Improved Icephobic Properties on Surfaces with a Hydrophilic Lubricating Liquid

Recent Developments in Slippery Liquid-Infused Porous Surface Coatings for Biomedical Applications

Slippery liquid-infused porous surface (SLIPS), inspired by the Nepenthes pitcher plant, exhibits excellent performances as it has a smooth surface and extremely low contact angle hysteresis. Biomimetic SLIPS attracts considerable attention from the researchers for different applications in self-cleaning, anti-icing, anticorrosion, antibacteria, antithrombotic, and other fields. Hence, SLIPS has shown promise for applications across both the biomedical and industrial fields. However, the manufacturing of SLIPS with strong bonding ability to different substrates and powerful liquid locking performance remains highly challenging. In this review, a comprehensive overview of research on SLIPS for medical applications is conducted, and the design parameters and common fabrication methods of such surfaces are summarized. The discussion extends to the mechanisms of interaction between microbes, cells, proteins, and the liquid layer, highlighting the typical antifouling applications of SLIPS. Furthermore, it identifies the potential of utilizing the controllable factors provided by SLIPS to develop innovative materials and devices aimed at enhancing human health.

Phase change surfaces with porous metallic structures for long-term anti/de-icing application

HYPOTHESIS

Ice mitigation has received increasing attention due to the severe safety and economic threats of icing hazards to modern industries. Slippery icephobic surface is a potential ice mitigation approach due to its ultra-low ice adhesion strength, great humidity resistance, and effective delay of ice nucleation. However, this approach currently has limited practical applications because of serious liquid depletion in the icing/de-icing process.

EXPERIMENTS

A new strategy of phase change materials (PCM)-impregnation porous metallic structures (PIPMSs) was proposed to develop phase changeable icephobic surfaces in this study, and aimed to solve the rapid depletion via the phase changeable interfacial interactions.

FINDINGS

Evaluation of surface icephobicity and interfacial analysis proved that the phase changeable surfaces (PIPMSs) worked as an effective and durable icephobic platform by significantly delaying ice nucleation, providing long-term humid tolerance, low ice adhesion strength of as-prepared samples (less than 5 kPa), and signally improved maintaining capacity of impregnated PCMs (less than 10 % depletion) after 50 icing/de-icing cycles. To explore the interfacial responses, phase change models consisting of the unfrozen quasi-liquid layer and solid lubricant layer at the ice/PIPMSs interfaces were established, and the involved icephobic mechanisms of PIPMSs were studied based on the analysis of interfacial interactions.

Reducing Ice Adhesion to Polyelectrolyte Surfaces by Counterion-Mediated Nonfrozen Hydration Water

Hydrophilic anti-icing coatings can be energy-effective passive solutions for combating ice accretion and reducing ice adhesion. However, their underlying mechanisms of action remain inferential and are ill-defined from a molecular perspective. Here, we systematically investigate the influence of the counterion identity on the shear ice adhesion strength to cationic polymer coatings having quaternary alkyl ammonium moieties as chargeable groups. Temperature-dependent molecular information on the hydrated polymer films is obtained using total internal reflection (TIR) Raman spectroscopy, complemented with differential scanning calorimetry (DSC) and ellipsometry. Ice adhesion measurements show a pronounced counterion-specific behavior with a sharp increase in adhesion at temperatures that depend on the anion identity, following the order Cl- < F- < SCN- < Br- < I-. Linked to the freezing of hydration water, the specific ordering results from differences in ion pairing and the amount of water present within the polymer film. Moreover, similar effects can be promoted by varying the cross-linking density in the coating while keeping the anion identity fixed. These findings shed new light on low ice adhesion mechanisms and may inspire novel approaches for improved anti-icing coatings.

Design, fabrication, and applications of bioinspired slippery surfaces

Bioinspired slippery surfaces (BSSs) have attracted considerable attention owing to their antifouling, drag reduction, and self-cleaning properties. Accordingly, various technical terms have been proposed for describing BSSs based on specific surface characteristics. However, the terminology can often be confusing, with similar-sounding terms having different meanings. Additionally, some terms fail to fully or accurately describe BSS characteristics, such as the surface wettability of lubricants (hydrophilic or hydrophobic), surface wettability anisotropy (anisotropic or isotropic), and substrate morphology (porous or smooth). Therefore, a timely and thorough review is required to clarify and distinguish the various terms used in BSS literature. This review initially categorizes BSSs into four types: slippery solid surfaces (SSSs), slippery liquid-infused surfaces (SLISs), slippery liquid-like surfaces (SLLSs), and slippery liquid-solid surfaces (SLSSs). Because SLISs have been the primary research focus in this field, we thoroughly review their design and fabrication principles, which can also be applied to the other three types of BSS. Furthermore, we discuss the existing BSS fabrication methods, smart BSS systems, antifouling applications, limitations of BSS, and future research directions. By providing comprehensive and accurate definitions of various BSS types, this review aims to assist researchers in conveying their results more clearly and gaining a better understanding of the literature.

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.

Reduced Ice Adhesion Using Amphiphilic Poly(Ionic Liquid)-Based Surfaces

Ice build-up on solid surfaces causes significant economic losses for a range of industries. One solution to this problem is the development of coatings with low ice adhesion strength. Amphiphilic poly(ionic liquid) (PIL)-based surfaces have been recently reported for antifogging/antifrosting applications. However, they have possible anti-icing properties through lowering the ice adhesion strength that have yet to be reported. Herein, we designed well-defined triblock copolymers composed of a polydimethylsiloxane component coupled with PIL segments of poly([2 (methacryloyloxy)ethyl] trimethylammonium chloride) (PMETAC), which were subsequently UV-cured with an oligo(ethylene glycol) dimethacrylate (OEGDMA) cross-linker. The structure-property relationships of the resultant semi-interpenetrating polymer networks (SIPNs) were investigated by varying the counterion (i.e., trimethylammonium bis(trifluoromethanesulfonyl)imide (TFSI^-)) and the content of the PIL segments and cross-linker. An ice adhesion strength as low as 13.3 ± 8.6 kPa was observed for the coating containing 12.5 wt % of PMETAC segment and 5 wt % of OEGDMA, which is one of the lowest values reported so far for the amphiphilic coatings. Characterization of the coatings in terms of surface features, wettability, and hydration states have enabled the elucidation of different deicing mechanisms. Self-lubrication due to the existence of nonfreezable bound water led to the obtained low ice adhesion strength. This work offers a new approach for the exploration of PIL-based icephobic coatings for practical applications.

Key Factors Affecting Durable Anti-Icing of Slippery Surfaces: Pore Size and Porosity

Slippery liquid-infused porous surfaces (SLIPSs) are widely used as an effective passive approach to reduce icing disasters. However, various porous structures make SLIPSs exhibit different droplet mobility and lubricant stability. Undoubtedly, the substrate surface has a great impact on the durable anti-icing of SLIPSs. Herein, surfaces with different pore sizes and porosities were prepared to study their effects on the performance of SLIPS. The results show that small pores and high porosity are beneficial for the preparation of durable anti-icing SLIPS. The small pore size (about 100 nm) has a strong capillary pressure on the lubricant, and the surface with high porosity (66%) possesses a large lubricant-liquid contact ratio. These two can greatly improve the lubricant stability of SLIPS and achieve rapid self-healing. The SLIPS prepared by a suitable porous surface shows excellent anti-icing performance in the simulated glaze ice and durable anti-icing ability in the long-term icing/deicing cycles. In detail, the prepared SLIPS experiences more than 140 icing/deicing cycles through four effective self-healing while maintaining extremely low ice adhesion (<20 kPa). This work proposes a certain improved SLIPS with small pores and high porosity to achieve excellent durable anti-icing performance, broadening the practical applications of SLIPS.

A Slippery Liquid-Infused Network-like Surface with Anti/De-icing Properties Constructed Based on the Phosphating Reaction

The excessive accumulation of ice seriously threatens various industrial facilities and production activities. Currently, slippery liquid-injected porous surfaces (SLIPS) have been developed as a new strategy for anti/de-icing; however, the lack of research on the adsorption and storage capacity for lubricating fluids has limited the development of SLIPS in the anti/de-icing field to some extent. In this work, a slippery liquid-infused phosphate network-like surface (SLIPNS) is prepared that adjusts the texture of the surface by varying the phosphating time to control the adsorption and storage of lubricating fluids. The as-obtained surface structure gives the SLIPNS excellent oil-storage/locked properties, can delay the freezing time of sessile droplets up to 436 s, which is almost 10 times that of an untreated aluminum sheet, and exhibits one-tenth the ice adhesion strength of untreated aluminum substrates (14.39 kPa). In addition, the SLIPNS shows effective durability and antifouling ability and has great potential in solving long-term anti/de-icing problems.

Total Liquid Transfer with Enhanced Contact Line Slippage

A new surface treatment method is developed to achieve total liquid transfer. The transfer process of a liquid droplet is recorded through high-speed photography and analyzed via image analysis to investigate the hydrodynamic interactions. For a pristine PMMA surface, a viscous and viscoelastic liquid facilitates transfer by increased viscous and inertial forces and delayed liquid bridge breakage but is limited by slow contact line slippage. Hydrophobic surface treatments can increase contact line slippage and the receding angle to achieve transfer ratios up to 98%. However, pinning and contact angle hysteresis from surface roughness features limit liquid transfer, especially for smaller droplets and higher separation velocities. A lubricant-infused surface treatment with PDMS and a thin layer of less viscous silicone oil provides a smooth, homogeneous surface with fast slippage, low contact angle hysteresis, and only a slight oil wetting ridge. Liquid could then transfer at high ratios (∼99.9%), regardless of droplet size and separation velocity. Finally, complete transfer liquid from indented cells is demonstrated to show the potential of this surface modification method for gravure printing.

Mechanical Durability of Low Ice Adhesion Polydimethylsiloxane Surfaces

Elastomeric surfaces and oil-infused elastic surfaces reveal low ice adhesion, in part because of their deformability. However, these soft surfaces might jeopardize their mechanical durability. In this work, we analyzed the mechanical durability of elastic polydimethylsiloxane (PDMS) surfaces with different balances between elasticity and deicing performances. The durability was studied in terms of shear/tensile ice adhesion strength before and after different wear tests. These tests consisted of abrasion/erosion cycles using standard procedures aimed to reproduce different environmental wearing agents. The main objective is to evaluate if our PDMS surfaces can become long-lasting solutions for ice removal in real conditions. We found that our elastic surfaces show excellent durability. After the wear tests, the ice adhesion strength values remained low or even unaltered. Although the oil-infused PDMS surface was the softest one, it presented considerable durability and excellent low ice adhesion, being a promising solution.

Frost Delay of a Water-Absorbing Surface with Engineered Wettability via Nonfreezing Water

Frost is common when a solid surface is subjected to a humid and cold environment. It can cause various inconveniences, complications, or fatal accidents. Water-repellent surfaces have demonstrated an antifreezing capability by enabling the water droplets to roll or bounce off before they freeze. However, these surfaces are often limited by their inability to shed the small water condensates, which can eventually grow and freeze. Recently, surfaces that can rapidly absorb and hydrogen bond with these water condensates have demonstrated significant delay in frost formation and growth. This is attributed to a lower freezing temperature of the absorbed water which makes it stay in a nonfreezing state. Herein, we report a surface with preferential wettability of water over oil (i.e., superhydrophilic and oleophobic wettability) that can significantly delay frost formation. The surface is fabricated by copolymerizing poly(ethylene glycol) diacrylate (PEGDA) and perfluorinated acrylate (1H,1H,2H,2H-heptadecafluorodecyl acrylate, HDF-acrylate) applied to a silane-grafted glass substrate (HDF-PEGDA). An HDF-PEGDA surface can quickly absorb condensed water which enables it to delay frost formation and growth for up to 20 min at a surface temperature of -35 °C. Also, the surface demonstrates that its frost-resistant capability remains almost unaffected even after being submerged in an oil bath due to its in-air oil repellency. Differential scanning calorimetry (DSC) measurements reveal that the significant quantity of absorbed water in an HDF-PEGDA surface remains in a nonfreezing state with a T_m value as low as -33 °C. A mathematical model that can predict the time at which the surface begins to be covered with frost is developed. Finally, an HDF-PEGDA is layered with a PEGDA copolymerized with sodium acrylate (Na-acrylate) that enables the continuous release of the absorbed water by posing forward osmotic pressure and regeneration of an HDF-PEGDA surface.

Kinetic Inhibition of Clathrate Hydrate by Copolymers Based on N-Vinylcaprolactam and N-Acryloylpyrrolidine: Optimization Effect of Interfacial Nonfreezable Water of Polymers

Amphiphilic polymers have now been designed to achieve an icephobic performance and have been used for ice adhesion prevention. They may function by forming a strongly bonded but nonfreezable water shell which serves as a self-lubricating interfacial layer that weakens the adhesion strength between ice and the surface. Here, an analogous concept is built to prevent the formation of clathrate hydrate compounds during oil and natural gas production, in which amphiphilic water-soluble polymers act as efficient kinetic hydrate inhibitors (KHIs). A novel group of copolymers with N-vinylcaprolactam and N-acryloylpyrrolidine structural units are investigated in this study. The relationships among the amphiphilicity, lower critical solution temperature, nonfreezable bound water, and kinetic hydrate inhibition time are analyzed in terms of the copolymer compositions. Low-field NMR relaxometry revealed the crucial interfacial water in tightly bound dynamic states which led to crystal growth rates changing with the copolymer compositions, in accord with the rotational rheometric analysis results. The nonfreezable bound water layer confirmed by a calorimetry analysis also changes with the polymer amphiphilicity. Therefore, in the interface between the KHI polymers and hydrate, water surrounding the polymers plays a critical role by helping to delay the nucleation and growth of embryonic ice/hydrates. Appropriate amphiphilicity of the copolymers can achieve the optimal interfacial properties for slowing down hydrate crystal growth.

Superhydrophobic versus SLIPS: Temperature dependence and the stability of ice adhesion strength

HYPOTHESIS

Ice adhesion to solids, being affected by the ice/solid intermolecular interactions and structure of ice surface layer is dependent on temperature, and ice surface layer equilibration time.

EXPERIMENTS

A new centrifugal method of shear ice adhesion strength measurement with accurate temperature control on each stage from ice formation on test surfaces to the adhesion measurement is applied to study ice adhesion to superhydrophobic and slippery surfaces. The determinative advantage of the developed method is related to monitoring in one experiment the ice detachment from numerous samples and accurate measuring the rotation frequency for each ice detachment.

FINDINGS

The following findings will be discussed.

Design of Icephobic Surfaces by Lowering Ice Adhesion Strength: A Mini Review

Ice accretion can lead to severe consequences in daily life and sometimes catastrophic events. To mitigate the hazard of icing, passive icephobic surfaces have drawn widespread attentions because of their abilities in repelling incoming water droplets, suppressing ice nucleation and/or lowering ice adhesion strength. As time elapses and temperature lowers sufficiently, ice accretion becomes inevitable, and a realistic roadmap to surface icephobicity for various outdoor anti-icing applications is to live with ice but with the lowest ice adhesion strength. In this review, surfaces with icephobicity are critically categorized into smooth surfaces, textured surfaces, slippery surfaces and sub-surface textured surfaces, and discussed in terms of theoretical limit, current status and perspectives. Particular attention is paid to multiple passive anti-icing strategies combined approaches as proposed on the basis of icephobic surfaces. Correlating the current strategies with one another will promote understanding of the key parameters in lowering ice adhesion strength. Finally, we provide remarks on the rational design of state-of-the-art icephobic surfaces with low ice adhesion strength.

Designing Self-Sustainable Icephobic Layer by Introducing a Lubricating Un-Freezable Water Hydrogel from Sodium Polyacrylate on the Polyolefin Surface

A convenient, environment-friendly, and cost-effective method to keep anti-icing for a long time was highly desirable. Slippery lubricant layers were regarded to be effective and promising for anti-icing on different surfaces, but the drought-out of lubricants and the possible detriments to the environment were inevitable. By combining super-high molecular weight sodium polyacrylate (H-PAAS) with polyolefin through a one-pot method, a self-sustainable lubricating layer with extremely low ice adhesion of un-freezable water hydrogel was achieved at subzero conditions. The lubricant hydrogel layer could auto-spread and cover the surface of polyolefin after encountering supercooled water, frost, or ice. Due to the reduction of storage modulus in the interface, the ice adhesion of the specimen surfaces was far below 20 kPa, varying from 5.13 kPa to 18.95 kPa. Furthermore, the surfaces could preserve the fairly low adhesion after icing/de-icing cycles for over 15 times and thus exhibited sustainable durability. More importantly, this method could be introducing to various polymers and is of great promise for practical applications.

Topology Reliable LCST-Type Behavior of ABA Triblock Polymer and Influence on Water Condensation and Crystallization

As a kind of smart material, thermoresponsive hydrogels are widely investigated and applied in many fields. Due to the limitation of the freezing temperature of the water, it is a challenge to further broaden their sol-gel transition temperature (T_gel ) range, especially below 0 °C. Herein, the lower critical solution temperature type of amphiphilic ABA triblock copolymers, synthesized via two-step reversible addition-fragmentation chain transfer (RAFT) polymerization is demonstrated. The hydrophilic A-block and the hydrophobic B-block are composed of poly(N,N-dimethylacrylamide) (PDMAA) and poly(diacetone acrylamide) (PDAAM), respectively. The degree of polymerization (DP) of both A-block and B-block shows a significant influence on the T_gel of triblock copolymer dispersion. By changing the length of these two blocks or physically blending these copolymers dispersions, the T_gel can be well adjusted in a temperature range from 45 to -10 °C. Moreover, When the T_gel is higher than 4 °C, the triblock copolymer coatings show a good anti-fogging property. And when the T_gel is around or lower than the freezing temperature of the water, aqueous dispersions of the triblock copolymer have an ice recrystallization inhibition activity, resulting in the decrease of average maximum grain size (MLGS) of ice crystal.

Robust icephobic coating based on the spiky fluorinated Al2O3 particles

Omniphobic and icephobic twin-scale surfaces based on the “urchin”-like fluorinated Al₂O₃ particles are presented. Combined effect of hierarchical topography and fluorination supplied to the surfaces omniphobic and icephobic properties. The study of the stability of the Cassie wetting state is reported. High apparent contact angles were accompanied with the low contact angle hysteresis and high stability of the Cassie air trapping wetting state. Time delay of the ice crystallization as high as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$88\pm 5$$\end{document}88±5 min was established when compared to the ice formation on flat aluminum and non-fluorinated “urchin”-like surfaces. Crystallized water droplets formed on the reported nano-structured surfaces were easily blown out by the air jet with the velocity of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$v=3.0\pm 1.0$$\end{document}v=3.0±1.0 m/s, (which is markedly lower than that common for exploitation of aircrafts and turbines). Heated “urchin”-like surfaces completely restored their omniphobic and icephobic surfaces after thawing. Qualitative analysis of water freezing is supplied.

Polysiloxane Nanofilaments Infused with Silicone Oil Prevent Bacterial Adhesion and Suppress Thrombosis on Intranasal Splints

Like all biofluid-contacting medical devices, intranasal splints are highly prone to bacterial adhesion and clot formation. Despite their widespread use and the numerous complications associated with infected splints, limited success has been achieved in advancing their safety and surface biocompatibility, and, to date, no surface-coating strategy has been proposed to simultaneously enhance the antithrombogenicity and bacterial repellency of intranasal splints. Herein, we report an efficient, highly stable lubricant-infused coating for intranasal splints to render their surfaces antithrombogenic and repellent toward bacterial cells. Lubricant-infused intranasal splints were prepared by creating superhydrophobic polysiloxane nanofilament (PSnF) coatings using surface-initiated polymerization of n-propyltrichlorosilane (n-PTCS) and further infiltrating them with a silicone oil lubricant. Compared with commercially available intranasal splints, lubricant-infused, PSnF-coated splints significantly attenuated plasma and blood clot formation and prevented bacterial adhesion and biofilm formation for up to 7 days, the typical duration for which intranasal splints are kept. We further demonstrated that the performance of our engineered biointerface is independent of the underlying substrate and could be used to enhance the hemocompatibility and repellency properties of other medical implants such as medical-grade catheters.

The effects of bio-inspired micro/nano scale structures on anti-icing properties

Ice formation and accumulation have detrimental effects on commercial surfaces and people's lives. The ice adhesion strength decreases with increasing surface hydrophobicity, and the superhydrophobicity of a surface can be constructed by a combination of low surface free energy and high surface roughness. Conversely, the characteristics of biological surfaces have aroused wide attention as a result of the superhydrophobicity of plants and animals, deriving from the synergistic effects of chemical compositions and multi-scale hierarchical structures. Therefore, inspired by bio-mimetic studies on biological surfaces, a lot of artificial bio-inspired superhydrophobic surfaces have been broadly designed and constructed. Herein, we aim to summarize the fundamental theories of surface wettability and recent progress in the fabrication of bio-inspired surfaces. The bio-inspired surfaces prepared by different facile methods not only have superhydrophobicity, but also have anti-icing/icephobic properties. In the end, some challenges and problems in the future study and advancement of bio-inspired superhydrophobic surfaces are proposed.

Biomimetic Brushlike Slippery Coatings with Mechanically Robust, Self-Cleaning, and Icephobic Properties

In this work, a facile strategy was proposed to prepare a series of brushlike thermoplastic polyurethane (TPU) coatings with mechanically robust, self-cleaning, and icephobic performance. Through a simple multicomponent click reaction of thiolactone with a diamine compound and mono-ethenyl-terminated polydimethylsiloxane (mono-ethenyl-PDMS), a diol with amide groups and flexible PDMS was synthesized, and a novel TPU could be obtained productively by a reaction of isocyanate and diol. The unique chain structure endowed TPU films with ascendant self-stratifying properties. During solvent vapor annealing, flexible PDMS chains migrated and enriched to the surface while urethane linkages with a strong interaction tended to locate at the substrate. Based on this, TPU-PDMS films exhibited mechanically robust property, and the tensile strength value of TPU-PDMS-3 showed a sharp increase to 48.62 MPa. The resultant TPU-PDMS-10 coatings exhibited a water repellent behavior and possessed superior movability of droplet water, and also the dirt on it could be readily removed by rinsing with water without leaving any traces. Furthermore, three different criteria were used to characterize the icephobic performance. The coatings exhibited a significantly lower freezing point (approximately -27 °C) of supercooled water, longer delay-icing time, and less ice adhesion shear strength. Therefore, these novel brushlike TPU coatings have tremendous potential applications.

Moth-Eye Mimicking Solid Slippery Glass Surface with Icephobicity, Transparency, and Self-Healing

Slippery liquid-infused porous surfaces (SLIPSs) have been actively studied to improve the limitations of superhydrophobic (SHP) surfaces, especially the defects of the nonwetting chemical coating layer and the weak mechanical robustness of surface micro/nanostructures. However, the SLIPSs also have several drawbacks including volatilization and leakage of lubricant caused by long-term usage. In this study, we suggest the use of icephobic, highly transparent, and self-healing solid slippery surface to overcome the limitations of both surfaces (SLIPS and SHP) by combining specific biomimetic morphology and intrinsic properties of paraffin wax. A moth-eye mimicking nanopillar structure was prepared instead of a porous structure and was coated with solid paraffin wax for water repellence. Moth-eye structures enable high surface transparency based on antireflective effect, and the paraffin layer can recover from damage due to sunlight exposure. Furthermore, the paraffin coating on the nanopillars provides an air trap, resulting in a low heat transfer rate, increasing freezing time and reducing adhesion strength between the ice droplet and the surface. The heat transfer model was also calculated to elucidate the effects of the nanopillar height and paraffin layer thickness. The antireflection and freezing time of the surfaces are enhanced with increase in nanopillar height. The paraffin layer slightly deteriorates the transmittance but enhances the icephobicity. The solar cell efficiency using a biomimetic solid slippery surface is higher than that of bare glass due to the antireflective effect. This integrated biomimetic solid slippery surface is multifunctional due to its self-cleaning, anti-icing, antireflection, and self-healing properties and may replace SLIPS and SHP surfaces.

Hydrogels as Durable Anti-Icing Coatings Inhibit and Delay Ice Nucleation

The accumulation of ice on surfaces brings dangerous and costly problems to our daily life. Thus, it would be desirable to design anti-icing coatings for various surfaces. We report a durable anti-icing coating based on mussel-inspired chemistry, which is enabled via fabricating a liquid water layer, achieved by modifying solid substrates with the highly water absorbing property of sodium alginate. Dopamine, the main component of the mussel adhesive protein, is introduced to anchor the sodium alginate in order to render the coating applicable to all types of solid surfaces. Simultaneously, it serves as the cross-linking agent for sodium alginate; thus, the cross-linking degree of the coatings could be easily varied. The non-freezable and freezable water in the coatings with different cross-link degrees all remain liquid-like at subzero conditions and synergistically fulfill the aim of decreasing the temperature of ice nucleation. These anti-icing coatings display excellent stability even under harsh conditions. Furthermore, these coatings can be applied to almost all types of solid surfaces and have great promise in practical applications.

A New Measuring System for the Determination of the Ice Adhesion Strength on Smooth Surfaces

To gain knowledge about cause-effect relationships for the adhesion of ice on surfaces with different chemical groups, we wanted to study the effect of thin polymer layers on the ice adhesion strength. To minimize the effect of roughness, smooth substrates that have generally relatively low ice adhesion strengths were chosen. To be able to obtain highly reproducible values for the region of low ice adhesion and to measure small differences of ice adhesion at surfaces with different chemical compositions, a new measuring system for the determination of the ice adhesion strength which is based on a modified spin-coater was developed. We show its technical potential on the basis of first results on pure silicon wafers and selected hydrophilic polymer layers. Furthermore, we investigated the effect of the water quality on the ice adhesion strength. The obtained data are discussed in the context of physicochemical properties of the layers and of the chemical characteristics of the used polymers.

Phosphonium Ionic Liquid-Infused Poly(vinyl chloride) Surfaces Possessing Potent Antifouling Properties

Microbial fouling is a costly issue, which impacts a wide range of industries, such as healthcare, food processing, and construction industries, and improved strategies to reduce the impact of fouling are urgently required. Slippery liquid-infused porous surfaces (SLIPSs) have recently been developed as a bioinspired approach to prevent antifouling. Here, we report the development of slippery, superhydrophilic surfaces by infusing roughened poly(vinyl chloride) (PVC) substrates with phosphonium ionic liquids (PILs). These surfaces were capable of reducing viable bacterial adherence by Staphylococcus aureus and Pseudomonas aeruginosa by >6 log10 cfu mL-1 after 24 h under static conditions relative to control PVC. Furthermore, we report the potential of a series of asymmetric quaternary alkyl PILs with varying alkyl chain lengths (C4-C18) and counteranions to act as antimicrobial agents against both Gram +ve and Gram -ve bacteria and illustrate their potential as antimicrobial alternatives to traditional fluorinated lubricants commonly used in the synthesis of SLIPSs.

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.

Reducing Static and Impact Ice Adhesion with a Self-Lubricating Icephobic Coating (SLIC)

Ice accumulation and adhesion can problematically occur on many engineering systems, such as electrical power networks, wind turbines, communication towers, and aircraft. An optional solution to these icing problems is the use of surfaces/coatings with low ice adhesion properties: Icephobic surfaces. Icephobic surfaces/coatings are very beneficial, as they facilitate the removal of ice or retard its formation and do not require the use of any sort of energy. A compact icing research tunnel (CIRT) was employed to measure ice tensile adhesion strength for both impact and static ice on a conventional metal surface (aluminum) and on a Self-Lubricating Icephobic Coating (SLIC) surface. The static ice consisted of deionized water slowly poured over the surface and left to be frozen on the test specimen surface at stationary conditions, while impact ice consisted of droplets of mean volumetric diameter (MVD) of 13 μm impacting the test specimen surface at a velocity of 40 m/s and freezing and accreting dynamically. The results revealed that static ice has an ice tensile adhesion stress higher than that of impact ice for the conditions used, consistent with previous studies. Additionally, a reduction of more than half was observed in ice tensile adhesion stress for SLIC compared to aluminum for both impact and static ice, and this performance stayed consistent even after multiple icing tests on the same sample. The SLIC coating hydrophobicity (roll-off angle and contact angle) also demonstrated resilience to icing and mechanical abrasion, confirming the self-healing properties.

From Extremely Water-Repellent Coatings to Passive Icing Protection—Principles, Limitations and Innovative Application Aspects

The severe environmental conditions in winter seasons and/or cold climate regions cause many inconveniences in our routine daily-life, related to blocked road infrastructure, interrupted overhead telecommunication, internet and high-voltage power lines or cancelled flights due to excessive ice and snow accumulation. With the tremendous and nature-inspired development of physical, chemical and engineering sciences in the last few decades, novel strategies for passively combating the atmospheric and condensation icing have been put forward. The primary objective of this review is to reveal comprehensively the major physical mechanisms regulating the ice accretion on solid surfaces and summarize the most important scientific breakthroughs in the field of functional icephobic coatings. Following this framework, the present article introduces the most relevant concepts used to understand the incipiency of ice nuclei at solid surfaces and the pathways of water freezing, considers the criteria that a given material has to meet in order to be labelled as icephobic and clarifies the modus operandi of superhydrophobic (extremely water-repellent) coatings for passive icing protection. Finally, the limitations of existing superhydrophobic/icephobic materials, various possibilities for their unconventional practical applicability in cryobiology and some novel hybrid anti-icing systems are discussed in detail.

Life and death of liquid-infused surfaces: a review on the choice, analysis and fate of the infused liquid layer

We review the rational choice, the analysis, the depletion and the properties imparted by the liquid layer in liquid-infused surfaces – a new class of low-adhesion surface.

Liquid infused surfaces with anti-icing properties

Ice accretion on solid surfaces, a ubiquitous phenomenon that occurs in winter, brings much inconvenience to daily life and can even cause serious catastrophes. Icephobic surfaces, a passive way of processing surfaces to prevent surface destruction from ice accumulation, have attracted much attention from scientists because of their special ice-repellent properties, and many efforts have been made to rationally design durable icephobic coatings. This review is aimed at providing a brief and crucial overview of ice formation processes and feasible de-icing strategies. Here, the excellent anti-icing performance of liquid infused surfaces (LIS) inspired from Nepenthes is emphatically introduced. After a short introduction, the recent progresses in ice nucleation theory and ice adhesion decrease mechanism are comprehensively reviewed to gain a general understanding of the long freeze process and low ice adhesion on LIS. Subsequently, the anti-icing performance of LIS is systematically evaluated from four aspects regarding water repellence, condensation-frosting, long freeze process, and low ice adhesion. Finally, this review focuses on discussing the advantages and disadvantages of LIS and the potential measures to eliminate and alleviate these drawbacks.

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.

Highly Stable Amphiphilic Organogel with Exceptional Anti-icing Performance

Various organogel materials with either a liquid or solid surface layer have recently been designed and prepared. In this work, amphiphilic organogels (AmOG) are innovatively developed from copolymer P(PDMS-r-PEG-r-GMA) and 2,2'-diaminodiphenyldisulfide via epoxy group addition reaction and then infiltrated with amphiphilic lubricants instead of traditional hydrophilic or hydrophobic lubricants. Because of synergistic effects of hydrophilic and hydrophobic segments of amphiphilic lubricants, the AmOG surfaces showed high stability and excellent anti-icing performance. The delay in the freezing point of water was 1000 s on AmOG surfaces, which is 40 times longer as compared to the untreated hydrophilic glass surface. More importantly, low ice adhesion strength (15.1 kPa) was observed on AmOG which remained about 40 kPa even after 20 icing-deicing cycles. The novel amphiphilic organogels provide a new idea to prepare long-term anti-icing materials for practical applications.

Icephobic Strategies and Materials with Superwettability: Design Principles and Mechanism

Ice formation and accretion on surfaces is a serious economic issue in energy supply and transportation. Recent strategies for developing icephobic surfaces are intimately associated with superwettability. Commonly, the superwettability of icephobic materials depends on their surface roughness and chemical composition. This article critically categorizes the possible strategies to mitigate icing problems from daily life. The wettability and classical nucleation theories are used to characterize the icephobic surfaces. Thermodynamically, the advantages/disadvantages of superhydrophobic surfaces are discussed to explain icephobic behavior. The importance of elasticity, slippery liquid-infused porous surfaces (SLIPSs), amphiphilicity, antifreezing protein, organogels, and stimuli-responsive materials has been highlighted to induce icephobic performance. In addition, the design principles and mechanism to fabricate icephobic surfaces with superwettability are explored and summarized.

Formation of Icephobic Surface with Micron-Scaled Hydrophobic Heterogeneity on Polyurethane Aerospace Coating

Development of an anti-icing surface on a desired industrial coating patch/object has been the persistent challenge to several industries, such as aviation and wind power. For this aim, performing surface modification to implement the icephobic property on existing commercial coatings is important for practical applications. This work accomplishes an icephobic coating overlying a PPG aerospace polyurethane coating. It manifests a clear capability to delay the formation of frost as well as to reduce the adhesion strength of ice. This icephobic coating is sustained by a unique hydrophobic heterogeneity in the micron-scale of segregation, which is realized through solution casting of a specific copolymer consisting of random rigid and soft segments, namely poly(methyl methacrylate) and poly(lauryl methacrylate-2-hydroxy-3-(1-amino dodecyl)propyl methacrylate), respectively. A wrinkled pattern developed over the coating is observed because of the diverse traits between these two segments. Besides, the OH/NH groups of the soft segment are crosslinked by a diisocyanate monomer upon drying and curing to strengthen the coating. More importantly, integration of a small dose of paraffin wax into the copolymer induces a spread of soft microdomains on the winkled pattern surface. It is hypothesized that these dual heterogeneous assemblies are responsible for the icephobicity since they instigate distinct interactions with condensed water droplets. Lastly, the thermoelectric cooling (Peltier effect) and the adhesion strength of ice on the typical coatings were assessed. This investigation also includes examination on the icephobic durability of coating, which is enhanced when a small amount of polyethylene oligomer is incorporated into the coating.

Passive Antifrosting Surfaces Using Microscopic Ice Patterns

Despite exceptional recent advances in tailoring the wettability of surfaces, to date, no engineered surface can passively suppress the in-plane growth of frost that invariably occurs in humid, subfreezing environments. Here, we show that up to 90% of a surface can exhibit passive antifrosting by using chemical or physical wettability patterns to template "ice stripes" across the surface. As ice exhibits a depressed vapor pressure relative to liquid water, these sacrificial ice stripes siphon the supersaturated water vapor to keep the intermediate surface areas dry from dew and frost. Further, we show that when these sacrificial ice stripes are elevated atop microfins, they diffusively coarsen in a suspended state above the surface. The suspended state of the coarsening ice results in a diffusive growth rate an order of magnitude slower than frost coarsening directly on a solid substrate and should also minimize its adhesive strength to the surface.

Frosting Behavior of Superhydrophobic Nanoarrays under Ultralow Temperature

Retarding and preventing frost formation at ultralow temperature has an increasing importance due to a wide range of applications of ultralow fluids in aerospace and industrial facilities. Recent efforts for developing antifrosting surfaces have been mostly devoted to utilizing lotus-leaf-inspired superhydrophobic surfaces. Whether the antifrosting performance of the superhydrophobic surface is still effective under ultralow temperature has not been elucidated clearly. Here, we investigated the frosting behavior of fabricated superhydrophobic ZnO nanoarrays under different temperature and different environment. The surface showed excellent performance in anticondensation and antifrosting when the surface temperature was approximately -20 °C. Although the frosting event inevitably occurs on all surfaces when the temperature is decreased from -50 to -150 °C, the frost accumulation on the superhydrophobic surfaces is always less than that on the untreated surfaces. Interestingly, the frost layer detaches from the surface within a short time and keeps the surface dry in the very beginning of the defrosting process. Further, there is no frost formation on the surface at -20 °C during 10 min testing when blowing compressed air and spraying methanol together or spraying methanol individually. It can reduce the height of the frost layer and increases the density when spraying methanol at -150 °C. Furthermore, the frost crystals on the top surface can been blown away due to the low adhesion of ice or frost. It provides a basic idea for solving the frosting problem under ultralow temperature while combined with other defrosting methods.

Durable Anti-Icing Coatings Based on Self-Sustainable Lubricating Layer

A versatile, convenient, and cost-effective method that can be used for grafting anti-icing materials onto different surfaces is highly desirable. Based on mussel-inspired chemistry, the anti-icing coating with extremely low ice adhesion is enabled by constructing a self-sustainable lubricating layer, achieved via modifying solid substrates with a highly hydrophilic conjugate of poly(acrylic acid)-dopamine. Both unfreezable and freezable water remain liquidlike at subzero conditions and synergistically fulfill the role of lubrication for reducing the ice adhesion. The anti-icing coatings show excellent stability in harsh environments and durability after the cross-linking. More importantly, this coating can be applied to various substrates and is of great promise for practical applications.

Robust Slippery Coating with Superior Corrosion Resistance and Anti-Icing Performance for AZ31B Mg Alloy Protection

Biomimetic slippery liquid-infused porous surfaces (SLIPSs) are developed as a potential alternative to superhydrophobic surfaces (SHSs) to resolve the issues of poor durability in corrosion protection and susceptibility to frosting. Herein, we fabricated a double-layered SLIPS coating on the AZ31 Mg alloy for corrosion protection and anti-icing application. The porous top layer was infused by lubricant, and the compact underlayer was utilized as a corrosion barrier. The water-repellent SLIPS coating exhibits a small sliding angle and durable corrosion resistance compared with the SHS coating. Moreover, the SLIPS coating delivers durable anti-icing performance for the Mg alloy substrate, which is obviously superior to the SHS coating. Multiple barriers in the SLIPS coating, including the infused water-repellent lubricant, the self-assembled monolayers coated porous top layer, and the compact layered double hydroxide-carbonate composite underlayer, are suggested as being responsible for the enhanced corrosion resistance and anti-icing performance. The robust double-layered SLIPS coating should be of great importance to expanding the potential applications of light metals and their alloys.

Icephobic Surfaces Induced by Interfacial Nonfrozen Water

It is known that smooth, hydrophobic solid surfaces exhibit low ice adhesion values, which have been shown to approach a lower ice adhesion strength limit (∼150 kPa) defined by the water receding contact angle. To overcome this limit, we have designed self-lubricating icephobic coatings by blending polydimethylsiloxane (PDMS)-poly(ethylene glycol) (PEG) amphiphilic copolymers into a polymer matrix. Such coatings provide low ice adhesion strength values (∼50 kPa) that can substantially reduce the lower bound of the ice adhesion strength achieved previously on smooth, hydrophobic solid surfaces. Different molecular mechanisms are responsible for the low ice adhesion strength attained by these two approaches. For the smooth hydrophobic surfaces, an increased water depletion layer thickness at the interface weakens the van der Waals' interactions between the ice and the polymeric substrate. For the self-lubricating icephobic coatings, the PEG component of the amphiphilic copolymer is capable of strongly hydrogen bonding with water molecules. The surface hydrogen-bonded water molecules do not freeze, even at substantial levels of subcooling, and therefore serve as a self-lubricating interfacial liquid-like layer that helps to reduce the adhesion strength of ice to the surface. The existence of nonfrozen water molecules at the ice-solid interface is confirmed by solid-state nuclear magnetic resonance (NMR) spectroscopy.

Suppression of Frost Nucleation Achieved Using the Nanoengineered Integral Humidity Sink Effect

Inhibition of frost formation is important for increasing efficiency of refrigeration systems and heat exchangers, as well as for preventing the rapid icing over of water-repellant coatings that are designed to prevent accumulation of rime and glaze. From a thermodynamic point of view, this task can be achieved by either increasing hydrophobicity of the surface or decreasing the concentration of water vapor above it. The first approach has been studied in depth, but so far has not yielded a robust solution to the problem of frost formation. In this work, we systematically explore how frost growth can be inhibited by controlling water vapor concentration using bilayer coatings with a porous exterior covering a hygroscopic liquid-infused layer. We lay the theoretical foundation and provide experimental validation of the mass transport mechanism that governs the integral humidity sink effect based on this coating platform as well as reveal intriguing sizing effects about this system. We show that the concentration profile above periodically spaced pores is governed by the sink and source concentrations and two geometrical parameters: the nondimensional pore size and the ratio of the pore spacing to the boundary layer thickness. We demonstrate that when the ratio of the pore spacing to the boundary layer thickness vanishes, as for the nanoporous bilayer coatings, the entire surface concentration becomes uniform and equal to the concentration set by the hygroscopic liquid. In other words, the surface concentration becomes completely independent of the nanopore size. We identified the threshold geometrical parameters for this condition and show that it can lead to a 65 K decrease in the nucleation onset surface temperature below the dew point. With this fundamental insight, we use bilayer coatings to nanoengineer the integral humidity sink effect to provide extreme antifrosting performance with up to a 2 h delay in nucleation onset at 263 K. The nanoporous bilayer coatings can be designed to combine optimal antifrosting functionality with a superhydrophobic water repelling exterior to provide coatings that can robustly prevent frost, rime, and glaze accumulation. By minimizing the required amount of antifreeze, this anti-icing method can have minimal operational cost and environmental impact.