Anti-icing performance on aluminum surfaces and proposed model for freezing time calculation

Freezing Characteristics of a Water Droplet on a Multiscale Superhydrophobic Surface

Superhydrophobic surfaces have the potential to retard ice formation owing to their super water-repellant nature arising from high static contact angle and low contact angle hysteresis. Most of the previous studies have focused on patterned surfaces with mono-scaled prismatic structures. In contrast, the freezing behavior on multiscaled rough superhydrophobic surfaces that are of practical significance is relatively little studied. This article presents, for the first time, the freezing dynamics of a water droplet interacting with multiscale fractal superhydrophobic surfaces which validates well with experimental measurements. It is shown that the dual effects of increased contact angle and poor interfacial conduction due to trapped air cavities within the roughness features of the superhydrophobic surface lead to increasing freezing time with increasing surface hydrophobicity, determined as a function of the fractal surface parameters. A comparison of the freezing dynamics of sessile droplets of identical contact angle on a smooth versus a rough superhydrophobic surface shows that interfacial asperity thermal resistance contributes to over 14% increase in the freeze time. It is further shown that by tailoring the multiscale characteristics, the freeze time may be increased by up to 7-fold compared to freezing on a smooth surface. The application of the numerical model to studying ice formation on several practical superhydrophobic surfaces of a range of metallic materials and fabrication methods is also discussed, which offers guidelines for the design of anti-icing surfaces in practice.

Advances in the Fabrication and Characterization of Superhydrophobic Surfaces Inspired by the Lotus Leaf

Nature has proven to be a valuable resource in inspiring the development of novel technologies. The field of biomimetics emerged centuries ago as scientists sought to understand the fundamental science behind the extraordinary properties of organisms in nature and applied the new science to mimic a desired property using various materials. Through evolution, living organisms have developed specialized surface coatings and chemistries with extraordinary properties such as the superhydrophobicity, which has been exploited to maintain structural integrity and for survival in harsh environments. The Lotus leaf is one of many examples which has inspired the fabrication of superhydrophobic surfaces. In this review, the fundamental science, supported by rigorous derivations from a thermodynamic perspective, is presented to explain the origin of superhydrophobicity. Based on theory, the interplay between surface morphology and chemistry is shown to influence surface wetting properties of materials. Various fabrication techniques to create superhydrophobic surfaces are also presented along with the corresponding advantages and/or disadvantages. Recent advances in the characterization techniques used to quantify the superhydrophobicity of surfaces is presented with respect to accuracy and sensitivity of the measurements. Challenges associated with the fabrication and characterization of superhydrophobic surfaces are also discussed.

Biomimetic superhydrophobic metal/nonmetal surface manufactured by etching methods: A mini review

As an emerging fringe science, bionics integrates the understanding of nature, imitation of nature, and surpassing nature in one aspect, and it organically combines the synergistic complementarity of function and structure–function integrated materials which is of great scientific interest. By imitating the microstructure of a natural biological surface, the bionic superhydrophobic surface prepared by human beings has the properties of self-cleaning, anti-icing, water collection, anti-corrosion and oil–water separation, and the preparation research methods are increasing. The preparation methods of superhydrophobic surface include vapor deposition, etching modification, sol–gel, template, electrostatic spinning, and electrostatic spraying, which can be applied to fields such as medical care, military industry, ship industry, and textile. The etching modification method can directly modify the substrate, so there is no need to worry about the adhesion between the coating and the substrate. The most obvious advantage of this method is that the obtained superhydrophobic surface is integrated with the substrate and has good stability and corrosion resistance. In this article, the different preparation methods of bionic superhydrophobic materials were summarized, especially the etching modification methods, we discussed the detailed classification, advantages, and disadvantages of these methods, and the future development direction of the field was prospected.

Correlations between rheological and mechanical properties of fructo-polysaccharides extracted from Ornithogalum billardieri as biobased adhesive for biomedical applications

Polysaccharides are extracted from Ornithogalum by maceration using different ultrasound (US) treatment times (0%US, 50%US, 100%US), and under optimized extraction conditions (OP%US). The total carbohydrates content (TCC) and proteins content of the extracts were determined. Data show that the extraction parameters significantly influence the extracts composition. Rheological measurements allowed determining the liquid, intermediate and gel states of the extract's solutions. The adhesion strength of the solutions was evaluated on paper and polylactide (PLA) substrates to evaluate their potential as environmentally friendly adhesive. OP%US presents the highest adhesion strength (1418.3 kPa) on paper, and is further tested on pork skin substrates. The adhesion strength is higher on skin/paper (870 kPa) than on skin/skin (411 kPa) substrate due to the capillary force of paper which allows penetration of adhesive into the micropores of paper. The correlation between rheological properties and adhesion strength indicates that the adhesion strength strongly depends on the state of adhesives and the substrate type. SEM analyses show that higher adhesion strength (intermediate and gel states) involves both cohesive and adhesive failure, whereas only adhesive failure is observed in liquid state on PLA substrates. Therefore, these polysaccharides extracts could be very promising as tissue adhesive in medical applications.

Fabrication and Characterization of Superhydrophobic Al-Based Surface Used for Finned-Tube Heat Exchangers

Enhancing the heat transfer performance of heat exchangers is one of the main methods to reduce energy consumption and carbon emissions in heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. Wettability modified surfaces developed gradually may help. This study aims to improve the performance of heat exchangers from the perspective of component materials. The facile and cost-effective fabrication method of superhydrophobic Al-based finned-tube heat exchangers with acid etching and stearic acid self-assembly was proposed and optimized in this study, so that the modified Al fins could achieve stronger wettability and durability. The effect of process parameters on the wettability of the Al fins was by response surface methodology (RSM) and variance analysis. Then, the modified fins were characterized by field-emission scanning electron microscopy (FE-SEM), 3D topography profiler, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), respectively. The durability of the superhydrophobic fins was investigated by air exposure, corrosion resistance, and mechanical robustness experiments. The RSM and variance analysis demonstrated that a water contact angle (WCA) of 166.9° can be obtained with the etching time in 2 mol/L HCl solution of 10.5 min, the self-assembly time in the stearic acid ethanol solution of 48 h, and drying under 73.0 °C. The surface morphology showed suitable micro-nano structures with a mean roughness (Ra) of 467.58 nm and a maximum peak-to-valley vertical distance (Rt) of 4.095 μm. The chemical component demonstrated the self-assembly of an alkyl chain. The WCAs declined slightly in durability experiments, which showed the feasibility of the superhydrophobic heat exchangers under actual conditions.

Superhydrophobic Candle Soot Coating Directly Deposited on Aluminum Substrate with Enhanced Robustness

In this study, superhydrophobic surfaces were developed by using a simple and environmentally friendly technique. The nano-network of candle soot (CS) as the byproduct of incomplete combustion of paraffin candle was directly coated onto both smooth and micro-rough aluminum (Al) substrates for various time periods of deposition. The simple technique of mechanical sanding was used to impart micro-rough structures onto Al substrates using different sandpaper grit sizes. The scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques were used to characterize the morphology and chemistry of the prepared surfaces. Wetting analysis of the prepared surfaces was performed by measuring both water droplet contact angle (CA) and sliding angle (SA). The prepared coatings showed superhydrophobic properties with high CAs and low SAs for CS surfaces coated on roughened Al substrates. Moreover, the robustness of the prepared surfaces was tested by continuous impingement of water droplets onto their surfaces from various heights. Post-testing wetting analysis showed that the micro-nano surfaces of candle soot coated on micro roughened Al substrates demonstrated improved robustness. These surfaces could be useful for self-cleaning, anti-corrosion and anti-icing applications.

Hydrophilic Slippery Surface Promotes Efficient Defrosting

Frost accretion occurs ubiquitously in various industrial applications and causes tremendous energy and economic loss, as manifested by the Texas power crisis that impacted millions of people over a vast area in 2021. To date, extensive efforts have been made on frost removal by micro-engineering surfaces with superhydrophobicity or lubricity. On such surfaces, air or oil cushions are introduced to suspend the frost layer and promote the rapid frost sliding off, which, although promising, faces the instability of the cushions under extreme frosting conditions. Most existing hydrophilic surfaces, characterized by large interfacial adhesion, have long been deemed unfavorable for frost shedding. Here, we demonstrated that a hydrophilic and slippery surface can achieve efficient defrosting. On such a surface, the hydrophilicity gave rise to a highly interconnected basal frost layer that boosted the substrate-to-frost heat transfer; then, the resulting melted frost readily slid off the surface due to the superb slipperiness. Notably, on our surface, the retained meltwater coverage after frost sliding off was only 2%. In comparison to two control surfaces, for example, surfaces lacking either hydrophilicity or slipperiness, the defrosting efficiency was 13 and 19 times higher and the energy consumption was 2.3 and 6.2 times lower, respectively. Our study highlights the use of a hydrophilic surface for the pronounced defrosting in a broad range of industrial applications.

Renewable Superhydrophobic Surfaces Prepared by Nanoimprinting Using Anodic Porous Alumina Molds

Renewable superhydrophobic surfaces based on laminated polymer films with nanopillar array structures were prepared. Polymer nanopillar arrays exhibiting superhydrophobic properties were prepared by nanoimprinting using anodic porous alumina as a mold. The hydrophobic properties of the obtained polymer nanopillar arrays could be controlled and optimized by changing the geometrical structures of anodic porous alumina molds used for nanoimprinting. The polymer films were laminated using a photocurable monomer. The ordered polymer nanopillar array structures could be maintained even after delamination of the films. Renewed polymer nanopillar arrays exposed by peeling off the upper films exhibited a water contact angle higher than 150°. Using this process, superhydrophobic surfaces could be obtained repeatedly by delamination of a film even when superhydrophobicity deteriorated with the collapse of surface patterns. The obtained renewable superhydrophobic surfaces can be used for various applications requiring high durability.

A Review of Fabrication Methods, Properties and Applications of Superhydrophobic Metals

Hydrophobicity and superhydrophobicity with self-cleaning properties are well-known characteristics of several natural surfaces, such as the leaves of the sacred lotus plant (Nelumbo nucifera). To achieve a superhydrophobic state, micro- and nanometer scale topography should be realized on a low surface energy material, or a low surface energy coating should be deposited on top of the micro-nano topography if the material is inherently hydrophilic. Tailoring the surface chemistry and topography to control the wetting properties between extreme wetting states enables a palette of functionalities, such as self-cleaning, antifogging, anti-biofouling etc. A variety of surface topographies have been realized in polymers, ceramics, and metals. Metallic surfaces are particularly important in several engineering applications (e.g., naval, aircrafts, buildings, automobile) and their transformation to superhydrophobic can provide additional functionalities, such as corrosion protection, drag reduction, and anti-icing properties. This review paper focuses on the recent advances on superhydrophobic metals and alloys which can be applicable in real life applications and aims to provide an overview of the most promising methods to achieve sustainable superhydrophobicity.