Strategies for anti-icing: low surface energy or liquid-infused?

Abstract

Anti-icing is of great importance in society since icing on facility surfaces may bring about serious disasters and economical losses in fields such as aerospace, transportation and electrical communication. Development of polymeric coatings with excellent anti-icing behaviours has been one of the practical topics in recent years. Control of the chemical compositions and topological surface structures is vital to anti-icing coatings. In this review, we summarize the recent progresses on polymeric anti-icing coatings prepared from low surface energy, hydrophilic or amphiphilic polymers. Surface characteristics of the anti-icing coatings including morphology, wettability and ice adhesion strength are discussed. Comparisons between representative studies, including low surface energy coatings and liquid-infused slippery surfaces will be highlighted, with emphasis on the polymer substrate properties and innovative aspects. This review is aimed at giving a brief and crucial overview of the strategies for preparation of icephobic coatings and fulfillment of the anti-icing behaviours.

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Bo Liu

Bo Liu received his bachelor's degree in the Faculty of Polymer Materials Science & Engineering from Hubei University in 2012 and master's degree from Hubei University in 2015. He is currently a PhD candidate in Tianjin University under the supervision of Professor Xiaoyan Yuan. His research interests focus on anti-icing coatings and antifreeze polymers.

Kaiqiang Zhang

Kaiqiang Zhang is currently a research engineer at the Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration of China. He received his master's degree in Tianjin University in 2016 under the supervision of Professor Xiaoyan Yuan. His research interests include icephobic coatings, anticorrosive coatings, living free-radical polymerization.

Chao Tao

Chao Tao received her bachelor's degree with honours in School of Materials Science and Engineering of Tianjin University in 2014, and is currently pursuing her master's degree under the supervision of Professor Xiaoyan Yuan. Her work focuses on anti-icing coatings.

Yunhui Zhao

Dr Zhao received her PhD degree in Tianjin University, China in 2002. She worked as an associate professor at School of Materials Science and Engineering in Tianjin University after a postdoc research in 2004. Her research interests involve functional and biomedical polymer materials.

Xiaohui Li

Dr Xiaohui Li received his PhD degree in Tianjin University, China in 2015 under the supervision of Professor Xiaoyan Yuan. Now he works at School of Materials Science and Engineering in Tianjin University as an engineer. His research interests focus on anti-icing coatings and anti-fogging coatings.

Kongying Zhu

Dr Kongying Zhu received his PhD degree in Tianjin University, China in 2012 under the supervision of Professor Xiaoyan Yuan. He has been working at Analysis and Measurement Center, Tianjin University since 2005. His research interests focus on solid-state nuclear magnetic resonance, multicomponent polymers, and anti-icing coatings.

Xiaoyan Yuan

Professor Xiaoyan Yuan started working at Tianjin University after she obtained her master's degree in 1988. She received her PhD degree from Tianjin University in 1997, and was promoted as a Professor in Polymer Materials in 2003. She ever worked as a research associate at the Hong Kong Polytechnic University from 1998 to 2001 with Prof. Arthur F. T. Mak, and visited Dr Qiaobing Xu at Tufts University from Mar. to Sept. in 2015. Her research interests focus on biodegradable biomaterials and anti-icing coatings and also include electrospinning, multicomponent polymers, and functional materials.

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1. Introduction

Icing on surfaces may cause severe accidents and disasters, such as hindering the normal operation of infrastructures, including aircrafts, ships, offshore oil platforms, wind turbines, power lines, dams, and telecommunication facilities.^1–3 Due to its great destructiveness and unforeseeable threat, great efforts have been dedicated to better understand the icing mechanism, and investigations on anti-icing surfaces have been conducted extensively in the past few decades.^4–7

Active de-icing and passive anti-icing are two strategies used in industries.⁸ Based on thawing or breaking the already formed ice layers, the former is energy-intensive and high cost.^1,9,10 In contrast, passive anti-icing is a more appealing and universal strategy for mitigating the icing problem, which is to engineer surfaces that depress both the ice quantity and the adhesion of the accreted ice.^11,12

Superhydrophobic surfaces are believed to effectively prevent ice formation by means of sliding or bouncing off water droplets before frozen.^13–18 Generally, artificial superhydrophobic surfaces are commonly fabricated via creating micro-/nanostructures on hydrophobic substrates^19,20 or chemically modifying a micro-/nano-structured surface with low surface energy materials.^21–24 Superhydrophobic surfaces with micro- or nano-scale roughness could prevent ice accumulation under certain conditions,^25–35 but can be detrimental in other environments.^12,36–39 Such as, under higher humidity conditions, moisture is facile to condense in the rough structure of superhydrophobic surface over time and droplet would accumulate rapidly and freeze to create ice layers.^40–42 Owing to mechanical interlocking between ice and the superhydrophobic surface, the ice adhesion strength value is several times higher than that on a smooth hydrophobic surface. Additionally, the micro-/nano-structure of superhydrophobic surfaces can be gradually destroyed during icing and de-icing.⁴

Therefore, two principal characteristics are required for an ideal anti-icing surface. One is to facilitate overcooled water droplets rolling off the surface, and the other is to weaken the ice adhesion for the accumulated ice on the surface.^43–45 Wang et al. introduced micro-anisotropic micropatterns on a superhydrophobic surface, and water microdroplets could coalesce and leap over the surface spontaneously along a prescribed direction.¹⁵ Cheng et al. introduced magnetic nanoparticles (Fe₃O₄) as the heat mediators in a superhydrophobic coating, and the outstanding photothermal and magnetothermal effects of the magnetic particles endowed the coatings with long time icing delay and thermal de-icing properties.¹⁶ Notably, the surface anti-icing properties were improved significantly with a hydrophilic or hydrophobic lubricating liquid.^46,47 For example, slippery liquid-infused porous surfaces (SLIPS) exhibited superior anti-icing and frost-resisting behaviours as compared to the superhydrophobic surfaces.⁴⁸ As one of most efficient and realizable methods for anti-icing, low surface energy coatings prepared by synthetic polymers or polymer/inorganic composites have been applied with encouraging results.^49–54 The surface ice nucleation rate on a hydrophilic solid surface was lower than that on a hydrophobic solid surface.^55,56 The amphiphilicity coatings, however, also showed distinguished anti-icing and frost-resisting performances.^57–59

In this review, we will summarize the strategies for anti-icing and highlight the anti-icing behaviors of low surface energy coatings and liquid-infused slippery surfaces. Polymer types, surface textures, surface chemical composition, wettability and ice adhesion strength are taken into account to explain the anti-icing behaviors. The current issues for potential researches of anti-icing coatings are suggested.

2. Low surface energy coatings

Ice exhibits a strong adhesion to materials, which largely depends on polar ice molecules interacting with the solid surface. Hydrogen bonding, van der Waals forces, and electrostatic interaction are principal physical mechanisms responsible for the ice adhesion on surface.^60–62 Among the three factors above, the direct electrostatic interactions have been found to be dominant. Materials with a lower permittivity would significantly reduce ice adhesion.^60–64 Therefore, an excellent icephobic coating made from low surface energy polymers would minimize the molecular interaction between water and the coating surface.^60,65–68 It was also reported that the ice adhesion strength of coatings was correlated with surface wettability, such as water contact angle (WCA), water contact angle hysteresis (CAH) and dynamic hydrophobicity.^69–72 Furthermore, the surface morphologies of coatings were concerned, which resulted in different ice-substrate contact areas and further affected ice adhesion strengths.^73–76

2.1. Fluoride-containing polymer coatings

Fluoropolymer, ascribed to their low permittivity (ε ≈ 2.1) and resultant extremely low surface energy, could be non-stick to water and become a good candidate as anti-icing materials.^60,70 Farzaneh et al. produced a polytetrafluoroethylene (PTFE) nanostructured surface by depositing colloidal PTFE nanoparticles on anodized aluminium to enhance surface roughness.^76,77 It was found that the ice adhesion strength value of PTFE nanostructured surface on aluminium was around four times lower than that on bare aluminium merely anodized 90 min.⁷⁷ Yang et al. studied the icephobic properties of fluoropolymer materials including pristine PTFE plates, sandblasted PTFE plates, fluorinated room-temperature vulcanized silicone rubber coating and fluorinated polyurethane coating.⁷⁸ It was found that fluoropolymer-based materials with relatively smoother surface could significantly reduce the ice adhesion strength. However, the hydrophobicity of the fluoropolymer materials with sub-micron surfaces descended at lower temperature and under higher humidity, leading to a significant increase of ice adhesion strength.⁷⁸

Compared with PTFE, the flexible dangling fluoroalkyl groups of poly(fluoroalkyl acrylate)s could affect the molecular aggregation and the surface properties.^79,80 Owing to the surface self-organization and self-assemble of fluorinated chains, the fluorinated polymer could endow a surface with lower surface energy and higher water contact angles. Particularly, with longer fluoroalkyl side groups, poly(fluoroalkyl acrylate)s exhibited crystallization properties, thus possess superior performances of chemical inertness, hydrophobicity and icephobicity.^80,81 Gleason et al. reported that the presence of the highly-crosslinked poly(divinyl benzene) (pDVB) network underneath a thin polyperfluorodecylacrylate (pPDFA) layer was apt to prevent inward reorientation of its fluorine groups when exposed to water, resulting in lower contact angle hysteresis (CAH) (Fig. 1).^81,82 The mobility of the fluorinated chains in the upper layer was limited by linking and prevented local remodelling of the surface when contacted with water. The ice adhesion strength was reduced by more than six-fold to around 200 kPa when steel or silicon substrates were coated with these fluorinated bilayer films as a result of the low surface energy, low CAH values and smoothness of the coatings.^81,82

Fig. 1 Schematics of bilayer films of 1H,1H,2H,2H-perfluorodecyl acrylate (pFDA) copolymerized with crosslinked divinylbenzene (DVB) synthesized via initiated chemical vapor deposition.^81,82 Reprinted with permission from ref. 82. Copyright 2013 American Chemical Society.

2.2. Silicon-containing polymer coatings

Polysiloxane (silicone) is another potential candidate for anti-icing. The interaction energy between the fluorocarbon polymer and water (30.49 mJ m⁻²) is about three times larger than that between the siloxane polymer and water (10.49 mJ m⁻²).⁸³ On the other hand, polysiloxane has a low T_g, and the polymer chains are believed to be flexible or soft under de-icing on the coating surface.^84,85 The dissimilar rheological-mechanical properties between ice and polysiloxane-based polymers resulted in low ice adhesion strength.^60,68,86,87 Therefore, silicone-based polymers are found to perform better than the PTFE-based ones in anti-icing behavior under certain conditions.

To investigate the relationship between ice adhesion strength with the surface structure of the copolymers, and to find out how the prepared polydimethylsiloxane (PDMS)-containing polyacrylate copolymers were potentially used for anti-icing, Yuan et al. synthesized polyacrylate-b-PDMS and polyacrylate-g-PDMS with diverse molecular weights of PDMS.⁴⁹ The results suggested that microphase separation appeared clearly in all the products, especially for the block copolymers. The PDMS chains aggregated on the top of the polymer surfaces caused by microphase separation could weaken the interaction between the polymer surface and ice (Fig. 2).⁴⁹

Fig. 2 (a) AFM topography and phase images of polyacrylate-b-PDMS. (b) Schematic drawing shows the surface structure of the copolymer coatings made from polyacrylate-b-PDMS or polyacrylate-g-PDMS. When the microphase separation occurs, the PDMS-rich phase locates on the top area and the polyacrylate-rich phase is beneath.⁴⁹ Reproduced with permission from ref. 49. Copyright 2013 Elsevier.

The obvious difference in moduli between ice and the “soft” surface could result in mismatch in strain when a force exerted to remove ice.⁸⁸ In contrast to “glassy” polymethylmethyacrylate, which showed negligible coating thickness dependence for ice adhesion strength, the coating thickness dependence was observed for PDMS elastomer (Sylgard 184). The peak removal force in shear was decreased 4 times (457 kPa to 115 kPa) as coating thickness increased from 18 to 533 μm (Fig. 3). The thickness was important because thicker coatings facilitated larger vertical displacements that led to building up stress at the frontier point or line rather than a plane.⁸⁸

Fig. 3 (a) Schematic diagram demonstration of stress building up at the interface plane and/or the front line or point during removal of a rigid, bonded object (ice) from a soft coating. (b) Peak removal force in shear as a function of coating thickness with 0.05 mm s⁻¹ probe speed.⁸⁸ Reprinted with permission from ref. 88. Copyright 2014 American Chemical Society.

2.3. Fluorosilicone copolymer coatings

Although PDMS-based polymers and fluoropolymers have been utilized for icephobic coatings individually, each of them has deficiencies in actual anti-icing applications. The former shows poor mechanical properties and low oil repellency, while the latter usually has a higher T_g value. Naturally, much attention has been paid to the comprehensive combination of fluorinated groups and siloxane compositions into synthetic materials. Thus, the synergistic effect of silicone and fluorine is expected to depress ice adhesion strength.^49,89 As far as we know, the self-assembly of polysiloxane in fluorinated block copolymers plays a significant role in the generation of low-energy surfaces, and the microphase separation generally occurs on the surfaces of the block polymers, which affects their surface properties. Therefore, the fluorosilicone block copolymers with integrated advantage of silicone and fluoropolymers could be potentially applied in icephobic coatings.^51,90,91 Yuan et al. synthesized polymethyltrifluoropropylsiloxane (PMTFPS)-b-polyacrylate block copolymers by free radical polymerization using PMTFPS macroazoinitiator (PMTFPS-MAI).⁹⁰ When the content of PMTFPS-MAI was 20 wt%, the delayed icing time of the copolymer film was 186 s at −15 °C, and the ice shear strength was 301 ± 10 kPa, significantly lower than that of polyacrylates (804 ± 37 kPa). The icephobicity of the copolymers was attributed primarily to the enrichment of PMTFPS on the film surface and the synergistic effect of both silicone and fluorine.⁹⁰

Besides, due to “flexible-hard” microphase separation and higher chemical composition ratio of fluorine to silicon, the interaction between ice and the copolymer surface could be weakened. The ice adhesion and shear strengths on the copolymers containing 15 wt% PDMS coating were 17 kPa and 187 kPa, respectively, exhibiting the outstanding icephobicity.⁹¹

POSS are molecules with special cage-like nano-scaled organic/inorganic hybrid structures. Importantly, owing to suitable substituent groups, POSS and its derivatives are demonstrated to be easily incorporated into various hybrid polymers.^52,92–96 The function of POSS in the coating is to facilitate microphase separation and increase the nano-scaled roughness.⁹³ Besides, due to POSS special micro-structure, the fluoropolymer chain could be fixed to the coating surface and was unable to migrate into the coating inside via rebounding.^94,95

Yuan et al. prepared fluorinated hybrid films composed of fluorinated polymethylsiloxane (PMHS-xFMA, x = 6, 13 and 17) and octavinyl-polyhedral oligomeric silsesquioxane (OVPOSS) for icephobic applications (Fig. 4).⁹² PMHS-xFMA was synthesized via hydrosilylation of polymethylhydrosiloxane (PMHS) with fluorinated methacrylates (xFMAs). The film prepared by PMHS-13FMA and 10 wt% of OVPOSS with proper roughness (90.2 nm) performed the lowest ice shear strength (188.2 ± 13.4 kPa) among all the samples. ​However, the fluorinated hybrid films with PMHS-17FMA presented higher ice shear strengths due to the stronger interfacial interactions between the film surface and ice/water.⁹²

Fig. 4 Synthesis of fluorinated polymethylsiloxane (PMHS-xFMA) and formation of hybrid icephobic films from PMHS-xFMA and octavinyl-polyhedral oligomeric silsesquioxane (OVPOSS) based on the addition reaction between vinyl group and Si–H group.⁹² Reproduced with permission from ref. 92. Copyright 2016 Elsevier.

The POSS-containing fluorosilicone pentablock copolymers, which were prepared by the further reversible addition–fragmentation chain transfer (RAFT) polymerization of HFBA, showed clear microphase separation.⁹⁴ The average roughness values of the copolymer films were enhanced by introducing POSS, and a certain POSS content led to a significant decrease of the receding contact angle. The block copolymers combining the advantages of POSS, PDMS and fluoropolymers could contribute to the decrease of ice shear strength.⁹⁴

In order to improve the icephobicity of the fluorosilicone block copolymer, POSS-containing fluorosilicone multi-block methacrylate copolymers [PDMS-b-(PMAPOSS-b-PFMA)₂] were synthesized via RAFT polymerization of MAPOSS with xFMA (x = 6 and 12) as different fluorinated side groups (Fig. 5).⁹⁵ The icephobicity results showed that the fluorosilicone block copolymer surfaces could decrease ice adhesion strength, but the water droplet could hardly slide off the copolymer surfaces. It was suggested that rapid dewetting surface could be achieved by increasing receding contact angle or by removing water droplets through minimization of contact angle hysteresis.⁹⁵

Fig. 5 Synthesis of PDMS-b-(PMAPOSS-b-PFMA)₂ via RAFT polymerization to characterize the ice shear strength of the fluorosilicone multi-block copolymer films.⁹⁵ Reproduced with permission from ref. 95. Copyright 2015 Elsevier.

UV-curing is efficient to produce crosslinked materials, due to the rapid curing rate at ambient temperature, as well as low energy consumption. Yuan et al. prepared UV-curable films containing thiol-terminated fluorosilicone methacrylate triblock copolymers [PDMS-b-(PFMA-SH)₂], thiol-functionalized PDMS (PDMS-SH) and OVPOSS for the icephobic application (Fig. 6).⁵² The PDMS-b-(PFMA-SH)₂ copolymers were synthesized via RAFT polymerization of 12FMA or 17FMA and followed by thiol-modification of end groups. Results of icephobic properties demonstrated that water droplets could rebound from the film surfaces at −15 °C, avoiding ice accretion, and the ice shear strengths on the film surfaces were lower than 210 kPa, only about 15% of the value on bare aluminium surface.⁵²

Fig. 6 Synthesis of UV-curable fluorosilicone triblock methacrylate copolymer films and high speed digital camera images of the dynamic behaviour of 7 mL water droplets dropping on the horizontal and 30° tilted 17F 50% surface (containing 50 wt% PDMS-b-(P17FMA-SH)₂ copolymer with respect to PDMS-SH) from a 10 cm height at 20 °C, −15 °C, −25 °C and −35 °C, respectively.⁵² Reproduced with permission from ref. 52. Copyright 2015 Royal Society of Chemistry.

Fluorinated methacrylate block copolymers containing POSS moieties were also synthesized by RAFT polymerization and transformed into thiolated-copolymers by aminolysis to adjust the self-assembly and modify surface morphology. Owing to the crystallization of perfluoroalkyl side groups, the coatings containing thiol-modified PMAPOSS-b-P17FMA-SH (S17F) exhibited excellent icephobicity, and water droplets could rebound completely at both normal and tilt modes before freezing even at −15 °C. The average ice shear strengths of the prepared UV-cured coatings were about eight-fold lower than that of bare aluminum, while the coating containing 5% S17F achieved the lowest ice shear strength of 105 ± 12 kPa in the research.⁹⁶

However, perfluoroalkyl acid released by oxidative degradation of fluoropolymers has an environmental risk for their possible bioaccumulation in wildlife and potentially harmful to human beings, and the use of perfluorooctanonic acid has been limited.⁹⁷ Besides, the organic solvent used during fabrication of anti-icing coatings was also detrimental to the environment. Thus, green methods of preparing anti-icing coatings are encouraged.

3. Liquid-infused slippery surfaces

Recently, liquid-infused slippery surfaces have gained a lot of interests because their remarkable anti-icing properties.^48,98–101 The bioinspired strategy of slippery liquid infused porous or texture surfaces were first created by Aizenberg et al. and adapted by other researchers.^98,102–106 Based on infused liquids, the liquid-infused slippery surfaces could be divided into two types, oil lubricating layer and aqueous lubricating layer.

3.1. Oil lubricating layered coatings

The nano-textured surfaces infused with low surface energy molecules, such as perfluorinated oil, has recently been proposed as a promising candidate for anti-icing.^48,99 Varanasi et al. have studied frost formation and ice adhesion on the perfluorinated oil lubricating layer surfaces.⁹⁹ It was suggested that the infused oil, which migrated from the wetting ridge and the substrate's texture to the frozen drop's surface, could inhibit ice formation.⁹⁹ The ice adhesion strength on the textured surfaces with thermodynamically stable oil lubricating layer was higher than that on surfaces with excess lubricant, and the ice adhesion strength decreased with increasing micropost texture density.⁹⁹ Ice nucleation could be inhibited by slippery liquid-infused porous surfaces because the surface with oil lubricating layer provided an ultrasmooth and chemically homogeneous interface and could presumably eliminate possible nucleation sites.^{55,56,100,107}

Sun et al. developed a bilayered anti-icing coating in which a permeable porous superhydrophobic layer separated antifreeze-infused superhydrophilic layer from the environment, which responded to surface icing by releasing antifreeze liquid.¹⁰⁸ Yin et al. fabricated a icephobic Fe₃O₄ nanoparticles–PDMS coating by using a polystyrene microsphere-assembled template and infiltrated with fluorinated polyether oil lubricants.¹⁰⁹ The anti-icing coating was integrated by self-lubrication and near-infrared photothermogenesis, and Fe₃O₄ nanoparticles afford high efficiency photothermal effect under near-infrared irradiation also for rapidly melting the accumulated ice.¹⁰⁹

Zhu et al. prepared a silicone-oil-infused PDMS coating with very low ice adhesion strength of 50 kPa, only about 3% of the value on a bare aluminum.¹¹⁰ Due to the low surface energy and the high mobility of silicon oil in the coating, the loose ice layer on the PDMS-coated surface could be formed (Fig. 7).¹¹⁰ Wang et al. prepared a durable organic gel anti-icing coating by swelling cross-linked PDMS network with liquid paraffin, and the ice adhesion was still ultralow (<10 kPa at −30 °C) after 35 cycles of icing/deicing and 100 days of exposure in ambient environment.¹¹¹ Urata et al. also reported that self-lubricating organogels used for anti-icing via releasing liquids from inner matrices to the outer surfaces through temperature changes.¹¹²

Fig. 7 Regeneration of the silicon-oil infused PDMS coating by the migration of silicon oil to the surface, and ice formation on the PDMS coated surface of an insulator under a condensing weather condition at −5 °C and saturated humidity.¹¹⁰ Reprinted with permission from ref. 110. Copyright 2013 American Chemical Society.

Although the liquid-infused slippery surface exhibited excellent anti-icing properties, the durability of oil lubricating layer is limited because quite a portion of the lubricant was lost via evaporation or wicking away with each frosting/defrosting or icing/de-icing cycle. Thus, its replenishment would become necessary for sustained anti-icing performance.^46,99,100 Moreover, the oil wicking away is expected to be detrimental to the environment. Therefore, more effort would be taken to make liquid-infused slippery coatings stable and durable.

3.2. Aqueous lubricating layered coatings

Wang et al. fabricated several kinds of anti-icing coatings with aqueous lubricating layers containing crosslinked poly(acrylic acid), hyaluronic acid or hydrophilic polyurethane particles.^113–115 Hygroscopic polymers crosslinked through poly(ethylene glycol diacrylate) could deliquesce and swell after absorbing water.¹¹³ When the temperature was sufficiently lowered and enough water was absorbed, the water swollen polymer network inside the micropores could migrate out of the surface. Then, the ice adhesion strength on this continuous self-lubricating aqueous liquid layer surface could reach 60 ± 16 kPa at −15 °C.¹¹³

Furthermore, a polyurethane and hydrophilic component dimethylolpropionic acid were used to prepare aqueous lubricating layer surface (Fig. 8).¹¹⁵ The polyurethane coatings with aqueous lubricating layer were prepared by spin-coating, and then thermally cured. The ice adhesion strength of these coatings could be lowered to 30.0 kPa, and it could be maintained at temperatures as low as −53 °C.¹¹⁵ The formed ice on this coating could be blown off by a wind action in the wind tunnel with a controlled temperature and wind velocity. The coatings with excellent anti-icing properties can be ascribed to the following reasons. The hydrophilic pendant groups of copolymers ionized in the water and it lowered the water activity, the ice or snow was melted and swollen once the particles were in contact with the ice or snow.¹¹⁵ The aqueous lubricating layer could eliminate the defects on the solid surface and make the surface much smoother, thus the mechanical interlocking between the ice and the surface texture is minimized.³⁸ Besides, the freezing point of the bound water in the aqueous lubricating layer is greatly lower than that of bulk water.¹¹⁶ Similarly, inspired by poison dart frogs, Rykaczewski et al. have developed an anti-icing coating by mimicking the bilayer skin architecture of a frog. The wick anti-icing coating with a permeable superhydrophobic surface on a anti-freezing agent (propylene glycol)-infused superhydrophilic layer could prevent glaze formation by shedding large impinging droplets.¹¹⁷

Fig. 8 The mechanism for the reduction of the ice adhesion by the introduction of an aqueous lubricating layer (a), and the ice adhesion strength of a polyurethane coating at different temperatures (b). The solid line is the ice adhesion strength, and the dash line is differentiated from the solid line. Regimes of I–III represent the aqueous lubricating layer disappeared with the decreasing of the temperature.¹¹⁵ Reprinted with permission from ref. 115. Copyright 2014 American Chemical Society.

In order to get optimal liquid-infused slippery layer surface, many factors influencing ice adhesion strength were taken into consideration, such as film thickness, feed molar ratios of monomers, temperature and so on. Wang et al. also used hyaluronic acid and dopamine in the anti-icing coating to form aqueous lubricating layer.¹¹⁴ The results indicated that the ice adhesion strength was the lowest (61 kPa) when the film thickness was 20 nm, and when the feed molar ratio (hyaluronic acid to dopamine) was 0.33.¹¹⁴ The ice adhesion strength was proportional to the ratio of viscosity of aqueous film according to the hydrodynamic lubrication mechanism.¹¹⁸ When the amount of dopamine were excess, the crosslinking density would be increased, which could greatly increase the viscosity of aqueous film and the ice adhesion strength.¹¹³ On the other hand, when the temperature was lower than the phase transition temperature (freezing point) of the bonding water around the hydrophilic groups of polymers, the aqueous lubricating layer disappeared due to frozen bonding water. Thus the ice adhesion strength increased sharply due to the mechanical interlocking between the ice and surfaces.^113,114,116 Wooley et al. discussed the molecular interactions that occur between water and the polymer networks comprised of hyperbranched fluoropolymers (HBFP) and poly(ethylene glycol) (PEG).⁵⁰ Due to the ability to depress the freezing/melting temperature of water, the crosslinked HBFP-PEG network film shown great potential as an anti-icing coating.⁵⁰

Ion polymers can also be used for anti-icing.^119,120 Chernyy et al. reported that superhydrophilic polyelectrolyte brushes incorporated mono-, bi-, and trivalent ions by ion exchange exhibited good anti-icing properties.¹¹⁹ The strongest kosmotropes that have a tendency to disrupt water hydrogen bonds, such as Li⁺ and Na⁺, were able to reduce ice adhesion strength by 40% and 25% to ca. 100 kPa and ca. 200 kPa, respectively at −10 °C due to hydration of ions.¹¹⁹ Meanwhile, Wang et al. reported that the ions could tune heterogeneous ice nucleation with counterions on polyelectrolyte brush surfaces with a large temperature window of up to 7.8 °C.¹²⁰

4. Conclusions

This paper highlighted and reviewed recent progresses in preparation and behaviours of anti-icing coatings. We divided it into two parts. The first part stated the fabrication of the low surface energy coatings and their anti-icing properties. Due to self-assembly properties, low surface energy polymers could give the coating with microphase separation, which caused decrease of the ice adhesion strength in comparison with a commercial polyacrylate–polyurethane coating.⁴⁹ Compared with the superhydrophobic surface, the nanostructure morphology of silicone-based polymers and fluoropolymers presented relatively smoother surfaces, and the effect of mechanical interlocking between ice and the copolymer surface on the ice shear strength could be negligible.^90,121

The second part illustrated the anti-icing/frost-resisting behaviours of liquid infused slippery surfaces including an oil lubricating layer or an aqueous lubricating layer. Nano-textured surfaces with an oil lubricating layer could remove ice through substantial oil migrating onto the surface. However, a significant portion of the lubricant was lost with icing/de-icing cycles, and it was not environment friendly. Generally, the durability of liquid-infused slippery surfaces could be the most challenging issue. The self-lubricating aqueous layer generated after the amphiphilic polymer deliquesced and swelled through absorbing enough water. Thus, the ice formed atop of coating surfaces could be removed easily by external force.¹¹⁵ However, the ice adhesion at enough low temperature increases sharply due to the disappearance of the self-lubricating liquid water layer. And, it could become difficult to remove ice, once the liquid water layer was frozen.

With respect to the requirements of environment-friendly, large-scale and practical applications, more attention should be paid to improve the stability and durability of anti-icing for polymer composites coatings in various environmental considerations. Mechanical strength of the coating (such as abrasive resistance, adhesive force) also needs to be discussed. Then, it is necessary to further clarify the nucleation and ice growth mechanism on the different kinds of coating surfaces. In addition, it is very necessary to further explore the interaction between free water/ice and the coating surface with low surface energy, surface topography or amphipathy. Based on the improvement of anti-icing comprehensive performance, the environmental problem should be taken into consideration during fabrication and application. Hence, precisely controlling the morphology of environment friendly polymer composite coatings for anti-icing is still challenging.

Acknowledgements

This work is financially supported by National Natural Science Foundation of China (No. 51273146) and Natural Science Foundation of Tianjin, China (No. 14ZCZDGX00008).

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