Preparation and Anti-icing Behavior of Superhydrophobic Surfaces on

May 30, 2013 - Eng. ACS Catal. .... In particular, fatty acid is the most attractive low-surface-energy material because it can overcome the distinct ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Preparation and Anti-icing Behavior of Superhydrophobic Surfaces on Aluminum Alloy Substrates Min Ruan,†,‡ Wen Li,*,‡ Baoshan Wang,*,† Binwei Deng,‡ Fumin Ma,‡ and Zhanlong Yu‡ †

College of Chemistry and Molecular Science, Wuhan University, Wuhan, China Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation, Hubei Polytechnic University, Huangshi, Hubei, China

Downloaded via UNIV OF THE SUNSHINE COAST on June 28, 2018 at 11:10:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

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 surfaces. We demonstrated that the SHP surfaces exhibited excellent anti-icing properties, i.e., from the room temperature of 16.0 °C, 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 °C to −6.1 °C. If such surfaces were tilted, the sprayed water droplets on the normal surfaces iced up at the temperature of −3.9 °C, but bounced off the SHP surface even as the temperature reached as low as −8.0 °C. The present study therefore suggests a general, simple, and low-cost methodology for the promising anti-icing applications in various engineering materials and different fields (e.g., power lines and aircrafts).

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 research2−6 owing to their fascinating water repellence and selfcleaning properties.7−9 Such SHP surfaces have shown potential applications in areas such as corrosion resistance, stain resistant textiles, drag-reduction, and inhibition of snow or ice adhesion.10 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,16 and fatty acid17 have been used to obtain low-surface-energy coatings in the fabrication of SHP surfaces. © 2013 American Chemical Society

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 lines21 can cause very serious problems. For example, the ice formation on power cables can result in power grid disruptions. Ice accretion on aircraft surfaces without anti-icing devices can significantly lower the safety and performance and 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 deicing 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 in the infrastructures dramatically, and passive deicing spends a lot of manpower, material resources, and time.26,27 Received: March 19, 2013 Revised: May 27, 2013 Published: May 30, 2013 8482

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

microscope (AFM; SPM-9500J3 from Shimadzu Co) was used for visualization of the surfaces operating in the contact mode. 2.2. The Fabrication of SHP Surfaces. A 10 × 10 × 0.10 mm aluminum alloy plate was polished mechanically using 2000# metallographic abrasive paper first, 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 (treated successively with 1 mol/L NaOH aqueous solution at 60 °C for 2 min, and then washed with deionized water and ethanol for 5 min each under ultrasonication) was used as the working electrode. The counter electrode was a Pt sheet. The distance between the cathode and the aluminum plate used as the anode was 50 mm. The electrolyte was a mixture solution with 100g/L H2SO4, 10g/L oxalic acid (C2H2O4), and 10g/L Glycerol (C3H8O3). The rough surface was electrochemically treated with a voltage of 2.0 V at a stable electro-polishing current density of 20 mA/cm2. The reaction times were 10 min, 30 min, 60 min, 90 min, and 120 min at room temperature, respectively. Then the aluminum alloy plate was washed ultrasonically with deionized water for 5 min, dried at 100 °C in air, and modified with 5 wt % ethanol lauric acid solution for 1.5 h. The CAs of the surfaces polished for different time of 10 min, 30 min, 60 min, 90 min, and 120 min are 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 60 min 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 an etching time of 60 min. From above CA and CAH results, we find out that the two-step electrochemical anodic oxidation method needs at least 60 min to obtain the SHP surface before being modified with 5 wt % ethanol lauric acid solution for 1.5 h. So using H3PO4 solution as the electrolyte, we can spend only 60 min and 120 min 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 0.3 mol/L H3PO4 aqueous solution for 60 min and 120 min, respectively. Then the aluminum alloy plate was washed ultrasonically with deionized water for 5 min, dried at 100 °C in air, and modified with 5 wt % ethanol lauric acid solution for 1.5 h. The CAs of the surfaces for 60 min and 120 min are 159.6° and 150.9°, respectively. The CAH of the surface that was polished for 60 min 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 40 mL of 37 wt % HCl, 192.5 mL of H2O, and 2.5 mL of 40 wt % HF) in a polyethene bottle at room temperature. The etching times were 0.5 min, 2 min, 6 min, 9 min, 12 min, 15 min, and 17 min, respectively. Then the plate was washed and dried at 100 °C in air and modified with 5 wt % ethanol lauric acid solution for 1.5 h. The CAs are 109.6°, 120.2°, 150.9°, 158.2°, 155.1°, 152.4°, and 151.1° for different times of 0.5 min, 2 min, 6 min, 9 min, 12 min, 15 min, and 17 min, 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 are 158.2° and 4.7° with an etching time of 9 min. 2.2.4. Two-Step Chemical Etching Method with FeCl3 and HCl. Similar to the above procedure, the prepared aluminum alloy plate was etched with chemical etchant of the aqueous solution containing 14 wt % FeCl3 and 3 wt % HCl at 40 °C water bath in a glass bottle. The etching time was 0.5 min, 1 min, 2 min, 5 min, 8 min, 11 min, 14 min, 17 min, 26 min, 29 min, and 32 min, respectively. Then the aluminum alloy plate was washed with deionized water as soon as possible for 5

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 “ice-phobic” materials because of their extraordinary water-repellency ability. In fact, it has recently been reported that SHP surfaces can reduce accumulation of snow and ice and even completely prevent the 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/nanorough hydrophobic coatings that 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. However, a thorough understanding on the anti-icing behavior of SHP surfaces has hardly been 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 qualitative, i.e., the above studies just compare the anti-icing capabilities 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. EXPERIMENT 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 produced by China Steel Aluminum Corporation were 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, and 97.17 Al. Both CA and CAH were measured on a Krű ss DSA 100 ContactAngle Goniometer (Krű ss GMBH, Hamburg, Germany) following standard procedures. Energy-dispersive spectroscopy (EDS) with an EDAX Genesis2000 detector was employed for the qualitative analysis of chemical composition. The microstructures of the metallic surfaces were observed using a scanning electron microscope (SEM; FEI Quanta 200, Holland). Spectral data were acquired with OPUS MIR Tensor 27 software version 4.0 (Bruker Optics, Inc.). An atomic force 8483

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

min, dried at 100 °C in air and then modified with 5 wt % ethanol lauric acid solution for 1.5 h. The CAs of the aluminum surfaces etched with FeCl3 and HCl by the 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 times of 0.5 min, 1 min, 2 min, 5 min, 8 min, 11 min, 14 min, 17 min, 26 min, 29 min, and 32 min, respectively. It can be seen that the CA of the surface increases to a peak immediately and then decreases slowly to a plateau with the etching time. The CA and CAH of the SHP surface at the peak is 156.8° and 4.9° with an etching time of 11 min. 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 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution at 40 °C water bath in a glass bottle. The etching times were 5 min, 8 min, 11 min, 14 min, 17 min, 20 min, 23 min, 40 min, 60 min, 90 min, and 130 min, respectively. Then the aluminum alloy plate was washed with deionized water as soon as possible for 5 min and dried at 100 °C. The CAs for the aluminum surfaces along with the etching time by the 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 times of 5 min, 8 min, 11 min, 14 min, 17 min, 20 min, 23 min, 40 min, 60 min, and 90 min, respectively. The trend is very similar 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 to 23 min and then decreases slowly to a plateau of 151.1° with an etching time to 90 min. 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

2.3. The Anti-icing Experiments. 2.3.1. Anti-icing Experiments by Normal Refrigerator. 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 °C to observe the antiicing behavior of a SHP surface. First, 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 5 min, 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 4 μL, which is fixed for all the experiments spraying for about 5 min, and finally took photos of the plates and qualitatively judged how much ice was 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, a tension dynamometer was used to measure the tensile force to take 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 complementary metal oxide semiconductor (CMOS) camera image acquisition system, and a PC (personal computer). The layout of the system is shown in Figure 1a. 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 the refrigerating box, which controls the temperature ranging from 2 °C to −10 °C by eight refrigeration films (model TEC1206, Thermoelectric Cooler). Additionally, 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 °C by the 1600 W small-sized industrial water-cooled circulating machine, namely, the second stage refrigerating part. As the cubic refrigerating box, an acrylic panel is used for the outer and inner layers, and a heat-insulated foam is used for the insulator. The inner layer of the front is arranged with two 2W light emitting diode (LED) lamps and a 4 cm ×4 cm 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 8 cm × 8 cm fans placed side by side for cold dispersion, a 11 cm × 18 cm cooling conduction block, six refrigeration films, two 4.2 cm × 12 cm × 1.1 cm water cooling heads, an aluminum plate with a thickness of 0.3 mm used for fixing the water cooling heads, 3 cm thick heat-insulated foam and acrylic panel, control board including the 32-bit STM32F107 (Synchronous Transfer Module) microprocessor controller, TLC5615DA (Digital to Analog) module, and a 320 × 240 liquid crystal display (LCD) 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 a diameter of 0.3 cm 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 4 μL. From the top to the bottom of the box, there is a 5 cm × 5 cm rotatable sample stage with the temperature sensor besides, a 5 cm × 12 cm aluminum plate for antiwater and wind, two 4 cm × 4 cm fans placed side by side for cold dispersion, two cooling conduction blocks, two refrigeration films, two 4.2 cm × 12 cm × 1.1 cm water cooling heads that connect with the two in the back through water pipes and finally join with the water-

Table 1. The CA, CAH, and Total Time Used for Fabricating SHP Surfaces by Different Methods method two-step anodicoxidation method with H2SO4, C2H2O4, C3H8O3 two-step anodic oxidation method with H3PO4 two-step chemical method with Beck’s dislocation two-step chemical etching method with FeCl3, HCl one-step chemical etching method with FeCl3, HCl

CA/°

CAH/°

total time/min

167.9

4.4

150

159.6

4.1

150

158.2

4.7

99

156.8

4.9

101

159.1

4.0

23

electrochemical anodic oxidation methods need at least 60 min to obtain an SHP surface before being modified with 5 wt % ethanol lauric acid solution for 1.5 h. 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 are 167.9° and 4.4°, respectively. The CA and CAH are 159.6° and 4.1° for the surface treated by 0.3 mol/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 the 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 9 min and obtained using the solution of 14 wt % FeCl3 and 3 wt % HCl as the etchant in a 40 °C water bath for 11 min 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 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution in a 40 °C water bath just for 23 min 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. 8484

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

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. cooled circulating machine used for cooling the hot end of the refrigeration films by water cooling method, a 0.3 mm aluminum plate used for fixing the water cooling heads, heat-insulated foam, an acrylic panel, and a stepper motor being used to rotate the sample stage. The wind speed sensor and humidity sensor are placed on the left side, while the atmosphere pressure sensor is fixed on the right side of the box. Figure 1b shows a photograph of the inside of the refrigerating box. The CMOS is installed on the XYZR 60 mm × 60 mm fine-tuning platform with high resolution of 2592 × 1944, 256 gray degrees, a capture speed of 6 fps, and 0.3−1 optical magnifying lens. To ensure there is no fog on the quartz glass due to temperature difference between the inside and outside of the refrigerating box, two high-speed 12 V0.48A fans are arranged at a 45° angle symmetrically outside the box. 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 °C to −10 °C 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 0−20 μL/s. The icing status can be captured through the camera for samples smaller than 2 cm × 2 cm 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 °C to −10.0 °C with 1 m/s wind

speed. We also put the two surfaces titled at 20° on the sample stage with 4 μL/s being the speed of water droplet and lowered the temperature from room temperature of 16.0 °C to −10.0 °C with 1 m/s wind speed at the same time. Then we observed the icing behaviors of the surfaces through the images recorded by the highspeed camera in the PC and processed the data through the software of the system.

3. RESULTS AND DISCUSSION 3.1. SHP Surface Characterizations. Figure 2a shows an SEM image with 10 000× magnification of the SHP aluminum 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 Figure 2b. It is the SEM image of the SHP surfaces for 60 min etching time with 10 000× magnification. Also, circular shallow holes with different sizes can be seen on this surface because the electrolyte of 0.3 mol/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). Figure 2c shows the SEM image with 4000× magnification of an aluminum alloy surface etched with the aqueous solution of FeCl3 and HCl by the two-step chemical etching method. From the surface microstructures, the chemical etching method is very different from the anodic oxidation method. There are no 8485

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

Figure 2. (a) SEM image of the SHP surface being anodically oxidized with the aqueous solution of 100g/L H2SO4, 10g/L C2H2O4 and 10g/L C3H8O3 as electrolyte by the two-step method with 10 000× magnification. (b) SEM image of the SHP surface being anodic oxidized with the solution of 0.3 mol/L H3PO4 as the electrolyte by the two-step method with 10 000× magnification. (c) SEM image of the SHP surface etched with the aqueous solution containing 14 wt % FeCl3 and 3 wt % HCl at 40 °C water bath by two-step method with 4000× magnification. (d) SEM image with 4000× magnification of aluminum alloy surface etched in the 1:1 volume mixture of 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution in a 40 °C water bath by the one-step method (I) for 5 min, (II) for 23 min, and (III) for 40 min.

deep or shallow holes on the surface, but with sizes of micro− nano structural cubes uniformly dispersing on it.

Figure 2d shows the SEM image with 4000× magnification of the aluminum alloy plates chemically etched with the mixture of 8486

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

structure with CA of 132.1° is plain regionally, which is not effective for superhydrophobicity. If the etching time is 23 min, a porous morphology can be clearly seen in the AFM image with a roughness of 328.7 nm, as Figure 3b shows, and the surface represents with the largest CA of 159.1°. If the etching time is lengthened to 40 min, the surface roughness decreases to 209.5 nm with CA of 152.3°, as Figure 3c shows. From the roughness, we can see that the surface with the biggest roughness exhibits the biggest CA, but the surface with the smallest roughness does not exhibit the smallest CA. It concludes that the CA of the surface is dependent on the roughness of it to some extent, but it is nonlinear with it. Besides roughness, we know the superhydrophobicity also has a relationship with the chemical compositions of the surface. Fourier transform infrared (FT-IR) and EDS spectroscopy were used to investigate the influence of it. Figure 4 shows the FT-IR spectrometer of the SHP aluminum alloy surface (bold line) being etched with the

FeCl3, HCl, and lauric acid ethanol solution by the one-step method. Panel (d, I) is the image for 5 min etching time. Panel (d, II) is for 23 min etching time. Panel (d, III) is for 40 min etching time. The flowerlike micronano cubic structures are dispersing on the surface, and the structure is more regular for the surface etched with 23 min than those with 5 min and 40 min. With the most suitable etching time of 23 min, the flowerlike protrusion structure is dispersed uniformly on the groove labyrinth structure, which mimics the SHP property. Figure 3 shows the AFM images of aluminum alloy surfaces etched with the mixture of FeCl3, HCl, and lauric acid ethanol solution by the one-step method. Figure 3a shows the AFM image of the surface being etched for 5 min with a root-meansquare (rms) roughness of 307.2 nm. It shows that the surface

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 a 1:1 volume mixture of 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution at 40 °C water bath for 23 min (thick line).

mixture of FeCl3, HCl, and lauric acid ethanol solution by the one-step method for 23 min. The dotted line presents the FTIR spectrometer of the normal surface without any treatment. The solid line corresponds to the FT-IR spectrometer of pure lauric acid. The two new bands at 2974 cm−1 and 2920 cm−1 of the SHP surface are identified as the stretching vibrations of −CH3− and −CH2− groups of the lauric acid, respectively.36 The bands at 1690−1790 cm−1 that correspond to the stretching of CO and at 1100 cm−1 ascribed to the C−OH stretching disappear in the FT-IR of the SHP surface, but two new bands at 1639 cm−1 and 1392 cm−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 Figure 5. Figure 5 shows the EDS spectra of the SHP aluminum alloy surface etched with a mixture of FeCl3, HCl, and lauric acid

Figure 3. The AFM image of aluminum alloy surface etched in a 1:1 volume mixture of 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution in a 40 °C water bath for different times: (a) for 5 min; (b) for 23 min; (c) for 40 min. 8487

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

two new elements of C and O that come from lauric acid that appear 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. Figure 6 shows the images clipped from the videos of a falling water droplet on the aluminum alloy surface on the microscopic level. Figure 6a 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°. Figure 6b reveals a 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 surface38−40 with a CA larger than 150°. The static behavior of water droplets on tahe general surface is showed in Figure 7a, and the droplets lay out at once because of the surface hydrophilicity. Also, if it is put parallel in Figure 7b or tilted at a 45° angle in Figure 7c 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 Figure 7d and even at −5.0 °C in Figure 7e in the refrigerator. The sphere shaped ices on the SHP surface are removed under gravity conditions, presenting distinguishable hydrophobicity.41 If the SHP plate is put at a 45° tilted angle in the refrigerator and sprayed with small water droplets for 5 min, little ice is seen on the SHP surface from Figure 7f. This happens because for a SHP surface, the small droplets will roll-off before it can ice on the surface. As Figure 7g,h shows, a hairline was put through the water droplet on the horizontal surface in the normal refrigerator, and the vertical tensile force to take 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 5 times higher than the SHP surface. Figure 8 shows three typical images comparing the anti-icing property using our designed system between the general aluminum alloy surface (left) and the SHP aluminum alloy surface (right), which are put in the refrigerating box side by side simultaneously. Figure 8a is the initial image of the surfaces, indicating that the water droplet is sticky on the general surface if the sphere is up on the SHP surface. The condition of the initial environment in the refrigerating box is at

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

ethanol solution by the one-step method for 23 min. The analysis results are listed in Table 2. It is known that the Table 2. EDS Spectrum Analysis Results of the SHP Aluminum Alloy Surface Etched in the 1:1 Volume Mixture of 40 wt % FeCl3, 6 wt % HCl Aqueous Solution, and 10 wt % Lauric Acid Ethanol Solution at 40°C water bath for 23 min sample

element

wt %

at %

normal surface without any treatment

MgK AlK CK OK MgK AlK

2.32 97.17 15.26 9.12 2.02 73.60

2.53 97.65 25.87 11.61 1.83 60.68

SHP surface

aluminum surface without any treatment is composed of 2.32 wt % Mg and 97.17 wt % Al. The Table shows that the prepared aluminum alloy SHP surface is composed of elements 15.26 wt % C, 9.12 wt % O, 2.02 wt % Mg, and 73.60 wt % Al. Obviously, the weight of Mg and Al decreased, and there are

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 23 min by the one-step method. 8488

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

Figure 7. Comparison of surface anti-icing property using a refrigerator between a general aluminum alloy surface and the SHP aluminum alloy surface treated by the 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 an aluminum alloy surface through a tension dynamometer.

pressure becomes 59.5% and 134.4 kPa, respectively, and the icing on the SHP surface begins at a temperature of −6.1 °C. The icing process also occurs from bottom to top and continues for 18 s. Figure 8c shows the state of the surfaces at 694 s. Figure 9 shows the anti-icing properties of the general and the SHP surfaces with an angle of 20° and being sprayed water droplets at the speed of 4 μL/s in the refrigerating box using our designed instruments. Figure 9a shows that the supercooled water droplets are sticky at the general aluminum surface and slide down slowly because of the droplet gravity, and finally freeze 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 °C. Figure 9b shows that the supercooled water droplets bounce off the SHP surface even the temperature decreases as low as −8.0 °C without ice formation, indicating ice-phobic behavior. These observations demonstrate that the SHP surface with the proper roughness of 328.7 nm 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 texture43 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.38 This can be interpreted by the Cassie equation45 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 a CA of θ′ traps air in the hollows, where the

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. (c) The state of the SHP surface beginning icing at −6.1 °C.

16.0 °C with humidity of 98.6% and pressure of 101.3 kPa. After 406 s, the humidity and pressure is 54.9% and 128.2 kPa, respectively, and the icing on the normal surface starts at this moment at the temperature of −2.2 °C. The transparent water droplet becomes white solid ice from bottom to top, and the icing process continues only for 9 s. Figure 8b shows the state of the surfaces at 415s. Then after 270 s, the humidity and 8489

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

the most effective for a large contact angle (CA, 159.1°) and a small CAH (4.0°). Furthermore, 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 328.7 nm 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 antiicing 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 °C, the untreated surface began to ice at a temperature of −2.2 °C after 406 s, and the process lasted for 9 s. By contrast, the SHP surface began to ice at a temperature of −6.1 °C after 270 s, and this process lasted for 18 s. If surfaces were titled at 20° in the box and a water droplet was sprayed at the speed of 4 μL/s, this droplet iced up on the untreated surface at −3.9 °C, but bounced off the SHP surface even as the temperature decreased to −8.0 °C.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: 780-492-1652; Fax: 780492-2200. Notes

The authors declare no competing financial interest.

■

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

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

liquid−solid interface has a CA of θ. The smaller the f and the bigger the θ, the bigger 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, and 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 iceadhesion properties.46 It is known that the surface energy of αAl2O3 is 2.6 J/m251 and it is 37 mJ/m2 for a lauric acid monolayer on mica,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 affinity for water/ice rapidly.

■

REFERENCES

(1) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (2) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Transparent superhydrophobic thin films with selfcleaning properties. Langmuir 2000, 16, 7044−7047. (3) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. Superhydrophobic perpendicular nanopin film by the bottom-up process. J. Am. Chem. Soc. 2005, 127, 13458−13459. (4) Li, W.; Amirfazli, A. Microtextured superhydrophobic surfaces: A thermodynamic analysis. Adv. Colloid Interface Sci. 2007, 132, 51−68. (5) Adamson, A. W. The solid−liquid interface-contact angle. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990; p 385. (6) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Super-waterrepellent fractal surfaces. Langmuir 1996, 12, 2125−2127. (7) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Superhydrophobic surfaces: From structural control to functional application. Mater. Chem. 2008, 18, 621−633. (8) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Super-hydrophobic surfaces: From natural to artificial. Adv. Mater. 2002, 14, 1857−1860. (9) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Bioinspired surfaces with special wettability. Acc. Chem. Res. 2005, 38, 644−652. (10) Saleema, N.; Sarkar, D. K.; Gallant, D.; Paynter, R. W.; Chen, X. G. Chemical nature of superhydrophobic aluminum alloy surfaces produced via a one-step process using fluoroalkyl-silane in a base medium. Appl. Mater. Interfaces 2011, 3, 4775−4781.

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 40 wt % FeCl3, 6 wt % HCl aqueous solution, and 10 wt % lauric acid ethanol solution at 40 °C water bath for 23 min was 8490

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491

Langmuir

Article

(11) Qian, B. T.; Shen, Z. Q. Fabrication of superhydrophobic surfaces by dislocation-selective chemical etching on aluminum, copper, and zinc substrates. Langmuir 2005, 21, 9007−9009. (12) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Super oilrepellent surfaces. Angew. Chem., Int. Ed. Engl. 1997, 36 (9), 1011− 1012. (13) Rao, A. V.; Gurav, A. B.; Latthe, S. S.; Vhatkar, R. S.; Imai, H.; Kappenstein, C.; Wagh, P. B.; Gupta, S. C. Water repellent porous silica films by sol−gel dip coating method. J. Colloid Interface Sci. 2010, 352 (1), 30−35. (14) Nakajima, A.; Abe, K.; Hashimoto, K.; Watanabe, T. Preparation of hard super-hydrophobic films with visible light transmission. Thin Solid Films 2000, 376, 140−143. (15) Saleema, N.; Sarkar, D. K.; Paynter, R. W.; Chen, X. G. Superhydrophobic aluminum alloy surfaces by a novel one-step process. ACS Appl. Mater. Interfaces 2010, 2, 2500−2502. (16) Sarkar, D. K.; Farzaneh, M.; Paynter, R. W. Superhydrophobic properties of ultrathin rf-sputtered Teflon films coated etched aluminum surfaces. Mater. Lett. 2008, 62, 1226−1229. (17) Wang, S. T.; Feng, L.; Jiang, L. One-step solution-immersion process for the fabrication of stable bionic superhydrophobic surfaces. Adv. Mater. 2006, 18, 767−770. (18) Wang, S. T.; Feng, L.; Liu, H.; Sun, T. L.; Zhang, X.; Jiang, L.; Zhu, D. B. Manipulation of surface wettability between superhydrophobicity and superhydrophilicity on copper films. Chem. Phys. Chem. 2005, 6, 1475−1478. (19) Gent, R. W.; Dart, N. P.; Cansdale, J. T. Aircraft icing. Philos. Trans. R. Soc., Sect. A 2000, 358, 2873−2911. (20) Dalili, N.; Edrisy, A.; Carriveau, R. A review of surface engineering issues critical to wind turbine performance. Renewable Sustainable Energy Rev. 2009, 13, 428−438. (21) Cober, S. G.; Isaac, G. A.; Strapp, J. W. Recent Canadian research on aircraft in-flight icing. J. Appl. Meteorol. 2001, 40, 1984− 2002. (22) Otta, S. P.; Rothmayer, A. P. Instability of stagnation line icing. Comput. Fluids 2009, 38, 273−283. (23) Xue, Y. S.; Fei, S. Y.; Pu, F. Q. Upgrading the blackout defense scheme against extreme disasters. Autom. Electr. Power Syst. 2008, 32, 1−5. (24) Hou, H.; Yin, X. G.; Chen, Q. Q. Review on the wide area blackout of 500 kV main power grid in some areas of south China in 2008 snow disaster. Autom. Electr. Power Syst. 2008, 32, 12−15. (25) Laforte, J. L.; Allaire, M. A.; Laflamme, J. State-of-the-art on power line de-icing. Atmos. Res. 1998, 46, 143. (26) Boinovich, L. B.; Emelyanenko, A. M. Anti-icing potential of superhydrophobic coatings. Mendeleev Commun. 2013, 23, 3−10. (27) U.S. Army Corps of Engineers, Cold Regions Research and Engineering Laboratory, Icing engineering, 2003; pp 1−8 (28) Boluk, Y.; Zhao.L. Aircraft anti-icing fluids formulated with nanocrystalline cellulose. Aircraft anti-icing fluids formulated with nanocrystalline cellulose. U.S. Patent, 8105430B2, 2012. (29) Louchez, P. R.; Laforte, J. L.; Bouchard, G. R.; Farzaneh, M. Laboratory evaluation of aircraft ground de/antiicing products. Int. J. Offshore Polar Eng. 1995, 5, 32−36. (30) Xiao, J.; Chaudhuri, S. Design of anti-icing coatings using supercooled droplets as nano-to-microscale probes. Langmuir 2012, 28, 4434−4446. (31) Cao, L. L.; Jones, A. K.; Sikka, V. K.; Wu, J. Z; Gao, D. Antiicing superhydrophobic coatings. Langmuir 2009, 25 (21), 12444− 12448. (32) Farhadi, S.; Farzaneh, M.; Kulinich, S. A. Anti-icing performance of superhydrophobic surfaces. Appl. Surf. Sci. 2011, 257, 6264−6269. (33) Liu, Z. L.; Gou, Y. J.; Wang, J. T.; Cheng, S. Y. Frost formation on a superhydrophobic surface under natural convection conditions. Int. J. Heat. Mass Transfer. 2008, 51, 5975−5982. (34) Liu, Z. L.; Zhang, X. H.; Wang, H. Y.; Meng, S.; Cheng, S. Y. Influences of surface hydrophilicity on frost formation on a vertical cold plate under natural convection conditions. Exp. Therm. Fluid Sci. 2007, 31, 789−794.

(35) Na, B.; Webb, R. L. A fundamental understanding of factors affecting frost nucleation. Int. J. Heat Mass Transfer 2003, 46, 3797− 3808. (36) Lummerstorfer, T.; Hoffmann, H. IR reflection spectra of monolayer films sandwiched between two high refractive index materials. Langmuir 2004, 20, 6542−6545. (37) Tao, Y. T. Structural comparison of self-assembled monolayers of n-alkanoic acids on the surfaces of silver, copper, and aluminum. J. Am. Chem. Soc. 1993, 115, 4350−4358. (38) Lee, Y.; Ju, K. Y.; Lee, J. K. Stable biomimetic superhydrophobic surfaces fabricated by polymer replication method from hierarchically structured surfaces of Al templates. Langmuir 2010, 26 (17), 14103− 14110. (39) Tong, S. R.; Wu, L. Y.; Ge, M. F.; Wang, W. G.; Pu, Z. F. Heterogeneous chemistry of monocarboxylic acids on α-Al2O3 at ambient condition. Atmos. Chem. Phys. Discuss. 2010, 10, 3937−3974. (40) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Newville, M.; Sutton, S. R.; Rivers, M. L. Structure of the hydrated αAl2O3 (0001) surface. Science 2000, 288 (10), 1029−1033. (41) Al-Abadleh, H. A.; Grassian, V. H. FT-IR study of water adsorption on aluminum oxide surfaces. Langmuir 2003, 19, 341−347. (42) Croutch, V. K.; Hartley, R. A. J. Adhesion of ice to coatings and the performance of ice release coatings. Coating Technol. 1992, 64, 41− 52. (43) Li, W.; Amirfazli, A. Microtextured superhydrophobic surfaces: A thermodynamic analysis. Adv. Colloid. Interface. 2007, 132, 51−68. (44) Jiang, L.; Zhao, Y.; Zhai, J. A lotus-leaf-like superhydrophobic surface: A porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew. Chem. 2004, 116, 4438−4441. (45) Cassie, A. B. D. Contact angles. Discuss. Faraday Soc. 1948, 3, 11−16. (46) Dotan, A.; Dodiuk, H.; Laforte, C.; Kenig, S. J. The relationship between water wetting and ice adhesion. Adhes. Sci. Technol. 2009, 23, 1907−1915. (47) Air Force aircraft and airfield deicing/anti-icing: The role of deicing and anti-icing in the Air Force, Fact Sheet Update, May 1998, http://www.p2pays.org/ref/01/00593.pdf. (48) Petrenko, V. Systems and methods for modifying ice adhesion strength. U.S. Patent, 2000, 6, 027-075. (49) McHale, J. M.; Navrotsky, A.; Perrotta, A. J. J. Effects of increased surface area and chemisorbed H2O on the relative stability of nanocrystalline γ-Al2O3 and α-Al2O3. Phys. Chem. B 1997, 101, 603− 613. (50) McHale, J. M.; Auroux, A.; Perrotta, A. J.; Navrotsky, A. Surface energies and thermodynamic phase stability in nanocrystalline aluminas. Science 1997, 277, 788−791. (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.

8491

dx.doi.org/10.1021/la400979d | Langmuir 2013, 29, 8482−8491