Hierarchical Superhydrophobic Surfaces Research Articles

The influence of submillimeter morphological variations on the wettability of WEDM-fabricated dual-scale roughness aluminum alloy 6082 surfaces

Deriving inspiration from natural hierarchical superhydrophobic surfaces, multi-scale structures were manufactured on AA6082 surfaces via wire electrical discharge machining (WEDM), featuring microscale texture due to spark erosion, superimposed upon a wide-range simple and more complicated geometries of submillimeter profiles. The effect that the higher-order scale morphologies had on wettability was investigated. The dual-scale morphology elevated the hydrophobicity of the surfaces compared to single-scale or unmodified surfaces, reaching superhydrophobicity (151°) in the case of a certain triangular profile. Rectangular and triangular profiles facilitated the higher contact angles, while re-entrant geometries were able to totally prevent cavity wetting. A correlation of static contact angle with roughness parameters of the larger scale such as Ra, Rz, Rp, Rsk, and Rku for certain geometry configurations was identified. Peak hydrophobicity resulted at Ra = 70 μm, Rz = 240 μm, and Rp = 160 μm concerning simple geometries. Negative Rsk and Rku > 1.5 affected negatively contact angle of samples. All investigated tested types were found to reach higher hydrophobicity at moderate drop volumes (5 μl). The fabricated samples were anisotropic in at least two directions, showing decreased hydrophobicity in the front, parallel to the groove direction. When tested in multi-directional dynamic tilting up to 90°, the more complicated geometries were able to retain resistance to spreading. All samples demonstrated superliquiphilicity with lower surface tension liquids, making them strong candidate in applications such as oil/water separation. Finally, all samples tested sustained their hydrophobic character subsequent to a 3-month atmospheric exposure period.

Experimental and theoretical study of hierarchical NiCo-TiO2 nanocomposite superhydrophobic surface for improving corrosion resistance

The superhydrophobicity of surfaces is a critical characteristic to prevent the corrosion problem, as a significant challenge under corrosive environments. In the present study, the nickel-cobalt (NC) alloy micro-nano hierarchical structure, along with the insertion of TiO2 nanoparticles (NCT) prepared by the economical and facile electrodeposition and myristic acid (MA) fatty acid physically adsorbed on the surface of NCT to reduce surface energy. The stability improvement of the prepared micro-nano scale composite due to the presence of TiO2 nanoparticles preserves the superhydrophobic feature created by the hierarchical structure and the fatty acid placed on the surface. The surface morphology and roughness, and the crystal structure of electrochemical deposited films were investigated using FE-SEM, AFM and XRD techniques, respectively. Also, mechanical and chemical stability, wetting behavior, and anti-corrosion properties were evaluated. The contact angle of 153° for the optimized Christmas tree-like nanocomposite (MA/NCT-2) implies the superhydrophobic property of the prepared coating. The excellent stability of MA/NCT-2 under corrosive different conditions, along with its supreme corrosion resistance confirm the potential of MA/NCT-2 for corrosion protection. Moreover, molecular dynamic (MD) simulations were performed in this first trial to determine wettability, intrinsic propagation mechanism, and structural evolution of water nanodroplets on copper (Cu) substrate and various coatings. The pristine substrate and NCT coating electrodeposited on the surface of Cu (NCT@Cu) were used to model the composite surfaces coated with MA. The last part concerned the study of the contact of this protective material with those surfaces and its possible improved performance as a superhydrophobic coating.

An energy-free strategy to elevate anti-icing performance of superhydrophobic materials through interfacial airflow manipulation

Superhydrophobic surfaces demonstrate excellent anti-icing performance under static conditions. However, they show a marked decrease in icing time under real flight conditions. Here we develop an anti-icing strategy using ubiquitous wind field to improve the anti-icing efficiency of superhydrophobic surfaces during flight. We find that the icing mass on hierarchical superhydrophobic surface with a microstructure angle of 30° is at least 40% lower than that on the conventional superhydrophobic plate, which is attributed to the combined effects of microdroplet flow upwelling induced by interfacial airflow and microdroplet ejection driven by superhydrophobic characteristic. Meanwhile, the disordered arrangement of water molecules induced by the specific 30° angle also raises the energy barriers required for nucleation, resulting in an inhibition of the nucleation process. This strategy of microdroplet movement manipulation induced by interfacial airflow is expected to break through the anti-icing limitation of conventional superhydrophobic materials in service conditions and can further reduce the risk of icing on the aircraft surface.

The thermodynamic stability of the Cassie–Baxter regime determined by the geometric parameters of hierarchical superhydrophobic surfaces

Understanding the geometric parameters of hierarchical superhydrophobic surfaces and their impact on the thermodynamic stability of the Cassie-Baxter regime are invaluable for surface wettability–related applications. Herein, we fabricated hierarchical micro-microstructured silicone rubber surfaces having superhydrophobic properties via an industrially applicable direct replication method. The mold inserts were fabricated using photochemical milling, laser ablation, and wet chemical etching to create different hierarchical levels. We calculated surface roughness ratios and equilibrium contact angles and considered the contribution of sub-microstructures in the wettability properties via physical and statistical analyses. Comparing the calculated theoretical wettability properties and experimental measurements revealed a good agreement among all samples because of the accurate predictions of the governing wetting regime. It was worthwhile insights into the design and fabrication of superhydrophobic structured surfaces. The presence of superimposed sub-microstructures produced desirable water-repellency properties because of the reduced solid–liquid contact area as low as 0.086. Given the primary importance of the Cassie–Wenzel wetting transition on the design and fabrication of superhydrophobic surfaces, we evaluated the thermodynamic persistence of the Cassie–Baxter regime by analyzing the energy barrier to be overcome by droplets with various volumes. Finally, we discuss the contribution of the dimensional parameters of microstructures on the stability of the Cassie–Baxter regime.

A Multiscale Strategy for Constructing Finned Microporous Superhydrophobic Surface for Highly‐Efficient Condensation Heat Transfer Enhancement Enabled by Lifespan Regulation of Droplets

AbstractSpontaneous droplet jumping on micro‐/nano‐structured superhydrophobic surfaces has been exploited as an efficient means for enhancing steam condensation heat transfer. However, the good performance of such surfaces quickly decays with raising the degree of subcooling, due to the mismatch between the characteristic length scales and droplet sizes when they grow up. Herein, a novel strategy for multiscale droplet regulation is proposed by combining sub‐millimeter fin structure with a hierarchical microporous superhydrophobic surface. A superior condensation heat transfer performance is attained on such hierarchical superhydrophobic finned tube (F‐SHB), in comparison to the baseline case of superhydrophobic non‐finned (SHB) tube under well‐controlled test conditions. Although the droplet jumping is not as vigorous as that on the SHB tube, the finned geometry of the F‐SHB tube leads to a condensation heat transfer enhancement even under high degrees of subcooling up to 36 K, because of the accelerated departure of large droplets by imposing Laplace force gradient in the presence of V‐shaped sub‐millimeter fins. This multiscale enhancement strategy is shown to enable a cascading regulation over the entire lifespan of condensate droplets. The fabrication of F‐SHB tubes is facile and easy to be scaled up, showing great potential in practical steam condensation applications.

Dropwise Condensate Comb for Enhanced Heat Transfer.

Dropwise condensation on superhydrophobic surfaces could potentially enhance heat transfer by droplet spontaneous departure via coalescence-induced jumping. However, an uncontrolled droplet size could lead to a significant reduction of heat transfer by condensation, due to large droplets that resulted in a flooding phenomenon on the surface. Here, we introduced a dropwise condensate comb, which consisted of U-shaped protruding hydrophilic stripes and hierarchical micro-nanostructured superhydrophobic background, for a better control of condensation droplet size and departure processes. The dropwise condensate comb with a wettability-contrast surface structure induced droplet removal by flank contact rather than three-phase line contact. We showed that dropwise condensation in this structure could be controlled by designing the width of the superhydrophobic region and height of the protruding hydrophilic stripes. In comparison with a superhydrophobic surface, the average droplet radius was decreased to 12 μm, and the maximum droplet departure radius was decreased to 189 μm by a dropwise condensate comb with 500 μm width of a superhydrophobic region and 258 μm height of a protruding hydrophilic stripe. By controlling the droplet size and departure on hierarchical micro-nanostructured superhydrophobic surfaces, it was experimentally demonstrated that both the heat transfer coefficient and heat flux could be enhanced significantly. Moreover, the dropwise condensate comb showed a maximum heat transfer coefficient of 379 kW m-2 K-1 at a low subcooling temperature, which was 85% higher than that of a superhydrophobic surface, and it showed 113% improvement of high heat flux or heat transfer coefficient when it was compared with that of the hierarchical micro-nanostructured superhydrophobic surface at a high subcooling temperature of ∼10.6 K. This work could potentially transform the design and fabrication space for high-performance heat transfer devices by spatial control of condensation droplet size and departure processes.

Hierarchical microporous superhydrophobic surfaces with nanostructures enhancing vapor condensation heat transfer

A new hierarchical superhydrophobic surface with micropores and nanoblades (MN SHS) is proposed to strengthen the jumping ability of droplets and maintain the high-efficiency droplet jumping condensation at the larger subcoolings. The results demonstrate that the jumping frequency and the proportion of multidroplets merging (n ≥ 8) on the MN SHS are higher than those on the nanostructured superhydrophobic surfaces (Nano SHS). Moreover, the maximum diameter of condensate droplets is only 150 μm, which is approximately 50% smaller compared to the Nano SHS. Spontaneous jumping of condensate droplets is observed on the 40PPI MN SHS (PPI: the average number of pores per inch.) at a subcooling of 16 K; while the droplet jumping condensation occurs at a maximum of 5 K on the Nano SHS. As the subcooling ranges from 5 K to 16 K, the heat transfer coefficient of MN SHS is increased by at least 26.4% compared to the Nano SHS. In addition, lattice Boltzmann model is adopted to simulate the self-propelled jumping behaviors on different structured surfaces. The energy conversion efficiency of drops on the MN SHS is higher compared to that on the Nano SHS. This work will contribute to the potential development of dropwise condensation in energy utilization, petrochemical engineering, thermoelectric cooling and aerospace thermal management systems.

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.

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Fabrication of superhydrophobic coatings by low-temperature sintering of Ag nanoparticle paste

Ag nanoparticle paste was used to fabricate mechanically durable superhydrophobic surfaces via sintering. A facile method was first adopted to synthesize the Ag nanoparticle paste. To obtain multi-level hierarchical superhydrophobic surfaces, a stainless steel mesh was introduced to the sintering process of the Ag nanoparticle paste. The morphology of the steel mesh was replicated onto the Ag surface under sintering pressure, forming a first-level surface roughness. Using low-temperature sintering, a bond was created between the Ag nanoparticles, enhancing the mechanical durability of the as-prepared surfaces. The gap between the Ag nanoparticles was retained under low pressure, leading to a high surface roughness, which is beneficial for preparing superhydrophobic surfaces. After modification with stearic acid, the as-fabricated Ag coatings exhibited good superhydrophobicity, long-term durability, and mechanical stability. The superhydrophobic Ag coatings can also be fabricated on different substrates successfully. Thus, this type of superhydrophobic coating has promising prospects in practical applications.

Second-Level Microgroove Convexity is Critical for Air Plastron Restoration on Immersed Hierarchical Superhydrophobic Surfaces.

Air plastrons trapped on the surfaces of underwater superhydrophobic surfaces are critical for their function. Fibrillar morphologies offer a natural pathway, yet they are limited to a narrow range of liquid-surface systems and are vulnerable to pressure fluctuations that irreversibly destroy the air layer plastron. Inspired by the convexly grooved bases of water fern (Salvinia) leaves that support their fibrous outgrowths, we focus on the effect of such second-level grooved structures or microgrooves on the plastron restoration on immersed three-dimensional (3D)-printed hierarchical surfaces. Elliptical, interconnected microgrooves are fabricated with varying surface curvatures to study the effect of their morphology. Immersion experiments reveal that the convex groove curvature stabilizes a seed gas layer (SGL) that facilitates plastron restoration for all immersed hydrophobic surfaces. Theoretical calculations and atomic-scale computations reveal that the SGL storage capacity that sets the SGL robustness follows from the liquid menisci adaption to the groove geometry and pressure, from micro- to nanoscales, and it can be further tuned using separated grooves. Our study highlights groove convexity as a key morphological feature for the design of second-level architectures for underwater air plastron restoration on hierarchical superhydrophobic surfaces.

Hierarchical Superhydrophobic Device to Concentrate and Precisely Localize Water-Soluble Analytes: A Route to Environmental Analysis.

An efficient superhydrophobic concentrator is developed using a hierarchical superhydrophobic surface on which the evaporation of a sessile droplet (6 μL) drives the nonvolatile elements it contains on a predefined micrometric analytical surface (pedestal of 80 μm diameter). This hierarchical silicon surface exhibits a surface texture made of etched nanopillars and consists of micropillars and guiding lines, arranged in radial symmetry around the central pedestal. The guiding lines ensure the overall convergence of the sessile droplet toward the central pedestal during evaporation. The nanopillar texturing induced a delay in the Cassie-Baxter to Wenzel regime transition, until the edge of the droplet reaches the periphery of the pedestal. Experiments performed with polymer microparticles suspended in ultrapure water or with DNA molecules solubilized in ultrapure water at sub-fM concentrations demonstrated that the totality of the nonvolatile elements in the liquid microvolume is delivered on or close to the pedestal area, in a very reproducible manner. The very high concentration capacity of the device enabled the discrimination of the degree of purity of ultrapure water samples from different origins. The concentrator also turned out to be functional for raw water samples, opening possible applications to environmental analysis.

How micropatterns affect the anti-icing performance of superhydrophobic surfaces

• Four dynamic-freezing modes are identified on superhydrophobic surfaces (SHSs). • The capillary evacuation contributes to an excellent anti-icing performance. • The coupling effects of nucleation and wetting dynamics are analyzed in detail. • A dimensionless number is put forward to optimally design anti-icing SHSs. Superhydrophobic surfaces (SHSs) have shown significant potential in the anti-icing field over the past decade. In this work, we conduct impact experiments of supercooled drops on SHSs with various micropillar arrays and show how micropatterns contribute to efficient anti-icing performances. Here, we found four dynamic-freezing modes: partial penetration, penetration with evacuation, penetration without evacuation, and internal rupture, and obtained the phase diagrams of these modes in terms of the structural parameters. The hierarchical SHSs, where penetration with evacuation and internal rupture can occur, present more excellent anti-icing performances than single-tier SHSs. The capillary evacuation is proved to play an important role in the reduction of contact time and contact area. A quantitative interpretation of the influence of the capillary evacuation on the contact time is obtained, which demonstrates the potential anti-icing ability of hierarchical SHSs. Coupling the effect of nucleation and wetting dynamics on hierarchical SHSs with capillary evacuation, we put forward a dimensionless number, combined the structural parameters and Weber number We , which is important for optimally designed anti-icing micropatterns of SHSs.

Superhydrophobic and Antibacterial Hierarchical Surface Fabricated by Femtosecond Laser

Superhydrophobic surfaces are important in many applications owing to their special properties such as self-cleaning, anti-icing, antibacterial, and anti-fogging. In this paper, a micro/nano hierarchical superhydrophobic surface with a low roll-off angle was created on 304 stainless steel. The water contact angle was measured to be 152° with a roll-off angle of 7.3°. Firstly, microscale bumps were created by femtosecond laser irradiation. Secondly, zinc oxide (ZnO) nanowires were fabricated on the laser-induced bumps using a hydrothermal synthesis method. Results show that after laser treatment and ZnO nanostructuring, the stainless steel surface became superhydrophobic. However, the roll-off angle of this hierarchical structure surface was larger than 90°. To reduce the surface activity, trimethoxy silane hydrophobic coating was applied. A 7.3° roll-off angle was achieved on the coated surface. The underlying mechanism was discussed. The hydrophobic ZnO structured surface can help prevent bacterial contamination from water, which is important for implants. Thus, for biomedical applications, the antibacterial property of this hierarchical surface was examined. It was found that the antibacterial property of sample surfaces with ZnO nanowires were significantly increased. The optical density (OD) of Escherichia coli (E. coli) attached to the original surface was 0.93. For the micro-structured surface (with bumps), the OD was 0.9, and for the hierarchical surface (with bump & nanowires), it was 0.54. For nanostructured ZnO nanowire surface, the OD was only 0.09. It demonstrates good antibacterial properties of ZnO nanowires.

Sandwich-Structured Surface Coating of a Silver-Decorated Electrospun Thermoplastic Polyurethane Fibrous Film for Excellent Electromagnetic Interference Shielding with Low Reflectivity and Favorable Durability.

Nowadays, high efficiency and low reflection electromagnetic interference (EMI) shielding materials have a wide potential application of electronic fields. However, it is still challenging to achieve long-term durability under external mechanical deformations or other harsh conditions. Herein, sandwich-structured surface coatings with a mixture of polydimethylsiloxane (PDMS)/carboxylated multiwalled carbon nanotube and magnetic ferriferous oxide nanoparticle hybrid fillers (MWCNTs-COOH/Fe3O4, MFs) are introduced onto a silver-decorated electrospun thermoplastic polyurethane (TPU) fibrous film to achieve both outstanding low reflective EMI shielding and favorable durability. The surface coatings not only act as an effective absorbing layer but also provide a micro-nano hierarchical superhydrophobic surface. The resultant film shows a remarkable conductivity (361.0 S/cm), an excellent EMI shielding effectiveness (SE) approaching 85.4 dB, and a low reflection coefficient value of 0.61. Interestingly, the obtained film still maintains an excellent EMI SE even after mechanical deformations or being immersed in strong acidic solution, alkaline solution, and organic solvents for 6 h. This work opens a new avenue for the design of low reflective EMI shielding films under harsh environments.

Water droplet bouncing on a hierarchical superhydrophobic surface fabricated by hydrothermal synthesis and ultraprecision machining

The ability of a water droplet to bounce on a superhydrophobic surface is an important indication of the surface wetting properties. In this study, a method integrating hydrothermal synthesis with ultraprecision machining (UPM) is reported for fabricating a hierarchical NiO-FAS-17 coated copper microgroove surface. Contact angles and water droplet bouncing behaviors were tested to evaluate the hydrophobicity of sample surfaces. Large contact angles over 160° and small contact angle hysteresis not exceeding 5° were recorded, showing good water repellency for the NiO-FAS-17 coating. The droplet bouncing results presented a larger first rebound height and stronger tendency of droplet break-up on the hierarchical NiO-FAS-17 coated microgroove surface in comparison with the NiO-FAS-17 coated flat surface, indicating a better hydrophobicity of the hierarchical surface. This reveals that the hydrophobicity of a surface can be enhanced by introducing UPM fabricated microgroove patterns into nanostructured surfaces that already possess good hydrophobicity.

Fast nucleation of water by self-arrangement of hydrophilic crystals on a hierarchically structured surface promoting coalescence-induced droplet jumping

The enhancement of the condensation efficiency is required for saving energy and resources in industries exploiting condensation. In this regard, the Laplace pressure-driven spontaneous movement of condensed droplets on hierarchical superhydrophobic surfaces has been exploited to increase the condensation efficiency via the fast removal of the condensates from the surfaces. Although the moving droplets enabled the nucleation sites to be exposed to vapor before the droplets were removed from the surface, nucleation after the exposure was delayed by vapor depletion around the moving droplets. Since the heterogeneous nucleation energy barrier depends on the surface wettability, we investigated the effect of the wettability of the nucleation sites on the nucleation after the nucleation sites were exposed owing to the moving droplets. We arranged hydrophilic crystals inside the inner tips of a zigzag-structured surface and adjusted the crystal size with capillary flow-induced self-arrangement. The crystal size was highly related to the nucleation period at the inner tips because the nucleation energy barrier decreased with increasing crystal size. Consequently, the nucleation period reduced by the adequately sized crystals promoted droplet jumping, thereby increasing the frequency and cumulative volume of the jumping droplets by up to 410% and 130%, respectively. Furthermore, we verified that the increased crystal size did not affect the jumping probability on the biphilic-zigzag surfaces; this is because the wetting transition induced by the V-shapes minimized the work of adhesion. These results provide insight into the relationship between the wettability and nucleation period on hierarchically structured surfaces and highlight the synergetic effect between the hierarchical structures and heterogeneous wettability, which promotes droplet jumping.

Hierarchical microcavity topography for enhancement of water vapor condensation heat transfer by regulating droplet dynamics and droplet size distribution

Manipulating droplet dynamics is of great importance in condensation heat transfer enhancement. Cavity topography was believed to be able to regulate droplet merging and self-removal via Laplace force imbalance by the cavity wall and the effect of spatial confinement and isolation. However, the size effect of the microcavity on the droplet dynamics behavior, droplet size distribution and condensation heat transfer performance are not sufficiently understood. In this paper, hierarchical microcavity-arrayed superhydrophobic surfaces with a nominal cavity diameter of 20, 35, 50 and 70 μm were fabricated. Condensation experiments were first carried out in the atmosphere under an optical microscope, and then in pure steam in a self-designed environmental chamber at 4000 Pa. Local dynamic behaviors and time-lapsed evolutions of droplets during condensation were observed. Droplet size distribution and jumping rate were statistically analyzed using ImageJ software. It was proved the smaller cavity size of 20 μm is beneficial to facilitate condensate droplet jumping and increase the density of small droplets. The surface coverage ratio on the microcavity-arrayed superhydrophobic surfaces was only half of that on the hydrophobic surface at steady state. Enhancement of pure steam condensation heat transfer can be realized by effectively regulating droplet dynamics using microcavity topographies at limited subcooling degrees.

Reproducible and fast preparation of superhydrophobic surfaces via an ultrasound-accelerated one-pot approach for oil collection

The currently available superhydrophobic coating techniques that utilize mussel-inspired polydopamine coating to construct hierarchical superhydrophobic structures require multiply procedures and are time-consuming processes. Here, we propose an ultrasound-accelerated strategy to various commercial sponges with hierarchically structured superhydrophobic surfaces by briefly submerging sponges into a slightly alkaline aqueous mixture comprising dopamine hydrochloride (DA) and dodecyltrimethoxysilane (DTMS) under ultrasound. The ultrasound is found significantly accelerate the hierarchical surface structures from PDA, while only minorly influences the hydrolysis of DTMS that hydrophobically modifies the surface of PDA. As a result, the hierarchical superhydrophobic surfaces are formed within 25 min. In addition, the superhydrophobicity of the surface can be recovered by repeating this ultrasound-assisted process, provided that the superhydrophobic feature is vanished or lost during their use in a harsh environment. These superhydrophobic sponges are superior materials for oil collection from water, in a durably robust, efficient and recyclable manner with multi-life span. This strategy presents a rather high efficient and time-saving process for constructing/recovering hierarchical superhydrophobic surfaces, which may be useful for the rapid engineering commercial materials with restorable superhydrophobic surfaces.

Volcano-like hierarchical superhydrophobic surface synthesized via facile one-step secondary anodic oxidation

Superhydrophobic surface has been extensively applied in the field of self-cleaning and corrosion prevention, and the common preparation route of the superhydrophobic surface involves the construction of rough structure and modification with low-surface-energy regents in multistep process. Herein, a facile one-step method was proposed to fabricate the superhydrophobic surface with the simultaneous construction and modification of hierarchical microstructure and surface, the volcano-like anodic aluminium oxide (AAO) -based superhydrophobic surfaces with controllable roughness were synthesized via secondary anodic oxidation (SAO) process of AAO film in an organic–inorganic sol–gel system. During the SAO process, the perfluorooctyl triethoxysilane, which covalently bonded onto the surface, resulted in the inhomogeneous distribution of surface free energy and heterogeneous generation of volcano-like structure. The hydrophobic ability was enhanced by the rough hierarchical micro-/nanostructure with low surface energy, and the TiO2/PFOTS-AAO-15 exhibited the optimal water repellence with a WCA of ~154°, self-cleaning performance and anticorrosion property in corrosive medium. This convenient method is anticipated to unlock various superhydrophobic surfaces on different substrates in the practical fields of self-cleaning and anticorrosion.