Preparation and Anti-icing Behavior of Superhydrophobic Surfaces on

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Fabrication and Anti-Icing Behavior of Superhydrophobic Surfaces on Aluminum Alloy Substrates Wen Li, MIN RUAN, BAOSHAN WANG, BIN DENG, fumin ma, and zhan yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la400979d • Publication Date (Web): 30 May 2013 Downloaded from http://pubs.acs.org on June 1, 2013

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Preparation and Anti-Icing Behavior of Superhydrophobic Surfaces on Aluminum Alloy Substrates Min Ruana, b, Wen Lib, *, Baoshan Wanga, *, Binwei Dengb, Fumin Mab, Zhanlong Yub a. College of Chemistry and Molecular Science, Wuhan University, Wuhan, China b. Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, Hubei Polytechnic University, Huangshi, Hubei, China

Abstract It has been expected that superhydrophobic (SHP) surfaces could have potential anti-icing applications due to their excellent water-repellence properties. However, a thorough understanding on the anti-icing performance of such surfaces has never been reported; even systematic characterizations on icing behavior of various surfaces are still rare because of the lack of powerful instrumentations. In this study, we employed the electrochemical anodic oxidation and chemical etching methods to simplify the fabrication procedures for SHP surfaces on the aluminum alloy substrates, aiming at the anti-icing properties of SHP surfaces of various engineering materials. We found that the one-step chemical etching with FeCl3 and HCl as the etchants was the most effective for ideal SHP surfaces with a large contact angle (CA, 159.1°) and a small contact angle hysteresis (CAH, 4.0°). To systematically investigate the anti-icing behavior of the prepared SHP surfaces, we designed a robust apparatus with a real-time control system based on the two stage refrigerating method. This system can monitor the humidity, pressure and temperature during the icing process on the 1

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surfaces. We demonstrated that the SHP surfaces exhibited excellent anti-icing properties, i.e., from the room temperature of 16.0℃, the icing time on SHP surfaces can be postponed from 406s to 676s compared to the normal aluminum alloy surface if the surfaces were put horizontally, and the icing temperature can be decreased from -2.2℃ to -6.1℃. If such surfaces were put tilted, the sprayed water droplets on the normal surfaces iced up at the temperature of -3.9℃, but bounced off the SHP surface even the temperature reached as low as -8.0℃. The present study therefore suggested a general, simple, and low-cost methodology for the promising anti-icing applications in various engineering materials and different fields (e.g., power line and aircrafts). Key words: superhydrophobic; anti-icing; aluminum alloys

*Corresponding author. Email: [email protected]; Phone: 780-492-1652; Fax: 780-492-2200

1. Introduction Since Barthlott et al.[1] revealed the famous lotus effect in 1997, superhydrophobic (SHP) surfaces with a high contact angle (CA) and a low contact angle hysteresis (CAH) have aroused tremendous interests from both industry and fundamental research[2-6] owning to their fascinating water repellence and self-cleaning properties[7-9]. Such SHP surfaces have shown the potential applications in areas such as corrosion resistance, stain resistant textiles, drag-reduction, and inhibition of snow or ice adhesion [10]. 2


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To achieve SHP surfaces, the general methods involve two steps, construction of rough structures and chemical modification with low-surface-energy materials. Many developed techniques have been reported to produce such rough structures, including chemical etching

[11]

, anodic oxidization

[12]

, sol-gel

[13]

, and phase separation

[14]

. To

fabricate surface microtextures, on one hand, various engineering materials such as metals and alloys are used as substrates. In particular, the preparations of SHP surfaces on aluminum alloys are very attractive because they exhibit various applications in the auto, aviation, and building industries. On the other hand, many chemical modification materials such as fluoroalkylsilane (FAS) [15], teflon fatty acid

[17]

[16]

, and

have been used to obtain low-surface-energy coatings in the fabrication

of SHP surfaces. In particular, fatty acid is the most attractive low-surface-energy material because it can overcome the distinct shortcoming of the expensive price [18]. Surface icing of aircrafts

[19]

, wind turbines

[20]

, and downed power lines

[21]

can

cause very serious problems. For example, the ice formation on power cables can result in the power grid disruptions. Ice accretion on aircraft surfaces without anti-icing devices can significantly lower the safety and performance result in loss of control. Additionally, visibility may be lost as ice adheres to the windshield and thrust may be reduced due to icing of the engine blade [22]. For power cables, basically, there are two kinds of de-icing methods [23-25]. One is to improve the circuit design criteria, and another is to use the technological equipment melting the ice. The various methods including new developed technologies in applications have certain effects, but the drawbacks are obvious. Improving design standards increases the investments 3


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in the infrastructures dramatically, and passive de-icing spends a lot of manpower, material resources and time [26, 27]. It is generally realized that icing occurs on a surfaces because the surface can absorb water and wet snow and then condense into ice below 0°. For this reason, the key to prevent icing is to resist water. If the water adsorption is avoided, the formation of frost or ice and accumulation of snow or slush on solid surfaces could be inhibited, and the icing problem should be solved from the origin

[28]

. Based on such

considerations, the so-called “ice-phobic” materials are developing [29]. It is expected that SHP surfaces are potential for the “ice-phobic” materials because of their extraordinary water-repellency ability. In fact, it is recently reported that SHP surfaces can reduce accumulation of snow and ice and to even completely prevent formation of ice [30]. For example, Cao et al.

[31]

reported direct experimental evidence

that hydrophobic nanoparticle- polymer composites can prevent ice formation upon impact of supercooled water both in laboratory conditions and in natural environments. Farhadi et al.

[32]

studies the anti-ice performance of several

micro/nano-rough hydrophobic coatings which were prepared by spin-coating or dip coating and used organosilane, fluoropolymer or silicone rubber as a top layer with different surface chemistry and topography. Liu et al. [33] reported a retardation of frost formation on SHP surfaces compared to that on bare copper surface and they believed that the role of surface roughness cannot be separated from that of surface chemistry. Liu et al.

[34]

and Na et al.

[35]

suggested that frost formation can also be delayed on

smooth hydrophobic surface compared to that on smooth hydrophilic surface. 4


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However, a thorough understanding on the anti-icing behavior of SHP surfaces has hardly presented in the above studies. In particular, to date, the mechanism responsible for such behavior has been unclear. In addition, no powerful specialized instruments have been developed to effectively characterize the icing behavior and anti-icing performance of SHP surfaces. This makes the present studies for the anti-icing behavior qualitatively, e.g., the above studies just compare the anti-icing capability of different surfaces through photos without quantitative data to compare the effectiveness of the SHP surfaces. As a result, the exact temperature and time for icing and anti-icing are still not available. In this work, we tried to find a simple approach to prepare SHP surfaces on aluminum alloys with a good icephobic property. Additionally, we designed a real-time control system to monitor the humidity, pressure and temperature based on the two stage refrigerating method to study the anti-icing behavior of the prepared SHP surfaces. 2. Experiments 2.1. Materials and instruments Lauric acid was purchased from the Shanghai Chemical Reagent Co., Ltd. China. The other experimental chemicals used with an analytic grade were purchased from the Tianjin Kermel Chemical Regent Co., China. Commercial 5052 aluminum alloys were produced by China Steel Aluminum Corporation was used. The chemical compositions (in wt.%) of the alloys are 2.32 Mg, 0.10 Si, 0.18 Fe, 0.01 Cu, 0.04 Mn, 0.16 Cr, 0.02 Zn, 97.17 Al. 5


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Both CA and CAH were measured on a Krőss DSA 100 Contact-Angle Goniometer (Krőss GMBH, Hamburg, Germany) following standard procedures. The EDS with an EDAX Genesis2000 detector is employed for the qualitative analysis of chemical composition. The microstructures of the metallic surfaces were observed using a SEM (FEI Quanta 200, Holland). Spectral data were acquired with OPUS MIR Tensor 27 software version 4.0 (Bruker Optics, Inc.). AFM (SPM-9500J3 from Shimadzu Co) is used for visualization of the surfaces operating in the contact mode. 2.2. The fabrication of SHP surface A 10×10×0.10 mm aluminum alloy plate was polished mechanically using 2000# metallographic abrasive paper firstly, and cleaned ultrasonically in sequence with alcohol and deionized water for 5 min respectively. The aluminum alloy plate was dried and prepared as a substrate for SHP surfaces. Then the plate was treated with the following different methods. After these treatments, the plate was rinsed ultrasonically for 5 min with ethanol to remove any residual dust particles from their pores, and dried at 100 °C in air. The static CAs were measured for these surfaces. 2.2.1. Two-step anodic oxidation method with mixture of H2SO4, C2H2O4 and C3H8O3 The prepared plate (was treated successively with 1mol/L NaOH aqueous solution at 60℃ for 2min, and then washed with deionized water and ethanol for 5min each under ultra-sonication) was used as the working electrode. The counter electrode was a Pt sheet. The cathode and the aluminum plate used as the anode with the distance of them was 50 mm. The electrolyte was a mixture solution with 100g/L H2SO4, 10g/L 6


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Oxalic acid (C2H2O4) and 10g/L Glycerol (C3H8O3). The rough surface was electrochemically treated with the voltage of 2.0 V at the stable electro-polishing current density of 20mA/cm2. The reaction time was 10min, 30min, 60min, 90min and 120min at the room temperature, respectively. Then the aluminum alloy plate was washed ultrasonically with deionized water for 5min, dried at 100 °C in air and modified with 5wt% ethanol Lauric acid solution for 1.5h. The CAs of the surfaces polished for different time of 10min, 30min, 60min, 90min and 120min is 126.2°, 140.3°, 167.9°, 147.6° and 145.8°, respectively. It is obvious that the CA increases to a peak of 167.9° slowly if the polishing time is up to 60min and then decreases slowly as the electrochemical treatment time being lengthened. The CA and CAH of the SHP surface at the peak is 167.9° and 4.4° respectively with the etching time of 60min. From above CA and CAH results, we find out that the two-step electrochemical anodic oxidation method needs at least 60min to obtain SHP surface before modified with 5wt% ethanol Lauric acid solution for 1.5h. So using H3PO4 solution as the electrolyte, we can spend only 60min and 120min to treat the aluminum surfaces to prepare the roughness. 2.2.2. Two-step anodic oxidation method with H3PO4 electrolyte solution As the above procedure, the aluminum alloy surface was electrochemically treated with the electrolyte of 0.3mol/L H3PO4 aqueous solution for 60min and 120min respectively. Then the aluminum alloy plate was washed ultrasonically with deionized water for 5min, dried at 100 °C in air and modified with 5wt% ethanol 7


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Lauric acid solution for 1.5h. The CAs of the surfaces for 60min and 120min are 159.6° and 150.9°, respectively. The CAH of the surface that polished for 60min is 4.1°. 2.2.3. Two-step chemical etching method with diluted Beck's dislocation etchant The prepared plate was etched chemically by immersing in a diluted Beck’s dislocation etchant (mixture of 40mL of 37wt% HCl, 192.5mL of H2O, and 2.5mL of 40wt% HF) in a polyethene bottle at room temperature. The etching time was 0.5min, 2min, 6min, 9min, 12min, 15min and 17min, respectively. Then the plate was washed and dried at 100 °C in air and modified with 5wt% ethanol Lauric acid solution for 1.5h. The CAs are 109.6°, 120.2°, 150.9°, 158.2°, 155.1°, 152.4° and 151.1° for different time of 0.5min, 2min, 6min, 9min, 12min, 15min and 17min, respectively. It is obvious that CA increases fast to a peak of 158.2° and then decreases slowly to 151.1° with the etching time. The CA and CAH of the SHP surface corresponding to the peaks is 158.2° and 4.7° with the etching time of 9min. 2.2.4. Two-step chemical etching method with FeCl3 and HCl Similarly as the above procedure, the prepared aluminum alloy plate was etched with chemical etchant of the aqueous solution containing 14wt% FeCl3 and 3wt% HCl at 40°C water bath in a glass bottle. The etching time was 0.5min, 1min, 2min, 5min, 8min, 11min, 14min, 17min, 26min, 29min and 32min, respectively. Then the aluminum alloy plate was washed with deionized water as soon as possible for 5min, dried at 100 °C in air and then modified with 5wt% ethanol Lauric acid solution for 8


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1.5h. The CAs of the aluminum surfaces etched with FeCl3 and HCl by two-step method are 111.6°, 115.7°, 138.2°, 149.0°, 151.9°, 156.8°, 154.2°, 151.5°, 151.7°, 148.7° and 148.5° for different etching time of 0.5min, 1min, 2min, 5min, 8min, 11min, 14min, 17min, 26min, 29min and 32min, respectively. It can be seen that the CA of the surface increase to a peak immediately and then decreases slowly to a platform with the etching time. The CA and CAH of the SHP surface at the peak is 156.8° and 4.9° with the etching time of 11min. 2.2.5. One-step chemical etching method with FeCl3 and HCl The etchant and the modification were mixed together in the one-step method which is unlike the two-step method. The prepared aluminum alloy plate was immersed into the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath in a glass bottle. The etching time was 5min, 8min, 11min, 14min, 17min, 20min, 23min, 40min, 60min, 90min and 130min, respectively. Then the aluminum alloy plate was washed with deionized water as soon as possible for 5min, dried at 100 °C. The CAs for the aluminum surfaces along with the etching time by one-step method using FeCl3 and HCl are 132.1°, 138.0°, 147.4°, 147.8°, 148.8°, 152.3°, 159.1°, 152.3°, 151.4° and 151.8° for different etching time of 5min, 8min, 11min, 14min, 17min, 20min, 23min, 40min, 60min and 90min, respectively. The trend is very similarly to the two-step method using the mixture solution of FeCl3 and HCl as the electrolyte. The CA increases to a peak of 159.1° quickly if the etching time is up 9


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to 23min and then decreases slowly to a platform of 151.1° with the etching time to 90min. The SHP surface with the biggest CA shows a CAH of 4.0°. The CA, CAH and the total time used for fabricating SHP surfaces by different methods are shown in Table 1. The two-step electrochemical anodic oxidation methods need at least 60min to obtain SHP surface before being modified with 5wt% ethanol Lauric acid solution for 1.5h. If the electrolyte is the mixture aqueous solution of 100g/L H2SO4, 10g/L Oxalic acid (C2H2O4) and 10g/L Glycerol (C3H8O3), the CA and CAH of the surface is 167.9° and 4.4° respectively. The CA and CAH is 159.6° and 4.1° for the surface treated by 0.3mol/L H3PO4 solution as the electrolyte. As the two-step chemical method, the etching time is shorter than that of electrochemistry if the modification process is the same as electrochemical method. The SHP surface can be archived using the Beck’s dislocation etchant with CA of 158.2° and CAH of 4.7° for only 9min and obtained using the solution of 14wt% FeCl3 and 3wt% HCl as etchant at 40°C water bath for 11min with CA of 156.8° and CAH of 4.9°. If the one-step chemical method is used, the SHP surface with CA of 159.1° and CAH of 4.0° is prepared by etching with the mixture solution of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath just for 23min in all. In other words, the chemical etching method is more effective than the electrochemical anodic oxidation method, and the one-step chemical method is more convenient than the two-step chemical method. 2.3. The anti-icing experiments 2.3.1. Anti-icing experiments by normal refrigerator 10


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To start an initial investigation and to gain a general insight, we used the normal refrigerator, and its chamber can be adjusted with a low temperature of -18.0℃ to observe the anti-icing behavior of a SHP surface. Firstly, we put the normal and the SHP surface on a horizontal big plat and dropped a water droplet on it respectively, then put the big plate in a refrigerator horizontally. After 5min, we took out the plate and investigated the shapes of ice on the two surfaces. Another way was to stick the normal and the SHP surface on a big plat with a tilted angle of 45° in a refrigerator, then a needle was fixed just above the surface with small water droplets with a droplet volume of 4µL which is fixed for all the experiments spaying for about 5min, at last took photos of the plates judged qualitatively how much ice on the two different plates. However, we can just investigate the differences of the initial and the final state of the water droplets on different surfaces to judge the anti-icing effect of the SHP surface. At the same time, in order to judge the adhesion strength of the ice on the aluminum alloy surfaces, the tension dynamometer was used to measure the tensile force to make the ice off the surface vertically. 2.3.2. Anti-icing experiments by a self-made real-time control system In order to obtain the mechanisms on the anti-icing behavior of the SHP surface, we designed an apparatus with a real-time control system based on the two stage refrigerating method. Such system can monitor the icing process of the water droplets on a material surface. This system includes a water-cooled circulating machine, a semiconductor refrigerating box, an environment parameter control and data acquisition controller, a CMOS (Complementary Metal Oxide Semiconductor) camera 11


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image acquisition system and a PC (Personal Computer). The layout of the system is shown in Fig. 1 (a). The water outlet and inlet pipe of the water-cooled circulating machine is connected with the inlet and outlet pipe of the semiconductor refrigerating box, respectively. The sensor wires of environmental temperature, humidity, wind speed and atmospheric pressure in the semiconductor refrigerating box are connected with the controller which communicates with the PC through the USB (Universal Serial BUS) line. The CMOS camera image acquisition system also communicates with the PC through the USB line. The first stage refrigerating part involves in the refrigerating box which controls the temperature ranging from 2℃ to -10℃ by eight refrigeration films with the model of TEC1206 (Thermoelectric Cooler). And the heat radiation of the heat end of the refrigeration films uses the water-cooled circulating system which can control the temperature between 10℃ and 20℃ by the 1600W small-sized industrial water-cooled circulating machine named the second stage refrigerating part. As the cubic refrigerating box, acrylic panel is used for the outer and inner layers and heat-insulated foam is used for insulator. The inner layer of the front is arranged with two 2W LED (Light Emitting Diode) lamps and a 4cm×4cm quartz glass is used as the observation window through the three layers. From the inner layer to the outer layer of the back of the box, they are two 8cm×8cm fans placed side by side for cold dispersion, a 11cm×18cm cooling conduction block, six refrigeration films, two 4.2cm×12cm×1.1cm water cooling heads, an aluminum plate with the thickness of 0.3mm used for fixing the water cooling heads, 3cm thick heat-insulated foam and 12


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acrylic panel, control board including the 32-bit STM32F107 (Synchronous Transfer Module) microprocessor controller, TLC5615DA (Digital to Analog) module and 320×240 LCD (Liquid Crystal Display) voltage conversion module. There is a microscopic water pump in the inner layer on the top surface of the box, and a water pipe with the diameter of 0.3cm passes through the pump and a needle is fixed at the end of the pipe being used to drop water with a droplet volume of 4uL. From top to bottom of the box bottom, they is a 5cm×5cm rotatable sample stage with the temperature sensor besides, a 5cm×12cm aluminum plate for anti-water and wind, two 4cm×4cm fans placed side by side for cold dispersion, two cooling conduction blocks, two refrigeration films, two 4.2cm×12cm×1.1cm water cooling heads which connect with the two of the back through water pipes and finally joint with the water-cooled circulating machine used for cooling the hot end of the refrigeration films by water cooling method, a 0.3mm aluminum plate used for fixing the water cooling heads, heat-insulated foam, acrylic panel and stepper motor being used for rotate the sample stage. The wind speed sensor and humidity sensor are placed on the left side when the atmosphere pressure sensor is fixed on the right side of the box. Fig.1 (b) shows the photograph of the inside of the refrigerating box. The CMOS is installed on the XYZR 60mm×60mm fine-tuning platform with high resolution of 2592×1944, 256 gray degrees, the capture speed of 6fps and 0.3-1 optical magnifying lens. To ensure there is no fog on the quartz glass due to temperature difference between inside and outside of the refrigerating box, two high-speed 12V0.48A fans are arranged at a 45º angle symmetrically outside the box. 13


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The upper computer controls the camera and captures the images through the special software designed by the BCB (Borland C++ Builder) programming language. This computer also controls and collects the box temperature, humidity, wind speed, atmosphere pressure and water droplet volume through serial communications. The system can accurately control the temperature ranging from 2℃ to -10℃ for 2L space, measure the humidity, atmospheric pressure and wind speed which is adjustable in the 0-1m/s range at real time, and adjust the speed of the water drop in the range of 0uL/s-20uL/s. The icing status can be captured through the camera for samples smaller than 2cm×2cm and the icing thickness can be calculated from the captured maps. We compared the anti-icing behavior by putting the normal and the SHP surfaces with a water droplet on horizontally on the sample stage in the refrigerating box and controlled the environmental temperature from room temperature of 16.0℃ to -10.0℃ with 1m/s wind speed. We also put the two surfaces titled at 20° on the sample stage with 4µL/s the speed of water droplet and lowered the temperature from room temperature of 16.0℃ to -10.0℃ with 1m/s wind speed at the same time. Then we observed the icing behaviors of the surfaces through the images recorded by the high-speed camera in the PC and processed the data through the software of the system. 3. Results and discussion 3.1. SHP surface characterizations Fig. 2 (a) shows the SEM image with 10000 magnifications of the SHP aluminum 14


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alloy surface treated by two-step anodic oxidation method using the mixture aqueous solution of H2SO4, C2H2O4 and C3H8O3 as the electrolyte. We can see that the rough surface is covered with the uniform cavities with some very small deep holes. There are many shallow holes with different sizes visible clearly on the surface in the SEM image from Fig. 2 (b). It is the SEM image of the SHP surfaces for 60min etching time with 10000 magnifications. And circular shallow holes with different sizes can be seen on this surface because the electrolyte of 0.3mol/L H3PO4 is gentler than that of the mixture aqueous solution of 100g/L H2SO4, 10g/L Oxalic acid (C2H2O4) and 10g/L Glycerol (C3H8O3). Fig. 2 (c) shows the SEM image with 4000 magnifications of aluminum alloy surface etched with the aqueous solution of FeCl3 and HCl by two-step chemical etching method. From the surface microstructures, the chemical etching method is very different from the anodic oxidation method. There are no deep or shallow holes on the surface, but with sizes of micro-nano structural cubes uniformly dispersing on it. Fig. 2 (d) shows the SEM image with 4000 magnifications of the aluminum alloy plates chemically etched with the mixture of FeCl3, HCl and Lauric acid ethanol solution by one-step method. Fig. (d, Ι) is the image for 5min etching time. Fig. (d, ΙΙ) is for 23min etching time. Fig. (d, ΙΙΙ) is for 40min etching time. The flowerlike micro-nano cubic structures are dispersing on the surface, and the structure is more regular for the surface etched with 23min than those with 5min and 40min. With the most suitable etching time of 23min, the flowerlike protrusion structure is dispersed 15


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uniformly on the groove labyrinth structure which behaviors the SHP property. Fig. 3 shows the AFM images of aluminum alloy surfaces etched with the mixture of FeCl3, HCl and Lauric acid ethanol solution by one-step method. Fig. 3 (a) shows the AFM image of the surface being etched for 5min with rms roughness of 307.2nm. It shows that the surface structure with CA of 132.1° is plain regionally which is not effective for superhydrophobicity. If the etching time is 23min, a porous morphology can be clearly seen in the AFM image with a roughness of 328.7nm, as Fig. 3 (b) shows, and the surface represents with the largest CA of 159.1°. If the etching time is lengthened to 40min, the surface roughness decreases to 209.5nm with CA of 152.3°, as Fig. 3 (c) shows. From the roughness, we can see that the surface with the biggest roughness exhibits with the biggest CA, but the surface with the smallest roughness does not with the smallest CA. It concludes that the CA of the surface is dependent on the roughness of it at some extent, but it is nonlinear with it. Besides roughness, we know the superhydrophobicity also have relationship with the chemical compositions of the surface. FT-IR and EDS spectroscopy were used to investigate the influence of it. Fig. 4 shows the FT-IR spectrometer of the SHP aluminum alloy surface (bold line) being etched with the mixture of FeCl3, HCl and Lauric acid ethanol solution by one-step method for 23min. The dotted line presents the FT-IR spectrometer of the normal surface without any treatment. The solid line is corresponding to the FT-IR spectrometer of pure Lauric acid. The two new bands at 2974cm-1 and 2920 cm-1 of the SHP surface are identified as the stretching vibrations of -CH3- and -CH2- groups of 16


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the Lauric acid, respectively

[36]

. The bands at 1690-1790cm-1 that corresponds to the

stretching of C=O and at 1100cm-1 ascribed to the C-OH stretching disappear in the FT-IR of the SHP surface, but two new bands at 1639cm-1 and 1392cm-1, which are identified as the asymmetric and symmetric stretching bands of the OCO group are found. It means that the Lauric acid does not interact with the surface physically but forms COO- to bond with the Al atom of the surface

[37]

. This can be further

confirmed by the EDS spectra, as shown in Fig. 5. Fig. 5 shows the EDS spectra of the SHP aluminum alloy surface etched with the mixture of FeCl3, HCl and Lauric acid ethanol solution by one-step method for 23min. The analysis results are listed in Table 2. It is known that the aluminum surface without any treatment is composed of 2.32wt% Mg and 97.17wt% Al. The Table shows that the prepared aluminum alloy SHP surface is composed of elements 15.26wt% C, 9.12wt% O, 2.02wt% Mg and 73.60wt% Al. Obviously, the weight of Mg and Al decreased and there are two new elements of C and O which come from Lauric acid appears on the treated SHP aluminum alloy surface. This means that Lauric acid has interacted with the atoms of the surface. This finding is also confirmed with the FT-IR results. 3.2. The anti-icing behavior of the SHP surfaces Fig. 6 shows the images clipped from the videos of a falling water droplet on the aluminum alloy surface on the microscopic level. Fig. 6 (a) shows the behavior of a droplet falling on the aluminum plate without any treatment during its first impact. The droplet adheres to the surface quickly with a CA less than 90°. Fig. 6 (b) reveals a 17


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significantly different behavior on a SHP aluminum surface treated by the one-step method and the water droplet rebounds upward elastically without leaving any residual traces on the surface and eventually comes to rest on the surface [38-40] with a CA larger than 150°. The static behavior of the water droplets on the general surface is showed in Fig. 7 (a), and the droplet lay out at once because of the surface hydrophilicity. And if it is put paralleled in Fig. 7 (b) or tilted at 45º angle in Fig. 7 (c) in a refrigerator, the frozen water droplets are still sticky to the surface, exhibiting an adhesional wetting mode on the surface. As for the SHP surface chemically etched by the one-step method, the static water droplets rest on it with spherical shape at room temperature in Fig. 7 (d) and even at -5.0℃ in Fig. 7 (e) in the refrigerator. The sphere shape ices on the SHP surface are removed under gravity conditions, presenting distinguishable hydrophobicity

[41]

. If the SHP plate is put at 45° tilted angle in the refrigerator and

sprayed with small water droplets on for 5min, little ice is seen on the SHP surface from Fig. 7 (f). This happens because for a SHP surface, the small droplets will roll-off before it can ice on the surface. As Fig. 7 (g) and (h) shown, a hairline was put through the water droplet on the horizontal surface in the normal refrigerator, and the vertical tensile force to make the ice off the normal surface and the SHP surface is 0.25N and 1.50N, respectively. Obviously, the adhesion strength of water droplets on the normal aluminum alloy surface is five times higher than the SHP surface. Fig. 8 shows three typical images comparing the anti-icing property using our designed system between the general aluminum alloy surface (left) and the SHP 18


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aluminum alloy surface (right) which are put in the refrigerating box side by side simultaneously. Fig. 8 (a) is the initial image of the surfaces, indicating the water droplet is sticky on the general surface if it is sphere up on the SHP surface. The condition of the initial environment in the refrigerating box is at 16.0℃ with humidity of 98.6% and pressure of 101.3kpa. After 406s, the humidity and pressure is 54.9% and 128.2kpa, respectively, and the icing on the normal surface starts at this moment at the temperature of -2.2℃. The transparent water droplet becomes to be the white solid ice from bottom to top and the icing process continues only for 9s. Fig. 8 (b) shows the state of the surfaces at 415s. Then after 270s, the humidity and pressure becomes to be 59.5% and 134.4kpa, respectively, and the icing on the SHP surface begins at the temperature of -6.1℃. The icing process also occurs from bottom to top and continues for 18s. Fig. 8 (c) shows the state of the surfaces at 694s. Fig. 9 shows the anti-icing properties of the general and the SHP surfaces with an angle of 20º and being sprayed water droplets on at the speed of 4µL/s in the refrigerating box using our designed instruments. Fig. 9 (a) shows that the super-cooled water droplets are sticky at the general aluminum surface and slides down slowly because of the droplet gravity and finally freezes with the shape of a fish whose head is at the bottom of the titled plate ad tail is at the top of the plate when the temperature decreases to -3.9℃. Fig. 9 (b) shows that the super-cooled water droplets bounce off the SHP surface even the temperature decreases as low as to -8.0℃ without ice formation, indicating the ice-phobic behavior. These observations demonstrate that the SHP surface with the proper roughness of 19


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328.7nm and modified with the low surface energy material of Lauric acid shows good superhydrophobicity and anti-icing property, that is, weak ice adhesion property [42]

. From the surface topography, the dual scale texture [43] with proper roughness can

trap air whose water CA is regarded to be 180°

[44]

through the dispersed protrusions

of asperities and make the droplet essentially rest on a layer of air interpreted

by

the

Cassie's

[38]

. This can be [45]

equation

cos θ ' = f cos θ + (1 − f ) cos 180° = f cos θ + f − 1 . f is the remaining area fraction, i.e., liquid-solid interface, if a rough hydrophobic surface with the CA of θ' traps air in the hollows, where the liquid-solid interface with a CA θ. The smaller of f and the bigger of θ are, the bigger of the θ ' It is found that a very good correlation exists between the contact angle and the ice reduction ability for the different surface coatings, that is, the bigger the contact angle is, the weaker the ice adhesion

[46]

. On a

molecular scale, electrostatic attraction, covalent bonding and Van der Waals forces contribute to the adhesion of ice. Water and ice are polar materials, the electrostatic force is considered to be significant, so solid ice will adhere to a metal surface for the same reason that liquid water does [47, 48]. Calorimetric studies of water adsorption on alumina suggest that the higher energy surfaces have the strongest affinity for water [49, 50]

, and the common hypothesis is that materials having poor chemical affinity with

water will also have weak ice-adhesion properties energy of α-Al2O3 is 2.6J/m2 mica

[51]

[46]

. It is known that the surface

and it is 37mJ/m2 for a Lauric acid monolayer on

[52]

, and it is obvious that the modification of Lauric acid on the aluminum

surface can reduce the surface energy dramatically, and then weaken the surface 20


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affinity for water/ice rapidly.

4. Conclusions In this study, the electrochemical anodic oxidation and chemical etching methods were used to simplify the fabrication procedures for SHP surfaces on the aluminum alloy substrates, and the one-step chemical etching with 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath for 23min was the most effective for a large contact angle (CA, 159.1°) and a small contact angle hysteresis (CAH, 4.0°). Furthermore, the SEM, AFM, EDS, FT-IR were employed to characterize the SHP surfaces and confirm that the Lauric acid O atoms formed Al-O bonds with the surface Al atoms and the surfaces with the biggest roughness of 328.7nm exhibited with the biggest CA and smallest CAH. A robust apparatus with a real-time control system based on the two stage refrigerating method was designed by us to systematically investigate the anti-icing behavior of the prepared SHP surfaces. This system can monitor the humidity, pressure and temperature during the icing process on the surfaces and it demonstrated that the SHP surfaces exhibited excellent anti-icing properties, i.e., if the untreated and SHP surfaces were put horizontally side by side in the refrigerating box at room temperature of 16.0℃, the untreated surface began to ice at the temperature of -2.2℃ after 406s and the process lasted for 9s. In contrast, the SHP surface began to ice at the temperature of -6.1℃ after 270s, and this process lasted for 18s. If surfaces were put titled with 20º in the box and a water droplet was sprayed at the speed of 4µL/s, this droplet iced up on the untreated surface at -3.9℃, but bounced 21


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off the SHP surface even the temperature decreased to -8.0℃.

Acknowledgments This work is financially supported by the Major Program of Education Bureau of Hubei Province, China (Z20104401), the Provincial Key Program of Natural Science Foundation of Hubei Province, China (2010CDA026), the NSFC (21174047, 21273166, 51272082, and 51202082)

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[51] Navrotsky, A. Energetics of nanoparticle oxides: interplay between surface energy and polymorphism. Geochem. Trans., 2003. 4(6), 34-37. [52] Bailey, A. I.; Kay, S. M. A direct measurement of the influence of vapour, of liquid and of oriented monolayers on the interfacial energy of mica. Proc. Roy. Soc. A. 1967, 301, 47-56.

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Figures USB conmmunication box body

water-cooled machine

outlet water controller pipe

two high speed fans camera lens sample stage camera quarz glass

inlet water pipe

XYZR microscopic platform

motor

PC machine USB

(a)

(b)

Figure 1. (a) The layout of the overall system based on two stage refrigerating method (b) The photograph of the inside of the refrigerating box.

29


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(a)

(b)

(c)

30


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(d,Ⅰ)

(d, Ⅱ)

(d, Ⅲ)

Figure 2. (a) The SEM image of the SHP surface being anodic oxidized with the mixture aqueous solution of 100g/L H2SO4, 10g/L C2H2O4 and 10g/L C3H8O3 as 31


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electrolyte by two-step method with 10000 magnifications; (b) The SEM image of the SHP surface being anodic oxidized with the solution of 0.3mol/L H3PO4 as the electrolyte by two-step method with

10000 magnifications; (c) The SEM image of

the SHP surface etched with the aqueous solution containing 14wt% FeCl3 and 3wt% HCl at 40°C water bath by two-step method with 4000 magnifications; (d) The SEM image with 4000 magnifications of aluminum alloy surface etched in the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath by one-step method (Ⅰ) for 5min; (Ⅱ) for 23min; (Ⅲ) for 40min.

32


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(a)

(b)

(c)

Figure 3. The AFM image of aluminum alloy surface etched in the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath for different time (a) for 5min; (b) for 23min; (c) for 40min.

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Figure 4. FT-IR spectrum of Lauric acid (solid line), the surface of aluminum alloy (dotted line) without any treatment, and the SHP aluminum alloy surface being etched in the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath for 23min(thick line).

34


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Figure 5. The EDS spectrum of the SHP surface of aluminum alloy surface being etched in the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath for 23min.

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(a)

(b)

Figure 6. (a) The dynamic process of a water drop on the aluminum alloy surface without any treatment; (b) The dynamic process of a water drop on the SHP aluminum alloy surface being treated for 23min by one-step method.

36


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a

d

b

e

h

g

Figure 7. Comparison of surface anti-icing property using refrigerator between general aluminum alloy surface and the SHP aluminum alloy surface treated by one-step chemical etching method. (a) Water droplet on the horizontal general surface at room temperature; (b) Water droplet on the horizontal general surface in a refrigerator; (c) The tilted normal aluminum alloy surface sprayed with small water droplets in a refrigerator; (d) Water droplet on the horizontal SHP aluminum alloy surface at room temperature; (e) Water droplet on the horizontal SHP aluminum alloy surface in a refrigerator; (f) The tilted SHP aluminum alloy surface sprayed with small water droplets in a refrigerator; (g) The water droplet on the surface with a hairline through it; (h) The measurement of the adhesion strength of a water droplet on aluminum alloy surface through a tension dynamometer.

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a

b

c

Figure 8. The comparison of surface anti-icing property using the artificial installment between the general aluminum alloy surface (left) and the SHP aluminum alloy surface (right). (a) The initial state of water droplet on the surfaces at room temperature; (b) The state of the general plate beginning icing at -2.2℃; (c) The state of the SHP surface beginning icing at -6.1℃.

38


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(a)

(b)

Figure 9. The comparison of surface anti-icing property with the plates titled at 20º in the refrigerating box using the artificial installment. (a) The super-cooled water droplets instantly freeze on the general surface at -3.9℃. (b) The super-cooled water droplets bounce off the SHP aluminum alloy surface treated by one-step method even at -8.0℃.

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Table 1. The CA, CAH and total time used for fabricating SHP surfaces by different methods. Method

CA/°

CAH/°

Total time/min

167.9

4.4

150

Two-step anodic oxidation method with H3PO4

159.6

4.1

150

Two-step chemical method with Beck's dislocation

158.2

4.7

99

Two-step chemical etching method with FeCl3, HCl

156.8

4.9

101

One-step chemical etching method with FeCl3, HCl

159.1

4.0

23

Two-step anodic oxidation method with H2SO4, C2H2O4, C3H8O3

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Langmuir

Table 2. EDS spectrum analysis results of the SHP aluminum alloy surface etched in the 1:1 volume mixture of 40wt% FeCl3, 6wt% HCl aqueous solution and 10wt% Lauric acid ethanol solution at 40°C water bath for 23min. Sample

Element

Wt%

At%

Normal surface without any

MgK

2.32

2.53

treatment

AlK

97.17

97.65

CK

15.26

25.87

OK

9.12

11.61

MgK

2.02

1.83

AlK

73.60

60.68

SHP surface

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Preparation and Anti-icing Behavior of Superhydrophobic Surfaces on

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