Superhydrophobic Aluminum Surfaces Research Articles

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.

Rapid fabrication of near superhydrophobic aluminium surface through nanosecond laser texture and heat treatment

Controlling surface wettability inspired by nano structure of lotus leaf has attracted much attention. The superhydrophobicity on the metals is significant for the applications including water repellence, self-cleaning, anti-corrosion, and reduction of drag. Herein a near superhydrophobic surface of 5051 aluminium has been fabricated by employing a combination of nanosecond laser texture and heat treatment, without using any toxic chemicals. The aluminium surface was textured by fibre nanosecond laser and then heat treated at 150 °C to create the near superhydrophobic surface. The evolution of heat treatment time with respect to contact angle measurements has been investigated. The contact angle became 0° when processed by two different kinds of laser textures (sinusoid pattern and grid pattern). The influence of heat treatment on the wettability of the laser textured aluminium surface was studied. The contact angle exhibits 134.88° and 136.97° for sinusoidal pattern and grid pattern with 72 hours heat treatment, respectively. The mechanism for fast wettability conversion time is discussed. This method is a rapid and environment-friendly process, which is feasible for fabrication.

Fast-freezing behavior of water droplet on subcooled superhydrophobic surface in DC electrostatic field

Water freezing in electrostatic field (EF) exists widely in industry and daily life. In this study, we visualized the freezing behavior of distilled droplet on the superhydrophobic aluminum-surface with varied surface temperatures and EF strengths. It is found that the presence of EF, either upward or downward, can promote the nucleation of water even on the superhydrophobic surfaces. Nucleation is more prominent in the upward EF than in the downward EF provided the same field strength. Stronger EF contributes to shorter freezing-delay time and thus accelerate the overall freezing, although the freezing time becomes longer in stronger EF. Particularly, the interflow of the freezing droplet in EFs was exploited by micro-particle image velocimetry (μPIV). The average velocity of the internal flow was observed to increase with larger EF, implying the intensified internal cooling of water. This work unravels the underlying mechanism of the accelerated freezing behavior of water droplet in EFs and provides new insights into the modern applications in anti-icing and refrigeration industry.

Robust and self-healing superhydrophobic aluminum surface with excellent anti-icing performance

Icing on the transmission line can cause disastrous consequence. At present, a lot of research has been conducted on related anti-icing technologies such as superhydrophobic surface (SHS). However, the application of SHS was limited by the poor durability. In this work, a durable superhydrophobic surface with an outstanding anti-icing property has been prepared via combining anodization and infusion of healing agent fluoroalkylsilane (FAS-17). The as-prepared SHS can efficiently prevent the icing process in glaze ice due to the effective repelling of the cooled water droplets at -8 °C. Beside, excellent anti-frosting performance has also been gained due to the self-transfer movement of condensation droplets. Furthermore, the as-prepared SHS surface can heal its superhydrophobicity after 8 times oxygen plasma or 64 times (7.68 m) of abrasion damage under the pressure of 12 kPa. The self-healing ability shows good stability even being heated at 80 °C after 30 days. The findings could provide developed insights into the design of high-performance anti-icing SHS based on the porous structure construction and self-healing modification.

Condensation performance of superhydrophobic aluminium surface material used for cooled ceiling panels under highly humid indoor conditions

The application of radiant cooling systems is very limited in hot and humid areas due to condensation. Research on superhydrophobic surface (SHS) materials has shown the potential of restricting the size of condensate drops on these materials, which provides possibilities for preventing dripping and thereby alleviating condensation risks for cooled ceiling panels, but there are few studies on the anti-condensation performance of these materials under the scale and conditions of building applications. An experimental study of condensation on superhydrophobic materials under indoor conditions is presented in this article. Two material samples with a size of 2.5 cm, including a superhydrophobic aluminum sheet and a pure aluminium sheet, were affixed on a cooled ceiling panel to perform the experiment under the following condition: temperature is 25°C ± 0.5°C, relative humidity is 80% ± 5%, and air dew point is 21.4°C. The panel was cooled by chilled water of 6°C for eight hours. The measured temperature on sample surfaces was about 13.5°C during the experiment. After eight-hour condensation, the diameter of drops on the superhydrophobic aluminum sheet was less than 150 μm, while the max drop on the pure aluminum sheet was near 4 mm. The results suggested that the size of condensate drops on superhydrophobic surface materials can be largely restricted during a long-time indoor operation below the dew point, which shows their potential for constructing condensation-free radiant cooling panels.

Facile fabrication of robust superhydrophobic aluminum surfaces with enhanced corrosion protection and antifouling properties

Despite the progress made on superhydrophobic (SHP) coatings, a key challenge is the ability to fabricate mechanically robust SHP surface. Here, we report findings of the fabrication and characterization of a mechanically robust SHP aluminum (Al) surface with enhanced corrosion resistance and reduced biofouling. The new coating consists of a top hydrophobic layer of hexamethyldisilazane modified nano-silica (low surface energy material) and a middle connecting layer of hydrolyzed glycidoxypropyltrimethoxysilane that is covalently bonded to the SHP surface and hydrophilic aluminum substrate. XPS analysis of the SHP coating confirms –O-Si-CH3 groups. SHP coating showed multi-scale fractal morphology with a fractal dimension of 2.2, and asperities of length scale 32 and 630 nm. The maximum water contact angle (WCA) of the SHP surface was 170 ± 1.5° with a sliding angle ~1°. The SHP surface showed good self-cleaning ability for both the hydrophobic and hydrophilic contaminant. Less than 5% detachment of coating after cross-hatch adhesion test (ASTM D3359-09) and the retention of superhydrophobicity up to 150 cm in the abrasion test (ASTM D4060), confirm the mechanical robustness of the fabricated superhydrophobic surface. A two order reduction in the corrosion current density, and a corrosion inhibition efficiency of ~99% was observed in SHP surface compared to the bare sample. SHP sample showed poor adhesion to gram-negative and gram-positive bacterial strains and the superhydrophobicity was retained even after folding, shear abrasion test, cross-hatch tape adhesion test, and immersion in 0.1 M NaCl solution. Several new insights into the relationship between surface topography, fractal dimensions, self-cleaning process, anticorrosion and bacterial attachment are obtained. Our work provides a framework to develop mechanically robust SHP coating with enhanced corrosion resistance, and antifouling capabilities.

A Superhydrophobic Self‐Cleaning and Anti‐Icing Aluminum Sheet Fabricated by Alkaline Solution

The aerospace industry has extremely strict requirements for metal materials, especially the stability of the material at a low temperature and in harsh environment. Herein, superhydrophobic anti‐icing aluminum sheets are fabricated with traditional hydrothermal reaction and modified by (heptadecafluoro‐1,1,2,2‐tetradecyl) triethoxysilane (AC‐FAS). Field‐Emission Scanning Electron Microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS) are used to analyze the surface morphology and chemical compositions. The satisfactory properties of superhydrophobicity and anti‐icing result from its dense microcosmic array structure and its low surface energy. The prepared superhydrophobic aluminum surface can realize wettability alteration, in which the contact angle increases from 118° on the original aluminum sheets to 155° with a rolling angle of 10°. In addition, the prepared superhydrophobic surface has anti‐icing and self‐cleaning ability. According to the aforementioned results, it is believed that the superhydrophobic aluminum sheets can be an excellent candidate in the aerospace field.

Tribological properties of superhydrophobic aluminium surfaces with extremely high and very low water adhesion

AbstractThe water adhesion of a superhydrophobic surface depends on its surface microstructure and chemical composition, which in turn affects its tribological properties. In this paper, a simple and environmentally friendly method was employed to fabricate superhydrophobic aluminium surfaces with different water adhesion. The results confirmed that the superhydrophobic aluminium surfaces with different water adhesion had different tribological properties under dry friction and water lubrication conditions. The sticky superhydrophobic sample with high adhesion had better wear resistance and lower friction coefficient than the slippery superhydrophobic one with very low adhesion. In addition, the related tribological properties were analyzed. This study is helpful to understand the tribological properties of superhydrophobic surfaces with different water adhesion more clearly, which is of great significance for their practical application.

Microsecond Laser Texturing of an Aerospace Grade Aluminum Alloy to Synthesize Superhydrophobic, Anti-Water Clogging Surfaces

Traditionally superhydrophobic surfaces are prepared by applying liquid repellant organic coatings or nano-based coatings. These superhydrophobic coatings are prone to wear and can be easily damaged by abrasion and cleaning. Recently researchers are switching interest to more efficient and promising technology of pulse laser texturing for engineering sub-micron topographies to have superhydrophobic surfaces. In this research, the micro-second Laser Pulses are used to feature sub-micron textures on titanium nitride coated aluminum and polished aluminum surfaces in order to achieve the water contact angle greater than 150°. Titanium nitride coated aluminum surface with scan line separation of 50 µm shows superior hydrophobicity having a water contact angle of 156º. These superhydrophobic aluminum surfaces have applications for anti-water clogging and anti-corrosion use.

Stable Superhydrophobic Aluminum Surfaces Based on Laser-Fabricated Hierarchical Textures

Laser-microtextured surfaces have gained an increasing interest due to their enormous spectrum of applications and industrial scalability. Direct laser interference patterning (DLIP) and the well-established direct laser writing (DLW) methods are suitable as a powerful combination for the fabrication of single (DLW or DLIP) and multi-scale (DLW+DLIP) textures. In this work, four-beam DLIP and DLW were used independently and combined to produce functional textures on aluminum. The influence of the laser processing parameters, such as the applied laser fluence and the number of pulses, on the resulting topography was analyzed by confocal microscopy and scanning electron microscopy. The static long-term and dynamic wettability characteristics of the laser-textured surfaces were determined through water contact angle and hysteresis measurements, revealing superhydrophobic properties with static contact angles up to 163° and hysteresis as low as 9°. The classical Cassie–Baxter and Wenzel models were applied, permitting a deeper understanding of the observed wetting behaviors. Finally, mechanical stability tests revealed that the DLW elements in the multi-scale structure protects the smaller DLIP features under tribological conditions.

Fabrication of biomimetic anisotropic super-hydrophobic surface with rice leaf-like structures by femtosecond laser

It was found that the anisotropic hydrophobicity of rice leaves was caused by the multi-level grooves and tiny papillary microstructures on their surface. Inspired by rice leaves, a rice leaf-like anisotropic super-hydrophobic aluminum surface was obtained by femtosecond laser raster scanning. The surface structure, chemical composition, contact angle and sliding angle of the structured aluminum surface have been investigated in detail. The results show that structured surface appears super-hydrophobic with sliding anisotropy. The contact angle reaches 153° for 5 μl water droplet. The sliding angle is about 9° along the groove direction and about 34° perpendicular to the groove direction for 10 μl water droplet. • The surface structures of the rice leaves were investigated and the mechanism of anisotropic hydrophobicity of rice leaves was revealed. • It achieves the processing of hierarchical grooves and tiny papillary microstructures in “one step”. • The dominant mechanism for the directional wetting was discussed.

A robust and anticorrosion non-fluorinated superhydrophobic aluminium surface for microplastic removal

Solid particulate pollutants such as microplastics constitute a global environmental issue in the 21st century. Many studies are exploring ways of removing these particles from marine environments such as seas and oceans. Here, we present a superhydrophobic surface obtained by combining anodisation and the liquid-phase deposition of lauric acid. The superhydrophobic surface was examined by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) to elucidate its hierarchical structure and wetting state, while time-of-flight secondary ion mass spectrometry (TOF-SIMS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) were applied to identify the chemical composition of the surface, which revealed that aluminium laurate decreased the surface free energy. As microplastics are usually found in saline water, it was important to study the anticorrosion properties of the surface. Polarisation curves of the anodised surface showed excellent anticorrosion properties in 3.5 wt% NaCl aqueous solution, which was enhanced by the superhydrophobic properties when the aluminium surface was anodised for 60 min. The functionalised surface was superhydrophobic (154°) and superoleophilic (0°). These wetting properties allowed the surface to remove microplastics from the NaCl aqueous solution with an efficiency higher than 99%. Thus, we present a novel application of a superhydrophobic and anticorrosive surface in the removal of microplastics. This has not been reported previously and provides a new scope for superwettable materials and their environmental applications.

The simulation of droplet impact on the super-hydrophobic surface with micro-pillar arrays fabricated by laser irradiation and silanization processes

Super-hydrophobicity is one of the significant natural phenomena, which has inspired researchers to fabricate artificial smart materials using advanced manufacturing techniques. In this study, a super-hydrophobic aluminum surface was prepared by nanosecond laser texturing and FAS modification in sequence. The surface wettability turned from original hydrophilicity to super-hydrophilicity immediately after laser treatment. Then it changed to super-hydrophobicity showing a WCA of 157.6 ± 1.2° with a SA of 1.7 ± 0.7° when the laser-induced rough surface being coated with a layer of FAS molecules. The transforming mechanism was further explored from physical and chemical aspects based on the analyses of surface morphology and surface chemistry. Besides, the motion process of droplet impacting super-hydrophobic surface was systematically analyzed via the optimization of simulation calculation grid and the simulation method of volume of fluid (VOF). Based on this simulation method, the morphological changes, the inside pressure distribution and velocity of the droplet were further investigated. And the motion mechanism of the droplet on super-hydrophobic surface was clearly revealed in this paper. The simulation results and the images captured by high-speed camera were highly consistent, which indicated that the computational fluid dynamics (CFD) is an effective method to predict the droplet motion on super- hydrophobic surfaces. This paper can provide an explicit guidance for the selection of suitable methods for functional surfaces with different requirements in the industry.

Fabrication of Sticky and Slippery Superhydrophobic Aluminum Surfaces Covered with Nanostructured Anodic Oxide

Anodizing aluminum in a concentrated pyrophosphoric acid solution leads to the formation of numerous anodic alumina nanofibers, and this nanofiber-covered surface shows superhydrophilicity measuring less than 10° in water contact angle. In contrast, as the alumina surface is modified with a self-assembled monolayer (SAM), the surface exhibits superhydrophobicity measuring more than 150°. Herein, we demonstrate the fabrication of sticky and slippery superhydrophobic aluminum surfaces by optimizing the nanostructure of anodic alumina nanofibers. Commercially available 5N aluminum and 3004 aluminum alloy were anodized at a constant voltage of 75 V for up to 60 min for the formation of anodic alumina nanofibers on the surface. The specimens, then, were immersed in a 1.5 mM 3,3,4,4,5,5,6,6,7,7,8,8,8,-tridecafluorooctylphosphonic acid (FOPA)/ethanol solution at 313 K for 48 h to form SAMs on the surface of the anodic oxide. The sliding behavior of the water droplet formed on the anodized surface was examined by advancing and receding contact angle investigations. Numerous anodic alumina nanofibers and nanofiber-tangled intermetallic particles were formed on the aluminum surface by anodizing in pyrophosphoric acid. As the nanofiber-covered aluminum alloys were modified with fluorinated phosphonic acid SAMs, the aluminum surface exhibits a superhydrophobic behavior with contact angles of more than 170°. The sliding behavior of the water droplet was drastically changed with the anodizing time, and the highly sticky and slippery aluminum surfaces could be fabricated by controlling the structure of alumina nanofibers (Figure 1). A superhydrophobic aluminum surface with different wettabilities including sticky and slippery properties was fabricated by the selective anodizing process. Figure 1

Rapid fabrication of anti-corrosion and self-healing superhydrophobic aluminum surfaces through environmentally friendly femtosecond laser processing.

The development of superhydrophobic metals has found many applications such as self-cleaning, anti-corrosion, anti-icing, and water transportation. Recently, femtosecond laser has been used to create nano/microstructures and wetting property changes. However, for some of the most common metals, such as aluminum, a relatively long aging process is required to obtain stable hydrophobicity. In this work, we introduce a combination of femtosecond laser ablation and heat treatment post-process, without using any harsh chemicals. We turn aluminum superhydrophobic within 30 minutes of heat treatment following femtosecond laser processing, and this is significantly shorter compared to conventional aging process of laser-ablated aluminum. The superhydrophobic surfaces maintain high contact angles greater than 160° and low sliding angles smaller than 5° over two months after the heat treatment. Moreover, the samples exhibit strong superhydrophobicity for various types of liquids (milk, coffee, CuPc, R6G, HCl, NaOH and CuCl2). The samples also show excellent self-healing and anti-corrosion properties. The mechanism for fast wettability conversion time is discussed. Our technique is a rapid process, reproducible, feasible for large-area fabrication, and environment-friendly.

Tunable Superhydrophobic Aluminum Surfaces with Anti-Biofouling and Antibacterial Properties

Surfaces in a hygiene critical environment can become potential reservoirs for transmission of pathogenic infections. Engineering surfaces with the tunable anti-biofouling and antibacterial properties could reduce infections particularly in hospitals and public transport hubs. In the present work, a facile two-step process has been deployed to fabricate a superhydrophobic and antibacterial aluminum surface by chemical etching, followed by passivation with low surface energy octyltriethoxysilane (OTES) molecules. The wettability and antibacterial properties of the OTES passivated aluminum was monotonically tuned by adding quaternary ammonium (QUATs) molecules. An anti-biofouling property of 99.9% against Staphylococcus aureus, 99% against Pseudomonas aeruginosa and 99% against E. coli bacteria, was achieved.

Study on dynamics and freezing behaviors of water droplet on superhydrophobic aluminum surface

In this work, a kind of superhydrophobic aluminum surface (treated-Al-SA) is fabricated by chemical etching using hydrochloric acid and hydrogen peroxide, and then treated by stearic acid. The surface of treated-Al-SA has typical hierarchical structure, which hold its durable superhydrophobicity under water droplet impact test, bending test, temperature test, humidity test and corrosive liquids test. Moreover, treated-Al-SA show excellent self-cleaning property at the different tilt angle under room temperature. Results on the dynamic behavior of water droplet is indicated the good anti-adhesion and superhydrophobicity of the treated-Al-SA. The whole freezing process of the water droplets on the surface of untreated-Al and treated-Al-SA is observed. The freezing time of treated-Al-SA is 59 s, which is longer than that of untreated-Al. It is estimated that the different anti-freezing behavior is caused by the thermal conductivity between the water droplet and plate, which is provided an insight into freezing characteristics of water droplet. This study is offered the novel opinion of the superhydrophobic aluminum surface produced by chemical etching, which may help the application of structured aluminum surface.

A durable, fluorine-free, and repairable superhydrophobic aluminum surface with hierarchical micro/nanostructures and its application for continuous oil-water separation

Artificial superhydrophobic surfaces have drawn a lot of attention owing to their multifunctional properties. However, the durability and environmental concerns of such surfaces are critical issues that have limited their widespread utilization in real-life applications. In this study, fluorine-free superhydrophobic aluminum surface was fabricated through a simple and cost-effective method. Its dual-scale micro-/nanostructures (MNS) was fabricated by combining chemical etching and hydrothermal processes. After modification with polydimethylsiloxane coating using a simple vapor deposition method, surface showed excellent superhydrophobicity, with a water contact angle of ~161° and sliding angle of 4°, and sustained its non-wetting properties against various food liquids. The effect of morphological change on the surface's wettability and the durability of the coating materials were investigated in sandpaper abrasion, tape-peeling, and blade-scratching tests. Compared with micro- or nanostructure surfaces, the MNS surface showed better robustness because of its dual-scale roughness. Furthermore, the abraded surface could effectively restore its superhydrophobicity after being recoated with PDMS. The surface also showed long-term superhydrophobic stability in an ambient environment for over 10 months. Finally, the method was applied to Al mesh with superhydrophobic/superoleophilic properties to separate various oil-water mixtures, yielding a high separation efficiency of up to 94% and outstanding reusability.