Superhydrophobic Aluminum Research Articles

Rational design on high-performance triboelectric nanogenerator consisting of silicon carbide@silicon dioxide nanowhiskers/polydimethylsiloxane (SiC@SiO2/PDMS) nanocomposite films

The relatively low output performance of triboelectric nanogenerator (TENG), which faces a challenge in performance improvement, limits its practical applications. Here, a high-performance TENG consisting of a silicon carbide@silicon dioxide nanowhiskers/polydimethylsiloxane (SiC@SiO2/PDMS) nanocomposite film and a superhydrophobic aluminum (Al) plate as triboelectric layers is demonstrated. The 7 wt% SiC@SiO2/PDMS TENG presents a peak voltage of 200 V and a peak current of 30 μA, which are ~ 300 and ~ 500% over that of the PDMS TENG, owing to an increase in dielectric constant and a decrease in dielectric loss of the PDMS film because of electric insulated SiC@SiO2 nanowhiskers. Furthermore, a 10 μF capacitor can be charged up to 3 V within ~ 87 s, which can be continuously operated on the electronic watch for 14 s. The work provides an effective strategy for improving output performance of TENG by adding core–shell nanowhiskers to modulate the dielectric properties of organic materials.Graphical abstract

Condensation heat transfer between a vertical superhydrophobic aluminum surface and moist air under natural convection

Condensation heat transfer at superhydrophobic surfaces has raised a lot of interest due to their potential application in atmospheric water harvesting and air-conditioning. In moist air, the condensation heat transfer using a superhydrophobic surface can be negatively affected by the existence of a large amount of non-condensable gas, namely, the dry air. Besides, the air flow pattern would substantially affect the surface condensation rate due to the convective mass transfer of water vapor. In this study, the condensation regime of a superhydrophobic aluminum surface vertically positioned in quiescent moist air was visualized and the condensation rate was experimentally investigated. Condensation experiments were also performed by using hydrophobic, hydrophilic, and superhydrophilic surfaces for comparison. The condensation rate of the superhydrophobic surface was consistent with that of other surfaces, though the condensation regimes of the four surfaces were quite different. The average deviation between the experimental condensation rate and the empirical correlation of natural convection heat and mass transfer was within 12%. The result indicates that vapor transfer induced by diffusion and convection in the boundary layer rather than the heat transfer resistance associated with condensate dominates the condensation heat and mass transfer between the moist air and the superhydrophobic surface.

Robust, Superhydrophobic Aluminum Fins with Excellent Mechanical Durability and Self-Cleaning Ability

The self-cleaning ability of superhydrophobic metal surfaces has attracted extensive attention. The preparation of superhydrophobic material using the coating method is a common processing method. In this experiment, aluminum fins were processed by laser etching and perfluorinated two-step coating. The aluminum surface was modified using a femtosecond laser and 1H,1H,2H,2H- perfluorooctane triethoxysilane (PFOTES). A superhydrophobic aluminum surface with excellent mechanical stability and self-cleaning properties was obtained with the superhydrophobic contact angle (WCA) of 152.8° and the rolling angle (SA) of 0.6°. The results show that the superhydrophobic surface has an excellent cleaning effect compared with an ordinary surface in unit time. Then, a wear resistance test of the superhydrophobic surface was carried out by using the physical wear method. The results show that physical wear had a low influence on the hydrophobic property of the specimen surface. Finally, the Vickers hardness analysis found that the superhydrophobic surface hardness was significantly better than the ordinary surface hardness compared with the superhydrophobic surface hardness. Based on the excellent self-cleaning properties, wear resistance, and robustness of superhydrophobic materials, the laser-etched and perfluorinated superhydrophobic aluminum fins designed and manufactured in this study have broad application prospects in improving the heat transfer efficiency of finned heat exchangers.

Robust superhydrophobic SiO2/epoxy composite coating prepared by one-step spraying method for corrosion protection of aluminum alloy: Experimental and theoretical studies

In this work, a robust epoxy adhesive (EP) @ superhydrophobic SiO2 (SH-SiO2)-based superhydrophobic coating was prepared on the aluminum substrate by a simple one-step spraying strategy. The prepared EP@SH-SiO2 coating exhibits hierarchical micro/nanostructures and is capable of maintaining a contact angle of more than 150° after 1000 cm of abrasion length and 80 tape-peeling repetitions. Molecular dynamics (MD) simulations and quantum mechanics calculations show that the improved mechanical resistance is attributed to the enhanced interfacial interaction between the EP@SH-SiO2 coating and the underlying substrate induced by the epoxy adhesive. Electrochemical measurements in 3.5 wt% NaCl solution show the corrosion inhibition efficiency of the EP@SH-SiO2 coating reaches up to 99.99%, and even after 8 days of immersion, the low frequency impedance modulus of the EP@SH-SiO2 sample is still ∼1 order of magnitude higher than that bare aluminum, demonstrating excellent long-term corrosion protection. The diffusion behavior of the corrosive medium in the EP@SH-SiO2 coating was revealed by MD simulation to support the experimental results. Additionally, the EP@SH-SiO2 coating exhibits good chemical stability and hydrophobic repair properties. This work will provide a new perspective for the preparation and study of robust superhydrophobic aluminum for corrosion protection properties.

Fabrication of Superhydrophobic Surface on Anodized Aluminum Through a Wet-Chemical Route

Fabrication of superhydrophobic surfaces on anodized aluminum substrates by wet-chemical grafting using cost-effective chemicals through a simple immersion process is described in this work. Formation of formate-alumoxane is possible by treating the anodized and sealed aluminum substrate with formic acid at around 50 °C. On treatment with sodium salts of higher-order carboxylic acids (stearic acid, lauric acid, and palmitic acid), the formate ions are replaced by higher-order carboxylates. A possible bonding mechanism of the longer chain carboxylic acids with aluminium surfaces has been suggested based on IRRAS and XPS studies. The as-prepared superhydrophobic aluminum substrates exhibited a static water contact angle of up to 167° with a sliding angle not higher than 6°, with decent resistance against abrasion in addition to good UV, environmental and thermal stabilities. Aluminium substrates of any size, shape and surface finish can be easily rendered robust and superhydrophobic without the use of expensive chemicals and sophisticated machinery.

Superhydrophobic and oleophobic microtextured aluminum surface with long durability under corrosive environment

Superhydrophobic (SHP) and oleophobic aluminum surfaces have been prepared through the combination of a scalable chemical microtexturing process and surface functionalization with long-chained polyfluoroalkyl moieties. The effect of an anodic layer on the microtextured surface has been assessed considering surface morphology, superhydrophobicity, surface mechanical properties and corrosion protection enhancement. The surface functionalization with polyfluoroalkyl moieties has been tackled in two different ways: (i) grafting of the polyfluoroalkyl moieties and (ii) deposition of a thin hybrid coating with low content of polyfluoroalkyl-containing compound. Aluminum surfaces showing high durability in salt spray environments, which maintain SHP and oleophobic properties at least up to 2016 h have been attained. Applications for this kind of surfaces range from easy-to-clean surfaces to anti-icing or anti-condensation functionalities that could be of interest for several sectors.

An electrothermal platform for active droplet manipulation.

The smart control of droplet transport through surface structures and external fields provides exciting opportunities in engineering fields of phase change heat transfer, biomedical chips, and energy harvesting. Here we report the wedge-shaped slippery lubricant-infused porous surface (WS-SLIPS) as an electrothermal platform for active droplet manipulation. WS-SLIPS is fabricated by infusing a wedge-shaped superhydrophobic aluminum plate with phase-changeable paraffin. While the surface wettability of WS-SLIPS can be readily and reversibly switched by the freezing-melting cycle of paraffin, the curvature gradient of the wedge-shaped substrate automatically induces an uneven Laplace pressure inside the droplet, endowing WS-SLIPS the ability to directionally transport droplets without any extra energy input. We demonstrate that WS-SLIPS features spontaneous and controllable droplet transport capability to initiate, brake, lock, and resume the directional motion of various liquid droplets including water, saturated NaCl solution, ethanol solution, and glycerol, under the control of preset DC voltage (∼12 V). In addition, the WS-SLIPS can automatically repair surface scratches or indents when heated and retain the full liquid-manipulating capability afterward. The versatile and robust droplet manipulation platform of WS-SLIPS can be further used in practical scenarios such as laboratory-on-a-chip settings, chemical analysis and microfluidic reactors, paving a new path to develop advanced interface for multifunctional droplet transport.

Experimental investigation on the impacting and freezing characteristics of water droplets on a PDMS-decorated superhydrophobic aluminum alloy surface

Due to the potential applications of superhydrophobic surfaces in water-repelling and anti-icing, it is of great interest to study the impacting and freezing processes of water droplets on superhydrophobic surfaces. In this study, the process of a water droplet impacting the PDMS-decorated superhydrophobic aluminum alloy (denoted as Al-PDMS) surface was studied, and the influences of a water droplet falling height and volume were systemically investigated with a high-speed camera. The results indicate that the impacting process can be categorized into four states, which are mainly affected by the dropping height. The bouncing processes of water droplets on horizontal superhydrophobic surfaces are studied by defining spreading factor α and rebounding factor β. Furthermore, the effect of the inclination angle φ of the superhydrophobic plate on the impacting behavior was investigated. The freezing processes of water droplets on superhydrophobic surfaces were also studied. The results showed that the delayed-icing time decreases with decreasing test plate temperature. Additionally, the volume and height of the frozen portion of the water droplets during freezing were investigated, and the movement characteristic of the freezing front was analyzed.

Hierarchical composite structure to simultaneously realize superior superhydrophobicity and anti-reflection

Superhydrophobicity and anti-reflection of metallic surfaces have potential applications, and obtaining superhydrophobicity on metallic surfaces by simple methods is still a challenge. The composite microstructure consisting of micro-protrusions, micro-lumps and particles was fabricated on aluminum surface by laser ablation. A layer of hydrophobic species that can prevent the surface from being wetted formed on the microstructure through organic adsorption. In particular, laser ablation parameters were optimized to achieve better superhydrophobicity and anti-reflection. The fabricated superhydrophobic surfaces showed contact angles > 165° for water and contact angles > 160° for high-concentration solutions and high-viscosity solutions. Conventional bounce contact time < 10 ms was exhibited for water droplets impacting on fabricated surfaces, especially, the shortest contact time reaching 9.4 ms. anti-reflection was also realized on the laser-ablated aluminum surface, and specular reflections were almost completely avoided by laser ablation structures. The mechanism of the transition from superhydrophilic to superhydrophobic on fabricated surfaces was discussed. Superhydrophobic aluminum surfaces with anti-reflection could be efficiently manufactured in a few hours without special equipment and harsh condition through the proposed two-step method.

A Novel Simple Fabrication Method for Mechanically Robust Superhydrophobic 2024 Aluminum Alloy Surfaces

The mechanical durability of a superhydrophobic aluminum alloy surface is an important indicator of its practical use. Herein, we propose a strategy to prepare a superhydrophobic 2024 aluminum alloy surface with highly enhanced mechanical durability by using a two-step chemical etching method, using a NaOH solution as the etchant in step one and a Na2CO3 solution as the etchant in step two. Robust mechanical durability was studied by static contact angle tests before and after an abrasion test, potentiodynamic polarization measurements after an abrasion test and electrochemical impedance spectroscopy tests after an abrasion test. Furthermore, the mechanism for enhanced mechanical durability was investigated through scanning of electron microscopy images, energy-dispersive X-ray spectra, Fourier transform infrared spectra and X-ray photoelectron spectra. The testing results indicate that a hierarchical rough surface consisting of regular micro-scale dents and some nano-scale fibers in the micro-scale dents, obtained with the two-step chemical etching method, contributes to highly enhanced mechanical durability. Meanwhile, the as-prepared superhydrophobic 2024 aluminum alloy surface retained a silvery color instead of the black shown on the superhydrophobic 2024 aluminum alloy surface prepared by a conventional one-step chemical etching method using NaOH solution as the etchant.

Superhydrophobic and Self-Cleaning Aluminum via a Rapid and Controlled Process

Superhydrophobic and self-cleaning materials have been an extensive topic of research owing to their corrosion resistance, antifouling, and anti-icing properties. In the past, superhydrophobicity in aluminum has been obtained by a variety of techniques involving etching by sandblasting, chemical etching, boiling in water, etc. followed by treatment with a fluorochemical and organic solution. However, these techniques are not controlled, might not be easy to implement on a large scale, and mostly make use of fluorochemicals, which are risky for human health and the environment. Therefore, there is a need for a rapid and controlled fabrication process that can provide patterned superhydrophobicity without compromising the environment or the human health. Here, a rapid, controlled, and environmentally friendly procedure for fabricating a superhydrophobic and self-cleaning aluminum surface has been developed, achieving a contact angle of 158.06° and a sliding angle of 1.94°. In the two-step process, a rough surface was obtained using the laser etching technique in under 3 min, which resulted in a precisely controlled pattern with a dual-scale roughness on the surface (∼20–30 μm). Finally, a low surface energy was imparted to the surface by treatment with stearic acid where a low concentration of 2 × 10–4 M and no more than 10 s of exposure was found to be sufficient. Unlike various superficially coated materials, this superhydrophobic surface is durable and does not lose its properties after dipping in water for up to 4 days, encountering fine sand or mud, or at extreme surrounding temperatures of −18 to 100 °C. Thus, this rapid and controlled process for obtaining superhydrophobic aluminum potentially can be deployed for large-scale, structural applications.

Efficient fabrication of ternary coupling biomimetic superhydrophobic surfaces with superior performance of anti-wetting and self-cleaning by a simple two-step method

Efficiently obtaining ideal superhydrophobic metallic surfaces through traditional laser ablation is still a challenge. Meanwhile, revealing the mechanism of the transition from superhydrophilic to superhydrophobic of laser-ablated metallic surfaces is required. Inspired by the lotus leaf, we fabricated the arrays of micro-protrusions on aluminum substrates by nanosecond laser ablation. A layer of hydrophobic species was formed on the micro-protrusions after organic adsorption process, which promoted the surface to obtain superior superhydrophobic, anti-wetting and self-cleaning properties. The principle of the transition from superhydrophilic to superhydrophobic during heat treatment process was revealed, and how organic adsorption temperature and time affect water contact angle and slide angle was clarified. We found that nano-particles can drastically reduce the solid–liquid fraction of the surface after analysis. The parameters including laser scanning space, organic adsorption time and heat treatment temperature were optimized to realize high-efficient and energy-saving production. Furthermore, superior wetting resistance to various solutions and the self-cleaning property for solid powders were experimentally indicated. Superhydrophobic aluminum surfaces could be efficiently fabricated in several hours without any expensive equipment, complicated process, strict environment and environmentally harmful chemical reagent by the proposed two-step method.

Femtosecond laser fabrication and chemical coating of anti-corrosion ethylene-glycol repellent aluminum surfaces

Ethylene glycol (EG) is widely used as a cooling agent in factories, aviation, and automobiles. However, its corrosive nature can create significant damage and reduce the life cycle of devices. This paper compares three different methods to achieve the superhydrophobic anti-corrosion aluminum surfaces for working in an engine coolant environment. The methods are combinations between femtosecond laser fabrication and chemical coatings using Polydimethylsiloxane (PDMS) and Triethoxy-1H,1H,2H,2H-tridecafluoro-n-octylsilane (TTFOS). All the samples show superhydrophobicity with contact angles greater than 150°, and sliding angles smaller than 10°. The superhydrophobic surface (SHS) show the best corrosion rate up to three orders of magnitude compared to flat surfaces when working in an engine coolant environment. Moreover, corrosion inhibition efficiencies of the fabricated surfaces impress the best results greater than 98%. The anti-corrosion behavior of the SHS is explained. After comparing the performances of the surfaces fabricated by three methods, the most suitable method is proposed to produce anti-corrosion aluminum surfaces for applications using engine coolant as an environment.

Optimization of coating parameters for fabrication of robust superhydrophobic (SHP) aluminum and evaluation of corrosion performance in aggressive medium

The huge interest in superhydrophobic surfaces originates from their potential engineering applications in corrosion protection, antifouling, oil recovery, self-cleaning, anti-adhesion, anti-icing, medical dressing, drag reduction and radiative cooling. Despite the progress made in developing superhydrophobic (SHP) coatings, a key challenge is the ability to fabricate mechanically robust SHP surfaces and a proper understanding of the corrosion performance of SHP surfaces in aggressive conditions. Here, we report a simple, rapid, inexpensive, scalable and eco-friendly method for fabrication of mechanically durable superhydrophobic (SHP) aluminum surfaces with self-cleaning properties using a scalable dip-coating method. The coating parameters were optimized to obtain a robust coating with desirable properties such as water contact angle (WCA), sliding angle, self-cleaning, abrasion and corrosion resistance. To obtain good adhesion between the substrate and the coating, chemical etching and hydroxylation of the Al surface were used. At an optimal withdrawal speed of the substrate in the precursor solutions, a maximum WCA of 172.3° ± 1.1° (SA ~ 1° ± 1°) was achieved. The hierarchical micro-nano roughness and the presence of the Si-(CH3)3 group were responsible for the superhydrophobic behavior. The existence of a covalent bond between the Al surface and the middle connecting layer and –O-Si-CH3 groups on the surface of the SHP Al are confirmed by LRS and XPS, respectively. The electrochemical studies in 3.5 wt% NaCl solution show that the Rct value of SHP Al during the initial state was ~2 orders higher than that of the bare sample. The Cdl value of SHP Al was 6 orders less than that of bare after 1 h of exposure in chloride solution. On the SHP surface, a droplet with an impinging velocity of 0.36 m/s showed approximately 7 bounces. The superhydrophobicity of the SHP sample was preserved up to 150 cm abrasion distance. The hierarchical surface morphology and superhydrophobicity (WCA ~151°) were retained by the SHP samples even after 120 h of immersion in 3.5 wt% NaCl, which indicates its suitability in seawater applications such as ships, underwater instruments and pipelines and marine structures. Further, our work provides a framework to develop mechanically robust SHP coating for various applications.

Wettability tailored superhydrophobic and oil-infused slippery aluminium surface for improved anti-corrosion performance

Inspired by the non-contact nature offered by the superhydrophobic surfaces of living creatures in nature, the superhydrophobic and slippery liquid-infused porous surfaces (SLIPS) have been fabricated to meet the engineering problems including metal corrosion. In this research, we fabricate the hydrophobic, superhydrophobic, hydrophobic lubricant-infused surface (H-SLIPS), and superhydrophobic lubricant infused surface (SH-SLIPS) onto aluminium alloy surfaces and explored their anti-corrosion properties. The hydrophobic and superhydrophobic surfaces are fabricated by hot-water treatment and silicon oil grafting onto the plain and superhydrophilic surface created by hot water treatment of the chemically etched aluminium surface, respectively. The corresponding SLIPS are fabricated by infusion of the silicone oil onto the prepared hydrophobic and superhydrophobic surfaces. Although the superhydrophobic surface exhibits self-cleaning behavior, the SLIPS shows excellent self-healing properties. In addition, the H-SLIPS shows better lubricant layer stability compared to the SH-SLIPS. The corrosion resistive and electrochemical impedance studies illustrate that H-SLIPS surfaces exhibit excellent anti-corrosive properties as compared to other substrates and show long-term performance, even after 30 days. Thus, the study illustrates that H-SLIPS performs better and the infused oil stability is a critical factor for applications like anti-corrosion properties.

A simple strategy towards construction of fluorine-free superhydrophobic aluminum alloy surfaces: self-cleaning, anti-corrosion and anti-frost

A simple strategy towards construction of fluorine-free superhydrophobic aluminum alloy surfaces: self-cleaning, anti-corrosion and anti-frost

Enhancing the cooling capacity of radiant ceiling panels by latent heat transfer of superhydrophobic surfaces

The cooling capacity of a radiant ceiling panel (RCP) is highly limited in hot and humid climates as the panel surface temperature must be controlled higher than the air dew point to avoid condensation. Previous research suggested that condensation risks can be mitigated by using superhydrophobic surface materials, making ceiling panels applicable with the surface temperature below the air dew point. In this study, the enhanced cooling capacity of RCP due to latent heat transfer is estimated. Firstly, the condensation heat transfer of a superhydrophobic aluminum surface under natural convection in humid air with subcooling degrees between 3 °C and 12 °C was investigated by experiments. The data proved that the heat and mass transfer analogy method can be used to well predict the condensation heat transfer of the superhydrophobic surface with an error within 15%. Based on the finding, the heat and mass transfer analogy and traditional empirical methods were employed to predict the total heat flux of RCP with latent heat transfer. The results indicated an enhancement of 3% to 500% for the total heat flux compared with RCP with only sensible cooling, depending on the panel surface temperature, air temperature and humidity. This study draws a clear picture of how the cooling capacity of RCP can be enhanced via a latent heat transfer process, and this can be realized not necessarily with net moisture removal capacities with hydrophobic surfaces due to the droplet jumping phenomenon and the due re-evaporation process, showing a hidden potential of radiant cooling panels.

Study on Frost-Suppression Characteristics of Superhydrophobic Aluminum Surface Heat Exchanger Applied in Air Source Heat Pump

In order to solve the frosting problem of air source heat pump (ASHP) outdoor heat exchange under low-temperature and low-humidity conditions, a superhydrophobic aluminum (Al) surface with a contact angle (CA) of 158.3° was prepared by chemical etching. The microscopic characteristics of droplet condensation and the freezing process of a superhydrophobic surface were revealed through visual experiments and theoretical analysis. On this basis, the frost-suppression effect of a superhydrophobic Al-based surface simulating the distribution of actual heat exchanger fins was preliminarily explored. The results demonstrated that, due to the large nucleation energy barrier and the coalescence-bounce behavior of droplets, the condensed droplets on the superhydrophobic surface appeared late and their quantity was low. The thermal conductivity of the droplets on a superhydrophobic surface was large, so their freezing rate was low. The frosting amount on the superhydrophobic Al-based surface was 69.79% of that of the bare Al-based surface. In turn, the time required for melting the frost layer on the superhydrophobic Al-based surface was 64% of that on the bare Al-based surface. The results of this study lay an experimental and theoretical foundation for the application of superhydrophobic technology on the scale of heat exchangers.

Superhydrophobic Aluminium Surface to Enhance Corrosion Resistance and Obtain Self-Cleaning and Anti-Icing Ability.

A facile environmentally acceptable surface roughening method using chemical etching in HCl/H2O2 followed by grafting with n-octyltrimethoxysilane (AS-8) and 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (FAS-8) was studied to fabricate a (super)hydrophobic aluminium surface. The ground aluminium surface after selected etching times (before and after grafting), was characterised using a contact profilometer, optical tensiometer, scanning electron microscope coupled with an energy-dispersive spectroscope and X-ray photoelectron spectroscope to evaluate surface roughness, wettability, surface morphology and composition. The durability of the grafted surface was tested using thermal and UV resistance tests. The corrosion properties were evaluated using potentiodynamic measurements and standard salts spray testing, ASTM B117-19. Finally, the self-cleaning and anti-icing abilities were assessed. The grafted aluminium surface with octyl- or perfluorooctyl silane reflected the highly hydrophobic (AS-8) and superhydrophobic behaviour (FAS-8). Moreover, the different behaviour of the octyl- or perfluorooctyl chain in the silane molecule on modified surface properties was also noticed because durability tests confirmed greater thermal, UV stability and corrosion resistance of FAS-8 compared to AS-8. The aluminium etched for 2 min and grafted with FAS-8 also demonstrated an excellent self-cleaning and anti-icing performance.

The Effect of Sugar Content for the Wettability of Superhydrophobic Aluminum Substrate

A chemical etching technique is used to prepare a superhydrophobic surface with a honeycomb rough structure on the aluminum surface. Use SEM, Optical contact angle meter and Surface tension detector to characterize the etched aluminum substrate. After the 8th etching, the surface of the sample showed the morphology of micro/nano-scale honeycomb pores and protrusions, and the water contact angle (WCA) is 135°. After being modified with octadecanethiol methanol solution, WCA is 153.1°. After modification, the contact angle of the sample surface decreases with the increase of the glucose solution concentration. When the glucose solution concentration reaches 1000 mg/L, the superhydrophobicity is lost.