Fabrication of a Highly Stable Superhydrophobic Surface with Dual-Scale Structure and Its Antifrosting PropertiesQinghua Zhang,* Biyu Jin, Bing Wang, Yuchen Fu, Xiaoli Zhan,* and Fengqiu Chen

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

ABSTRACT: The stable superhydrophobic aluminum alloy surfacewith dual geometric architectures was prepared by a combination ofsimple processes of chemical etching, dip-coating, and modification offluorosilicone. The Al surface with 20 min of acid etching and nanosilicadip-coating has the best superhydrophobicity, which showed watercontact angles (WCAs) of >157° and water sliding angles (SAs) of <1°.The superhydrophobic surface showed excellent antifogging, antifrost-ing, and delayed icing performances, compared to hydrophobic andhydrophilic Al surfaces. Furthermore, the superhydrophobicity of as-prepared surfaces is mechanically durable after 11 tape tests and 120 cmwear (under a pressure of 0.8 kPa). The strong interfacial interactionsamong the SiO2 nanoparticles, fluorosilicone-modified polyester resin,and the Al surface contributed to superior abrasion resistance. Thismethod could provide a facile, low-cost, and stable route to fabricate alarge-area superhydrophobic Al surface for application in various harsh environments.

1. INTRODUCTIONFog, frost, and ice formation and accumulation on the facilitysurfaces are known to cause serious economic and safetyproblems, such as ceasing the operation, impairing theefficiency, and even paralyzing the entire system, whichwould cause irreversible catastrophic results, especiallyregarding application in aircrafts, warcrafts, power lines,transportation, telecommunications equipment, dams, andheat exchangers.1−4 For example, air-source heat pumps aregenerally placed outdoors to heat inner spaces and water; thesepumps would lose efficiency significantly in cold climates,because of the insulating effect of frost or ice layers, andconsequently increase the pressure drop in channels.5−7 Evenworse, once the condensate droplets freeze to form a frost/icelayer, rain droplets would freeze immediately when theyimpinge upon the cold surface, which may exacerbate the icingprogress.8 Therefore, numerous efforts have been made inrecent decades to search for freeze-delay or antifrost/anti-icing,antiwater condensation materials, which can effectively preventice/frost/fog formation.9−17 Relevant results have proved thatthe superhydrophobic materials are promising candidates.18−22

By now, massive superhydrophobic surfaces have beenfabricated on different substrates, based on the combinationof surface with micronano structures and chemical composi-tions of low surface energy using diverse synthetic methods.23,24

However, such techniques are still at their infant stage andsuffer many shortcomings.25,26 Wang et al. presented a flexiblesuperhydrophobic surface with a hierarchical structure via thecombination of soft-lithography and hydrothermal methods.Although the surface displayed robust anti-icing performance,the fabrication process is so complicated and harsh (the heattreatment temperature is 350 °C) that large-scale production

cannot be realized.27 Guo et al. prepared microstructured/nanostructured surfaces using mechanical steps and crystalgrowth methosd, which can delay icing for ∼7000 s at −10 °C.However, the “Achilles’ heel” of this method is that the crystalgrowth process is time-consuming (15 h).28

As the third-richest element contained in the Earth’s crust,aluminum and its alloys possess dominant mechanical andphysical advantages, such as superior ductility, high strength/weight ratio, easy formability, unique electronic and thermalconductivity, nonferromagnetic properties, excellent corrosionresistance, a nontoxic nature, and easy recyclability.29−32

Therefore, their broad usage can be observed in a wide rangeof applications, especially in transportation aerospace, sensors,and household goods.33−36 However, the intrinsic hydro-philicity of Al hampers its application in clammy environments,to a large extent. To address this difficulty, many super-hydrophobic surfaces with superior water repellent propertieshave been fabricated on aluminum and its alloys. To date, avariety of synthetic techniques to construct surface roughness,such as chemical etching,37,38 dip coating methods,33,39

electrochemical anodization,34 laser marking approaches,35

and combinations of those methods,36 have been developed.Nevertheless, most of the methods mentioned above requiremultiple steps and high energy cost. Among these, chemicaletching is usually used in combination with dip coatingmethods, which are relatively easy to operate. For example,Entezari et al. obtained superhydrophobic aluminum surfaces

Received: November 30, 2016Revised: February 15, 2017Accepted: February 21, 2017Published: February 21, 2017

Article

pubs.acs.org/IECR

© XXXX American Chemical Society A DOI: 10.1021/acs.iecr.6b04650Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

using simple immersion and ultrasound approaches andmodification of the rough surfaces with stearic acid.33 Lei etal. hydrothermally treated Al in a La(NO3)3 aqueous solutionto fabricate ginkgo-leaf-like nanostructures, then modified alow-surface-energy compound (Actyflon-G502),39 while thehigh cost and deleterious nature of the rare-earth-metal saltsimpede the application. In another work, He et al. fabricatedhierarchically structured porous Al surfaces by simplyimmersing them in a hot water bath, followed by modificationwith 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17),while the hot water temperature is hard to accurate control forindustrial production.40 As is well-known, low-surface-energycompounds, such as fluorosilane,35−37,39,41 long-chain alkanes(acids),33,34,42 and silicone acrylic resins,43 were mostlyphysically covered on the dual-scale structures, to reduce thesurface energy, resulting in low adhesion to the substrates.According to our previous work,44 a fluorosilicone-modifiedpolyester resin with superior thermal and surface performancewas obtained. After being modified with fluorosilicone, thepolyester resin became more hydrophobic and oleophobic.Moreover, it possessed prominent nonsticking, high startingdecomposition temperature, antiyellowing, and higher hardnessproperties, which could broaden the practical application.Above all, the fluorosilicone-modified polyester resin washydroxyl-terminated, which could react with the nanoparticles,the Al surface, and itself together, thereby enhancing themechanical properties.For outside use, it is crucial for the superhydrophobic

metallic surface to survive when subjected to water scouring,sand storms, sweeping, and long-term mechanical wear.Actually, many superhydrophobic coatings are incapable ofhaving practical applications mainly ascribed to their inherentweakness toward mechanical abrasion, particularly the nano-scale roughness.25 For many reported superhydrophobicsurfaces, slight attrition could destroy the superhydrophobicity,and some of them could not even withstand fingertouching.31,45,46

In this paper, the aluminum plates are etched by immersingin a hydrochloric acid, resulting in the formation of amicrotextured surface composed of irregular, stairlike protru-sions. The etched plates then were dip-coated with SiO2 togenerate a micronanoscale rough structure. After being sprayedwith fluorosilicone-modified polyester resin (F20SP70) toreduce the surface energy, the superhydrophobic Al plateswere obtained. In order to systematically optimize thefabrication parameter to prepare the dual-scale structure, theeffects of etching time on surface morphology and wettabilitywere investigated. Meanwhile, the antifogging, antifrosting, anti-icing, and mechanical durability (e.g., sandpaper abrasion andtape peeling) of the superhyrdophobic Al surfaces werediscussed in detail. Moreover, the mechanism of the prominentstability of the as-prepared superhydrophobic surface wasexplained. To the best of our knowledge, there is very limitedscientific investigation on superhydrophobic Al surfaces withgreat mechanical stability, antifogging, antifrosting, as well asanti-icing performance.

2. EXPERIMENTS2.1. Materials. An aluminum plate with 99.6% purity was

purchased from Pingyin Xing’an Aluminum Co. Ltd.(Shandong, China). Hydrochloric acid, toluene, ethanol, andsilica with an average particle size of 15 nm were purchasedfrom the Shanghai Chemical Reagent Co., Ltd. (China).

Fluorosilicone-modified polyester resin (denoted as F20SP70)was synthesized by following the previously reported method.44

All reagents were analytical grade and used as received withoutany further purification.

2.2. Procedures. 2.2.1. Etching Process. The aluminumplates (20 mm × 20 mm × 1 mm) were ultrasonically cleanedsuccessively in toluene and ethanol for 10 min, respectively,then dried in air at room temperature. Afterward, the cleanedaluminum plates were immersed in hydrochloric acid solutions(3M) for different times (10, 15, 20, and 25 min) to introducemicroprotrusions. The samples were then ultrasonically cleanedwith deionized (DI) water for 10 min and desiccated in avacuum dryer at 100 °C.

2.2.2. Dip-Coating Process. The dried aluminum plates wereimmersed in silica ethanol solution (mass fraction of 1%) for 2min, after being taken out from the solution and heat-treated at300 °C for 120 min micronanoscale roughness structuressurface were obtained. For the purpose of simplifying thefollowing discussion, different samples listed in Table 1 werenamed based on the synthesis process.

2.2.3. Modification Process. In order to endow the surfacewith superhydrophobicity, the pretreated substrates weresprayed with F20SP70 using an airbrush (2 bar workingpressure, nozzle 0.5 mm, and stand-off distance 400 mm) at aspeed of ∼5 cm/s, then the coated substrates were cured in avacuum dryer at 150 °C for 3 h.

2.3. Wettability and Surface Morphology. Watercontact angles (WCAs) were measured using an opticalcontact-angle goniometer (Model CAM 200, KSV Instruments,Helsinki, Finland) at room temperature and averaged over fivedifferent spots on each specimen to get a reliable value. In thecase of superhydrophobic surfaces, water droplets of smallvolume would adhere tightly to the syringe and were hardlyable to be placed on the surface; thus, 7 μL droplets were usedfor the measurement of equilibrium contact angles. Watersliding angles (SAs) were measured by adjusting the homemademutable plane slowly from 0° to higher angles until a 10 μLdroplet start to roll off, the inclined angle was recorded as thesliding angle. The roughness of the surface was measured byatomic force microscopy (AFM) operated by Multi Mode(Veeco, USA) in tapping mode. The scan ranges were 10 μm ×10 μm. The surface morphologies of Al surface were observedby scanning electron microscopy (SEM) (Model Nanosem 430,FEI Co. Ltd.).

2.4. Mechanical Stability Tests. Mechanical stabilities ofthe surfaces were evaluated by applying the tape-peeling testand the abrasion test. The tape peeling test was conducted withpressing and then peeled off the adhesive tape on the as-prepared superhydrophobic surface, according to method B ofASTM Standard D3359-09ε2. A rectangle eraser was loading onthe tape, to ensure the compact adhesion of it.The abrasion test was carried out under the following

protocol. The surface was placed on 400 grit SiC sandpaper

Table 1. Symbols for Samples Prepared under DifferentConditions

sample sample description

Al-E20-SiO2 20 min etched Al surface coated with SiO2

Al-E0-SiO2−FS 0 min etched Al surface coated with SiO2 and F20SP70Al-E10-SiO2−FS 10 min etched Al surface coated with SiO2 and F20SP70Al-E20-SiO2−FS 20 min etched Al surface coated with SiO2 and F20SP70

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under a loading of 50 g, and pulled in one direction at a speedof ∼60 mm/s at 20 cm increments until an ultimate distance of120 cm was reached. The variation of WCA values during eachabrasion test pause were recorded.2.5. Antifogging and Antifrosting Properties. The

specimen was placed on a cooling stage that was connectedwith a coldtrap (Model C203W), and could precisely maintainits temperature at −2 °C (antifogging test) and −12 °C(antifrosting test), the temperature of the surfaces wasinspected, using a digital temperature sensor, and themeasurement error was ±1%. The spray of a humidifier wasused to generate condensed water droplets on the surface. Thehumidity was controlled within the range of 60%−95%. Thefrosting processes were monitored and collected by a digitalcamera.2.6. Anti-icing Properties. The icing delay time (IDT) of

water droplets on superhydrophobic surfaces was tested usingthe Model CAM200 system and the Model C203W system. Itwas performed by controlling the temperature of the C203Wsystem at −18 °C, then a 2 μL water droplet was placed ontothe Al surface until it reached thermal equilibrium. During themeasuring process, given the difference in the refractive indexesbetween water and ice, the delay time was defined as the timethat the water droplet began to change from being transparentto being completely frozen and undergoing deformation. Thehumidity of the environment was controlled at 45% ± 5%.The ice adhesion strength was measured by the method

described in the literature.47 This adhesion was considered tobe the shear strength needed to remove the ice from thecoating. Bottomless cuvettes filled with the liquid water wereplaced onto the Al surfaces with different etching times, whichwere then maintained at −36 °C for 4 h to form the ice. Adynamometer was used to measure the adhesion; each valuewas averaged over three tests on each specimen.

3. RESULTS AND DISCUSSION

3.1. Preparation of Superhydrophobic Surfaces onAluminum Substrates. The process of fabricating super-hydrophobic surface on Al substrates is illustrated in Scheme 1,including chemical etching in HCl solution, dip-coating nano-SiO2 to create hierarchical structures, and then subsequentmodification with F20SP70 resin to reduce the surface energy.The etching process is based on the replacement reactionbetween H+ and aluminum, which is spontaneous and rapidunder ambient conditions. The strong acid atmosphereprovided a stable replacement environment in which aluminumcould be easily dissolved.Within three simple procedures, the aluminum substrates

converted from commonly hydrophobic (WCA = 91°, SA =66°) to superhydrophobic (WCA = 157°, SA = 1°) in just a fewminutes. The WCAs and SAs of the specimen with differentetching times were measured to determine the wettability.According to Figure 1, the WCAs and SAs were varied withetching time. The WCA of the Al-E-SiO2-FS surface gradually

increased as the etching time increased and reached the highestpoint of 157° at 20 min and then flopped down. Inversely, theSA decreased from 66° to the lowest point of 1° and increasedwith further etching afterward. This can be illustrated asfollows. The dual structure on the surface can trap abundant airin the space between the rough structure and the water droplet;therefore, the water droplet mostly contacts with air rather thanthe solid Al surface, making it difficult for the droplet tooverspread the surface. However, when the etching timeincreased to 25 min, the dual structure was damaged, whichleads to a decrease in the contact angles. In comparison, thesurfaces without dip-coated nano-SiO2 obviously showed worsewetting behavior and could not be superhydrophobic over theentire investigated etching time ranges in this paper. Theimprovement in the hydrophobicity of the surface could bemainly attributed to the micronanoscale structures obtainedusing the dip-coating process. Nevertheless, the coverage ofnanoparticles was insufficient to achieve superhydrophobicity.The groups such as −CF2 and −CF3 contained in F20SP70 onthe modified surface have extremely low surface energies, whichinduce surface superhydrophobicity. From the chemicalstructure of the F20SP70 resin presented in our previouswork,44 it also played the role of a binder to combine thenanoparticles with each other and firmly fixed them on the Alsurface through the terminated hydroxyl during the curingprocess under high temperature.The dynamic process of a water droplet on the super-

hydrophobic aluminum surface was displayed in Figure 2. An Alsurface etched for 20 min was confirmed to have the bestsuperhydrophobic performance with the highest WCA and thelowest SA; therefore, we chose it to further examine the water-adhesion property. A 7 μL droplet was applied as a probe(Figure 2a); it was difficult to exude the drop from the needleand incline it to slide away when the needle moved down tosqueeze it, because of the low water adhesion. As the needlemoved up, it was lifted from the surface without any trace. Thisphenomenon implied that etching for 20 min, dip-coating, and

Scheme 1. Schematic Illustration of the Fabricating of the Superhydrophobic Aluminum Surface

Figure 1. Water contact angles and sliding angles on the specimen Alsurfaces with different etching times after subsequent dip-coating andmodification processes.

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modification processes endowed the Al surface with exceptionalsuperhydrophobicity.The high-speed camera was supplied to record the roll-off

behavior of a water droplet.48 As shown in Figure 2b, a dropletrolled off immediately within 0.25 s once it was deposited ontothe Al surface, because of the extremely low adhesion force.This illuminated the belief that a superhydrophobic surface wasbeneficial to duly remove impacting water droplets, so that thedroplets were difficult to condense on the coating. Thisprovided evidence for superhydrophobic surfaces possessingantifogging, antifrosting, and anti-icing properties.3.2. Microstructure. To obtain a rough structure on the Al

surface, the samples were chemically etched in hydrochloricacid for 0, 10, 15 and 20 min. The surface morphologies of the

aluminum surfaces through acid etching and dip-coatingprocesses are demonstrated in Figure 3. According to Figure3a, an untreated Al plate was presented, except for somescratches on it. When etching for 10 min (Figure 3b), thealuminum oxide film on the surface was destroyed and themicroscopic defects on the Al surface with relatively higherenergy were preferentially dissolved, a few deep voids with sizeson the order of hundreds of micrometers were partiallydistributed on these etched aluminum surfaces. As the etchingproceeded (Figure 3c and 3d), the entire amount of aluminumwas uniformly etched and formed fractal-like pits andprotrusions, which were consisted of 1−2 μm stair-stepstructures. The corrosion morphology is believed to bedetermined by the relative rates of the preferential nucleation

Figure 2. Dynamic process of a water drop on the superhydrophobic aluminum surface etched for 20 min (a) water-adhesion measurement, (b)water droplet roll-off experiment. The time sequence was displayed in the top left of the images.

Figure 3. SEM images of aluminum alloy with different etching times: (a) 0 min, (b) 10 min, (c) 15 min, (d) 20 min, and (e) 25 min; labels withsubscripts “1”, “2”, and “3” indicate that the photomicrographs have magnifications of 1000×, 5000×, and 10 000× of etched aluminum withoutnano-SiO2, respectively. The figure labels with a “4” subscript indicate photomicrographs with a 10 000× magnification of etched aluminum withnano-SiO2 deposited (the red rectangle on the upper right corner of each photomicrograph has a magnification of 50 000 magnification).

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of unit pits and the movement of the monatomic strides acrossthe surface.41 While further etching would destroy thestructural integrity, it was apparent from Figure 3e thatexcessive etching would lead to a loss in fractal-like structureand smooth scallops, which would decrease the roughness ofthe surface and ultimately decline the hydrophobicity.Comparing the etched surfaces in series 3 and 4 in Figure 3,The stair-step structures in series 3 became vague and wascovered with a layer of tiny white SiO2 particles with sizes onthe order of dozens to hundreds of nanometers (4 series)which constituted the micronanoscale structure after the dip-coating process. This hierarchical structure is in favor ofsuperhydrophobicity after a reduction in the surface energy,while the surface with a microstructure cannot. The aluminumsurfaces treated by hydrochloric acid had large amounts ofhydroxyl, which reacted together via hydroxyl on single oragglomerated nano-SiO2 surfaces through a dehydrationreaction under 300 °C, contributing to the stability ofmicronanoscale structure and strengthening the hydrophobic-ity. The variations in microstructure between the samplesetched for different time conformed that microstructured Alsurfaces with different surface roughness could be achieved byvarying the etching time.

Three-dimensional (3D) atomic force microscopy (AFM)images of the sample are shown in Figure 4. The root-mean-square (RMS) roughness of the Al surface etched for 0, 10, 15,20, and 25 min were 25.47, 184.54, 267.93, 242.5, and 144.17nm, respectively, indicating that the roughness of Al surfaceincreased with the increasing etching time at certain ranges. Asthe etching time extended, the microstructure was moreuniform so that the WCA was 157 °C and the SA was 1 °Cwhen the etching time is 20 min, although the RMS of the Alsurface is <15 nm. When the etching time increased up to 25min, the surface was etched immoderately, leading to a fierceincrease in the SA and the degradation of hydrophobicity. Theaforementioned results illustrated that the etching processcould create a rough structure but excessive etching wouldreduce the roughness, which was extremely compatible with theSEM analysis.

3.3. Antifogging Properties. We evaluated the antifog-ging performance of the Al surfaces by storing them at −2 °Cand controlled the humidity within a range of 60%−95%.Unsurprisingly, fog formed on the four different surfaces after120 min in the experiment (Figure 5A). The Al-E20-SiO2

substrate was hydrophilic, on which the droplets converged intodepressed large ones and filled the entire surface. As the WCAincreased, the condensed droplets became smaller and had a

Figure 4. AFM images of aluminum alloy with different etching times: (a) 0, (b) 10, (c) 15, (d) 20, and (e) 25 min.

Figure 5. (A) Photographs of different horizontal aluminum surfaces that were fogged for 120 min; (B) fog weight of different aluminum surfacesafter 120 min of frosting (20 min etched, unmodified surface (condition a); 0 min etched, modified surface (condition b); 10 min etched, modifiedsurface (condition c); and 20 min etched, modified surface (condition d)).

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tendency to be spherical. The fog weight of different aluminumplates after 120 min of fogging are shown in Figure 5B. The Al-E20-SiO2 and Al-E20-SiO2−FS substrates were covered with66.1 and 26.7 mg of water, respectively. These resultsmanifested that the low-surface-energy resin could significantlydecrease fog quality by 59.6%. Regardless of the complicatedfog formation process, the various antifogging performancecould due to the fact that the more hydrophobic the material,the lesser the rate of water cooling.12 The two hydrophobicsurfaces (Al-E0-SiO2−FS and Al-E10-SiO2−FS) had littledifference on the fogging conditions. Nevertheless, longertimes were needed to form the initial fog on the super-hydrophobic surface, so that the Al-E20-SiO2−FS substrate wascovered with the least droplets.3.4. Antifrosting Properties. In order to investigate the

antifrosting properties, the frost formation process on differentAl plates were observed using optical photographs (Figure 6). Itwas easily observable that the frost crystals appeared muchearlier on the Al-E20-SiO2 substrate (Figure 6a), which was ahydrophilic surface. Previous research clarified that waterdroplets were mainly contacted with the air trapped by therough structures rather than the solid substrates on thehydrophobic surfaces.49 Since the heat-transfer coefficient of airis much less than that of a solid, based on the condition that thetotal heat transfer between the cold Al plates and the waterdroplets was the same, there is no doubt for the delay offrosting on the Al-E20-SiO2−FS substrate, which performed thebest superhydrophobicity. Despite this observation, all theplates that were ultimately covered with a thin frost layer ∼30min later in Figure 6, the Al-E0-SiO2−FS and Al-E10-SiO2−FS

substrates possessed almost the same amount of frost, while theamount of frost on the Al-E20-SiO2−FS substrate wasapparently less than the other. The frost weight of differentaluminum plates after 50 min of frosting are shown in Figure7A. The Al-E20-SiO2 and Al-E20-SiO2−FS substrates werecovered with 72.7 and 47.7 mg of frost, respectively. The resultselucidated that spraying the fluorosilicone-modified polyesterresin to reduce surface energy could lead to a 34% decrease infrost quality.Moreover, the Al-E20-SiO2−FS substrate still showed

hydrophobicity after the freezing−thawing process (Figure7B). The melted frost shrank into spherical droplets on thesurface that could easily slide down with the wind blowing orthe aluminum plates inclined. While the water droplets spreadout on the hydrophilic Al-E20-SiO2 substrate and formed alayer of water film. With the enhancement of the hydro-phobicity, melted frost converged into small droplets thatdispersed on the surface. Meanwhile, the amount of the dropletfrom melted frost decreased as the hydrophobicity increased,and only some sporadic droplets appeared on the Al-E20-SiO2−FS substrate. These results revealed that superhydro-phobic surfaces could retard the appearance of frost in earlystage under the temperature of −12 °C, and could promptlyrestore its hydrophobicity after heat defrosting. This stablecharacter was important for the surface to be applied inpractical situations.In this research, frost formation on Al plates is a typical

heterogeneous nucleation process; the size of the particlespresent on the plates has important influence for the kinetics ofthe ice nucleation.50 According to classic heterogeneous

Figure 6. Photographs of different vertical aluminum surfaces during the frosting process: (left) 20 of frosting, (middle) 30 min of frosting, and(right) 50 min of frosting). [Legend: (a) 20 min of etching, unmodified surface; (b) 0 min of etching, modified surface; (c) 10 min of etching,modified surface; and (d) 20 min of etching, modified surface.]

Figure 7. (A) Ice weight of different aluminum surfaces after 50 min of frosting; (B) photographs of aluminum surfaces after defrosting with differenthydrophobicities. [Legend: (a) 20 min of etching, unmodified surface; (b) 0 min of etching, modified surface; (c) 10 min of etching, modifiedsurface; and (d) 20 min of etching, modified surface.]

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nucleation theory, the free-energy barrier (ΔGc) for nucleationaround a spherical particle of radius R is

Δ = ΔG G fc chomo

where ΔGchomo is the free-energy barrier of heterogeneous ice

formation, f is a factor varies from 0 to 1, defined as

= + − + − − + −

+ − −

⎜ ⎟ ⎜ ⎟ ⎜ ⎟

⎜ ⎟

⎛⎝

⎞⎠

⎡⎣⎢

⎛⎝

⎞⎠

⎛⎝

⎞⎠

⎤⎦⎥

⎛⎝

⎞⎠

fmx

wx x m

wx m

w

mx x mw

12

12

12

2 3

32

1

3 3 3

2

x is the relative particle radius (x = R/rc), m = cos θflat, w = (1 +x2 − 2xm)1/2, and rc is the radius of the critical nucleus, which isdefined as

γν= −Δ

rG

2c

(Here, γ is the water-ice interfacial tension, and ν is the watermolar volume.) T is the frosting temperature, Tm is the icemelting temperature, and Cp is the water heat capacity. In thiswork, γ ≈ 0.034 J m−2, ν ≈ 1.8 × 10−5 m3 mol−1, T = 261.15 K(−12 °C), Tm = 273.15 K (0 °C), Cp ≈ 75.3 J mol−1 K−1. Usingthe expression

Δ ≈ − +−

⎡⎣⎢⎢

⎛⎝⎜⎜

⎞⎠⎟

⎤⎦⎥G C T

TT

TT

ln1p

m

m

under the aforementioned experimental conditions, rc iscalculated to be 60.8 nm.According to previous research, as the surface particle radius

R increases, f monotonically decreases, which indicates that theenergy barrier falls off. Because the icing probability is inverselyrelated to the free-energy barrier, it is easily observed thatetching and dip-coating processes endow the surface with

micronanoscale particle diameters, which can augment theenergy barrier and restrain the water droplets nucleation.Consequently, the superhydrophobic Al surface is moreinclined to impede frosting and icing. This is consistent withthese experimental observations.42

3.5. Anti-icing Properties. The superior anti-icing proper-ties of the as-prepared Al surfaces were mainly evaluated by twoaspects, including crystallization delay of water droplets and lowadhesion to ice.48 In this section, the freeze delay time ofindividual water droplets on different Al surfaces was tested;photographs taken during the entire freezing process are shownin Figure 8. The droplets were all transparent and the center ofthem reflected light, forming a bright point initially. As timemoved on, the droplets gradually became opaque, such that thelightspot vanished. When the droplets were completely iced,they deformed from a spherical dome and formed sharpcorners. The recorded photographs demonstrated that thehydrophilic surface was quickly iced within 7 s, while thehydrophobic surfaces can prolong the IDT, in a way, by 118and 236 s, respectively. As expected, the superhydrophobicsurface with outstanding antifogging and antifrosting perform-ance were iced after 450 s, which is 64 times faster than thehydrophilic surface. This supported the belief that thesuperhydrophobic surface that we fabricated could delay iceformation, because of the tardy heat conduction and high free-energy barrier of nucleation that we previously described.51,52

In practical application, although the superhydrophobicsurface exerts strong ice delay properties, the ice forming isinevitable when the outdoor temperature below 0 °C for a longtime. To scientifically assess the anti-icing ability, we measuredthe ice adhesion strength at −36 °C using a homemade device(Figure 9). The ice adhesion on the hydrophilic surface was thehighest, with a force value of 1.364 N. For the hydrophobicsurfaces, the ice adhesion strength were 0.638 and 0.609 N,respectively. The minimum value on a superhydrophobic

Figure 8. Photographs of individual water droplets on (a) 20 min of etching, unmodified surface; (b) 0 min of etching, modified surface; (c) 10 minof etching, modified surface; (d) 20 min of etching, modified surface during the entire freezing process.

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surface was observed at a force value of 0.377 N. Because of thelow contact area, ice formed on the superhydrophobic surfacewas loose and fluffy, in that the ice adhesion decreased by morethan 72%.3.6. Mechanical Durability. The easy wiping and poor

durability are the two main notorious problems for mostmacronanoscale structures of superhydrophobic surfaces.35

Sometimes, even a slight strike or friction could destroy thesurface structure, thus disabling the superhydrophobicity. Inorder to assess the mechanical durability of preparedsuperhydrophobic surfaces, the Al-E20-SiO2−FS substrate wastested through tape-peeling and coarse-sandpaper-abradingexperiments. The tape-peeling experiment revealed that thesuperhydrophobic aluminum surface could sustain super-hydrophobicity after testing with adhesive tape (Scotch-600)at least 11 times. Figure 10A presents the WCAs, as a functionof the times peeled the tape. The water repellency merelyexhibited a slight decline (from 160° to 152°) after scaling 11times; however, it still within the lotus effect classification.To further demonstrate the mechanical stability, we chose

400 grit SiC sandpaper to evaluate the durability of thecondition d sample. The influence of abrasion on samples isdetermined by many factors, such as pressure, abrasiondistance, and the abrasion surface.53 Figure 10B showed theevolution of WCAs upon dragging in one direction with aspeed of 6 mm/s with weights of <50 g. The WCA almostremained the same above 150° after dragging for a length of120 cm. Those results indicated that the as-prepared aluminumsuperhydrophobic surface was superior to other reportedworks,39 which is quite significant for practical applications.

The robustness of the aluminum superhydrophobic surface isprimarily derived from the following three aspects:• First, creation of hierarchical roughness with two-scale

unevenness.33 The aperture between microprotrusions couldtrap the nanoparticles during stability tests to protect themfrom being carried off.• Second, the F20SP70 resin, which is rich in hydroxyl

groups and covered the superhydrophobic surface, couldenhance the adhesion between the nanoparticles and thesubstrates, while protecting the surface structure againstexternal forces for the first time, so that it can withstand theterrible environment, to a considerable extent.• Third, the interfacial interactions among the SiO2, the low-

surface energy molecules, and the aluminum surface throughthe hydroxyl groups have a great contribution to the structuraland compositional stability.During the dip-coating process, nano-SiO2 gained plenty of

hydroxyl groups that could participate in cross-linking reactionsto form tough and stable bonding, while a pure Al surface couldonly react with a few F20SP70 to form relative weak bonding.Meanwhile, the Si atom has a unique extranuclear electronicarrangement, with an octet of electrons filling the 2s22p6

outermost shell, which is inertial to the formation of hydrogenbonds with interfacial water molecules, resulting in the inherenthydrophobicity. However, the Al atom is deficient of twoelectrons in its three sp2-hybrid orbitals, which has a tendencyto capture two electrons from interfacial water molecules inorder to achieve a stable state, resulting in the inherenthydrophilicity. The inherent hydrophobicity property of SiO2 isalso conducive to sustaining the superhydrophobicity.39

4. CONCLUSIONS

In summary, stair-step microstructures covered with SiO2nanoparticles were constructed on an aluminum surface via asimple etching method, accompanied by a deposition process.Further modification with a self-made low-surface-energyfluorosilicone-modified polyester resin (F20SP70) improvedthe superhydrophobicity (with a water contact angle (WCA) of157° and a sliding angle (SA) of 1°). The as-preparedsuperhydrophobic aluminum surface showed remarkabledurability, based on mechanical stability tests, which was dueto the micronanoscale surface structure, the strong and stablebonding between SiO2 particles, and F20SP70 molecules. Thesuperhydrophobic surface was effective in retarding fog andfrost formation, as well as delaying ice formation and decreasingice adhesion, which showed excellent antifogging, antifrosting,and anti-icing characteristics, compared to hydrophobic and

Figure 9. Ice adhesion strength of the Al surface with different etchingtimes: (a) 20 min of etching, unmodified surface; (b) 0 min of etching,modified surface; (c) 10 min of etching, modified surface; and (d) 20min of etching, modified surface.

Figure 10.Water contact angles (WCAs), as a function of (A) the number of tape tests and (B) abrasion length for 20 min of etching on a modifiedsuperhydrophobic surface.

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hydrophilic Al surfaces. Since the as-prepared superhydropho-bic surfaces are robust in a clammy environment, it is expectedto be used in aplications such as airplanes, air conditioning, andheat exchangers. Moreover, the fabricating approach for such arobust superhydrophobic surface is maneuverable and universalto various metals and alloys.

■ AUTHOR INFORMATIONCorresponding Authors*Tel.: +86-571-8795-3382. Fax: +86-571-8795-1227. E-mail:qhzhang@zju.edu.cn (Q. Zhang).*Tel.: +86-571-8795-3698. Fax: +86-571-8795-1227. E-mail:xlzhan@zju.edu.cn (X. Zhan).ORCIDQinghua Zhang: 0000-0003-1350-6388NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the National Natural ScienceFoundation of China (Award Nos. 21676248, 21576236, and21476195) and the Zhejiang Provincial National ScienceFoundation of China (Grant No. Y14B060038).

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