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Biomimetic Superhydrophobic Engineering Metal Surface with Hierarchical Structure and Tunable Adhesion: Design of Microscale Pattern Wei Jiang, Ming Mao, Wei Qiu, Yingming Zhu, and Bin Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03936 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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Industrial & Engineering Chemistry Research

Biomimetic Superhydrophobic Engineering Metal Surface with Hierarchical Structure and Tunable Adhesion: Design of Microscale Pattern Wei Jiang1, *, Ming Mao1, Wei Qiu1, Yingming Zhu2, Bin Liang1 1

Multi-Phases Mass Transfer and Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu, 610065, China

2

Insitute of New Energy and Low-carbon Technology, Sichuan University, Chengdu, 610065, China

Abstract: Lotus leaves and rose petals are both typical natural superhydrophobic surfaces, with low and high adhesion, respectively. This fact inspires us to prepare superhydrophobic surfaces with different levels of adhesion on iron by mimicking their hierarchical structures through three simple steps: abrasion, calcination, and modification. A uniform and stable superhydrophobic iron surface with excellent adaptability and wearability can be obtained, and its adhesion is tunable. The results confirmed that superhydrophobicity and adhesion are both dependent on the synergy of the microscale and nanoscale patterns of the hierarchical structure generated by the designed abrasion and thermal treating. The adhesion level can be controlled by simply adjusting the abrasion program to obtain the desired microscale pattern with a proper ratio of height-to-width of the microstructure. This easy, inexpensive, and

*

Corresponding author. Tel.: +86-28-85990133; fax: +86-28-85460556. E-mail address: [email protected] 1

ACS Paragon Plus Environment


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clean three-step method is widely applicable for different engineering metals and alloys and suitable for large-scale production.

Keyword: Superhydrophobicity; Tunable adhesion; Microscale pattern; Abrasion; Hierarchical structure; Nanoscale pattern; Engineering metal; Biomimetic surface

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1. Introduction Engineering metals such as Fe, Cu, Al, Ni, and Ti are widely used in many different industries. Treating these metals to have superhydrophobic surfaces with various levels of adhesion endows them with desirable qualities such as self-cleaning, anti-fogging, anti-icing, anti-corrosion, anti-fouling, microfluidic chip, drag reduction, and no lost transport of liquid1-12. Superhydrophobicity is a special property of solid surfaces with a static water contact angle (CA) larger than 150°13-15. Adhesion is an important characteristic of superhydrophobic surfaces, as it directly determines the water dynamic behavior of a superhydrophobic surface, and can be characterized by CA hysteresis (CAH), rolling angle (RA), and adhesion force of water droplets16-23. Superhydrophobicity and adhesion are both dependent on the surface geometrical structure and chemical composition24-27. Since metal surfaces are usually hydrophilic, two strategies can be adopted to fabricate superhydrophobic surfaces on them28: creating rough topographic features on substrates followed with hydrophobic surface modifications, and depositing hydrophobic materials with rough surface textures. According to the abovementioned strategies, constructing appropriate and stable microstructures is crucial to obtaining superhydrophobic metal surfaces with different adhesion. Currently, numerous approaches including template method, sol-gel process, chemical vapor deposition, lithography, evaporation-induced phase separation, electrospinning, anodization, etching, and electrochemical method29-33 have been employed to build hierarchical rough structures on metal surfaces that mimic 3



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hierarchical structures of natural superhydrophobic structures such as lotus leaves or rose petals. The synergy between micro- and nanoroughness can result in high CAs with strong34-36 or weak adhesion37-39, and even tunable adhesion40-43 by tailing the hierarchical structure27, 44-47. However, these reported methods reported are characterized by tedious procedures, complicated strategies, corrosive or poisonous etchants, and expensive equipment and modifiers. These inherent drawbacks limit their large-scale application for the future. Thus, it is necessary to develop a simple, cheap, and clean process to prepare hierarchical

structures

on

metal

surfaces

for

large-scale

fabrication

of

superhydrophobic engineering metals. Although many attempts have been made to mimic the hierarchical structures that exist in nature, these studies have primarily focused on constructing and carefully adjusting nanoscale patterns within the hierarchy. Applying this concept of adjusting the microscale pattern of a hierarchical morphology to construct a superhydrophobic surface with tunable adhesion to large scale applications could potentially be very valuable and is worth investigating. However, this research should be conducted within the context of specific examples such as natural masterpieces, lotus leaves, and rose petals. In this study, the fresh lotus leaf (Nelumbo nucifera Gaertn) and rose petal (Rosa rugosa Thunb), which are typical natural superhydrophobic surfaces with low and high adhesion respectively, were characterized to observe the difference between their microscale structures in detail. Figure 1-A indicates that water droplets on the lotus leaf roll towards the center or away from it owing to the shape of the leaves; however, 4


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on the rose petals (Figure 1-B), they stop arbitrarily on the surface irrespective of whether the petal is horizontal, curved, or inclined. The CA values of the lotus leaf and rose petal were 160.5±1° and 153.2±1°, as shown in Figure 1-C and Figure 1-D, respectively. The RA values shown in Figure 1-E and Figure 1-F were 3±1° and 180°, which exhibit negligible adhesion and strong adhesion, respectively. Environmental scanning electron microscope photographs of a lotus leaf and rose petal in Figure 1-G and Figure 1-H, respectively, reveal that their surfaces represent typical hierarchical structures. The lotus leaf is combined with sparse 5–10 µm microscale papillary hills with about 100 nm nanoscale protruded tubules on the papilla. A fractal analysis of the lotus SEM image revealed the fractal dimension to be D = 2.1594. The equation shown below48 was used to correlate CA. cos θ= (L/l)D-2 cos θ0. (1) In the self-similarity range, L and l were valued as the average size of the microscale and nanoscale pattern, respectively, by Nano Measurer 1.2 software. The L and l of the lotus leaf were 7.5618 and 0.1565 µm respectively. Then, the correlated CA was found to be 159.3°, with a slight deviation of 160.5°. Atomic force microscope (AFM) analysis was employed to determine the height and width of the space between these papillary hills. The average height and width of the lotus leaf were 6.0351 and 8.1818 µm, respectively. The ratio of height-to-width (H/W), reached a value of 0.7376. This means the microscale pattern of the lotus leaf surface sharply fluctuates. With this structure, it is easy to form a discontinuous triple-phase contact line that holds air to form an air-film, and exhibits low adhesion. 5



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On the contrary, the rose petal surface distributed dense 15–20 µm papillary hills with ca. 1 µm nanoscale folds on the papilla. The correlated CA was 151.6 °, which was slightly less than the measured value of 153.2°. The values of D, L, and l were determined as 2.1072, 9.6782, and 0.4553 µm. The AFM results revealed the average height and width to be 1.0315 and 3.6542 µm respectively. The H/W had a value of 0.2768. Such a relatively smooth surface results in a continuous triple-phase contact line, increases the contact area and energy barrier, and exhibits high adhesion. The results shown in Figure 1-I and Figure 1-J suggest that the microscale papilla height and distance could be used to control adhesion. This discovery inspired a straightforward approach

to manipulate

the

microscale

pattern to

obtain

superhydrophobic surface with tunable adhesion. Constructing microscale papilla with dimensions of about 5-20 µm and a nanoscale pattern with dimension of tens of nanometer is feasible to obtain the hierarchical structure for achieving superhydrophobic surface, and manipulating the height of microscale papilla and their distance can tune the adhesion. Current machining technology can easily achieve 10 µm accuracy on metal surfaces, which is the same order of magnitude as the microscale patterns of the lotus leaf and rose petal. Furthermore, a simple application of abrasion or grinding can be used to obtain microscale scratches. If the abrasion direction in intentionally crisscrossed, several microscale pitches or papillary hills can be achieved on the cross-sections of scratches and regarded as the desired microscale patterns for the hierarchical structure.

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Geometrical parameters such as depth and distance can be simply controlled by changing the abrasion program. Another useful enlightening fact is that some metals automatically form nanostructures in the form of metal oxides on the surface after calcination in air under high temperatures. We found that simple calcination in muffle can generate hematite nanoscale patterns on the surface of pure iron49. Superhydrophobic Zn50 and Sn51 have been prepared using thermal oxidation to obtain nanoscale structures on surfaces. This fact suggests that nanoscale patterns of hierarchical structures can be easily obtained through simple calcination. Moreover, a bioinspired hierarchical structure on metal surface can be easily constructed and used as a substrate to administer superhydrophobic surface treatments after modification with low-surface-free-energy substances. This approach was attempted in this study using iron as a typical engineering metal. Superhydrophobicity was achieved on iron’s surface using the following three-step method: abrasion, calcination, and modification. The effects of the resulting microscale and nanoscale patterns on superhydrophobicity and adhesion were investigated. Curbing the microscale pattern of the hierarchical structure via intentionally adjusting the abrasion program was used to control the level of adhesion. Other engineering metals were also attempted to determine the general applicability of this three-step method.

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2. Experiments 2.1 Materials Commercially available Fe (purity ≥ 99.5%), Ni (purity ≥ 99.9%), Ti foils (purity ≥ 99.9%), 316 stainless steel (06Cr17Ni12Mo2) and tinplate, were purchased from Shanghai Shengzhuo Materials Co., Ltd. (China). Al foils (purity ≥ 99.5%) was obtained from Tianjin Bodi Materials Co. Ltd. (China). Cu foils (purity ≥ 99.8%) were purchased from Dongguan Mingjue Metal Materials Co., Ltd. (China). The waterproof abrasive papers were Matador produced by Starcke GmbH & Co. KG (Germany). Ethanol, solid paraffin, and oleic acids were obtained from Chengdu Changzheng Glass Co. Ltd. (China). 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS-17) was purchased from Wuhan Silworld Chemical Co.,Ltd. (China). Octadecanethiol was obtained from Aladdin-e.COM. All other chemical reagents, including isopropyl alcohol, ethanol, acetone, and sodium dodecyl sulfate, were analytic-grade and obtained from Kelong Chemicals Corp. (Chengdu, China). All reagents were used without further treatment unless otherwise specified. 2.2 Abrasion process Metal foils including Fe, Cu, Ti, Ni, and Al, as well as stainless steel and tinplate, were ultrasonically cleansed with acetone, isopropyl alcohol, ethanol, and deionized water sequentially. After that, metal foils was abraded manually by metallographical sandpapers with different mesh. Abrasion was conducted on a weighbridge to control the abrading force strength. Criss-cross abrasion was used to form microscale patterns, 8


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and one horizontal and vertical abrasion was defined as one abrasion times. After specified abrasion times with specific sandpaper to obtain metal foils with desired microscale patterns, abraded metal foils was blown by a blower to remove the metal debris, and cleansed again with deionized water and ethanol, sequentially. For example, a typical superhydrophobic iron surface with low adhesion was abraded by a 400 mesh sandpaper at 40 abrasion times; a typical superhydrophobic iron surface with high adhesion was abraded by a 1500 mesh sandpaper at 40 abrasion times. 2.3 Calcination process Calcination was carried out in air atmosphere in a tube furnace (MXG1200-60, Shanghai Micro-X Furnace CO. Ltd.). Abraded metal foil after cleansing was directly placed into the furnace. Temperature was raised to the specific temperature with given heating rate. After reaching the temperature, the calcination process was timed. For example, Fe foil was calcined at 400 °C with heating rate of 8 °C/min, and kept for 4 hours to obtain desired nanoscale patterns on surface. Cu foil was calcined at 300 °C with heating rate of 3 °C/min, and kept for 2 hours. Al foil was calculated at 450 °C with heating rate of 5 °C/min, and kept for 6 hours. Ti foil was calcined at 550 °C with heating rate of 5 °C/min, and kept for 6 hours. Ni foil was calcined at 700 °C with heating rate of 5 °C/min, and kept for 8 hours. Stainless steel foil was calcined at 650 °C with heating rate of 3 °C/min, and kept for 9 hours. Tinplate treatment program was same to Fe. After that, the samples after thermal treating was cooled in furnace.

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2.4 Modification process Modification was carried out with two methods: immersion method and vaporization method. The former was used to load FAS-17, octadecanethiol, oleic acid and sodium dodecyl sulfate. The latter was used to load paraffin and lard, and the schematic diagram was shown in Support Information. For FAS-17 modification, the solution was premade by mixing 1 g of FAS-17 and 99 g of ethanol for 3 h under continuous stirring at a rate of 100 rpm. The calcined metal foil with hierarchical structures was immersed into the FAS-17 solution at ambient temperature for 12 h. After that, the metal foil was washed with ethanol and dried in a drying chamber at 100 °C for 1 h. For octadecanethiol, oleic acid and SDS, the modifiers were dispersed and dissolved into ethanol at the concentration of 1.0 wt.% to obtain corresponding solutions. The as-prepared solution were dropped onto the surface of calcined metal foil until a liquid film covering the entire surface. After 12 h standing for natural drying, the metal foils were dried in a oven at 70 °C for 10 minutes. After cooling, the final superhydrophobic products modified with different modifiers were obtained. For wax and lard, the solid modifiers were heated beyond their melting point, changing into liquid state and being heated continuously. Abraded metal foils were hung above the heated modifers with their abraded surface downwards. The suspended height of foils was 1cm. Vapor raised from heated wax or lard condensed on the surface of metal foils. After a specific fumigated time, final superhydrophobic metal surface was obtained after cooling. 10


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2.5 Determination of wettability and adhesion Static contact angles, advancing angle, receding angle and rolling angle of different metal samples were measured with a contact angle meter (Powereach, Model JC2000C1) with deionized water (5µL) at the room temperature. Five points of each sample were tested, and the average value was calculated as the determined CA, advancing angle and receding angle. Value of contact angle hysteresis was the calculated difference between advancing angle and receding angle. Determined RA was the average value of five tests. Adhesion force was estimated by droplet volume deformation and maximum volume when hung on the surface. Motion of water droplets on samples were recorded by a high-speed video camera (Photron, FASTCAM Mini UX50) with photographic speed of 1,000 frame/s. 2.6 Characterization The surface morphology of the metal foils were conducted by a scanning microscopy system (SEM) (JEOL, Model JSM-7500F). The surface topology and root-mean-square (RMS) roughness were recorded by an atomic force microscopy (AFM) (Asylum RESEARCH, Model MFP3D) images. The crystal structure of samples was examined by a X-ray diffraction (XRD) (Karaltay ,Model DX2700) . The successful loading of different modifiers were identified by judging the absorption spectra using a Fourier transform infrared spectroscopy (FT-IR)(Perkin Elmer, Model Spectrum Two , L1600300).

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2.7 Calculation of fractal dimension of rough surface Here the calculation of fractal dimension was determined by the box-counting dimension method. The concrete steps of calculated the box-counting dimension was carried out by the method of pixel-covering method: a) After binarization of a SEM image, each pixel point of images were transformed into two colors, black and white. A data matrix was obtained, which ranks number corresponded to the ranks number of the binary figure; b) Dividing the obtained data matrix into several chunk, and making raw and column number of each chunk were k. Marking the quantity of single-valued chunks that only contain 0 or 1 as Nk. Usually value of k was given as 1, 2, 4, ..., 2i. Thus the box number, N1, N2, N4, ..., N2i was obtained by counting the quantity of single-valued chunks. c) Plotting logk against logNk in the log-log coordinate plane. Conducting the linear fit of all the data points with least squares method, the obtained negative value of the fitted slope, D, was the physical box-counting dimension of this SEM image. Program edited with MATLAB was included in Support Information.

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3. Results and Discussion 3.1 Implementation of bioinspired hierarchical structure design The hierarchical structures on the iron surface were established by combining abrasion and calcination. Figure 2-A to Figure 2-D show different surface morphologies of origin, abrasion, and calcination, and the two-step treated iron foil by abrasion and calcination, respectively. Inserted CA photos confirm that untreated smooth pure iron is with a CA of 92.6±2°, as shown in Figure 2-A. After simple abrasion, the CA decreased to 50.7±2° sharply because of the ca. 10 µm microscale scratches and pitches constructed by abrasion (Figure 2-B). On the other hand, the CA value of the iron surface after direct calcination for 4 h at 500 °C in Figure 2-C decreased dramatically to 9.7±1°. This significant decrease in CA was attributed to the disordered Fe3O4 nanoflakes (Figure S1) in situ generated by the thermal oxidation reaction of the iron surface in air, which were chaotic in arrangement, but uniform in size. The size of the thermal-generated nanoflakes was ca. 1 µm in length and ca. 50 nm in thickness. If the iron foil was abraded and calcinated in sequence, the CA value decreased to 3±1°, which meant that a superhydrophilic surface was successfully obtained. This attractive transformation ascribed to the synergy of microscale and nanoscale patterns of a hierarchical structure on surface, which can be observed in Figure 2-D. Undulating microscale grooves and bumps comprised the microstructure, and clustered nanoflakes constituted the nanostructure. This hierarchical architecture provided a basis for achieving superhydrophobicity on the iron’s surface. 13



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3.2 Superhydrophobicity with tunable adhesion A typical fluoroalkylsilane with low surface free energy， 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17), was loaded to modify the hierarchical structures on the iron surface to obtain a superhydrophobic surface. The Fourier transform infrared spectroscopy (FT-IR) results shown in Figure S2 confirmed the successful loading of modifiers. Figure 3 depicts the typical final superhydrophobic surface with low adhesion (SSLA). The SEM results in Figure 3-A confirmed that, after modification, no observable change of hierarchical structure occurred. The widths and depths of the microstructures were 8-15 µm. Nanoflakes with a thickness of 50 nm and length and depth of 1-2 µm migrated into clusters, covering the entire iron surface. Figure 3-B exhibits the superb and uniform superhydrophobicity of the prepared low-adhesion sample. Water was found to maintain a shirked aspheric shape on the surface. When water was dropped within the ditches of the iron sample, it rapidly rolled away along the groove without any residual (Figure 3-C). The CA value in Figure 3-D determined was 161.2±1°. The CAH of 2.1° in Figure 3-E confirmed its negligible adhesion. A series of pictures of the downward and upward movement of a water droplet on the sample shown in Figure 3-F suggested a negligible adhesion that hardly retarded the motion of the water droplet or withheld any water on the surface. High-speed photographs shown in Figure 3-G confirmed the bouncing behavior of water droplets dropped on SSLA.

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A typical superhydrophobic surface with high adhesion (SSHA) is exhibited in Figure 4. The SEM photo in Figure 4-A shows that the nanoscale pattern of the prepared sample had no observable change relative to SSLA, but its microscale pattern significantly changed to a relatively flat and shallow structure with widths and depths of approximately 1 µm. The sample with a large area shown in Figure 4-B exhibited uniform superhydrophobicity. However, water tightly clung to the inverted sample shown in Figure 4-C, thereby confirming the strong adhesion. The CA value determined from Figure 4-D was 158.3±1°. The measured CAH in Figure 4-E was 91.0°, confirming its high adhesion. In Figure 4-F, the high adhesion was reaffirmed since the water droplets adhered to the surface at arbitrary angles despite their distorted shapes due to gravity. Figure 4-G exhibits hanging water droplets with gradually increasing volume, confirming the high surface adhesion. Furthermore, the maximum adhesion force can be estimated as 195.8 µN with this test. A high-speed video taken of a water droplet dropped vertically (Figure 4-H) on such a surface exhibited a notable stopping effect of SSHA upon impact.

3.3 Adaptability of the prepared superhydrophobic iron surface The adaptability of the prepared SSLA and SSHA was investigated with respect to water pH levels, which was adjusted by adding H2SO4 or NaOH. The results, shown in Figure 5-A, confirmed that the pH level had almost no impact on the water wettability or adhesion behavior of the SSLA. However, the SSHA revealed different results. When the solution had pH values of 2 and 1, the CA value changed to 152±1° 15



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and 136±2°, respectively. The RA value also lowered from 180° to 85° and 33°, respectively. It was surmised that the strong acidity destroyed the microstructure since the SSHA had no air film in its microscale pattern to prevent direct contact between the acid and the surface. The temperature adaptability of the prepared sample was also assessed for a temperature range of -5°C to 200°C. Glycerol, another liquid with a repellent effect on superhydrophobic surfaces, was used to determine the wettability on the sample for avoiding freezing and vaporization of water. The results, shown in Figure 5-B, confirmed that SSLA maintained its superhydrophobicity from 157°C to 164° over the entire temperature range. However, the RA sharply decreased from 18° to 5° when the temperature increased from -5°C to 30°C, and gradually decreased to 3° over the subsequent temperature range. This interesting scenario ascribed to the increasing viscosity of glycerol under relatively low temperatures. On the contrary, for SSHA, the CA increased from 150° at -5°C to 155° over 15°C through 45°C and back down to lower than 150° at 70°C. The RA value maintained an angle of 180° from -5°C through 65°C, and sharply decreased to 90° at 70°C. Such a sudden change was interesting, and could be explained by the transformation model from the Wenzel state to the Cassie state under high temperatures due to the establishment of a vapor film in the hierarchical structure by the thermal expansion of the impregnating liquid, thereby decreasing the effective contact rough structure.

3.4 Stability and mechanical durability 16


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Service life is an important performance parameter of superhydrophobic surfaces for industrial applications. In this study, the stability of the prepared samples exposed to air and NaCl solution was determined. When exposed to outdoor air, the SSLA maintained its superhydrophobicity up to 120 days as shown in Figure 5-C, and after washing with ethanol its recovered CA value exceeded 150°. The SSHA lost its superhydrophobicity after 80 days due to its high affinity for contamination, but also easily recovered its superhydrophobic state after ethanol-washing. The adhesion tendencies were found to be similar to those of wettability. No significant changes of the exposed surface in the SEM graphs were observed. However, when samples were immersed in 1 M NaCl solution, the results were different. As shown in Figure 5-D, the SSLA maintained its superhydrophobicity for 5 days. Conversely, the SSHA displayed relatively worse stability with stable time of 3 days. The changes in adhesion were consistent with these results. SEM pictures of the treated sample confirmed that corrosion of nanostructure resulted in quick irreversible performance decay. These samples also exhibited excellent stability in pure water and a 1M NaOH solution for over 30 days; however, in a 0.1M HCl solution, their superhydrophobicity lasted less than 5 min due to strong corrosion (Figures S3 to S5). The different anti-corrosion performance of SSLA and SSHA should ascribe to the different microstructure for holding air, which can effectively isolate corrosive aqueous liquid from metal surface, and slow down the corrosion rate.

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The anti-abrasion property was assessed via two typical methods: adhesive tape and sand abrasion. When subjected to over 1,000 repeated tests of pasting and tearing polyacrylate adhesive tape, the superhydrophobicity and adhesion of the samples remained almost unchanged. When 2 N of vertical pressure was applied, the paste-wear limits of SSLA and SSHA decreased to 11 and 4 as shown in Figure 5-E, respectively. The RA value of SSLA sample increased dramatically with increasing paste-wear cycles, and finally reached 180° after losing its superhydrophobicity. The SEM results confirmed that residual adhesive and torn nanoscale structures resulted in a loss of superhydrophobicity and enhanced adhesion. Figure 5-F exhibits the change in superhydrophobicity and adhesion with a scouring of dropped sand. The results confirmed that SSLA tolerated the erosion of more than 0.9 kg of sand dropped from a height of 10 cm, whereas the SSHA only withstood 0.1 kg of sand erosion. Obvious damage to the surface structure was observed in the SEM graphs. The third ultrasonic method also proved the strong wearability of the SSLA and SSHA, which withstood 16 h and 8 h, respectively, of 50 W ultrasonic percussion without significant performance decay (Figure S6). All the results proved the superb mechanic wearability of the superhydrophobic iron surface, which originated from the high wear-resistance of metal oxides layer formed in situ on the metal surface.

3.5 Applicability

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Since the prepared SSLA and SSHA were verified to possess excellent adaptability, stability, and durability, it was necessary to corroborate its applicability for various relevant applications. All attempts proved to be successful, as shown in Figure 6. The self-cleaning test results in Figure 6-A showed that SSLA was able to successfully self-clean, whereas SSHA failed because the water droplets stopped on the dirty surface. The SSLA and SSHA were used for vapor condensation and icing, with untreated pure iron as a reference. The results of the samples are shown side-by-side in Figure 6-B. The dropwise condensation and anti-icing properties of the superhydrophobic surface were confirmed by the fact that the condensed water droplets and frozen ice grains were smaller and less prevalent on the SSLA, and were larger and more prevalent on the SSHA. The serial photos in Figure 6-C exhibit the transportation of a ca. 10 µL water droplet from the SSLA to the SSHA with a superhydrophobic surface with moderate adhesion (SSMA). This lossless delivery could potentially transport and sample water using very little energy. Figure 6-D depicts the detailed process of the precipitation reaction between an AgNO3 solution and a NaCl solution within a droplet on the SSHA. In the reaction, the initial transparent NaCl solution and the final AgCl suspension maintained their spherical shapes. The total precipitation process took place in these immobilized droplets without rolling or spreading. When two solution droplets were preplaced on the SSHA and dragged towards each other with a syringe until they met, the same reaction occurred, as shown in Figure 6-E. Replacing the syringe with a SSLA plate to convey liquid, the AgNO3 solution droplets quickly rolled and hit the preplaced 19



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NaCl solution droplets accurately (Figure 6-F). This phenomenon inspired the concept of designing an in situ batch or continuous microreactor. The buoyancy of an iron sheet also was reinforced by its superhydrophobicity. As shown in Figure 6-G, SSLA, pure iron, and SSHA, all floated on the water. However, their buoyancies determined via a weighting method were found to differ, 4.1709 N/cm2 of pure iron, 4.3463 N/cm2 of SSHA, and 5.1185 N/cm2 of SSLA.

3.6 Universality of the three-step method It is attractive and significative to extend the simple three step method to other engineering

metals

and

modifiers.

Therefore,

varied

modifiers

including

octadecanethiol, lard, oleic acid, wax, and sodium dodecyl sulfate (SDS), were utilized to conduct modification of hierarchical iron surface under the enlightenment of lotus leaf and rose petal (Figure S7). Other common engineering metals Al, Cu, Ti, and Ni, as well as typical alloys 316 stainless steel and tinplate were used as the substrates in this three-step method. Figure 7-A shows the superhydrophobicity of the iron surface modified with different substances. The results of FT-IR analysis confirmed successful loading of these different modifiers (Figures S8 to S12). All modified iron surfaces showed excellent superhydrophobicity with CAs greater than 155°. These results affirmed that modifier type has no effect on creating superhydrophobic surfaces as long as their surface free energy is low enough.

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Similarly, hierarchical metal oxide structures on Al, Cu, Ti, and Ni surfaces, as well as stainless steel and tinplate surfaces, were successfully developed following abrasion and calcination (Figures S13 to S18). Their fine hierarchical structures and corresponding wettability and tunable adhesion, resulting from modification with FAS-17, are exhibited in Figures 7-B to 7-E in sequence. Although the shape, size, and substance of the nanoscale patterns were different, they all demonstrated excellent superhydrophobicity and had similar microscale patterns. This fact confirmed that it was not the substrate material or the nanoscale pattern, but rather the microscale pattern of the hierarchical structure that was the critical factor required to achieve superhydrophobicity and controlled adhesion. At same time, these results confirmed that this simple method of preparing superhydrophobic surfaces has universal application.

3.7 Influence of microscale patterns Since the importance of the microscale pattern of the hierarchical structure was established,

it

was

necessary

to

identify

its

mechanism

of

action

on

superhydrophobicity and adhesion. Here, two parameters of abrasion, the mesh of the sandpaper and the times of abrasion, were changed to obtain different microscale patterns to perform a comprehensive investigation. The abrasion times was defined by the number of crisscross motions since abrasion was manually performed. The CA and RA of all obtained samples were determined and plotted as shown in Figures 8-A and 8-B, respectively. 21



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The CA trend shown in Figure 8-A confirmed that superhydrophobic status could be achieved for all sandpapers. When using the same sandpaper, the CA value increased with increasing abrasion. The maximum CA value, 165±1°, corresponded to the largest abrasion times with intermediate mesh sandpaper of 600 mesh. The corresponding particle size of sand was 10–14 µm, which was equivalent to the size of microscale patterns of the lotus leaf and rose petal. For too-large or too-small mesh sandpaper, more abrasion was needed to obtain superhydrophobic status. However, the trend for adhesion was different. It was observed that high adhesion only occurred during the initial stages of abrasion. The highest adhesion occurred during the initial stage of abrasion with the largest or the smallest mesh sandpaper. The lowest adhesion occurred during the initial stage of abrasion with 600 mesh sandpaper. As the abrasion times increased, the adhesion decreased significantly no matter what mesh sandpaper was employed. The sample with the smallest adhesion was obtained after 80 abrasion cycles with 600 mesh sandpaper. Similar trend of CAH and adhesion can be obtained as Figures S19 and S20. Therefore, it could be concluded that, during the initial stage of abrasion, moderate mesh sandpaper with sand particle sizes of 10–15 µm could easily fabricate a superhydrophobic surface with a relatively large CA and low adhesion. Oversized and undersized mesh sandpaper tended to prepare superhydrophobic surfaces with relatively small CA and high adhesion. However, with increased abrasion times, superhydrophobic surfaces with large CA and low adhesion could be obtained.

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Fractal and AFM analysis were conducted to reveal the change in microscale patterns and determine its effect on superhydrophobicity and adhesion. The correlation results are shown in Figure 8-C. Almost all values correlated by fractal analysis were observed near the diagonal line, which implied negligible deviation. The maximum relative deviation was only 6%, confirming the accuracy of fractal theory for explaining superhydrophobicity. If the nanoscale patterns obtained by the three-step method were similar, then the wettability mainly depended on the microscale patterns. The relationship between the adhesion and microscale patterns was also investigated. An example of the statistic procedure used in the analysis is shown in Figure S21. Figure 8-D exhibits the space widths and heights of all samples determined by AFM. It can be noted that all SSHA were observed on the lower portion below the H/W of 0.4. The AFM graph confirmed the existence of shallow grooves and flat morphology. For SSLA, all points were observed on the upper portion with a height-to-width ratio of 0.7. The samples with moderate adhesion were distributed in the area between ratios of 0.4 and 0.7. From these results, it could be concluded that lower height-to-width ratios resulted in higher adhesion, and larger height-to-width ratios resulted in lower adhesion.

3.8 Influence of nanoscale patterns Since the nanoscale patterns generated in the thermal-treating step were assumed to be similar, it is necessary to confirm this assumption. Samples prepared by 23



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undergoing 60 abrasion cycles with 600 mesh sandpaper to yield a typical SSLA were selected as the research object. Two operation parameters of the calcination step, temperature and time, were varied to investigate their influence on nanostructure and the corresponding superhydrophobicity and adhesion. Figure 9-A exhibits the superhydrophobicity and adhesion trends as functions of calcination temperature after 2 hour calcination. The CA value was observed to increase significantly with increasing calcination temperature. The abraded iron surface transformed to superhydrophobic status under a temperature of 300°C. However, the RA displayed different trends. When the calcination temperature was lower than 300°C, the treated iron surface demonstrated strong adhesion. With increasing calcination temperature, the RA dramatically decreased to 2° under and over 400°C. Further increasing the temperature to 600°C caused the Fe3O4 layer to detach from the iron substrate surface. Serial SEM photos in Figure 9-B exhibit the change in nanoscale patterns with respect to temperature. Under 200°C, only abrasion scratches were observed without any nanostructure, and the CA was only 136±1°. Under 300°C, sparse Fe2O3 nanoneedles (Figure S22) were generated, resulting in superhydrophobicity with a CA value of 156±1°. Further increasing the calcination temperature to 400°C, these nanostructures converted into Fe3O4 nanoflakes, larger in size and more densely distributed, increasing the CA value to 160±1°. Further increasing temperature to 450°C, the nanoflakes grew even larger with a CA value of 161±1°. The generation of

24


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this nanostructure provided high roughness for superhydrophobicity and enough air for low adhesion. The effect of calcination time on wettability and adhesion was relatively obvious. As shown in Figure 9-C and 9-D, with extension of thermal-treating time, more and more Fe3O4 nanoflake structures were generated (Figure S23). Consequently, the CA value increased from 148° at 0 h to 161° at 2 h, and maintained a value of 161° in the following stage. In the same period, the RA decreased from 180° at 0 h to 2° at 2 h. Thus, it could be concluded that the generated nanostructure during the calcination step also effected the superhydrophobicity and adhesion significantly. However, since the required nanoscale pattern has been generated after only 2 h of calcination at 400°C, further increasing the calcination temperature and time did not cause significant changes. The microscale patterns of the hierarchical structure possessed better operating sensitivity than the nanoscale patterns to produce a superhydrophobic surface and specific adhesion in this case.

3.9 Mechanism of superhydrophobicity and adhesion tuning As the results show, superhydrophobicity of the prepared iron sample was bestowed by the synergy of the microscale and nanoscale pattern of the hierarchical structure. However, the adhesion tuning highly depended on the design and fabrication of the microscale pattern. The working mechanism of the hierarchical structure obtained by the three-step method for superhydrophobicity and adhesion is depicted by Figure 10. 25



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Van der Waals forces between the water and surface are regarded as the main source of the adhesion. Since the nanoscale patterns of different samples were consistent, it is reasonable to assume that the van der Waals force per unit area at micro-level dimension is constant. With the change in the height-to-width ratio of the microscale patterns, H/W, the different abraded surfaces exhibit a large or small surface area. Here, the capillary effect is not considered since all microscale cross-shaped grooves are interconnected. Water invades the microscale patters due to gravity, although surfaces with low surface energy modifiers are hydrophobic, and its penetrating depth only depends on the pattern width and water mass. The contacting area size between the water and surface determines the adhesion behavior of the droplets. Thus, in case A, the microscale pattern of the SSHA is relatively shallow due to low H/W. This means that water can maintain contact with all surfaces of microsized grooves including the bottom and sidewalls to form a continuous triple-phase contact line, expelling air, and exhibiting high adhesion52, 53. If the contact area is large enough, the van der Waals force generated between the water and micro-level surface can overcome the gravity of the water droplets, G, and cause them to hang upside down. This case is similar to the rose petal, and can be described as the Wenzel state. However, if adequate contact area is not provided with a moderate H/W ratio, the resultant force of van der Waals force F cannot overcome the gravity caused by water’s weight. Now the scene becomes case B, SSMA, where water droplets can adhere to the surface and roll down the surface with a specific inclination angle. At 26


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this time, the resultant force F is equal to the component force of gravity G'. Moderate adhesion can be observed. This wetting status can be considered as the transitional state between the Wenzel and Cassie states. On further increasing the H/W ratio, the microscale pattern assumes a deep shape. Water cannot invade the total space completely. Too-small contact area results in insufficient adhesion. At this moment, water can roll away freely without considerable resistance. This SSLA is representative of case C, which is similar to the lotus leaf. Since the space of the nanoscale patterns is not filled by water, the air is restrained, causing the formation of a discontinuous triple-phase contact line52, 53. Thus, a Cassie state is formed to obtain superhydrophobicity without considerable adhesion. Now, all the characteristics and preparation parameters of SSHA, SSMA and SSLA are summarized for purposes of comparison in Table 1. It can be concluded that the parameter, H/W, can be used as the criteria to judge the adhesion type of a superhydrophobic surface, and used to design a superhydrophobic surface with specific adhesion.

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4. Conclusion In conclusion, the present work demonstrated an easy, clean, and inexpensive method to construct a hierarchical structure for obtaining a stable and durable superhydrophobic surface with tunable adhesion based on the bioinspiration of the lotus leaf and rose petal. By carefully adjusting the abrasion program, the adhesion of a superhydrophobic surface on an engineering metal can be controlled. To achieve this, a hierarchical structure was built via abrasion to obtain a microscale pattern with the appropriate height-to-width ratio, which was used as substrate to grow nanoscale patterns in the calcination step, to produce the required roughness and contact area. This universal method can be applied in fabrication of superhydrophobic surfaces regardless of which engineering metal or modifier is employed. This method is suitable for large-scale production and is promising for diverse, attractive, and useful applications in various industries and daily life.

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Supporting Information The Supporting Information gives the XRD patterns of original untreated iron, abraded iron, directly calcined iron surface, and two-step treated iron surface after calcination and abrasion (Figure S1); FT-IR analysis of modified iorn surface with FAS-17 (Figure S2); Wettability and adhesion of SSLA and SSHA immersed in pure water, in NaOH (1M) solution, in HCl (0.1M) solution, and with ultrasonic method (Figure S3-S6); FT-IR analysis of lotus leaf and rose petal; FT-IR analysis of modified iorn surface with lard, with octadecanethiol, with oleric acid, with paraffin wax, and with sodium dodecyl sulfate (Figure S7-S12); XRD pattern of fresh and calcined Al, Ni, Ti, Cu, 304 stainless steel, and tinplate foil and FT-IR analysis of modified samples with FAS-17 (Figure S13-S18); CAH change with respect to abrasion program (Figure S19); Determined adhension force change with respect to abrasion

program

(Figure

S20);

AFM

result

of

a

typical

as-prepared

superhydrophobic surface and its height and width of a crosssection (Figure S21); XRD patterns of calcined iron foil under different temperature and at different time under 400 °C (Figure S22-S23); The schematic diagram of the vaprotion method for loading wax and lard (Program 1); The calculation program of fractal dimension edited with MATLAB (Program 2). All of Thess materials are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements

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We appreciated the financial support from the National Natural Science Foundation of China (No. 21476146).

Author contributions W.J wrote the paper, designed the work and directed the experiments; M.M prepared the samples and carried out the experiments; W.Q assisted the experiments; Y.Z. co-supervised the project; B.L contributed the manuscript revision; and all authors discussed the results and commented on the manuscript.

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Figures

Figure 1. Superhydrophobicity and Natural hierarchical structure of lotus leaf

and rose petal. (a) Photo of lotus leaf after the rain. (b) of rose petal. (c) the CA of lotus leaf. (d) of rose petal. (e) the RA of lotus leaf. (f) of rose petal. (g) ESEM of lotus leaf. (h) of rose petal. (i) AFM and fractal analysis of lotus leaf. (j) of rose petal.

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Figure 2. Construction of hierarchical structure on iron surface. (a) original untreated iron surface and its wettability. (b) crisscrossed abraded iron surface and its hydrophilicity. (c) directly calcined iron surface and its hydrophilicity. (d) two-step treated iron surface and its superhydrophilicity.

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Figure 3. Superhydrophobic iron surface with low adhesion. (a) Microscale pattern and nanoscale pattern of as-prepared SSLA. (b) uniform large-area SSLA. (c) grooves made of SSLA for water transportation. (d) CA. (e) CAH. (f) up-and-down motion of a water droplet on SSLA. (g) bouncing behavior of water droplets on SSLA.

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Figure 4. Superhydrophobic iron surface with high adhesion. (a) Microscale pattern and nanoscale pattern of as-prepared SSHA. (b) uniform large-area SSHA. (c) water droplet upside down on an inverted SSHA. (d) CA. (e) CAH. (f) hung water droplet on SSHA with arbitrary angle. (g) water hung upside down with increasing volume. (h) sticking effect of water droplets on SSLA.

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Figure 5. Adaptability, stability and durability of SSLA and SSHA. (a) wettability and adhesion of SSLA and SSHA with respect to pH value. (b) to surface temperature. (c) to time after exposing in air. (d) to time after immersing in NaCl solution. (e) to paste-wear times with adhesive tape method. (f) to sand mass with sand abrasion method.

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Figure 6. Application of as-prepared SSLA and SSHA. (a) self-cleaning of SSLA and water adhesion on SSHA. (b) vapor condensation and water icing on different samples. (c) no loss water transportation from SSLA to SSHA. (d) precipitation reaction happened in droplet microreactor on SSHA. (e) dragging water with a syringe needle to trigger the reaction on SSHA. (f) conveying water with a SSLA to trigger the reaction on SSHA. (g) floating of different samples on water surface and the buoyancy test.

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Figure 7. Universality of the three-step method. (a) as-prepared superhydrophobic iron surface with five different modifiers. (b) microscale pattern, nanoscale pattern and CA of as-prepared superhydrophobic Al surface. (c) Ni surface. (d) Ti surface. (e) Cu surface. (f) 316 stainless steel surface. (g) tinplate surface.

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Figure 8. Effect of microscale patterns resulted from abrasion program. (a) wettability change with respect to abrasion program. (b) adhesion change with respect to abrasion program. (c) CA correlation with fractal analysis. (d) influence of microscale pattern height-to-width ratio on adhesion.

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Figure 9. Effect of nanoscale patterns resulted from thermal-treating program. (a) wettability and adhesion with respect to calcination temperature. (b) nanoscale pattern and CA under different calcination temperature. (c) wettability and adhesion with respect to calcination time. (d) nanoscale pattern and CA under different calcination time.

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Figure 10. Mechanism analysis of microscale pattern on wettability and adhesion. (a) low H/W ratio microscale pattern of SSHA. (b) moderate H/W ratio microscale pattern of superhydrophobic surface with moderate adhesion. (c) high H/W ratio microscale pattern of SSLA.

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Tables Table 1. Summary of three-step method for preparing SSHA, SSMA and SSLA Type

Wettability

Adhesion

H/W ratio

SSHA

CA

Biomimetic Superhydrophobic Engineering ... - ACS Publications

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