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1.School of Mechanical Engineering and Automation，Beihang University，Beijing 100191，P.R.China；2.Fluids &Thermal Engineering Research Group，Faculty of Engineering，University of Nottingham，Nottingham NG7 2RD，UK

Abstract: Icing on the surface of aircraft will not only aggravate its quality and affect flight control，but even cause safety accidents，which is one of the important factors restricting all-weather flight.Bio-inspired anti-icing surfaces have gained great attention recently due to their low-hysteresis，non-stick properties，slow nucleation rate and low ice adhesion strength.These bio-inspired anti-icing surfaces，such as superhydrophobic surfaces，slippery liquid-infused porous surfaces and quasi-liquid film surfaces，have realized excellent anti-icing performance at various stages of icing.However，for harsh environment，there are still many problems and challenges.From the perspective of bioinspiration，the mechanism of icing nucleation，liquid bounce and ice adhesion has been reviewed together with the application progress and bottleneck issues about anti-icing in view of the process of icing.Subsequently，the reliability and development prospect of active，passive and active-passive integrated anti-icing technology are discussed，respectively.

Key words：mechanical manufacturing and automation；anti-icing of aircraft；superhydrophobic surface；slippery liquid-infused porous surface；electrothermal coating

0 Introduction

Ice accretion has become an urgent challenge in aerospace，energy and mechanics fields，which greatly harms the reliability and safety of equipment.Due to the impact of raindrops under low tempera?ture condition or the supercooled droplets in the at?mosphere during flight，ice will easily accumulate on aircraft surface and greatly change the aerody?namic shape of aircraft［1-2］.The most serious parts of aircraft for ice accumulation include the leading edg?es of the wings and empennages，the propellers and hubs on fixed-wing aircraft，the engine nacelles，the rotor blades on helicopters，and the windshields［3］.Thus，various anti-icing strategies have been pro?posed and applied to aircraft，including mechanical deicing，thermal output anti-icing and antifreeze liq?uid technologies［4-5］.The above strategies relying on external energy input are generally called traditional active anti-icing technologies.However，there are several disadvantages that cause the traditional ac?tive anti?icing strategies unsuitable for present air?craft requirements，such as enormous energy con?sumption，elaborate supporting systems and serious environmental pollution.This imposes restrictions on the development and application of active anti?ic?ing systems.

Due to low-hysteresis，non-stick properties（rolling angle（RA）lower than 5°），slow nucle?ation rate and low ice adhesion strength，superhy?drophobic surface（SHS）and slippery liquid-infused porous surface（SLIPS）have been deemed as two of the most excellent candidates for new generation anti-icing surface in aerospace［6］.SHS is inspired from the surface of lotus leaves［7］，butterfly wings［8］，rice leaves［9］，water strider legs［10］，etc.，and the latter from Nepenthes［11］.Both SHS and SLIPS mainly rely on interface characteristics to re?alize anti-icing properties，thus called passive anti-ic?ing strategies，which can effectively assist the exist?ing anti-icing systems getting rid of energy limita?tion and environmental pollution.However，the pas?sive bio-inspired anti-icing surfaces are extremely easy to lose efficacy due to microstructure breakage or chemical property degradation in harsh flight envi?ronment.

To solve the shortcomings of passive and ac?tive anti-icing strategies，researchers combine the novel active anti-icing surfaces and bio-inspired low adhesion surfaces，and the combination effectively reduces the anti-icing energy consumption and im?proves the interface performance.Therefore，the in?tegrated anti-icing strategies have become an impor?tant trend in the development of bio-inspired anti-ic?ing system in aerospace［12-16］.Herein，this review will systematically introduce recent anti-icing re?search from the aspect of theories，strategies and progresses based on bio-inspired anti-icing surfaces.In view of the processes of icing on the surface of aircraft，the existing strategies are divided into two types：（1）Lessening ice accumulation by slacken?ing ice nucleation or shortening the contact time of droplets，（2）weakening the ice adhesion by trans?forming the contact interface between ice and sub?strate.The mechanism of ice nucleation，liquid bounce and ice adhesion has been reviewed togeth?er with the application progress and bottleneck is?sues about anti-icing，such as durability，energy consumption and multi-function compatibility.We hope this review will contribute to profoundly un?derstanding anti-icing mechanism and provide reli?able guidance for the next-generation anti-icing sys?tem.

1 Anti?icing Mechanism

1.1 Slackening ice nucleation and shortening droplet contact time

Icing refers to the process that droplet sponta?neously forms ice cores internally，and ultimately grows with reduction of temperature［17］.Gibbs’hy?pothesis points out nucleation is necessary during phase transitions，such as freezing［18-19］.The nucle?ation process of droplet can be classified into homo?geneous nucleation and heterogeneous nucleation，and the latter is more ubiquitous in real life.The ho?mogeneous nucleation occurs at the temperature ap?proximately below -40 ℃［20］.The classical nucle?ation theory is shown as

whereγl?v，γs?l，γs?vare the surface tension coeffi?cients of liquid-gas，solid-liquid and solid-gas，re?spectively；r*is the radius of curvature of droplets，andθthe contact angle（CA）of droplets on the sur?face，as shown in Fig.1（a）.Eq.（1）shows that the free energy required for icing is decreased with the decrease of the CA，which demonstrates the signifi?cance of CA for icing.Thus，the CA is intuitive indi?cator for assessing the ice resistance of a surface（Fig.1（a））.SHS exhibits the ability for inhibiting the heterogeneous nucleation of droplets on the surface due to larger CA in static environment［21?22］，which can be demonstrated by Wenzel model（Fig.1（b））and Cassie-Baxter model（Fig.1（c））［23-24］.From the perspective of energy，the edge or defect region ex?hibits a lower energy barrier for heterogeneous nucle?ation，resulting in easier icing and then subsequently expanding outwards in the form of ice bridges［17，25］.As for SLIPS，lubricant liquid infiltrates into the mi?cro/nano?structure instead of air and generate contin?uous and homogeneous lubricant phase on the sur?face［26］（Fig.1（d））.This will result in homogeneous nucleation of droplets with higher energy barrier，which delays or even inhibits the heterogeneous nu?cleation of static droplets on the surface［27?29］.

In static environment，ice mass and area will be effectively reduced by inhibiting heterogeneous nucleation.However，for dynamic icing process，such as the real flight condition，the supercooled droplets hit the cold surface at a high speed and nu?cleate instantly，resulting in ice accretion.The drop?let impacting and freezing processes on the cold sur?face can be divided into four stages（Fig.1（e））：Contact and nucleate；ice nucleus growing；recede and oscillate；and completely frozen.Droplet within a certain range of Weber number（We）will rebound when impacting on bio-inspired surfaces，such as SHS and SLIPS.

In cold environment，droplet rebounding or sticking dynamics is associated with viscosity and nucleation［30］.Maitra et al.［31］found that increased viscous effects significantly influenced all stages of impact dynamics，in particular，the impact and me?niscus impalement behavior.Recently，Zhang et al.［32］proposed that the phenomenon of liquid adhe?sion on the surface under low temperature was caused by ice nucleation rather than enhanced vis?cous effect.As same as SHS，lubricant film on SLIPS is easily deformed and extruded until being function failed when droplet impacting on the sur?face with highWe.The loss of interface characteris?tic results in droplet pinning on the surface and in?creases the contact area between the droplet and the substrate，which enhances heat transfer efficient，leading to heterogeneous ice nucleation easily［33］.Bird et al.［34］proved that a small fraction of heat was transferred between the droplet and the surface dur?ing the droplet impacting and rebounding，and quan?titatively estimated how much heat was transferred.This phenomenon promotes droplet nucleation and causes the droplet to ultimately fail to bounce.Fur?thermore，when a relatively hot droplet impacts on a cold surface，the droplet easily condenses and eventually fully infiltrates within the cavities at the micro-nano structure leading to bouncing failure［35］.

Thus，strengthening droplet bounce behaviors and obstructing heat transfer between droplet and substrate can avoid droplet nucleation and decrease ice accretion on the surface.Superhydrophobic sur?face exhibits special droplet bounce behavior，which endows the interface with great anti-icing abili?ty［36-39］.The behavior mainly includes the coales?cence and jumping of condensed droplets and bounc?ing behaviors of microdroplets impacting on the sur?face［8，40］.Microstructures on the surface can offer high velocity and energy conversion efficiency for droplets during the process of spontaneous coales?cence and jumping.Liu et al.［41］designed a microanisotropic SHS，using the work of adhesion as the steering force for leaping microdroplets，and achieved the leaping of droplets in a guided lateral direction on SHS without any external forces.Lu and Wang et al.［42-43］studied the energy conversion efficiency and the droplet velocity during the bounc?ing process on various microstructures respectively.Liu et al.［44］observed that a special pancake bounce phenomenon occurred as the millimeter-sized drop?lets impacting on the nanostructure-modified conical pillar array surface whenWewas 10.The structure could further reduce the contact time to 3.4 ms（Fig.1（f））.The capillary energy stored at the bot?tom of the droplet can be converted into kinetic ener?gy，which is enough for the droplet to bounce in the shape of pancake［45?46］.Bird et al.［47］demonstrated that liquid mass could be redistributed when impact?ing on SHS with ridged structures leading to the morphology transformation of droplet hydrodynam?ics，which broke the lower bound on the impact time of ordinary SHS to 7.8 ms（Fig.1（g））.Shen et al.［48］revealed the lower bound on the impact time was 5.5 ms on SHSs with multi ridged struc?tures（Fig.1（h））.

Fig.1 Schematic of droplet morphology and freezing processes and mechanism of shortening the contact time of droplets on SHS

Droplets exhibit lesser pinning when impacting on SLIPS compared with SHS.Yeganehdoust et al.［49］compared the behaviors of droplets when im?pacting on SHS and SLIPS by simulation，and pro?posed that the microdroplets penetrated the micro?structure on SLIPS was much weaker than that on SHS at highWe（We≈160）.Biroun et al.［50］of?fered an effective way of applying surface acoustic wave technology along with SLIPS to reduce the contact time and alter the droplet rebound angles.These phenomena provide an idea for the subse?quent suppression of heat transfer and phase change of the supercooled droplet impacting on surfaces with a high speed.

1.2 Build ice?phobic interfaces

Bio-inspired anti-icing surface has been proved to effectively delay the occurrence of icing in static environments，however，in harsh flight environ?ment，the function will gradually fail and generate massy ice accretion with time.Generating isolation between the ice and the surface has been deemed as an effective and promising strategy to solve this problem［25，51］.Thus，ice adhesion strengthτice=F/Ais the key parameter for measuring the ability of ice-phobic of these surfaces，which represents the required force（F）for removing ice with unit area（A）on the surface［52-53］.The surface with ice adhe?sion strength lower than 100 kPa are recognized as ice-phobic surface［54］.Ice-surface isolation forms can be divided into two categories according to the pres?ence or absence of heat input.The passive forms without heat input include hard-hard interface crack（on ordinary surfaces），Cassie ice falling off（on air film），ice slipping away（on oil film），and hard-soft interface crack（on soft substrate），as shown in Fig.2（a）.Additionally，combining bio-inspired sur?face properties with the soft substrate is promising to promote crack initiation，further reducing the force required［55-56］.Golovin et al.［52，57］reported an anti-icing material with low interfacial toughness（LIT），which significantly reduced the force re?quired to remove a large area of ice，and confirmed that the force was independent of the ice area（Fig.2（b））.In Fig.2（b），Lcis the critical bonded length at which a transition between the interfacial cohe?sive strength mode and bonding energy mode of fail?ure occurs andГthe interfacial toughness.Under the action of external force（e.g.gravity，wind，etc.），local deformation stress concentration oc?curred on the surface of the low interface toughness material and interface micro-cracks are generated，after which the force required for the expansion of micro-cracks is no longer affected by the ice area.Compared with the super-wetting interface，the elastomer surface can promote the occurrence of in?terfacial cracks and accelerate the debonding of the ice by means of the obvious mismatch of the elastic moduli of the substrate and the ice［57-58］.Thus，on the soft substrate，τice?soft=，whereEis the substrate modulus，Gthe surface energy，athe to?tal crack length，andΛthe non-dimensional con?stant related to interface geometric configuration［59］.According to different test forms，ice adhesion ex?perimental devices can be divided into centrifugal type，parallel push type，etc.Ice adhesion experi?ments need to be standardized，such as the environ?ment of the icing process，the geometric parameters of the ice，the kinematic parameters of the testing process，etc.This standardization helps to compare the ice adhesion results from different laboratories，and establish an industry standard that can measure the anti?icing ability of the surface［60］.However，the research of ice behavior on the soft substrate is still limited to the macroscopic measurement of icing ad?hesion［61-62］.To further research the ice desorption behavior and crack formation mechanism on various interfaces is necessary and urgent for anti-icing field.

Fig.2 Schematic of ice isolation forms

The active forms accompanying by ice melting process include slipping away（isolated by heat-in?duced water film），melting frost self-jump［36］and spontaneous Wenzel to Cassie-Baxter transition［63］（Fig.2（c））.Ref.［51］revealed the bio-inspired antiicing synergistic mechanism of“heat-induced water film isolation”.The contact interface switched from“solid-solid contact” into“solid-liquid-solid con?tact”when heating，which greatly weakened the ice adhesion force（Fig.2（d））.In addition，the sponta?neous motion phenomenon of frost on SHS during the melting process has also attracted attention［64-65］.Refs.［36，66］revealed the mechanism of melting frost self-jumping phenomenon from the energy point of view，and proposed a frost directional selfjumping control method based on anisotropic macromicro structure.Generally，droplets turn from Cassie-Baxter model to Wenzel model during icing and the process is hard to recover to Cassie-Baxter model after melting.Zhong et al.realized the sponta?neous transformation from Wenzel state to Cassie-Baxter state at the interface between ice and the sur?face during the ice-melting cycle［63］.The reversible switching was attributed to the bubbles in the ice drop which rapidly hit the micro-nano structure un?der the action of Marangoni force（Fig.2（e））.Thus，the combination of active and passive anti-ic?ing strategies is significant and meaningful for antiicing in aerospace.

2 Anti?icing Strategies

2.1 Lessening ice accumulation

The strategies of lessening ice accumulation can be divided into solid-solid（SHS），solid-liquid（SLIPS）or solid-biology（Antifreeze protein，AFP）contact form based on the substrate.

Compared with ordinary surfaces，SHS mini?mizes the effective contact area between super?cooled droplets and cold solid substrates，inhibiting the formation of ice［67-68］.Ref.［25］constructed a ro?bust SHS based on fiberglass cloth（FC）with mi?cro-texture and adhesive.The surface exhibited sig?nificantly icing delay and greatly reduced ice accre?tion in dynamic ice test compared with origin sub?strate（Fig.3（a））.In addition，high mechanical strength substrate and strong adhesive strengthen the abrasion resistance of the surface.

SLIPS is covered by a low surface energy lubri?cant film，delaying the heterogeneous nucleation of ice.Yin et al.［69］demonstrated a scalable and repro?ducible coating method to create a slippery surface on aluminum（Fig.3（b））.Compared with pure and smooth PDMS film without pores fluorination（SF）and lubricant fluorinated porous film（FF），droplets on fluorinated porous film infiltrated with perfluoro polyether lubricant（LF）could easily slide off from the surface before freezing，significantly reducing ice accretion.The ice on LF also could be easily re?moved from the surface by gravity at low tilt angles after partially melting.Qian et al.［70］constructed SLIPS with pressure responsive property by a facile template strategy.The surface regulated the porous structure under external pressure，which effectively reduced the loss of lubricant and droplets can easily shed on the as-prepared surface results in a short contact time，avoiding ice nucleation.Irajizad et al.［71］reported a new magnetic slippery surface（MAGSS）with anti-icing at -34 ℃and excellent icing delay ability（Fig.3（c））.In addition，the sur?face also exhibited extremely low ice adhesion strength（about 2 Pa）and stability in shear flows up to Reynolds number of 105.However，loss of lubri?cant will cause the function failure of SLIPS under dynamic environments，especially at high speed and shear force situations.To improve this shortcom?ing，researchers adopted non-flowing media instead of flowing to enhance the stability of the surface，in?cluding polyelectrolyte brushes and AFP grafted sur?face，both of which exhibited excellent ice delay properties［72-75］.The combination of polymer brush and ion endows the surface with unique capabilities of inhibiting ice nucleation and propagation.Wang et al.［75］revealed the inner mechanism of ions at polyelectrolyte brush（PB）for controlling the ice nucleation and propagation processes，further rein?forcing the significance of ion in inhibiting ice forma?tion（Fig.3（d））.

AFP in polar animals and plants has two char?acteristics of thermal hysteresis activity：Reduction of recrystallization，ice crystal growth inhibi?tion［72-73］.He et al.［74］prepared a multifunctional antiicing hydrogel by combining antifreeze proteins with polydimethylsiloxane-grafted polyelectrolyte hydro?gels（Fig.3（e）），which can simultaneously achieve inhibition of ice nucleation（ice nucleation tempera?ture below -30 ℃），prevention of ice growth（ice propagation rate below 0.002 cm2/s），and reduction of ice adhesion（below 20 kPa）.Wang et al.［76］de?signed a material with cold adaptation by combining hydrogel and AFP.The surface exhibited excellent anti?icing performance at low-temperature conditions.

Fig.3 Strategies to reduce ice accumulation

2.2 Weakening ice adhesion

The existing anti-icing strategies for weakening the ice adhesion can be classified into passive and ac?tive approaches［14，77-78］.Passive strategies rely on special micro-nano structures，such as SHS and SLIPS.Active anti-icing methods mainly prevent ice accretion with an external energy source［79-80］.Both surfaces have been deemed outstanding candi?dates for anti-icing due to their low ice adhesion［80-81］.

2.2.1 Passive strategies for weakening ice adhe?sion

As shown in Figs.1（b，c），air film traps into the micro-/nano-structure when the droplet contacts with SHS.In the process of icing，a nano-air film also will be trapped and generate“air film isola?tion”，leading to low ice adhesion on SHS，which has been observed by cryo-focused ion beam-assist?ed scanning electron microscopy［39］.The existence of a large amount of air pockets confirmed that the frost at this time was in the Cassie state（so-called“Cassie ice”），which intuitively explained the rea?son for low adhesion of ice on the nanotextured SHS.Chen et al.［82］fabricated a robust micro-nanonanowire triple structure-held PDMS superhydro?phobic surface（PDMS-MNN），which showed a long-term anti-icing performance with high deicing robustness and low ice adhesion strength（Fig.4（a））.In Fig.4（a），Rf means the product of surface roughness and substrate area fraction.Shen et al.［83］proposed that the icing-delay time of a droplet on SHS was many times longer，and the ice adhesion strength on SHS was greatly reduced，which was attributed to the Cassie wetting state of a droplet on the surface（Fig.4（b））.In Fig.4（b），SS，MS，NS，and MNS refer to Ti6Al4V substrate surfaces without modification，micrometer-scale pit surface with FAS-17 modification，nanowire structured sur?face with FAS-17 modification，and micropitnanowire structured surface with FAS-17 modifica?tion，respectively.Based on this theory，Zhou et al.［84］constructed an anti-icing concrete material which realized good anti-icing and easy deicing abili?ty（deicing stress：0.06 MPa）.However，it is still controversial in some papers whether SHS can re?duce ice adhesion，which can be explained by the gradual damage of the surface structure，the con?densation of water in the microstructures［85］，and the mechanical interlocking between the ice and the surface texture［86］.

Except from SHS，novel slippery surfaces，such as SLIPS and liquid-like surface，exhibit excel?lent ability for reducing ice adhesion.Chen et al.［87］fabricated a robust anti-icing coating with a self-lu?bricating liquid water layer（SLWL）where the ice adhesion was one order of magnitude lower than that on SHS，exhibiting excellent capability of selfhealing and abrasion resistance（Fig.4（c））.Wang et al.［88-89］proposed an anti-icing coating with self-lubri?cating water layer，which formed a water lubricating layer between the ice and the solid surface.The asprepared surface could remain liquid at -42 ℃and greatly reduces the adhesion of the ice.Liu et al.［90］incorporated a binary liquid mixture with an upper critical melting temperature into reversibly ther?mosecreting organogel（RTS-organogel）to obtain a SLIPS with the temperature-controlled phase iso?lation of solution（Fig.4（d））.The surface exhibited extreme low ice adhesion strength，and the ice adhe?sion strength decreased with the decrease of the mass ratio of silicone oil to liquid paraffin（S/P）.Zhao et al.［91］observed the behavior of ice adhesion on liquid-like surface and demonstrated the macro?scopic evidence of the liquid-like of surface-tethered poly（dimethylsiloxane）brushes.Whereas ice per?manently detaches from solid surfaces when subject?ed to sufficient shear，commonly referred to as the material’s ice adhesion strength，adhered ice in?stead slides over PDMS brushes indefinitely.The liquid-like surface exhibited extreme low ice adhe?sion strength of 0.3 kPa.Chernyy et al.［92］construct?ed various polyelectrolyte brush surfaces on glass surfaces，and found that Li+polyelectrolyte brush surfaces reduced the ice adhesion by 40% at -18 ℃and by 70% at -10 ℃，while Ag+ions reduced ice adhesion by 80% at-10 ℃（Fig.4（e））.

Fig.4 Strategies to reduce ice adhesion without additional energy input

2.2.2 Active and integrated strategies for weak?ening ice adhesion

Passive strategies have shown excellent water repellency and low ice adhesion in laboratory envi?ronment.However，these strategies exhibit serious limitations in dynamic environments，such as severe flight icing condition with high speeds，low tempera?ture［86，93］.The contact form between the ice and SHS will become mechanical interlocking/solid?sol?id contact.Meanwhile，the low ice adhesion effect of SLIPS is greatly dependent on the amount of oil in the liquid film which is extremely easy to de?crease gradually with the icing?deicing cycles［94］.The active and integrated strategies generate a wa?ter lubricating layer on the contact interface to form a“heat?induced water film isolation”，assist the pas?sive anti?icing surface to play a role，and ensure a low adhesion effect.The existing active strategies rely on external energy input to maintain a liquid lay?er for completely anti?icing，which also have been adapted in commercial.

The novel active anti-icing methods mainly in?clude electrothermal，photothermal，magneto-ther?mal and several synthetical anti-icing meth?ods［51，61-62，95-100］.Compared with the traditional builtin heating sheet，the novel conductive heating coat?ing can directly act on the icing interface with high heat transfer efficiency and rapid response time，which mainly relays on conductive polymer or mixed metal nanoparticles，carbon nanotubes，gra?phene and other conductive nanoparticles［61-62］.Ref.［51］obtained an electric heating anti-icing SHS by combining carbon nanotube and hydrophobic sub?stances.The surface realized excellent anti-icing ability with low energy consumption（Fig.5（a）），which reduced the energy consumption by 58%compared with the traditional electric heating meth?od.The coating has been widely adapted by various aircraft wings，engine inlets，wind turbine blades，and other surfaces for anti-icing.In addition，Cheng et al.［95］designed a new nano-integrated coating with superhydrophobicity，magnetocaloric effects and wetting stability by combining magnetic nanoparticles（MNP）with fluorine-containing copo?lymers.Under the alternating magnetic field of 7.8 kW，the surface raised from 24 ℃to 44 ℃at 25 s.Meanwhile，the temperature could rise more than 10 ℃ at 1 min under 75 W infrared light（Fig.5（b））.

Fig.5 Novel active and integrated strategies for weakening ice adhesion by“heat?induced water film isolation”

The above-mentioned ways need energy from external input by the equipment.However，sun?light is a natural source of heat，which can be fully used for surface heating for active anti-icing with energy-saving，thus，photothermal surface has at?tracted researchers’attention gradually［99］.Wu et al.［96，100］obtained a low-cost，high-efficiency super?hydrophobic photothermal surface based on candle soot，which exhibited fast heating characteristics under sunlight，inhibiting icing and rapidly melting frost on the surface（Fig.5（c））.Liu et al.［97］pre?pared a carbon nanotube-modified fluoropolyacry?late-based SHS，which had both photothermal and electrothermal effects to ensure energy-saving and reliable deicing under different sun irradiations（Fig.5（d））.Zhang et al.［98］used an ultrafast pulsed laser deposition method to prepare a photo?thermal superhydrophobic coating，which could maintain excellent photothermal conversion，drop?let self-clearing，anti-icing and frost-resistant prop?erties in extreme environments of low temperature and high humidity.

Above all，we can see that various active antiicing methods can effectively achieve surface anti-ic?ing.The integration of active and passive strategies can realize long-term，high-efficient and energy-sav?ing anti-icing，which have been deemed as nextgeneration anti-icing technology［12-16］.

3 Challenges and Trends of Bio?in?spired Anti?icing/De?icing Strat?egies

Biomimetic passive and active anti-icing strate?gies have shown excellent results，but their largescale applications on aircraft are still subject to chal?lenges，such as poor durability，large energy con?sumption requirements，and multi-function compati?bility problems.The researches on overcoming these challenges will be the development trend of the bio-inspired anti-icing technology.

As above mentioned，active anti-icing methods mainly prevent ice accretion with an external energy source［79?80］，but require complex regulatory equip?ment to prevent overheating or excess energy con?sumption under various cold loads［77］.Passive strate?gies rely on special micro?nano structures but their poor durability and fragility cause function failure in dynamic environments［19，74，101］.Poor tolerance，large energy consumption，and multi-function com?patibility have become urgent problems for next gen?eration functional surface for anti-icing.

3.1 Robustness enhancement of passive anti?ic?ing surfaces

The passive bio-inspired surfaces realize excel?lent anti-icing performance by gas film/liquid film within micro-nano structures.However，surfaces will fail during icing cycles due to nanostructures with poor mechanical durability and lubricants that easily to evaporate or lose［85，102?103］.On SLIPS，the droplets would pull up and can even become com?pletely cloaked by lubricant，which can accelerate lubricant depletion from the textured surface and cause pinning［104?105］.Even more fatal for SHS and SLIPS，once the droplet enters the micro-nano structure and nucleates into ice，the newly formed ice will interlock with the structure，making the ice more difficult to move［37，53］.Moreover，the ice-wind tunnel tests also revealed that the SHS cannot with?stand high-speed，low-temperature and high-humidi?ty conditions during flight［105-106］.

To promote the application of bio-inspired antiicing performance in flight environments，it is neces?sary to ensure the reliability of micro-nano structure and interface film.The general robustness enhance?ment strategies for SHS can be divided into micro?structures protection［107?109］，reinforcement by adhe?sive［25，110］，and homogeneous structure［111?115］.Deng et al.［107］combined the superhydrophobic property of nanostructures with the durability of microstructures（Fig.6（a））.A SHS with strong wear resistance was prepared on the glass substrate on which the mi?crostructures acted as“armor”was used to protect the fragile nanostructure inside the frame.Zhong et al.［116］designed a three-scale integrated micro/nano?structured SHS on the metal surface by ultrafast la?ser ablation，chemical oxidation and other methods，which exhibited excellent Cassie state stability，high critical Laplace and low ice adhesion strength.Lu et al.［110］used commercial adhesives to bond the super?hydrophobic paint composed of perfluorosilane?coat?ed titanium dioxide nanoparticles to various sub?strates and promote robustness.These surfaces maintained their water repellency after finger?wipe，knife?scratch，and even 40 abrasion cycles with sandpaper.Combined the micro?texture of the fiber and the strong adhesion of the adhesive，our group［25］prepared a glass fiber cloth?adhesive?based SHS，which immensely enhanced the sandpaper abrasion resistance.Without additional heating，the obtained surface showed significantly icing delay un?der dynamic conditions（Fig.6（b））.Peng et al.［115］designed an all-organic，flexible superhydrophobic nanocomposite coatings by combining polytetrafluo?roethylene，perfluoropolyether（Krytox oil）and flu?orinated epoxy（denoted as PKFE coating）with strong mechanical robustness under cyclic tape peels and Taber abrasion（Fig.6（c））.Our group［111］used fluorinated epoxy resin and carbon/PTFE particles to prepare the multifunctional anti?icing/de?icing coating，which exhibited excellent robust superhy?drophobic property by repeated sandpaper abrasion and tape peeling tests.

On the other hand，the general robustness en?hancement strategies for SLIPS can be divided into lubricant holding structures［117?120］，phase?changeable or solid lubricants［121?123］，and slow release or contin?uous delivery structure of lubricant/antifreeze［124］.Wang et al.［125］constructed a uniform WO3nanofi?ber network layers on the stainless steel which pos?sessed highly lubricant holding ability and realized long-term stability of SLIPS（Fig.6（d））.Liu et al.［90］incorporated a binary liquid mixture with an upper critical melting temperature into an organogel（Fig.4（d））to achieve a temperature-controlled phase iso?lation of solution infiltration gels.As a result，the surface friction coefficient changed from 0.4 to 0.03 and the ice adhesion strength fell down below 1 kPa.Our group［124］added phase-change lubricant micro?crystalline wax into the elastomer network to con?struct a temperature responsive slippery surface whose slippery ability could gradually increase with temperature rising（Fig.6（e））.The combination of elastomer and phase change lubricant enhances the lubricant stability of SLIPS，and now the mass loss is extremely tiny under the erosion of the water va?por two?phase flow.Sun et al.［124］deeply studied the way in which the poison dart frog stores and se?cretes toxins，then proposed and constructed a dou?ble?layer with different infiltration properties as shown in Fig.6（f）.

Fig.6 Robustness enhancement strategies for passive anti-icing surfaces

However，the robustness of passive anti-icing surface is still not enough for aerospace require?ments，it is necessary for developing anti-icing sur?face with higher mechanical strength.

3.2 Active?passive integrated anti?icing/de?ic?ing strategies for energy saving under flight conditions

In aerospace，the power consumption of the an?ti-icing system is required to be as small as possible due to the limited power supply and cost on the air?craft.Excessive anti-icing energy consumption will not only bring unnecessary weight burden to the air?craft，but also be detrimental to energy saving and environmental protection.

To solve the problem of high energy consump?tion，we designed a integration surface with super?hydrophobicity and electro-thermal ability［25］.The surface realized reducing over 50% energy consump?tion for dynamic anti-icing，as shown in Fig.7（a）.Meanwhile，we fabricated a slippery liquid?infused porous electric heating coating（SEHC）which ex?hibited lower ice adhesion compared with superhy?drophobic porous electric heating coating（SHP）and porous electric heating coating（PEHC）.And we also realized the“heat-induced water film isola?tion”anti-icing and energy-saving synergies with the“oil film isolation”state（Fig.7（b））［51，126］.In addi?tion，researchers have devoted themselves into de?signing the surface with bio-inspired interface charac?teristics and strong photothermal effect for anti-icing with none-consumption［127-130］.Ref.［111］prepared a multifunctional anti?icing/de?icing coating with su?perhydrophobic passive anti?icing and electrother?mal/ photothermal active de?icing properties and the synergistic ice?phobic mechanism was validated by characterizing the different freezing states of water droplets on surfaces（Fig.7（c））.In Fig.7（c），EPD means electric heating power density.

Fig.7 Active and passive integrated anti-icing strategies

However，for energy saving，the synergy of ac?tive and passive anti-icing strategies requires the sur?face with fast heating rate and durable interface char?acteristic.Thus，the integration of interfacial and thermodynamic properties needs to be further pro?moted and improved to fit various requirements in aerospace.

3.3 Multi?function compatibility of anti?icing surfaces on aircraft

Considering the multi-function requirements of the aircraft surfaces，such as transparency of wind?shield，flexibility of moving parts，temperature/flow field/ice sensing ability of airfoils，etc.，anti-ic?ing systems on aircraft should coordinate with other functions.The multi-function compatibility of antiicing surface becomes a new development trend.

Zhou et al.［131］fabricated a kind of flexible hy?drophobic MXene-based transparent conducting films based on the excellent electrical conductivity of two-dimensional Ti3C2Tx（Fig.8（a））.The ob?tained film exhibited the balanced optical and electri?cal properties with a low electrical resistance of 35.1 Ω/sq and corresponding transmittance of 33.4%，showing the rapid steady Joule heating per?formance at safe voltages（about 100 ℃at 13 V）.We prepared an electric heating anti-icing coating with intelligent self-controlling heating effect and solid-liquid interface conversion effect by combining elastomeric polymer network and phase change lu?bricant（Fig.8（b））［125］.Compared with the matched sample（MS），the lubricant induced a temperaturecontrolled transition from a solid state to a slippery surface，reducing the icing adhesion.Furthermore，the positive temperature coefficient effect endowed the coating with an intelligent self-controlling heat?ing effect.Under the conditions of different cold loads and initial heat input，it showed excellent ener?gy saving and temperature self-regulation perfor?mance.In addition，anti-icing for irregular surfaces is also significant for aerospace，especially for en?gine intake.We designed a novel sandwich structur?al electric heating coating，which realized electric heating properties and high efficiency of anti-icing/de-icing for miniature complex components［132］.

Fig.8 Novel multi-functional active anti-icing/de-icing coatings

With the expansion of application environ?ments，the requirements for function surface have become stricter.Anti/de-icing surface is demanded not only to accomplish its original function but also to realize intelligent self-regulation，environment sensing and integrated design based on multidisci?plinary intersection.

4 Conclusions

Aircraft anti-icing technology has become a necessary requirement to ensure all-weather flight safety.The development of large-scale UAVs，stealth aircraft，and integrated material skins makes the need for weight reduction and efficiency enhance?ment of anti-icing technology more urgent.The pe?culiar evolution of nature has captured endless imagi?nation and constantly enlightens our scientific work.The development of bio-inspired anti-icing coating technology has made great progress，and the design and development methods for micro-nano multi-lev?el structure and low surface energy materials have become more mature.“Quasi-liquid film/heat-in?duced water film isolation effect”can achieve the goal of significantly reducing ice adhesion.The inte?grated design method provides a feasible solution for the integrated anti-icing and energy-saving and effi?ciency enhancement of aircraft and becomes a new way for high-efficiency anti-icing applications，such as large UAVs，stealth aircraft，and integrated skins.

The bio-inspired anti-icing coating technology is still facing challenges to realize their actual appli?cation on aircraft.The research direction to over?come these bottlenecks has become the develop?ment trend of the bio-inspired anti-icing coating tech?nology.Mechanical durability enhancement，active/passive integrated energy-saving strategy，multifunction compatibility as well as the large scale/low cost manufacture are the main topics of bio-inspired anti-icing surfaces development，in terms of applica?tion on aircraft.