Anti-icing performance of superhydrophobic aluminum alloy surface and its rebounding mechanism of droplet under super-cold conditions

Accepted Manuscript Anti-icing performance of superhydrophobic aluminum alloy surface and its rebounding mechanism of droplet under super-cold conditions

Yan Liu, Xinlin Li, Yuying Yan, Zhiwu Han, Luquan Ren PII: DOI: Reference:

S0257-8972(17)31065-4 doi:10.1016/j.surfcoat.2017.10.032 SCT 22797

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

29 June 2017 28 September 2017 9 October 2017

Please cite this article as: Yan Liu, Xinlin Li, Yuying Yan, Zhiwu Han, Luquan Ren , Anti-icing performance of superhydrophobic aluminum alloy surface and its rebounding mechanism of droplet under super-cold conditions. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2017.10.032

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ACCEPTED MANUSCRIPT Anti-icing performance of superhydrophobic aluminum alloy surface and its rebounding mechanism of droplet under super-cold conditions Yan Liua,*, Xinlin Lia, Yuying Yanb, Zhiwu Hana, Luquan Rena

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a. Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, China; b. Energy & Sustainability Research Division, Faculty of Engineering, University

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of Nottingham, UK

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Abstract

This work presented a facile and environment friendly method to

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fabricate the biomimetic superhydrophobic surfaces with micro-nano

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hierarchical structure on aluminum alloy successfully. It has shown a high contact angle of 164 ° and low slide angle of 2 °. Both static and dynamic

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analyses have been carried out to testify its excellent anti-/deicing

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property by mimicking natural environment. A mechanical model is built to analysis the dynamic process of supercold water droplets with different

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pH values impacting on the cold superhydrophobic surface in detail.

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Furthermore, this process is also analyzed at the aspect of energy. The framework of the present work provides an insight for the design and analysis of superhydrophobic surfaces for ice prevention in harsh environment. Key words: Anti-icing, Superhydrophobic, Aluminum alloy, Rebounding mechanism, Static and dynamic analysis 1. Introduction

ACCEPTED MANUSCRIPT Ice accumulation is very common phenomenon in nature especially in winter. However, it can cause many problems, great economic loss and even disasters.[1-6] such as ice accumulation on the inner wall of refrigerators, power lines, and aircrafts. There are many methods to solve

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the thorny problem, which can be classified into two strategies, i.e., 7-10]

The passive

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passive and active anti-icing/deicing strategies.[2,

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icing/deicing, including thermal melting, manual deicing, mechanical vibration and so on, have been widely used in the aircraft industry, power

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transmission and refrigerators. However, these methods not only rely on

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continuous energy inputs and special equipment of different conditions, but induce environmental contamination. Therefore, the advantages of

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active strategies[11, 12] have attracted wide attentions of researchers for its

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environment friendly, low costs and energy consumptions. Generally, bare surface can absorb water and wet snow which is able

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to condense into ice below 0 °C, and then icing occurs on the surfaces. If

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water and wet snow can be repelled immediately, instead of sticking on the surface, the formation of frost or ice and accumulation of snow or slush are able to be avoided.[13-15] Inspired by many plants and insets, such as lotus leaves[16], rose petals[17], butterfly wings[18] and legs of water striders[19], superhydrophobic surfaces with anti-/de-icing property have been proved to have great importance in fundamental research and potential for industrial applications,[13, 20-22] which is recognized as contact

ACCEPTED MANUSCRIPT angle > 150 º and contact angle hysteresis < 10 º. Wettability[23-28] is considered to be an important factor, which determines the feasibility of icing. Both of surface free energy and surface morphology contributes to non-wettability of surface. Up to now, many methods have been reported

electrochemical deposition,[31,

32]

hydrothermal reaction,[33,

34]

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30]

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for constructing superhydrophobic surfaces, such as colloidal template,[29,

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self-assembly,[35-37] which require either very complicated techniques or costly sophisticated devices and could only be applied in the laboratory in

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small-scale. As a very common technology, anodic oxidization has been

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widely used in practical engineering. Lee et al.[38] prepared a superhydrophobic surface by anodic oxidization and proved that changing

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the structure from nanopore to nanopillar arrays at the surface caused a

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dramatic increase in the receding angle and a decrease in the hysteresis of water contact angles. Ruan et al.[39] presented two strategies, the

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electrochemical anodic oxidation and chemical etching methods, to

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simplify the fabrication procedures for superhydrophobic surfaces on the aluminum alloy substrates, and anodic aluminum oxidation surface was endowed with excellent superhydrophobicity and icephobicity. Most of these superhydrophobic surfaces were oxidized firstly and modified by low-surface-energy substance afterwards.[40, 41] Compared with Song et al.[42], we, in this paper, presented a simple approach to prepare superhydrophobic surfaces on aluminum alloys by

ACCEPTED MANUSCRIPT combining laser processing and anodic oxidization. It has shown a high contact angle of 164 ° and low slide angle of 2 °. Both static and dynamic analyses have been carried out to testify its excellent anti-/deicing property by mimicking natural environment. From the static analysis, the

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as-prepared surface was proved to dramatically decrease the possibilities

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of ice covering and exhibit good anti-fouling properties tested with

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super-cooling mud mixture drops, so that they can be widely applied in complicated environment. From the dynamic analysis, these surfaces

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showed different surface adhesion to the different pH values droplets.

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And the impacting process was analyzed in the aspect of mechanics and energy conservation.

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2. Experimental

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2.1 Materials

7075 Al alloy sheets (0.4 wt% Si, 0.5 wt% Fe, 2.0 wt% Cu, 0.3 wt%

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Mn, 2.9 wt% Mg, 0.28 wt% Cr, 6.1 wt% Zn, 0.2 wt% Ti, with balance

No.

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being Al) with the size of 20 mm × 20 mm × 1 mm, sand paper No. 400, 800

and

No.

1500,

acetone,

ethanol

and

FAS-13

((CH3CH2O)3SiCH2CH2(CF2)6F) (99%, MERYER CO.,LTD) were used for experiments reported in this paper. 2.2 The experimental process 7075 Al alloy sheets were polished with 500 #, 800 # and 1500 # emery papers in turn, and then cleaned with acetone and ethanol in an

ACCEPTED MANUSCRIPT ultrasonic bath for 10 min respectively. And then all substrates were irradiated with the area of 20 mm × 20 mm by pulsed fiber laser to form the micro-column array structure. High power density was guaranteed by the F–θ lens, which focuses the laser beam on a small spot. Afterwards

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the samples were cleaned with acetone and ethanol in an ultrasonic bath

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for 15 min respectively. Finally, anodic oxidation processing was

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performed at room temperature. The uniform electrolyte solution was the mixture of 0.05 mol/L aluminum nitrate (V1) and 1 %wt FAS-13 was

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poured into ethanol (V2) with different volume ratio (η= V1: V2 = 1:2,

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1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10). Then as-prepared substrate was used as anode, and two aluminum alloy plates (30 mm × 30 mm) were

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taken as cathode in an electrolyte cell, respectively. The distance between

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the cathode and the anode was 20 mm, to which a direct current (DC) voltage of 20 V was applied at a stable electro-polishing current density

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of 20 mA/cm2 in 2 hours. After electrolysis, the as-prepared substrate was

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rinsed in distilled water several times to remove the residual liquid and then dried at 60 ℃ for 30 min. 2.3 Characterization The surface morphology was analyzed by scanning electron microscopy (SEM, EVO 18, ZEISS), and the surface composition was detected by X-ray photoelectron spectroscopy (XPS, SPECS XR50, Japan). The surface wetting behavior is assessed by the water contact

ACCEPTED MANUSCRIPT angle (CA) which is collected by a contact angle meter (JC2000A Powereach, China) with sessile drop method at ambient temperature of 23±2 °C and the relative humidity of 53±5 %. Water droplets with the volume of 3 µL were carefully dropped onto the surfaces in five different

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positions to obtain the average static contact angle value. The infrared

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spectrum of the samples were recorded with a Fourier Transform-Infrared

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(FTIR, JACSCO, Japan) spectrometer at a resolution of 2 cm-1. 2.4 Anti-icing property

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A self-established machine, which was composed of a cold stage,

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digital micro-syringe, high-speed camera and a computer, was employed to acquire the dynamic statistics during the impacting process on

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superhydrophobic surfaces, as shown in Figure S2. The droplet impact

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and spreading on the surface is recorded by a high-speed camera (model NX7-S2 from IDT) with a frame rate of 5000 fps and a pixel resolution of

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7.38 µm (12x zoom lens, Navitar). The droplet is illuminated by a LED

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lamp (model LLS2 from SCHOTT) with 5 µs exposure time per frame. The digital micro-syringe was able to precisely control the diameter of droplets of different pH values in 3 mm by connecting with computer. When the droplets was impacting on the as-prepared samples, the temperature of which was kept by the cold stage under 0 °C, all the data was captured by the high-speed camera and collected by computer spontaneously.

ACCEPTED MANUSCRIPT The ice adhesion strength was tested by a self-assembled apparatus, as shown in Figure S6, which consists of a cooling stage, a chamber, and a motion stage with force transducer as depicted in a previous paper. Nine samples were clamped to a custom-built sample holder fixed on cooling

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stage, respectively. Ice columns were formed by syringing 1 mL of

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ultrapure water into quartz cuvettes (10 mm × 10 mm × 25 mm) that were

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placed on each sample surface and were kept at the testing temperature of -20 °C for 90 min to ensure a complete icing process. The chamber was

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purged continuously by a flow of nitrogen gas into it to avoid frosting on

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the sample surfaces. When testing, the force probe was kept 1 mm above the sample surface and the shearing velocity was 500 µm/s. The peak

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value of the shear force detaching the ice column from each sample

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surface was recorded for calculation. The ice adhesion strength was

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obtained by averaging nine testing results.

3. Results and discussion

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3.1 Surface morphology Scanning electron microscopy (SEM) was employed to investigate the surface morphology of as-prepared substrates. Fig.1 (a) shows the SEM image of the sample surfaces after anodization. An array of lotus leaves-like structure can be found, and the distance between each two center of mastoids is 100 µm. It is found that some sections of the surface

ACCEPTED MANUSCRIPT appears to be bright white, which attributes to the defects generating during the anodic oxidation. After the mastoid was investigate in high magnification as shown in Fig.1 (b), many balls and bars in nano-scale can be found to cover all of the mastoids and gaps. Therefore, a complex

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hierarchical structure was built up on the aluminum alloy successfully.

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Wettability is a very important factor to test the feasibility of icing. As

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shown in Fig.1 (d), a water droplet of 3 µL was employed to test its contact angle and sliding angle, which exhibits a typical spherical shape

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on the as-prepared surfaces. And the as-prepared superhydrophobic

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surfaces by anodic oxidation (η=1:8) shows the best superhydrophobic property with its contact angle (CA) of 164 ° ± 1 ° and sliding angle (SA)

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of 2 ° ± 1 °, respectively. An important factor contributing to the excellent

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superhydrophobicity of as-prepared surface must attribute to lotus leaves-like structure and surface roughness. The hierarchical structure

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dramatically decreased the water-solid contact area and increased more

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interspaces for trapped air, so that water is not easy to penetrate into the air-cushion and exhibits as a typical spherical shape. To further investigate the changes of surface roughness, laser scanning confocal microscopy (LSCM) was employed. Figure S3. (a) shows the surface confocal images of radiated surfaces before anodic oxidation with a root-mean square (rms) roughness of 20.7 nm, which presented superhydrophilicity for its high surface energy. However,

ACCEPTED MANUSCRIPT radiated surfaces after anodic oxidation, whose confocal images was shown in Figure S4. (b), rendered superhydrophobicity with a root-mean square (rms) roughness of 13.4 nm, due to the existence of FAS-13 in the electrolyte. Therefore, surface roughness is only one factor that

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contributes to superhydrophobicity of the surfaces, and chemical

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composition of surfaces is another important factor.[43-45]

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3.2 Chemical characterization

The presence of C, F, Si and Al on the aluminum alloy surfaces

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modified by FAS-13 was revealed by X-ray photoelectron spectroscopy

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(XPS) investigations. Fig.2 shows the full-spectrum of the as-prepared surfaces and three strong peaks of Al 2p, Si 2p, C 1s, O 1s and F 1s,

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which are found to increase significantly compared to the untreated 7075

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Al alloy surface. Binding energies of F1s is clearly visible at 690 eV as shown in the Fig.2 inset (a). Two peaks correspond to the C1s binding

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energies as shown in the Fig.2 inset (b): the principal one at BE = 291.8

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eV corresponds to C-C bonds and the secondary peak at BE = 284.9 eV can be ascribed to C-F bonds. [46] In addition, Fig.3 shows the FT-IR spectrum of the Aluminum alloy surfaces after anodic oxidization. Two absorption peaks were found at 2922.15 cm−1 and 2850.79 cm-1, which are due to the asymmetric and symmetric stretching vibrations of methylene groups of FAS-13, respectively. Appearance of absorption peak at 1384.88 cm-1 is due to

ACCEPTED MANUSCRIPT symmetrical deformation vibration of methyl groups, and the absorption peak of Si−O bands were found at 1060.84 cm-1. Moreover, the absorption peak at 1624.06 cm-1 is attributed to the stretching vibrations of C−F bands. In addition，a strong absorption peak is found at 3435.22

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cm-1 which is due to physically adsorbed water. In conclusion, a

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self-assembled molecular film of FAS-13((CH3CH2O)3SiCH2CH2(CF2)6F)

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is successfully constructed on the as-prepared surfaces. 3.3 Anti-icing properties

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Ice accumulation has caused a variety of problems for modern society,

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both to the industry and daily lives. Due to physical properties of water, ice accumulation can be caused by different types of water, such as snow

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(ice crystals) and sleet (frozen drops) precipitation. These types of solid

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ice precipitates can easily accumulate on the high power lines, insulators, offshore platform, and other mechanical components exposed in the air.

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Once this type of ice accumulates successfully, great danger is going

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to appear. Considering such an extreme condition, an experiment has been conducted by mimicking the cold weather condition. Both of the bare substrates and superhydrophobic substrates as well as the experimental platform were put in refrigerator for 12 hours to ensure they were completely cold. And all the substrates were transferred onto the cooling experimental platform in air atmosphere when they were under experiment. Then a spoon of snow was put on the substrates immediately.

ACCEPTED MANUSCRIPT As can be seen in Fig.4, snow melted gradually and entirely stuck on the bare surface under the tilting angle of 10 °, which was able to form glaze ice under low temperature in a few minutes. However, the melted snow slid off the superhydrophobic surfaces immediately in a form of ice water

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mixture before it iced. The smallest sliding angle was investigated by

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gradually decreasing the inclination angle of the sample, as shown in

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Fig.4 (b) to (e). As the sliding angle was decreased to 1 °, the ice water mixture was stocked and suspended on superhydrophobic surface

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presented as ellipsoid shape. More interestingly, the ellipsoid-shaped

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water drop was expelled from the superhydrophobic surface after a small external perturbation - blowing with wind at a speed of 0.05m/s - was

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applied. By comparing from Fig.4 (a) to (e), the melting time of ice

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superhydrophobic surface is relatively longer than on bare aluminum alloy, which indirectly indicates that the air cushion on superhydrophobic

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surface is able to prevent heat conduction. Therefore, these surfaces can

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dramatically decreased the possibilities of ice covering and widely applied in complicated environment. In addition, the ice adhesion on the superhydrophobic surfaces by anodic oxidation (η=1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10) have been investigated. As shown in Figure S7, it shows that the as-prepared superhydrophobic surface (η= 1:8) presents the lowest ice shear force, which is 3 times lower than the as-prepared superhydrophobic surface (η= 1:2).

ACCEPTED MANUSCRIPT In most cases, the water in natural circumstances is not as pure as that used in laboratory, so that it is important to testify its anti-icing property by mimicking natural conditions. An experiment was employed to test the anti-icing property of as-prepared superhydrophobic surfaces if the water

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had been contaminated. A certain amount of soil collected outdoors was

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added into 40 mL water by continuously stirring, and then the beaker was

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transferred into the refrigerator until the ice water mixture emerged, as shown in Fig.5c. Both of the bare substrates and superhydrophobic

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substrates were completely cooled in refrigerator for 12 hours. And then

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all of the cold substrates were transferred onto a cooling platform at a tilting angle of 10 °. When the cold mud mixture was dropped onto the

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bare surface, they were stuck immediately, by which the surface was

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contaminated, as shown in Fig.5 b.1 and b.2. However, the super-cooling mud mixture drops were immediately repelled from superhydrophobic

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surfaces, instead of sticking and contaminating the surfaces, as shown in

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Fig.5 a.1 to a.5. Thus, the as-prepared superhydrophobic surface still exhibits Cassie state even tested by the super-cooling mud mixture drops, and the bare surface shows Wenzel state, as shown Fig.5 a and b, respectively. Typically, impacts of liquid drops onto a solid substrate are divided into three or four stage, i.e. spreading, contracting or rebounding, and depositing, which depends on the impact velocity, drop size, liquid

ACCEPTED MANUSCRIPT properties, and surface properties (roughness and wettability).[47, 48] The dynamic characters of a droplet of different pH values is very important to investigate when impacting on a cold superhydrophobic surfaces. These characteristics can well reveal the stability of superhydrophobic

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surfaces under extreme conditions. A self-established machine, which was

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composed of a cold stage, digital micro-syringe, high-speed camera and a

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computer, was employed to acquire the dynamic statistics during the impacting process on superhydrophobic surfaces, as shown in Figure S2.

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The digital micro-syringe connected to the computer was employed to

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precisely control the droplets diameter of different pH values of 3 mm. When the droplets were impacting on the as-prepared samples, the

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temperature of which was kept by the cold stage under 0 ℃, all the data

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was captured by the high-speed camera and collected by computer spontaneously. The droplet has an initial diameter D0, reproducible with a

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relative error of 1 %. When the droplet was released from 230 mm, the

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impact velocity reached 2.1 m/s accelerated by gravity with a relative error of 2 %. Obviously, all of the droplets of different pH values, as shown in Figure S5, experienced five stages on cold superhydrophobic surface, i.e. contacting, spreading, contracting, rebounding and depositing. In addition, due to the dynamic nonwetting behavior of superhydrophobic surfaces, most of the droplets impacting on superhydrophobic surfaces at a high speed exhibits corona splash and receding breakup at the spreading

ACCEPTED MANUSCRIPT stage and contracting stage, respectively. It is essential to note that the droplets of pH value between 3 and 12 experienced more than one rebound cycle before it deposited on the cold superhydrophobic surface, while the pH value out of this range only experienced one rebound cycle.

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And the droplets of pH 4 were found to rebound five times.

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During the impacting process, two important parameters should be

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considered, maximum spreading diameter Dmax and maximum rebounding height Hmax of the droplets. The two parameters can directly reflect the

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dynamic characters of different pH droplets on the cold superhydrophobic

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surface. As it is difficult to measure the actual value of Dmax and Hmax, normalized spreading ratio β = Dmax/Ls and normalized rebounding ratio λ

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= Hmax/Ls are defined, in which Ls presents the length of substrates.

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Normalized spreading ratio β and normalized rebounding ratio λ in Fig.6(a) were employed to indirectly represent the maximum spreading

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diameter and maximum rebounding height of different pH value droplets,

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respectively. β and λ are relatively low at the extreme conditions, while the strong peak is found to be not at neutral solution but around weak acid. In general, this trend is very close to parabola. Amazingly, the increase in maximum spreading diameter is corresponding to the decrease in maximum rebounding height and vice versa. This process involves complex quality and energy loss. To well explain this process, we established a mechanical model, as shown in Fig.6 (b). After water

ACCEPTED MANUSCRIPT droplet was released at a certain height, it was able to gain a certain speed and energy under the action of gravity acceleration finally. Once the droplet contacts with the superhydrophobic surface, water droplet still remains spherical for the affection of intermolecular cohesion defined as

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f1, which exists permanently. At such a high speed, however, the

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impacting water is not going to stop but continue to spread along the

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surface. Due to the existence of discontinuous liquid-solid-vapor contact line, the spreading water is suspended by the thrust f2 generated by the

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trapped air in the micro gaps. At the meantime, the spreading water,

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because of the existence of surface adhesion, is also affected by viscous resistance f3, which is positively correlated with surface contact area. The

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spreading velocity defined as v2 will gradually decrease affected by

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intermolecular cohesion f1 and viscous resistance f3. After spreading water reaches the maximum diameter, the spreading velocity v2 decrease to 0

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m/s, but the contracting velocity v3 begins to increase gradually at the

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same time. As the viscous resistance f3 is positively correlated with surface contact area, both of the spreading velocity v2 and the contracting velocity v3 present the non-linear growth. Finally, the droplet rebounds from the surface at the velocity v4. Therefore, it can be supposed that the viscous resistance f3 is mainly affected by the liquid-solid contact area. Decreasing liquid-solid contact area is helpful to reduce viscous resistance f3, so that micro-nano hierarchical structure is proved to be

ACCEPTED MANUSCRIPT endowed with the lowest surface adhesion.[49, 50] According to Newton`s law, it can be conclude that 𝑚

𝐷𝑚𝑎𝑥/2

∫ ∫ 0

𝑎𝑖 𝑑𝑚 𝑑𝑥 = 𝑓2 + 𝑓3

0

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Where ai is the acceleration of spreading and contracting stage. Thus, the viscous resistance f3 can directly affect the water acceleration

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and water velocity of spreading and contracting stage.

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It is proved that a droplet impacting on a solid surface follows the energy

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conservation law.[51] There are four forms of energy should be taken into account, i.e. the kinetic energy, capillary or surface tension energy, energy

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loss due to viscous dissipation and heat loss in the liquid-solid-vapor interface. The actual impacting process of droplet was captured and

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shown in the insets of Fig.6 (a).

The kinetic energy before impact KE1 is (1)

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𝐾𝐸1 = (𝜋/12) ∙ 𝐷03 𝜌𝑉12

The surface tension energy before impact SE1 and the surface tension

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energy at maximum spreading SE2 are shown in follows, respectively. 𝑆𝐸1 = 𝜋𝐷02 𝛾𝐿𝑉 𝑆𝐸2 = 𝑆𝐿𝑉 ∙ 𝛾𝐿𝑉 + 𝑆𝑆𝐿 ∙ (𝛾𝑆𝐿 − 𝛾𝑆𝑉 )

(2) (3)

Where D0 is the initial drop diameter prior to impact, ρ is the liquid droplet density, 𝛾𝐿𝑉 is the surface tension of water droplet, 𝛾𝑆𝐿 is the surface tension between solid surface and droplet surface in the three-phase line, 𝛾𝑆𝑉 is the surface tension of solid surface in the

ACCEPTED MANUSCRIPT three-phase line, SLV is the contacting area of droplet surface with the vapor phase, while SSL is the contacting area of droplet surface on the solid surface. The loss of energy due to viscous dissipation is define as follows: [52] 𝑡

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W = ∫0 𝑚𝑎𝑥 ∫Ω 𝜏𝑖 𝑑Ω𝑑𝑡 ≈ 𝜏𝑖 Ω𝑡𝑚𝑎𝑥

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𝜏𝑖 = 𝜇(𝑉𝑖 /𝛿)2

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2 Ω = (π/4)𝐷𝑚𝑎𝑥 ∙𝛿

(4) (5) (6) (7)

𝑡𝑚𝑎𝑥 = (8/3) ∙ (𝐷0 /𝑉𝑖 )

(8)

𝐷𝜇 = 𝜇/𝜌

(9)

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δ = (2𝐷𝜇 ∙ 𝑡𝑚𝑎𝑥 )1/2

Therefore, we can conclude that

(10)

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2 W = √3 ∙ 𝜋 ∙ (𝜇 ∙ 𝐷0 ∙ 𝑉𝑖 ∙ 𝜌)1/2 ∙ 𝐷𝑚𝑎𝑥

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Where 𝜏𝑖 is the mean value of the viscous dissipation energy per unit time and volume[53], Ω is the volume where viscous dissipation occurs,δ

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is the boundary layer thickness, µ is the viscosity, Vi is the impact velocity,

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tmax is the time at maximum spreading[52] According to the energy conservation law, the energy balance equation between state before impacting and state at maximum spreading is KE1 + SE1 = SE2 + W + Q

(11)

Where Q is the heat loss in the liquid-solid-vapor interface. From the analysis above, it can be deduced that 2 SE2 = KE1 + SE1 - √3 ∙ 𝜋 ∙ (𝜇 ∙ 𝐷0 ∙ 𝑉𝑖 ∙ 𝜌)1/2 ∙ 𝐷𝑚𝑎𝑥 - Q

(12)

ACCEPTED MANUSCRIPT The surface tension energy at maximum spreading SE2 as potential energy will finally affect the maximum rebounding height Hmax, which presents quadratic relationship with Dmax. As a result, the maximum rebounding

which perfectly conforms to the curves in Fig.6 (a).

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Conclusion

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height Hmax can be deduced to show quadratic relationship with Dmax,

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In this study, the biomimetic superhydrophobic surface with micro-nano hierarchical structure was successfully fabricated by

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combining the electrochemical anodic oxidation and laser ablation on the

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aluminum alloy substrates, and provided insights to the fabrication of superhydrophobic surfaces in a facile and environment friendly strategy.

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The water contact angle and slide angle was shown to be 164 ° and 2 °,

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respectively. By static and dynamic analyses, the as-prepared superhydrophobic surface was proved to endow with excellent

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anti-/deicing property in natural environment. From the static analysis,

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these surfaces were able to dramatically decrease the possibilities of ice covering and exhibit good anti-fouling property tested with super-cold mud mixture drops, so that they can be widely applied in complicated environment. Moreover, a mechanical model was built to systematically analysis the dynamic impacting process of different pH values super-cold droplets on cold superhydrophobic surface. Consequently, the maximum rebounding height Hmax can be deduced to show quadratic relationship

ACCEPTED MANUSCRIPT with Dmax, which perfectly conforms to the experimental data. The framework of the present work provide an insight for the design and analysis of superhydrophobic surfaces for ice prevention in harsh environment.

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ACKNOWLEDGEMENTS

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The authors thank the National Natural Science Foundation of China (Nos. 51775231, 51475200 and 51325501), Science and Technology

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Development Project of Jilin Province (No.20150519007JH) and 111

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project (B16020) of China. Reference

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Fig.1 SEM image of as-prepared superhydrophobic surfaces by anodic

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oxidation ( η =1:8); (a) low magnification SEM image of surface

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panorama; (b) high magnification SEM image of one single microtrabeculae; (c) high magnification SEM image of the top surface of

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the single microtrabeculae; (d) Water contact angles (CA) of the

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superhydrophobic surfaces by anodic oxidation (η=1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10) and its corresponding slide angle (SLA).

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Fig.2 XPS spectra of the as-prepared superhydrophobic aluminum alloy

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surface of (a) full-spectrum and (b) C 1s.

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Fig.3 FT-IR spectrum of the Aluminum alloy surfaces after anodic

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oxidization.

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Fig.4 Snow melting experiment (a) on cold bare surface at tilting angle of

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10°and (b-e) cold as-prepared superhydrophobic surfaces at tilting angle

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of 10 °, 5 °, 3 °and 1 °, respectively.

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Fig.5 Anti-fouling experiment with super-cooling mud mixture drops on cold as-prepared superhydrophobic surfaces at tilting angle of 10 ° (a.1 to

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a.5) which exhibits Cassie state (a) and cold bare surface at tilting angle of 10 ° (b.1 and b.2) which shows Wenzel state (b); and the super-cooling

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mud mixture in a beaker (c).

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Fig.6 Different normalized spreading ratio 𝛃 = 𝑫𝒎𝒂𝒙 /𝑳𝒔 normalized

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rebounding ratio 𝛌 = 𝑯𝒎𝒂𝒙 /𝑳𝒔 indicate the maximum spreading diameter and maximum rebounding height of droplets in different pH

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value, respectively (a). The mechanical model was built to analysis the

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dynamic process of supercold water droplets with different pH values impacting on the cold superhydrophobic surface (b). Insets are the images of water droplets of contacting, contracting and rebounding stage.

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Graphic Abstract

ACCEPTED MANUSCRIPT Highlights 1. The samples were fabricated by anodic oxidation and laser ablation. 2. Static and dynamic analyses were applied to testify its anti-icing property.

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3. A mechanical model is built to analysis rebounding process in cold

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conditions.