fabrication and characterization of superhydrophobic surfaces on aluminum alloy substrates

Accepted Manuscript

Title: Fabrication and Characterization of SuperhydrophobicSurfaces on Aluminum Alloy Substrates

Author: F.Y. Lv P. Zhang

PII: S0169-4332(14)02138-2DOI: http://dx.doi.org/doi:10.1016/j.apsusc.2014.09.147Reference: APSUSC 28803

To appear in: APSUSC

Received date: 6-7-2014Revised date: 3-9-2014Accepted date: 22-9-2014

Please cite this article as: F.Y. Lv, P. Zhang, Fabrication and Characterization ofSuperhydrophobic Surfaces on Aluminum Alloy Substrates, Applied Surface Science(2014), http://dx.doi.org/10.1016/j.apsusc.2014.09.147

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http://dx.doi.org/doi:10.1016/j.apsusc.2014.09.147

http://dx.doi.org/10.1016/j.apsusc.2014.09.147

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Fabrication and Characterization of Superhydrophobic Surfaces on Aluminum

Alloy Substrates

F. Y. Lv, P. Zhang∗

Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, 200240, China

ABSTRACT

Superhydrophobic surfaces have potential anti-icing applications in industries and daily life. In the present study,

we combine the methods of chemical etching and surface modification with 1H, 1H, 2H,

2H-Perfluorooctyltriethoxysilane (POTS) which has very low surface energy to simplify the fabrication

procedures for superhydrophobic surfaces on aluminum alloy substrates. The results show that the contact angle

(CA), rolling angle (RA) and contact angle hysteresis (CAH) of superhydrophobic surfaces etched with 8.0 wt%

HCl aqueous solutions are 162.5°, 1.9° and 1.1°, respectively; the apparent surface free energies (ASFEs) of

superhydrophobic surfaces increase with the decrease of surface temperature; the freezing time of water droplets

on the superhydrophobic surfaces is retarded by 1568 s, and the temperature drops to as low as -11.9 ℃. The

results indicate that the superhydrophobic surfaces exhibit excellent anti-icing properties.

Keywords

Superhydrophobic surface; Ice nucleation; Freezing time delay; Anti-icing properties; Apparent surface free

energy

1. Introduction

Superhydrophobic surfaces with high contact angles (CAs), low rolling angles (RAs) and contact angle

hysteresis (CAH) have many potential applications in industries because they show the characteristics of low ice

adhesion force to surfaces and self-cleaning. The fabrication of superhydrophobic surfaces generally includes two

∗ Corresponding author. Tel: +86-21-34205505; Fax: +86-21-34206814. E-mail address: [email protected] (P. Zhang)

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steps: first, construction of micro/nanostructured rough surfaces like lotus leaves; second, modification of surfaces

using chemical substances with low surface energy. Many methods were used to fabricate the hierarchical

structured superhydrophobic surfaces, such as anodic oxidization [1-4], phase separation [5], sol-gel processing

[6], chemical vapor deposition [7], plasma etching [8], chemical etching [9], and so on. Various materials such as

plastics, glass and metals were used as substrates to fabricate artificial superhydrophobic surfaces [10,11], and

some chemical substances with low surface energy including silane [12,13], Teflon [14,15], fluoroalkylsilane

[16-19], n-octadecyl mercaptan [20] and fatty acid [21-23] were used in the modification process.

It is well known that icing occurs on normal surface because it can absorb water easily. Superhydrophobic

surfaces can avoid freezing or delay the formation of ice on the surfaces to a certain degree because these surfaces

have extreme water-repellent properties. The icing experiments conducted by Cao et al. [24] demonstrated that the

surfaces coated with low surface energy polymer could effectively prevent ice formation both under laboratory

conditions and in field tests. Some researchers found that the freezing time of water droplet on superhydrophobic

surface was retarded significantly [25-27], which was explained as that the nucleation energy barriers for

superhydrophobic surfaces were much higher than those for normal surfaces.

Boinovich et al. [9] fabricated superhydrophobic surfaces on stainless steel substrates by combining the

solution-immersion and deposition of mixture containing silica nanoparticles and hydrophobic agent. Li et al. [28]

prepared superhydrophobic CuO surfaces with tunable adhesion through combining both solution-immersion

process and self-assembly of fluoroalkylsilane. The morphologies of CuO surfaces were controlled by adjusting

the immersion time, growing temperature, and solution compositions. Surfaces which exhibited

superhydrophobicity were prepared by Ruan et al. [29] on aluminum alloy substrates through chemical etching

and modification with lauric acid. Many methods to fabricate superhydrophobic surfaces on metal substrates were

reported in the literature, but the methods might be complex. A very simple, inexpensive and environment-friendly

method is necessary to fabricate the superhydrophobic surface on aluminum alloy substrates. However, the

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superhydrophobic surfaces on aluminum alloy substrates are of potentially wide application in heat transfer

devices. The freezing time of water droplet on superhydrophobic surface can be extremely delayed during the

freezing process, which is essential for the applications in preventing icing and frost. It is very important to

understand the relationship among apparent surface free energy (ASFE), surface temperature and

superhydrophobicity so as to obtain the surfaces with excellent superhydrophobicity properties on aluminum alloy

substrates for applications.

In the present study, a very simple method is proposed to fabricate superhydrophobic surfaces on aluminum

alloy substrates through chemical etching and self-assembling a thin film of fluoroalkylsilane. The variation of

ASFEs of superhydrophobic surfaces with temperature is obtained. A cooling apparatus with temperature control

and data acquisition system is built to investigate the anti-icing properties of the prepared superhydrophobic

surfaces. The freezing properties of water droplets on various surfaces are investigated in the experiments, and the

results are discussed in detail.

2. Experiments for superhydrophobic surface and anti-icing

2.1. Materials

Hydrochloric acid with an analytic grade was purchased from Shanghai Chemical Reagent Co., Ltd. China. The

1H, 1H, 2H, 2H-Perfluorooctyltriethoxysilane (POTS) with purity of 97.0% was purchased from Alfa Aesar. The

grade of aluminum alloy is 6061 with the chemical compositions (in wt%) of 0.15 Cu, 0.15 Mn, 0.8 Mg, 0.25 Zn ,

0.04 Cr, 0.15 Ti, 0.4 Si, 0.7 Fe, and 97.36 Al.

2.2. Procedures of fabricating superhydrophobic surface

The aluminum alloy plates with the dimension of 50 × 30 × 3 mm were used as substrates and sandblasted

with silica sand particles of 120 grit size (particle diameter of 125 μm). All the aluminum substrates used in the

experiments were not polished. The pristine and sandblasted substrates were cleaned ultrasonically in sequence by

acetone, ethanol and distilled water for about 15 min until the dusts and grease were removed. Afterwards, the

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cleaned substrates were immersed in hydrochloric acid aqueous solution with the concentrations of 6.0 wt%, 8.0

wt%, 10.0 wt% and 12.0 wt% at room temperature and etched for about 25 min, respectively. Then, the substrates

were rinsed and ultrasonically washed with distilled water to remove dust particles on the porous texture surfaces.

The ethanol solution of POTS with the concentration of 1.0 wt% was hydrolyzed by adding threefold mole of pure

water and a small drop of acetic acid to adjust the pH value to about 4, which could promote the hydrolysis and

suppress the dehydration synthesis of fluorine silanol groups [30]. Then, the solution was stirred at 120 rpm with

an agitator at room temperature for 5 hours. In order to self-assemble a monolayer of fluoroalkylsilane film, the

pretreated substrates were immersed in the hydrolyzed POTS solution for 45 min at room temperature. The

substrates were washed with ethanol and baked at 120 ℃ for 60 min after the substrates were removed from the

modification solution containing POTS. Then, the superhydrophobic surfaces were obtained. The

superhydrophobic surfaces etched with hydrochloric acid aqueous solution were denoted as samples 1, 2, 3 and 4

for the concentrations of 6.0 wt%, 8.0 wt%, 10.0 wt% and 12.0 wt%, respectively, and the pristine and sandblasted

surfaces were labeled as samples 5 and 6, respectively.

2.3. Characterization

The morphologies of surfaces were characterized with scanning electron microscope (Sirion 200, FEI, America)

after the chemical etching treatment process. The organic functional groups linked to the micro/nanostructured

surfaces after the modification process were examined through Microscopes Fourier transform infrared (FT-IR)

spectrometer (Nicolet iN10 MX, ThermoFisher, America). Water CAs, RAs and CAHs were measured with a

video-based contact angle measuring device (OCA-20, DataPhysics GmbH, Germany) at room temperatures

following the standard procedures. Ultrapure water droplet of 5.0 μL in volume was gently placed on each sample

surface at five different locations with automatic syringe controlled by the computer. The water CA was the

average value of five different locations. The RAs were obtained when the water droplet of 6.0 μL in volume

rolled off the tilted sample surfaces. The CAH was measured by adding and absorbing water at a speed of 0.5 μL/s

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through the needle fixed on the syringe.

2.4. Experiments of anti-icing property

In order to reveal the anti-icing properties of superhydrophobic surfaces, an experimental apparatus equipped

with temperature control, image acquisition and data collection systems was built. The icing processes of water

droplets on sample surfaces were monitored by a CCD camera. This apparatus included thermostatic bath,

thermoelectric cooler (TEC2-254010, China), T-type thermocouples, Agilent data acquisition (34970A, Agilent

Technologies, America), CCD camera (GRAS-50S5C-C, Point Grey, Canada), and computer. The temperature

was measured using the thermocouples with the uncertainty of 0.5 ℃, and the images were taken by the CCD

camera at a frame speed of 7.5 fps. The schematic diagram of the experimental apparatus is shown in Fig. 1. The

thermostatic bath was used to provide cooling for thermoelectric cooler fixed on the bottom of the chamber

through heat conduction of aluminum block, and the temperature of the chamber was controlled at a temperature

range of about 20.0 to 2.5 ℃ by the thermoelectric cooler for different experimental runs. The samples were

fixed on the surface of the thermoelectric cooler using thermally conductive silicone grease. The polymethyl

methacrylate (PMMA) was used as the inner and outer walls of the chamber covered by thermal insulation

material to reduce the heat leak. The top transparent cover was used as the incident window for light and the

window on the side wall was used for visual observation by the CCD camera.

A water droplet of 10.0 μL in volume with the initial temperature at 20.0 ℃ was placed on the sample surface

in the chamber using pipettor through a small hole on the top cover after the chamber was flushed with nitrogen

gas to remove the air and avoid the frost formation on the sample surface. Two thermocouples were fixed on the

surfaces of the thermoelectric cooler and sample, respectively, and one thermocouple was suspended in the

chamber near the sample surface. To investigate the freezing time delays of water droplets, the freezing

experiments of water droplets were conducted on the superhydrophobic, pristine and sandblasted surfaces. The

initial temperatures of the sample surfaces and the environment within the chamber were 12.0 ℃ and 20.0 ℃,

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respectively. The temperatures of the samples 1, 2, 3, 4, 5 and 6, which were controlled by the thermoelectric

cooler in the freezing experiments, were about -10.3 ℃, -11.9 ℃, -11.6 ℃, -11.1 ℃, -7.5 ℃, and -6.6 ℃,

respectively. The data were collected using Agilent data acquisition and recorded by the computer.

3. Results and discussion

3.1. Characterization of superhydrophobic surfaces

The morphologies of superhydrophobic and superhydrophilic surfaces etched with HCl aqueous solution appear

to be similar because the monolayer adsorption of hydrophobic chemical reagent does not change the surface

textures [31]. The SEM images of aluminum alloy substrate etched with 8.0 wt% HCl aqueous solution are

illustrated in Fig. 2(a), which show uniform micro/nanostructures, and the sizes of the protrusions with different

shapes range from 4.0 to 16.0 μm in Fig. 2(a,I). The long flower petal-like nanostructures with plenty of

needle-like pricks are observed in Fig. 2(a,III). The average size of flower petal-like nanostructures is 249.0 nm.

The randomly distributed micro/nanostructures can be observed from the images etched with 6.0 wt% and 10.0

wt% HCl aqueous solution in Fig. SI1(a,I), (a,II), (b,I) and (b,II) in the Supporting Information, respectively. The

short flower petal-like nanostructures with only slight needle-like pricks appear on the texture surface, as shown

in Fig. SI1(b,III). But for the surface etched with 6.0 wt% HCl aqueous solution, the needle-like pricks are not

observed on the texture surface, as shown in Fig. SI1(a,III). The sizes of the microstructures for the surface etched

with 10.0 wt% HCl aqueous solution range from 6.0 to 16.0 μm, and the average size of nanostructures is about

303.0 nm. It is shown in Fig. 2(b,III) that the average dimension of the rectangular cavities is about 501.0 nm.

Both the flower petal-like nanostructures and needle-like pricks are not observed in Fig. 2(b,III) because the

micro/nanostructured morphologies of the texture surface are destroyed with the increase of concentration of HCl

aqueous solution to 12.0 wt% (corresponding to the images in Fig. 2(b)). It can be seen from Fig. SI1(d) in the

Supporting Information that the sandblasted surface shows slight flaw structures, but it is very smooth for the

pristine surface shown in Fig. SI1(c). The morphologies of surfaces also demonstrate that the etching method with

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HCl aqueous solution is effective to form the hierarchical structures on aluminum surface.

The wettability of the superhydrophobic surfaces is characterized with water CAs. Water CAs, RAs and CAH

on superhydrophobic and hydrophilic surfaces are presented in Table 1. It is shown that the CAs of

superhydrophobic surfaces etched with HCl aqueous solution corresponding to the concentrations of 6.0 wt%, 8.0

wt%, 10.0 wt% and 12.0 wt% are 158.3°, 162.5°, 161.3° and 157.9°, respectively. The RAs and CAHs of

superhydrophobic surfaces are smallest for sample 2, closely followed by those of samples 3 and 4, and the largest

are for sample 1. The water CAs change from 43.5° to 162.5°, demonstrating that the change in wettability from

hydrophilicity to superhydrophobicity is influenced by the micro/nanostructured morphologies and POST film.

The average water CA of 162.5° on sample 2 is higher than that on samples 1, 3 and 4, revealing that flower

petal-like nanostructures with a plenty of needle-like pricks play key roles in improving the water CA. The RAs

and CAHs demonstrate that water adhesion force to the texture surface of flower petal-like nanostructures with a

plenty of needle-like pricks is smaller than that of the surface structures without needle-like pricks. It can be

concluded that the optimal concentration of HCl aqueous solution is 8.0 wt%. The CAs of the superhydrophobic

surfaces are all above 150°, and the RAs are very small because air can be trapped in micro/nano-binary structures

of the texture surfaces. These surface morphologies reduce the contact area between water droplet and surface.

The durability of superhydrophobic surfaces is characterized through the variation of CAs with time, as shown in

Fig. SI2 in the Supporting Information, demonstrating that the CAs of samples 2, 3 and 4 are all above 150°

during a period of 90 days, which indicates that the properties of superhydrophobicity can be maintained for a

long time.

It is well known that the superhydrophobicity depends not only on the micro/nanostructures but also on the

chemical compositions of the surface. The FT-IR spectra of superhydrophobic surface on aluminum alloy

substrate modified with POTS is shown in Fig. 3. The red line corresponds to the FT-IR spectrum of POTS, and

the blue line represents the FT-IR spectrum of superhydrophobic surface etched with HCl aqueous solution and

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coated with POTS. The wide band at 3392 cm-1 represents the stretching vibration of -OH group existed on the

superhydrophobic surface. The two bands at about 2925 cm-1 and 2851 cm-1 of superhydrophobic surface are

ascribed to asymmetric and symmetric stretching vibrations of -CH2- group, respectively. The bands at 1552 cm-1

and 1414 cm-1 might correspond to the asymmetric and symmetric stretching vibration of -COO- which comes

from the interference of carboxylate in the modification solution added with a small of amount of acetic acid to

adjust the pH value. The band at about 1055 cm-1 of superhydrophobic surface is identified as the stretching

vibrations of C-F group of the monolayer film. The band at 955 cm-1 is likely contributed by the Si-O-Al vibration,

indicating that POTS is covalently bounded to aluminum surface [32,33]. It is indicated that the molecule of

POTS dose not interact with the aluminum surface physically but chemically bonds to the aluminum surface.

3.2. Anti-icing properties of superhydrophobic surfaces

The shapes of water droplet of 10.0 μL on superhydrophobic surface at different temperatures are shown in Fig.

4(a) and (b). The white spot on the top front surface of the water droplet is attributed to the light reflection. The

white spot at the bottom of water droplet in Fig. 4(a) is caused by the reflection of air layer between droplet and

solid surface [34]. As can be seen from Fig. 4(a) and (b), the water droplet placed on the superhydrophobic surface

displays spherical shape at room temperatures, but the spherical crown of water droplet appears on the

superhydrophobic surface at -11.6 ℃. The change of water CA (corresponding volume is 10.0 μL) with the

temperature is illustrated in Fig. 4(c). It is shown that the CA decreases from about 155.0° to 150.0° when the

temperature decreases in the first icing experiment. As shown in Fig. 4(c), the CA decreases significantly from

155.0° to 132.0° when the temperatures of sample surface vary from 31.0 to -11.0 ℃ after several freezing

experiments. The obviously decreasing trend appears between 25.0 ℃ and -2.5 ℃, which might be due to the

water film spreading on the interface; the hydration of hydrogen bonding on the superhydrophobic surface; the

hydration of residual hydroxyl groups on the superhydrophobic surface [35]. The surface tension of water and the

ASFE of superhydrophobic surface increase with the decrease of temperature. The water repellent properties of

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superhydrophobic surface decrease under subzero temperatures [31,36]. The ASFEs of samples are obtained in the

experiments to investigate the relationship between ASFEs of superhydrophobic surfaces and temperatures

[37-39].

The equation used to estimate ASFE of solid surface can be written as [40]:

2(1 cos )(2 cos cos )

L AS

R A

γ θγθ θ+

=+ +

(1)

where θA and θR are advancing and receding CAs of liquid with the surface tension γL , respectively. γS denotes the

surface energy of the thin film on the solid surface. The ASFEs are calculated using equation (1) with advancing

and receding CAs of ethylene glycol. The advancing and receding CAs are obtained through adding and absorbing

ethylene glycol. Variation of ASFEs of superhydrophobic surfaces is shown in Fig. 5, and the values are listed in

Table SI1 in the Supporting Information. It can be seen that the variation trends of ASFEs are almost consistent

for samples 1, 2, 3 and 4. The ASFEs of surfaces reaches the minimum of 2.13 mJ/m2 (sample 2) at the

temperatures above 6.0 ℃, and the ASFEs increase significantly below 6.0 ℃. The ASFE of superhydrophobic

surfaces coated with ODMCS, DMDCS and FDDCS from the literature [40] is illustrated in Table SI2 in the

Supporting Information, respectively, and the values were also calculated using the same equation shown in

equation (1) in [40]. The ASFEs change from 0 to 0.15 mJ/m2 for the superhydrophobic surfaces with the

microstructure size varying from 2 to 32 μm. The ASFE of fluorocarbon FC-932 surface was investigated by

Chibowski [41], where the calculated result of ASFE was 2.14 mJ/m2. The variation trends of ASFE in Fig. 5

reveal that the ASFE increases with the decrease of surface temperatures. The CA decreases with the increase of

ASFE, which is attributed to the sample surface easily wetted once the ASFEs are higher, making the

superhydrophobicity of surface decline markedly.

The freezing time delay of water droplet is defined as the time from the turning on the thermoelectric cooler to

the onset of freezing of droplets. The initial temperature in the chamber is about 20.0 ℃ after the air in the

chamber is replaced by the nitrogen gas. In order to obtain the freezing time delay, the statistic average value is

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used as the freezing time delay for all samples. The statistic diagrams of freezing time delays are illustrated in Fig.

SI3 in the Supporting Information, which indicate that the freezing time delays of droplets on superhydrophobic

surfaces show a certain of randomness, but the freezing time delay of droplets on pristine and sandblasted surfaces

changes in a narrow range. The stochastic characteristic of freezing of superhydrophobic surfaces might be

attributed to the non-uniformly distributed micro/nanostructures on the surfaces.

The freezing time delays in the present experiments are listed in Table 2, and the results from literature are

shown in Table SI3 in the Supporting Information. It is seen that the present superhydrophobic surfaces show

good anti-icing properties compared with those in the literature. As shown in Table 2, the freezing of water

droplets on pristine and sandblasted surfaces starts at 280 s and 159 s, corresponding to the temperatures of -7.5

℃ and -6.6 ℃, respectively, and the freezing processes for the pristine and sandblasted surfaces last for only 10 s

and 8 s, respectively. Freezing occurs at 1281 s, 1386 s and 1297 s on the surface of samples 1, 3 and 4 after the

experiments starts, respectively. The initial surface temperature for samples 1, 3 and 4 is 12.0 ℃ and then reduce

to -10.3 ℃, -11.6 ℃ and -11.1 ℃, respectively, and the whole freezing processes last for only 18 s, 18 s and 16

s, respectively. The temperature at icing starting of droplet on sample 2 is as low as -11.9 ℃, corresponding to the

freezing retard time and the whole freezing process time of 1568 s and 18 s, respectively. The experimental results

mentioned above demonstrate that the freezing is retarded significantly for superhydrophobic surfaces compared

with pristine and sandblasted surfaces.

The freezing time delay is directly proportional to the performance of superhydrophobic surface in the freezing

experiments, and the results agree well with the reports of other researches [25,36,42-45]. The water droplets on

both superhydrophobic (sample 2) and hydrophilic (sample 6) surfaces, which are shown in the sequence images

of Fig. 6, lose the transparency starting from the bottom to the top of the droplet. The solidification phenomenon

occurs firstly at the interface of liquid-solid but not in the internal or air-liquid interface of the droplet. The reason

of this might be the heterogeneous nucleus which is produced at the interface of liquid-solid, and the nucleation

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energy barriers at the liquid-solid interface are much lower than that at other places of droplet. The freezing

process reveals that the key factors contributing to the heterogeneous nucleation might be the lower temperature at

the interface and micro/nanostructures on the surfaces. The contact area between water droplet and surface

decreases with the increase of CA, which makes the heterogeneous nucleation occur more difficultly. The

experimental results reveal that heterogeneous nucleation energy barriers increase with the increase of the CA,

which agrees well with the earlier results [31,46]. The onset of freezing on superhydrophobic and sandblasted

surfaces is shown in Fig. 6(a,I) and (b,I). As shown in Fig. 6(a,II) and (b,II), a small sharp-pointed protrusion

appears on the top of the ice droplet in the final stage of the solidification of droplet, and the ice droplet forms a

peach-like shape body eventually. The sharp-pointed protrusion is also observed in previous studies [42,47,48].

In order to obtain the real contact area between water droplets and sample surfaces, the real contact areas for

hydrophilic and superhydrophobic surfaces are calculated through the relationship among CA, projected area and

roughness (hydrophilic surface) or fraction of liquid-solid (superhydrophobic surface). For hydrophilic surface

corresponding to Wenzel state, the rough structures are wetted by water droplet, and the relationship between the

apparent CA on rough surface and the CA on smooth surface is expressed as [49]:

cos cos srθ θ= (2)

r

p

SrS

= (3)

where θ and θs represent the apparent CA on rough surface and CA on smooth surface, respectively. Sr and Sp

correspond to the real contact area and the projected surface area, respectively. r denotes the real contact area

divided by projected area [50]. The real contact area is calculated from the projected contact area which is

measured from the sequence images by equations (2) and (3). Whereas, the water droplet suspends on the

micro/nanostructures of surface because the air layer trapped in the porous structures restricts the contact between

water and surface in the Cassie-Baxter state [51]. The real contact area on superhydrophobic surface is obtained

through the following equations:

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cos (cos 1) 1sfθ θ= + − (4)

r pS fS= (5)

where θ and θs represent the CAs for the superhydrophobic and smooth surfaces, respectively. f denotes the

fraction of liquid area to solid area on superhydrophobic surface.

The relationship among freezing time delay, real contact area and CA is displayed in Fig. 7. The freezing time

delays for superhydrophobic surfaces (samples 1, 2, 3 and 4) are larger than those for samples 5 and 6, which

might be attributed to the micro/nanostructures on the superhydrophobic surfaces. The freezing time delay

gradually increases with the increase of CA. The freezing time delays for pristine and sandblasted surfaces are

very small, indicating that these surfaces are easy to trigger the nucleation formation of ice crystals and have

lower nucleation energy barriers. The real contact area between the droplet and surface decreases with the increase

of CA, leading to the increase of the freezing time delay.

The freezing time delays of water droplets on superhydrophobic surfaces corresponding to samples 1, 2, 3 and 4

are 1281 s, 1568 s, 1386 s and 1297s, respectively. The freezing time of droplets on different superhydrophobic

surfaces is very short and shows almost no difference. The smaller contact area related to the larger CA weakens

the heat transfer from the cold sample surface to droplet, and the smaller contact area postpones the cooling and

prolongs the freezing time of droplet on the surfaces. These results indicate that superhydrophobic surfaces can

significantly retard freezing. The freezing time delay of sample 2 is the longest in the four superhydrophobic

samples, which is attributed to the flower petal-like nanostructures with a mass of needle-like pricks on the

surface of sample 2. The freezing process on superhydrophobic surface (sample 2) is displayed in Fig. 8, where

the thermocouple for monitoring the temperature is fixed in the vicinity of the water droplet on the sample surface.

It can be seen from the figure that the temperature decreases smoothly without any fluctuation except around 1680

s. The temperature of sample surface in the inset of Fig. 8 shows a slight fluctuation and maintains at about -12.87

℃ before 1665 s. The onset of freezing in Fig. 8 is at 1665 s which is different from the statistic value of freezing

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time delay from many experiments, and the freezing process lasts for 18 s, and the corresponding temperature

ascends due to the release of latent heat of solidification. Water droplets suspend on the top of the

micro/nanostructures, and the air layer is trapped in the porous structures at the interface of contact area, which

reduces the heat transfer and retards the freezing. The superhydrophobic surfaces can retard the freezing compared

with the pristine and sandblasted aluminum alloy surfaces, demonstrating that superhydrophobic surfaces can be

used to prevent icing in practical applications.

4. Conclusions

In the present study, superhydrophobic surfaces on aluminum alloy substrates are fabricated using the simple

chemical etching and surface modification. The superhydrophobic surface with the CA as high as 162.5°, the RA

as low as 1.9° and the CAH as low as 1.1° shows excellent anti-icing properties etched with 8.0 wt% HCl aqueous

solution for 25 min and then modified with POTS. The CAs are all above 150.0° during a period of 90 days,

which reveals that durability of superhydrophobic surface is very good. SEM characterization demonstrates that

the aluminum alloy surface has a large number of flower petal-like nanostructures with a plenty of needle-like

pricks. The ASFEs of superhydrophobic surfaces increase with the decrease of surface temperature. The

superhydrophobic surface of sample 2 starts to freeze at -11.9 ℃ after 1568 s from the beginning of the

experiments, but the sandblasted surfaces begin to ice at -6.6 ℃ after only 159 s, demonstrating that the

superhydrophobic surfaces have extremely excellent anti-icing properties compared with the pristine and

sandblasted aluminum alloy surfaces.

Supporting information

The apparent surface free energy (ASFE) calculation process and the related results are provided in the

Supporting Information, and the ASFE from the literature is also shown. The SEM images of the pristine and

sandblasted surfaces, durability of superhydrophobic surfaces and the statistic diagrams of freezing time delays of

water droplets are shown in the Supporting Information.

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Acknowledgements

This research is jointly supported by the National Natural Science Foundation of China under the Contract No.

51176109 and the NSFC-JSPS cooperative project under the contract No. 51311140169.

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Table 1 Water CAs, RAs and CAHs on different sample surfaces.

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

CA (°) 158.3 ± 0.3 162.5 ± 0.9 161.3 ± 0.4 157.9 ± 0.3 69.4 ± 0.6 43.5 ± 2.1

RA (°) 4.1 ± 0.6 1.9 ± 0.6 2.3 ± 0.9 3.3 ± 0.9 >90 >90

CAH (°) 3.3 ± 0.3 1.1 ± 0.4 1.3 ± 1.0 2.7 ± 1.1 33.8 ± 2.3 26.9 ± 2.8

Table 2 Temperature, freezing time delay and freezing process time of droplets on different sample surfaces.

Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6

Temperature (℃) -10.3 -11.9 -11.6 -11.1 -7.5 -6.6

Freezing time delay (s) 1281 ± 366 1568 ± 260 1386 ± 276 1297 ± 289 280 ± 29 159 ± 19

Freezing process time (s) 18 18 18 16 10 8

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Fig. 1. The schematic diagram of the experimental setup

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(a,I) (a,II) (a,III)

(b,I) (b,II) (b,III)

Fig. 2. SEM images of aluminum alloy substrate surfaces etched with HCl aqueous solution at room temperature.

The concentrations of HCl aqueous solution for (a) and (b) are 8.0 wt% and 12.0 wt%, respectively. The

magnifications for (I), (II), (III) are 1000, 10000, 100000, respectively.

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4000 3500 3000 2500 2000 1500 1000

955

1552 10551414

28512925

Wavenumber (cm-1)

POTS Superhydrophobic surface3392

Tra

nsm

ittan

ce

Fig. 3. FT-IR spectra of superhydrophobic surface and POTS. The blue line represents the FT-IR spectrum of

superhydrophobic surface etched with HCl aqueous solution and coated with POTS. The red line corresponds to

the FT-IR spectrum of POTS.

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(a) (b)

35 30 25 20 15 10 5 0 -5 -10 -15130

135

140

145

150

155

160 Repeated First Time

Con

tact

Ang

le (D

eg.)

Temperature (oC)

(c)

Fig. 4. Images of water droplet on superhydrophobic surface of aluminum etched with 8.0 wt% HCl aqueous

solution. (a) Room temperature; (b) The temperature of the surface is -11.6 ℃; (c) The relationship between

water CA of superhydrophobic surface and temperature (the volume of water droplet is 10.0 μL).

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-15 -10 -5 0 5 10 15 20 250.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

App

aren

t sur

face

free

ene

rgy

(mJ/

m2 )

Temperature (oC)

Sample 1 Sample 2 Sample 3 Sample 4

Fig. 5. Variations of ASFE of superhydrophobic surfaces with temperature ranging from 24 to -12 ℃. Ethylene

glycol is used as the probe liquid.

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(a,I) 0s (a,II) 18s

(b,I) 0s (b,II) 8s

Fig. 6. Sequence images of freezing process of a single tap water droplet on different surfaces. (a) and (b)

represent the samples 2 and 6 corresponding to the temperatures of -11.9 and -6.6 ℃, respectively. (I) and (II)

correspond to the initial and final stages of freezing process, respectively.

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40 50 60 70 156 158 160 162 1640

200400600800

100012001400160018002000

Freezing time delay Contact area

Contact angle (Deg.)

Free

zing

tim

e de

lay

(s)

Sample 60.000.050.100.150.20

3.00

4.00

5.00

6.00

7.00

Sample 1

Sample 2Sample 3Sample 4

Sample 5

Contact area (m

m2)

Fig. 7. Freezing time delay and contact area vs. CA for different samples.

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0 400 800 1200 1600 2000

-12.0

-8.0

-4.0

0.0

4.0

8.0

12.0

Tem

pera

ture

(o C)

Time (s)

1600 1650 1700 1750 1800-13.00

-12.95

-12.90

-12.85

-12.80

-12.75

Fig. 8. Temperature history of cooling process of sample 2. The inset corresponds to the enlarged part marked by

red circle.

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Graphical abstract

A very simple chemical etching method is employed to fabricate the superhydrophobic surface on aluminum surface, and the prepared superhydrophobic surface with very low ASFE can significantly retard the freezing time.

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Highlights

The fabricated superhydrophobic surface displayed excellent superhydrophobicity after 90 days.

The apparent surface free energies increased with the decrease of surface temperature.

The freezing time was significantly retarded on superhydrophobic surface.

fabrication and characterization of superhydrophobic surfaces on aluminum alloy substrates

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