A Family of Frost-Resistant and Icephobic Coatings

Hypothesis: A sustainable dynamic anti-icing surface with the potential for rapid rechargeability

Ice accumulation poses significant challenges across numerous industries. While dynamic anti-icing surfaces (DAIS) have shown potential in mitigating ice formation and adhesion, their practical use is often limited by the rapid diffusion of liquids and lack of reusability. Overcoming these limitations is crucial to addressing the environmental and economic issues related to ice management. In this study, we introduce a novel approach by incorporating β-cyclodextrin (β-CD) into polydimethylsiloxane (PDMS) silicone rubber, enabling the creation of a sustainable DAIS with the potential of rapid rechargeability. The multiple hydroxyl groups present on the outer surface of β-CD facilitate dipole-dipole interactions and hydrogen bonding, particularly with polar molecules like ethanol and isopropanol. This transforms the surface into a rechargeable system, capable of restoring its low ice adhesion functionality within just 10 min after liquid replenishment. When ice forms on the surface, the system dynamically responds to environmental changes via concentration gradients, controlling the release of liquids and altering surface characteristics. These retained liquids effectively lower the freezing point, melt the ice, and disrupt the ice structure, converting the solid-liquid interface into a liquid-liquid interface. The DAIS effectively alter the ice-substrate interaction and enhance performance at temperatures as low as -18 ℃. By optimizing the β-CD mass ratio and liquid treatments, especially with isopropanol, we achieved an ultra-low ice adhesion strength of 0.6 kPa, which remains stable even after 35 days. This study presents a significant advancement in the development of sustainable, rapidly rechargeable DAIS, offering immense potential for applications in various industries.

Discontinuity-enhanced icephobic surfaces for low ice adhesion

HYPOTHESIS

Passive low ice-adhesion surfaces are frequently composed of soft materials; however, soft materials potentially present durability issues, which could be overcome by fabricating composite surfaces with patterned rigid and soft areas. Here we propose the innovative concept of discontinuity-enhanced icephobic surfaces, where the stress concentration at the edge between rigid and soft areas, i.e. where discontinuities in elasticity are located, facilitates ice detachment.

EXPERIMENTS

Composite model surfaces were fabricated with controlled rigid-soft ratios and discontinuity line lengths. The ice adhesion values were measured while recording the ice/substrate interface, to unravel the underpinning ice detachment mechanism. The experiments were complemented by numerical simulations that provided a better understanding of the ice detachment mechanism.

FINDINGS

It was found that when a surface contains rigid and soft areas, stress is concentrated at the edge between soft and hard areas, i.e. at the discontinuity line, rather than all over the soft or rigid areas. An unexpected non-unidirectional crack propagation was observed for the first time and elucidated. When rigid and deformable materials are present, the crack occurs on the discontinuity line and propagates first on rigid and then on soft areas. Moreover, it was demonstrated that an increase in discontinuities promotes crack initiation and leads to a reduction of ice adhesion.

Scalable Preparation of Liquid Infused Coatings for Lubrication of 103 m2 Dry Ski Slopes

To facilitate effective training for freestyle skiers on artificial dry ski slopes, it is crucial to reduce the friction coefficient of the slopes and closely match it with that of snow. Traditional lubrication methods, such as water or soapy water, come with multiple disadvantages, including water waste, which leads to environmental pollution, short-lived effectiveness, and high costs. In this study, we have successfully developed a method for the scalable preparation of a liquid-infused coating (LIC) by tandem spraying inexpensive and environmentally friendly SiO2 particles and silicone oil lubricants. Experimental results showed that the resulting LIC is capable of imparting slippery properties to various surfaces, regardless of the surface chemistry. Moreover, the presence of LIC could reduce the friction coefficient significantly. By carefully regulating the surface composition, we achieved a friction coefficient of 0.059 between a snowboard and the LIC-functionalized ski slope, closely matching that between the snowboard and snow in a typical skiing competition venue (∼0.06). We successfully applied LIC onto 103 m2 dry ski slopes, providing a training ground for professional freestyle skiers.

One-step synthesis of functional slippery lubricated coating with substrate independence, anti-fouling property, fog collection, corrosion resistance, and icephobicity

Ranging from industrial facilities to residential infrastructure, functional surfaces encompassing functionalities such as anti-fouling, fog collection, anti-corrosion, and anti-icing play a critical role in the daily lives of humans, but creating these surfaces is elusive. Bionic dewetting and liquid-infused surfaces have inspired the exploitation of functional surfaces. However, practical applications of these existing surfaces remain challenging because of their inherent shortcomings. In this study, we propose a novel functional slippery lubricated coating (FSLC) based on a simple blend of polysilazane (PSZ), silicone oil, and nano silica. This simple, nonfluorine based, and low-cost protocol promotes not only hierarchical micro-nano structure but also favorable surface chemistry, which facilitates robust silicone oil adhesion and excellent slippery properties (sliding angle: ∼1.6°) on various solid materials without extra processing or redundant treatments. The highly integrated competence of FSLC, characterized by robustness, durability, strong adhesion to substrates, and the ability for large-area preparation, render them ideal for practical production and application. The proposed FSLC holds outstanding application potentials for anti-fouling, self-cleaning, fog collection, anti-corrosion, and anti-icing functionalities.

Photo-thermal waxgels with fast wax layer regeneration ability for anti-icing

Waxgels are known for their unique ability to generate sacrificial wax layers during anti-icing. To address the severe slow regrowth of the wax layer, here, carbon black is incorporated in the waxgel network to endow photothermal function. The rate of the regrowth of the wax layer is raised by >6 times under natural light conditions. Meanwhile, the photothermal waxgel showed improved anti-icing performances in terms of delayed ice formation and lower ice adhesion strength.

Adhesion of impure ice on surfaces

The undesirable buildup of ice can compromise the operational safety of ships in the Arctic to high-flying airplanes, thereby having a detrimental impact on modern life in cold climates. The obstinately strong adhesion between ice and most functional surfaces makes ice removal an energetically expensive and dangerous affair. Hence, over the past few decades, substantial efforts have been directed toward the development of passive ice-shedding surfaces. Conventionally, such research on ice adhesion has almost always been based on ice solidified from pure water. However, in all practical situations, freezing water has dissolved contaminants; ice adhesion studies of which have remained elusive thus far. Here, we cast light on the fundamental role played by various impurities (salt, surfactant, and solvent) commonly found in natural water bodies on the adhesion of ice on common structural materials. We elucidate how varying freezing temperature & contaminant concentration can significantly alter the resultant ice adhesion strength making it either super-slippery or fiercely adherent. The entrapment of impurities in ice changes with the rate of freezing and ensuing adhesion strength increases as the cooling temperature decreases. We discuss the possible role played by the in situ generated solute enriched liquid layer and the nanometric water-like disordered ice layer sandwiched between ice and the substrate behind these observations. Our work provides useful insights into the elementary nature of impure water-to-ice transformation and contributes to the knowledge base of various natural phenomena and rational design of a broad spectrum of anti-icing technologies for transportation, infrastructure, and energy systems.

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.

Predictive model of ice adhesion on non-elastomeric materials

HYPOTHESIS

When ice accumulates on a surface, it can adversely impact functionality and safety of a platform in infrastructure, transportation, and energy sectors. Despite several attempts to model the ice adhesion strength on ice-shedding materials, none have been able to justify variation in the ice adhesion strength measured by various laboratories on a simple bare substrate. This is primarily due to the fact that the effect of underlying substrate of an ice-shedding material has been entirely neglected.

EXPERIMENTS

Here, we establish a comprehensive predictive model for ice adhesion using the shear force method on a multi-layered material. The model considers both shear resistance of the material and shear stress transfer to the underlying substrate. We conducted experiments to validate the model predictions on the effect of coating and substrate properties on the ice adhesion.

FINDINGS

The model reveals the importance of the underlying substrate of a coating on ice adhesion. Most importantly, the correlation between the ice adhesion and the coating thickness are entirely different for elastomeric and non-elastomeric materials. This model justifies different measured ice adhesion across various laboratories on the same material and elucidates how one could achieve both low ice adhesion and high mechanical durability. Such predictive model and understanding provides a rich platform to guide the future material innovation with minimal adhesion to the ice.

An Extreme-Environment-Resistant Self-Healing Anti-Icing Coating

Anti-icing coatings on outdoor infrastructures and transportations inevitably suffer from surface injuries, especially in extreme weather events (e.g., freezing weather or acid rain). The coating surface damage can result in anti-icing performance loss or even icing promotion. The development of anti-icing coatings that enables self-healing in extreme conditions is highly desired but still challenging. Herein, an extreme-environment-resistant self-healing anti-icing coating is developed by integrating fluorinated graphene (FG) into a supramolecular polymeric matrix. The coating exhibits both anti-icing and deicing performance (ice nucleation temperature is ≈-30.3 °C; ice shear strength is ≈48.7 kPa), mainly attributable to the hydrophobic FG and silicone-based supramolecular material. Notably, owing to the crosslinking polymeric network with various dynamic bonds, this coating can sustain anti-icing/deicing performance after autonomous self-healing under harsh conditions including low temperature (-20 °C), strong acid (pH = 0), and strong alkali (pH = 14) environments. This coating paves the way to meet the anti-icing demand in open air, especially for the infrastructures in polar regions or acid/alkali environments.

Superhydrophobic microstructures for better anti-icing performances: open-cell or closed-cell?

Based on geometrical characteristics, all surface microstructures are categorized into two types: closed-cell and open-cell structures. Closed-cell structures are well-known to have more stable and durable superhydrophobicity at room temperatures. However, in low-temperature environments where massive environmentally induced physical changes emerge, whether closed-cell surfaces can maintain good anti-icing performances has not yet been confirmed, and thus how to design optimal superhydrophobic anti-icing microstructures is rarely reported. Here, we apply an ultrafast laser to fabricate superhydrophobic surfaces with tunable patterned micro-nanostructures from a complete closed-cell to different ratios and to a complete open-cell. We discover that droplets on closed-cell structures completely degrade to the high-adhesion Wenzel state after icing and melting cycles while those on the open-cell structures well recover to the original Cassie-Baxter state. We propose an improved ideal gas model to clarify the mechanisms that the decreased air pocket pressure and the air dissolution on closed-cell structures induce easy impalement during icing and the difficult recovery during melting, paving the way for optimizing the anti-icing structure design. The optimized open-cell surfaces exhibit over 33 times lower ice adhesion strengths (1.4 kPa) and long-term icephobic durability (<20 kPa after 33 deicing cycles) owing to the increased air pocket pressure at low temperatures. Significant dewetting processes during condensation endow the open-cell structures with more remarkable high-humidity resistance and anti-frosting properties. Our study reveals the general design principle of superhydrophobic anti-icing structures, which might guide the design of superhydrophobic anti-icing surfaces in practical harsh environments.

Fracture-controlled surfaces as extremely durable ice-shedding materials

Icing imposes a significant burden on those living in cold climates, with negative impacts on infrastructure, transportation, and energy systems. Over the past few decades, a wide range of materials with ice-shedding characteristics have been developed, including surfaces that are non-wetting/hydrophobic, liquid-infused, stress-localized, and those with low interfacial toughness. Although many of these materials have demonstrated low ice adhesion in a laboratory setting, none have achieved widespread practical adoption. This is primarily a result of the fact that they tend to have very low durability, limiting their applicability. Thus, the primary challenge in developing ice-shedding materials is finding materials with both low ice adhesion AND good durability. Here, we introduce the concept of a so-called "fracture-controlled surface." Through coordinated mechanical and chemical heterogeneity in the material structure, we affect the interfacial crack nucleation and growth on these surfaces. Through this controlled process, fracture-controlled surfaces exhibit both low ice adhesion and very high mechanical durability. Measurements of the durability of these surfaces indicate performance that is three orders of magnitude greater than other state-of-the-art ice-shedding materials. Physically, via mechanical heterogeneity of the material, we pre-specified the crack nucleation coordinates at the interface and guided the crack growth in an interfacial plane, with no kinking in other directions. This helps to maximize the energy that goes towards crack nucleation and growth. A detailed mathematical model is developed to predict adhesion of external solid objects on these materials. The model suggests that an elastic matching criterion is required to achieve minimal adhesion of solid objects on these materials. Fracture-controlled surfaces provide a rich material platform to guide future innovation of materials with minimal adhesion while having very high durability.