Ice accretion by spraying supercooled droplets is not dependent on wettability and surface free energy of substrates

Transformation of Fly Ash-Based Oxide Particles into a Functional Silica-Alumina Aerogel and Its Potential Application as an Anti-icing Surface

Lightweight, surface hydrophobic, highly insulating, and long-lasting aerogels are required for energy conservation and ice-repellent applications. Here, we present the conversion of fly ash to a silica-alumina aerogel (SAA) by utilizing its high silica content. The extracted silica component replaces expensive precursors typically used in conventional aerogel production. Ice adhesion performance was compared to that of polypropylene (PP), an insulating commodity polymer. First, we removed some salt impurities and heavy metals via water and alkaline washing protocols. Then, we produced SAA via the ambient pressure drying method by using trimethylchlorosilane (TMCS) as an adhesion promoter. The newly produced SAA has a surface area of 810 m2 g-1 and shows hydrophobic properties with a contact angle of 140 ± 5°. The thermal conductivity of SAA is 0.0238 W m-1 K-1 with C P = 1.1922 MJ m-3 K-1. The ice adhesion strength of the PP substrate was calculated as 188.30 ± 51.24 kPa, while the ice adhesion strength of the SAA was measured as 1.21 ± 0.40 kPa, which was about 150 times lower than that of PP. This indicated that SAA had icephobic properties since ice adhesion strength was less than 10 kPa. This study demonstrates that fly ash-based SAA can be utilized as an economical material with a large surface area and exceptional thermal insulation capacity and is free of harmful compounds (heavy metals), making it potentially suitable as an anti-ice thermal insulation material.

Modified Amber Particles as a Stabilizer to Construct Oil-in-Water Pickering Emulsions for Improved Thermal Stability of Acorus tatarinowii Essential Oil

Lingzhu Pulvis is a classic formulation for treating febrile convulsions in children. However, Acorus tatarinowii essential oil (AT-EO) in this prescription is prone to volatilization and oxidation, compromising the efficacy and quality control of this formulation. Herein, based on the concept of "combination of medicine and adjuvant", Pickering emulsion technology was applied to enhance the stability of AT-EO using modified amber as a stabilizer. Amber was a resinous medicinal powder in Lingzhu Pulvis and was modified into a suitable stabilizer for Pickering emulsion through surface modification. A thermal stability study indicated that Pickering emulsion, stabilized by modified amber, exhibited a higher retention rate of AT-EO and lower levels of peroxide value and malondialdehyde content compared to those of the pure AT-EO group after heat treatment at 40 °C for 1, 3, and 8 h. Additionally, component analysis in content and composition revealed that the volatile components of AT-EO in the Pickering emulsion were more stable during the thermal treatment process. This study convincingly illustrates the potential of a Pickering emulsion stabilized with modified medicinal powders to improve the thermal stability of the essential oil.

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.

Interdependence of Surface Roughness on Icephobic Performance: A Review

Ice protection techniques have attracted significant interest, notably in aerospace and wind energy applications. However, the current solutions are mostly costly and inconvenient due to energy-intensive and environmental concerns. One of the appealing strategies is the use of passive icephobicity, in the form of coatings, which is induced by means of several material strategies, such as hydrophobicity, surface texturing, surface elasticity, and the physical infusion of ice-depressing liquids, etc. In this review, surface-roughness-related icephobicity is critically discussed to understand the challenges and the role of roughness, especially on superhydrophobic surfaces. Surface roughness as an intrinsic, independent surface property for anti-icing and de-icing performance is also debated, and their interdependence is explained using the related physical mechanisms and thermodynamics of ice nucleation. Furthermore, the role of surface roughness in the case of elastomeric or low-modulus polymeric coatings, which typically instigate an easy release of ice, is examined. In addition to material-centric approaches, the influence of surface roughness in de-icing evaluation is also explored, and a comparative assessment is conducted to understand the testing sensitivity to various surface characteristics. This review exemplifies that surface roughness plays a crucial role in incorporating and maintaining icephobic performance and is intrinsically interlinked with other surface-induced icephobicity strategies, including superhydrophobicity and elastomeric surfaces. Furthermore, the de-icing evaluation methods also appear to be roughness sensitive in a certain range, indicating a dominant role of mechanically interlocked ice.

Gypsum Seeding to Prevent Scaling

Eutectic freeze crystallization (EFC) is a novel separation technique that can be applied to treat brine solutions such as reverse osmosis retentates. These are often a mixture of different inorganic solutes. The treatment of calcium sulphate-rich brines using EFC often results in gypsum crystallization before any other species. This results in gypsum scaling on the cooled surfaces of the crystallizer, which is undesirable as it retards heat transfer rates and hence reduces the yield of other products. The aim of this study was to investigate and understand gypsum crystallization and gypsum scaling in the presence of gypsum seeds. Synthetic brine solutions were used in this research because they allowed an in-depth understanding of the gypsum bulk crystallization process and scaling tendency without the complexity of industrial brines. A cooled, U-shaped stainless-steel tube suspended in the saturated solution was employed as the scaling surface. This was because a tube-shaped surface enabled the introduction of a constant temperature cold surface in the saturated solution and most industrial EFC crystallizers are constructed from stainless steel. Gypsum seeding was effective in decreasing the mass of scale formed on the heat transfer surface. The most effective seed loading was 0.25 g/L, which reduced scale growth rate by 43%. Importantly, this seed loading is six times the theoretical critical seed loading. The seeding strategy also increased the gypsum crystallization kinetics in the bulk solution, which resulted in an increase in the mass of gypsum product. These findings are relevant for the operability and control of EFC processes, which suffer from scaling problems. By using an appropriate seeding strategy, two problems can be alleviated. Firstly, scaling on the heat transfer surface is minimised and, secondly, seeding increases the crystallization kinetics in the bulk solution, which is advantageous for product yield and recovery. It was also recommended that the use of silica as a seed material to prevent gypsum scaling should be investigated in future studies.

Fabrication of C-Doped Titanium Dioxide Coatings with Improved Anti-icing and Tribological Behavior

The accumulation of ice on solid surfaces can cause serious losses and accidents. Current anti-icing/deicing coatings find it difficult to maintain their properties under frequent mechanical wear. In this work, the aerosol-assisted chemical vapor deposition method was employed to prepare C-doped titanium dioxide coatings that have both photothermal properties and excellent wear resistance. The temperature of the sample surface reached 37.6 °C after 2 sun (2 kW/m²) irradiation for 30 min at -30 °C. Compared to a pristine titanium dioxide coating, the C-doped titanium dioxide coatings demonstrated superior antifriction behavior, with the friction coefficient reduced by 47% under dry conditions. These wear resistance properties make C-doped titanium dioxide coatings highly suitable for a range of outdoor applications, especially for anti-icing purposes.

Tailoring the Icephobic Performance of Slippery Liquid-Infused Porous Surfaces through the LbL Method

There has been increasing interest in recent years in identifying an ice-removal procedure that is low cost and scalable and consumes a negligible amount of energy in order to prevent catastrophic failures in outdoor structures. One of the potential solutions to the structural problems caused by frigid and icy conditions is the use of slippery liquid-infused porous surfaces (SLIPS) to effect passive ice removal using easy, economical, and energy-free means. This work takes advantage of the highly flexible layer-by-layer (LbL) technology to customize and design surfaces that have a high degree of roughness using negatively and positively charged polyelectrolytes and negatively charged silica nanoparticles (NPs). SEM (scanning electron microscopy) images represent the silica nanoparticles deposition on the surface of the thin film. The roughness of these thin films has been demonstrated by AFM (atomic force microscopy) investigation. The main characteristics of these surfaces are their high contact angle and low water contact angle hysteresis, which is achieved by the fluorinated lubricant that is infused in the pores of the films. The ice adhesion strength of the thin films was measured using a home-built normal mode tensile test in an environmental chamber, which confirmed the icephobicity of the surface as having an adhesion strength of less than 5 kPa, implying that this surface is an excellent candidate for passive removal of ice. The thin films were aged for up to 100 days, and the results showed that the thin film could reduce the ice adhesion strength by 65%, even after this period. The ice adhesion strength of the thin film after icing/deicing cycles showed that 80% of the icephobicity of the thin film had been preserved even after 50 cycles.

Hard Quasicrystalline Coatings Deposited by HVOF Thermal Spray to Reduce Ice Accretion in Aero-Structures Components

Weather hazards, in particular icing conditions, are an important contributing factor in aviation accidents and incidents worldwide. Many different anti-icing strategies are currently being explored to find suitable long-lasting solutions, such as surface engineering, which can contribute to reduce ice accumulation. Quasicrystals (QCs) are metallic materials, but with similar properties to those of ceramic materials, such as low thermal and electrical conductivities, and high hardness. In particular, QCs that have low surface energy are commercially used as coatings to replace polytetrafluoroethylene (PTFE), also known as Teflon, on frying pans, as they do not scratch easily. PTFE exhibits excellent anti-wetting and anti-icing properties and therefore QCs appear as good candidates to be employed as ice-phobic coatings. Al-based QCs have been applied by High Velocity Oxyfuel (HVOF) thermal spray on typically used aeronautic materials, such as Ti and Al alloys, as well as steels. The coatings have been characterized and evaluated, including the measurement of hardness, roughness, wetting properties, ice accretion behavior in an icing wind tunnel (IWT), and ice adhesion by a double lap shear test. The coatings were studied, both as-deposited, as well as after grinding, in order to study the effect of the surface roughness and morphology on the ice accretion and adhesion properties. The QC coating was compared with PTFE and two polyurethane (PU)-based commercial paints, one of them known to have anti-icing properties, and the results indicate an ice accretion reduction relative to these two materials, and ice adhesion lower than bare AA6061-T6, or the PU paint in the ground version of one of the two QCs. Since the QC coatings are hard (GPa Vickers hardness > 5), a durable behavior is expected.

Practical Applications of Superhydrophobic Materials and Coatings: Problems and Perspectives

Synthetic superhydrophobic (SH) surfaces were developed after 1990s, and the number of publications in this field is around 13 500 at present. However, the industrial production of SH coatings is very unsatisfying after the intensive research activity in the last two decades. The main reason is the loss of the water repellence properties when SH surfaces are exposed to outdoor conditions due to their weak mechanical properties and contamination from the medium which removes the initial SH properties. In this Feature Article, we focus on the scientific and technical reasons which prevent the application of the SH surfaces in our daily lives by highlighting some well-known but mostly overlooked problems in this area. (The synthesis methods of SH surfaces are not the subject of this article since they were reviewed previously in very good articles.) The basic contact angle science and the issue of the cancellation of the Wenzel and Cassie-Baxter equations are reviewed in the first part. The issues of the expensive and small-scale SH surface preparation problems, the difficulties in obtaining a transparent SH surface, the troubles arising from the water vapor condensation on an SH surface, the lack of robustness and abrasion resistance of most of the SH surfaces, the drawbacks of the fabricated self-healing SH surfaces, the short useful service life of self-cleaning SH surfaces due to surface contamination, and the ineffective anti-icing SH coatings are reviewed in the following text. Some important problems affecting the unsuccessful industrial applications of the SH surfaces are discussed critically in the Conclusions and Outlook section. Finally, some proposals are presented for future directions on the synthesis and applications of SH surfaces.

Icephobic performance of one-step silicone-oil-infused slippery coatings: Effects of surface energy, oil and nanoparticle contents

HYPOTHESIS

State-of-the-art superhydrophobic surfaces (SHSs) usually do not function in high humidity and frosty climate conditions. Lubricant-infused slippery surfaces (LISSs) with a homogeneous and ultraslippery surface are expected to be a reliable icephobic technique. Hence, the fabrication of simple and scalable bioinspired LISSs is important for practical applications.

EXPERIMENTS

Durable one-step LISSs consisting of silicone oil and polymer mixtures were fabricated. A grid map based on added oil and silica nanoparticles was developed to tune wettability, morphology, and slippery behavior of surfaces. A similar framework for ice adhesion of lubricant-infused coatings was also presented for the design of optimal icephobic materials.

FINDINGS

LISSs with slight hydrophobicity yield slippery properties, resulting in an order of magnitude lower ice adhesion compared to SHSs. The stable 20-w% silicone-oil-infused slippery coating with slight hydrophobicity and silica nanoparticles was found to be effective in anti-icing. The nanoparticles firmly anchor the oil overlayer and eliminate contamination by drying the surface. The LISSs made of polymers with surface energy ranging from 29 to 31 mJ/m² show the potential to achieve low ice adhesion. As a result, the use of systematic frameworks highlights the role of material parameters. One-production strategy can be broadly used to design icephobic materials.