain plant leaves, such as the lotus leaf, are known to be superhydrophobic due to the hierarchical structures on the leaf surfaces. In this paper, two kinds of superhydrophobic plants leaves with hierarchical structured surface, including aged lotus and loropetalum chinense leaves, were introduced. Further, the surface structural models were established for the introduced leaves corresponding to their surface micromorphologies. More importantly, the theoretical modes for predicting sliding angles of liquid droplets on the introduced leaf surfaces were formulated. In addition, the role of surface parameters in determining the superhydrophobicity of hierarchical structured leaves was demonstrated effective. The results of this paper could be used as a guidance for designing desired superhydrophobic property of hierarchical surfaces.

Synergistic Superhydrophobic and Photodynamic Cotton Textiles with Remarkable Antibacterial Activities.

Pathogenic bacterial contamination is at the root of many persistent and chronic bacterial infections. Synergistic superhydrophobic surfaces functionalized with a cationic antimicrobial agent (e.g., quaternary ammonium components) show promising antimicrobial efficacies, but are inherently contradictory in simultaneously maintaining excellent surface liquid repellency and bactericidal activity due to the compromised low surface energy stemming from the introduced hydrophilic biocides. Herein, we present a synergistic antibacterial cotton textile with superhydrophobic bacterial repellency and photodynamic bactericidal activity for bothStaphylococcus aureus and Escherichia coli. The modified cotton textile was constructed by integrating tunable micro/nanoscale roughness, hydrophobic photosensitizer chlorin e6, and surface perfluorination. The triple-scale structured superhydrophobic surfaces exhibited ≲90% reductions in both waterborne and airborne bacterial adhesions. Subsequently, after being exposed to visible light for 45 min, the synergistic surface demonstrated complete inactivation (100% killing) against residual bacterial cells via photodynamic bactericidal capacities. Moreover, the triple-scale structured superhydrophobic surface displayed totally suppressed whole blood adhesion, suggesting potential in preventing plenty of undesirable biomedical forms of contamination. This synergistically antibacterial surface not only improves the antibacterial efficiency but also leads to long-lasting antimicrobial performance, each of which is highly desirable in combating bacterial infections.

Influence of micro-structured superhydrophobic surfaces on nucleation and natural convection in a heated pool

This work experimentally explores sub-boiling pool nucleation on micro-structured superhydrophobic surfaces. All surfaces tested were submerged in a 20 mm deep pool of water and heated from below to maintain a constant surface temperature, while the side walls of the pool were insulated, and the top was covered. Three thermocouples positioned in the pool obtain the average pool temperature. A heat flux sensor is placed directly beneath the surface to measure the heat flux supplied to the pool. Free convection heat transfer coefficients are obtained for the sub-boiling temperature range of 40–90 °C. Six surface types are studied: smooth hydrophilic, smooth hydrophobic, superhydrophobic with rib/cavity structures, superhydrophobic with rib/cavity structures and additional sparsely spaced ribs to close off the cavities, circular posts, and circular holes. It is found that structured superhydrophobic surfaces provide cavities for nucleation to occur. More dissolved air effervesces from the water as the surface temperature increases due to an increased level of supersaturation and convection. The nucleation leads to large air bubble formations that reduce the overall convection coefficient when compared to the smooth surfaces. For the rib/cavity structured surfaces, the bubbles form in an anisotropic manner and are aligned with the surface structure. More bubbles are observed on the superhydrophobic surfaces where the cavities are bounded. Since water’s ability to dissolve air is dependent on temperature, heat and mass transfer cannot be treated independently on any of the superhydrophobic surfaces studied here.

Re-entrant Cavities Enhance Resilience to the Cassie-to-Wenzel State Transition on Superhydrophobic Surfaces during Electrowetting.

Electrowetting-based droplet actuation has applications in digital microfluidics. Mobility of droplets on surfaces can be enhanced using structured superhydrophobic surfaces that offer inherently low adhesion to droplets in the Cassie state. However, these surfaces must be designed to prevent transition to the Wenzel state (in which droplets are immobile) at high electrowetting actuation voltages. The electrowetting behavior of cylindrical microposts and mushroom-shaped re-entrant microstructures, both of which afford excellent superhydrophobicity, is investigated and compared. A surface-energy-based model is employed to estimate the energy barrier for the Cassie-to-Wenzel transition and thus the electrowetting voltage required to initiate this transition. The mushroom structures are predicted to be more resilient to transition (i.e., transition occurs at a voltage that is up to 1.5 times higher) than microposts. Both types of microstructured surfaces are fabricated and electrowetting experiments performed to demonstrate that mushroom structures indeed inhibit the Cassie-to-Wenzel transition at voltages that induce such transition on the cylindrical microposts.

A comparative analysis of the effective and local slip lengths for liquid flows over a trapped nanobubble

The gas–liquid interfaces distributed on a superhydrophobic (SHP) surface promote the effective slip and might result in significant drag reduction desirable in many applications. While the slippage of water past gas–liquid interfaces on structured SHP surfaces has attracted wide attention, the slip behavior at gas–liquid interfaces trapped by the wettability step still remains unclear. Using molecular dynamics simulations, we first demonstrated that the three-phase contact line can be pinned on a smooth substrate of mixed wettability. We then numerically investigated slip flows over smooth surfaces with flattened gas bubbles trapped by the wettability step. It was found that the local slip length is relatively large at the gas–liquid interface and its spatial distribution becomes asymmetric due to shear-induced deformation of the attached bubble, while the effective slip length remains nearly constant. With increasing gas areal fraction, the local and effective slip lengths become larger, especially in the case of a stripe-like continuous gas–liquid interface where the interface curvature in the flow direction is absent.

A study about the influence of single-scale and dual-scale structures on surface wettability

Surface morphology is known as the key factor to obtain lyophobic surface. This paper illustrates two superhydrophobic surfaces with different surface morphologies including single-scale nanorod structure and dual-scale flower-like structure. We compared contact angles of many liquids and dynamic behavior of water droplet impinging on the nanorod structured surface to those of on the flower-like structured surface. It was found that both the single-scale nanorod structure and the dual-scale flower-like structure can achieve superhydrophobicity. However, the dual-scale flower-like structure is superior to the single-scale nanorod structure in terms of repelling the droplet with low surface tension. In addition, we investigated stability and corrosion resistance of these two superhydrophobic surfaces, showing that both the single-scale nanorod and the dual-scale flower-like structured superhydrophobic surfaces had excellent long-term stability and thermal stability. Nevertheless, the dual-scale flower-like structured superhydrophobic surface was more stable under outside vibration and had better corrosion resistance than the single-scale nanorod structured superhydrophobic surface.

A 3-D model for thermodynamic analysis of hierarchical structured superhydrophobic surfaces

Lotus leaves and rose petals are the two typical natural examples with superhydrophobicity, also known as the lotus effect and petal effect, respectively. It is generally recognized that the hierarchical structures on their surfaces are responsible for the lotus effect and petal effect. However, the relations between surface geometries and the corresponding wetting states have not been understood completely. In this paper, a 3-D model is built to thermodynamically analyze the wetting behavior of hierarchical structured surfaces. The free energy (FE) and free energy barrier (FEB) of different wetting states are analyzed in detail. Furthermore, equilibrium contact angle (CA), receding and advancing CAs, as well as contact angle hysteresis (CAH) can be predicted by this approach. More importantly, the critical Nano pillar height used for characterizing the transitions between the two wetting states is clearly defined in terms of geometric parameters of solid surfaces, and the final stable wetting state dominated by surface roughness and chemical composition can be determined based on the obtained results. The approach presented in this study can also be used as a guidance for fabricating superhydrophobic surfaces.

Facile fabrication of hierarchical structured superhydrophobic surface and its ultra dynamic water repellency

Here we presented an ultra water-repellent surface with the ternary composite structures, expecting to provide an inspiration to the application research. Our strategy is to design macro ridge structures on the superhydrophobic surface, which is prepared by modifying the composite micro-nanoscale hierarchical structures with FAS-17. The effect of micro-nanoscale hierarchical structures on the non-wettability were firstly analyzed and examined for achieving a perfect unity of microstructures and property. On this basis, we designed the macro ridge structures and obtained the final sample surface, which could not only display a strong superhydrophobicity with the apparent contact angle reaching 161° and the contact angle hysteresis reducing to 1.5°, but also exhibit the capacity to compel the impact water droplet to bounce off within an extreme short time (5.5ms).

Verifying the deicing capacity of superhydrophobic anti-icing surfaces based on wind and thermal fields

Abstract Based on the high icing-delay performance and low ice adhesion strength, the anti-icing potential of superhydrophobic surfaces has been well investigated over the past years. The aim of this work was to verify the deicing capacity of superhydrophobic surfaces under the conditions of wind and thermal fields, expecting to promote its engineering applications. We took various wetting surfaces ranging from hydrophilic to superhydrophobic as research objects, and discussed the deicing properties of these sample surfaces based on the wind and thermal fields. The results indicated that under the condition of wind field, the micro-nanoscale hierarchical structured superhydrophobic surface exhibited a remarkable deicing property (no matter what the freezing temperature was), due to the time required for blowing away ice on the surface being least. However, the specific micro-nanoscale hierarchical structures on the superhydrophobic surface also induced a poorly melting ice property owing to the lower heat transfer efficiency caused by trapped air pockets. Furthermore, the superhydrophobic surface displayed a robust durability under the condition of 30 icing/deicing cycles. Therefore, we believe that the wind field deicing is an ideal choice as the assistant deicing approach considered in the practical applications of the anti-icing superhydrophobic surfaces.

Biomimetic one step fabrication of manganese stearate superhydrophobic surface as an efficient barrier against marine corrosion and Chlorella vulgaris-induced biofouling

Marine corrosion and biofouling on metallic materials are sticky problems. Herein, we prepared hierarchical structured superhydrophobic (SHPB) surfaces via a versatile one step electrodeposition approach. The synthetic SHPB surface was employed as an efficient barrier against corrosion and Chlorella vulgaris-induced biofouling. The surface morphology and chemical compositions were analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrophotometer (FTIR) and Energy-dispersive spectrum (EDS). Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and immersion test in Chlorella vulgaris-inoculated culture medium were carried out to evaluate the anti-corrosion and anti-biofouling performance of the obtained SHPB surface. The results demonstrated that the synthetic SHPB surface exhibits great enhanced corrosion resistance and biofouling mitigation. Moreover, the as-fabricated SHPB surfaces retain superhydrophobicity in wicked environment such as strong acid and alkali conditions, showing good chemical stability. We believe that the SHPB surfaces over metallic substrates provide a potential and worthful strategy for marine corrosion and biofouling.

On jet impingement and thin film breakup on a horizontal superhydrophobic surface

When a vertical laminar jet impinges on a horizontal surface, it will spread out in a thin film. If the surface is hydrophobic and a downstream depth is not maintained, the film will radially expand until it breaks up into filaments or droplets. We present the first analysis and model that describes the location of this transition for both isotropic and anisotropic structured superhydrophobic (SH) surfaces. All surfaces explored are hydrophobic or SH, where the SH surfaces exhibit an apparent slip at the plane of the surface due to a shear free condition above the air filled cavities between the structures. The influence of apparent slip on the entire flow field is significant and yields behavior that deviates notably from classical behavior for a smooth hydrophilic surface where a hydraulic jump would form. Instead, break up into droplets occurs where the jet’s outward radial momentum is balanced by the inward surface tension force of the advancing film. For hydrophobic surfaces, or SH surfaces with random micropatterning, the apparent slip on the surface is uniform in all directions and droplet breakup occurs in a circular pattern. When alternating rib/cavity microstructures are used to create the SH surface, the apparent slip varies as a function of the azimuthal coordinate, and thus, the breakup location is elliptically shaped. The thin film dynamics are modeled by a radial momentum analysis for a given jet Weber number and specified slip length and the location of breakup for multiple surfaces over a range of jet Weber numbers and realistic slip length values is quantified. The results of the analysis show that the breakup radius increases with increasing Weber number and slip length. The eccentricity of the breakup ellipse for the rib/cavity SH structures increases with increasing Weber number and slip length as well. A generalized model that allows prediction of the transition (break-up) location as a function of all influencing parameters is presented. Model results are compared to experimental measurements with very good agreement.

Effect of hierarchical structured superhydrophobic surfaces on coherent structures in turbulent channel flow

In this study, the coherent structures, which were effected by superhydrophobic (SH) surfaces in turbulent channel flow, were investigated by time-resolved particle image velocimetry (TRPIV). Hierarchical structured SH surfaces which have micro-nano secondary structures were applied in the experiment. Drag reduction rate is approximately 11% at Reθ≅1300, which is based on momentum thickness. Coherent structures (low-speed streak, hairpin vortex and hairpin vortex packets) with scale information were obtained by conditional sampling and averaging method. Compared to the hydrophilic (PH) surfaces, the conditional averaged hairpin vortex structures of SH surfaces have a lager spacing of vortex legs, a smaller angle of inclination along streamwise and spanwise directions. Accordingly, a smaller-scale hairpin vortex packet with three swirling motions was emerged. Through the comparative research on angle of inclination, the lifting movement of vortex in wall-normal direction was also weaker than PH case. These shows that the existence of SH surfaces restricts the development of coherent structures and induces drag reduction effects.