Surfaces with Sustainable Superhydrophobicity upon Mechanical

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Surfaces with Sustainable Superhydrophobicity upon Mechanical Abrasion Xue Bai, Chao-Hua Xue, and Shuntian Jia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08672 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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ACS Applied Materials & Interfaces

Surfaces with Sustainable Superhydrophobicity upon Mechanical Abrasion Xue Bai,† Chao-Hua Xue,*, †, ‡ Shun-Tian Jia† †

College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an

710021, China ‡

College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and

Technology, Xi’ an 710021, China

ABSTRACT: Surfaces with sustainable superhydrophobicity have drawn much attention in recent years for improved durability in practical applications. In this study, hollow mesoporous silica nanoparticles (HMSNs) were prepared and used as reservoirs to load dodecyltrimethoxysilane (DDTMS). Then superhydrophobic surfaces were fabricated by spraying coating of HMSNs with DDTMS as particle stacking structure and polydimethylsiloxane (PDMS) as hydrophobic interconnection. The mechanical durability of the obtained superhydrophobic surface was evaluated by a cyclic sand abrasion. It was found that once the surface was mechanically damaged, new roughening structures made of the cavity of the HMSNs would expose and maintain suitable hierarchical roughness surrounded by PDMS and DDTMS, favouring sustainable superhydrphobicity of the coating. The surfaces could sustain superhydrophobicity even after 1000 cycles of sand abrasion. This facile strategy may pave the way to the development of robust superhydrophobic surfaces in practical applications. KEYWORDS: superhydrophobic, surface, sustainable, mechanical abrasion, HMSNs 1

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1. INTRODUCTION Nature is an endless source of inspiration for the fabrication of novel functional materials.1 Inspired by the extreme water-repellency and self-cleaning property of the lotus leaf in the natural world, artificial superhydrophobic surfaces, which display an apparent water contact angle greater than 150°, and little sticking to water droplets, have aroused great attention not only because of its scientific background but also for wide range of technological applications including self-cleaning,2-6 anti-fouling,7-9 anti-icing,10-12 and oil-water separation.13-15 After decades of study, it has been revealed that the interplay of proper surface roughness and low-surface-energy materials can give rise to superhydrophobic surfaces.16-18 Nevertheless, the major obstacle to the deployment of the artificial superhydrophobic surfaces in practical applications is their susceptibility to mechanical abrasion.19-22 On one hand, mechanical abrasion on the superhydrophobic surfaces could destroy the microscopic rough structures that are essential for superhydrophobicity.23-25 On the other hand, mechanical abrasion can also remove most of the hydrophobic surface layer on the superhydrophobic surfaces, thus leading to a decline in their non-wetting property.26-27 For the above-mentioned reasons, sustaining the superhydrophobicity upon abrasion is of significance for the wide application of such surfaces. Maintaining low-surface-energy material and hierarchical micro/nanoscale roughness upon mechanical abrasion is one of the important approaches to tackle the mechanical durability of the surperhydrophobic property of surfaces.28-34 And a few of works have been reported in recent years. For example, Jin et al.35 prepared 2


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superomniphobic fluorinated silica aerogels and tested the mechanical stability of the surfaces by abrasion with sandpaper. The superomniphobicity of the materials could maintain even after removal of the uppermost layer upon mechanical abrasion. The reason for that was the nanoporous self-similar aerogel structure that essentially maintained the desired topography even upon abrasion, and in addition by deposition of fluorinated surfactant in the interior of the aerogel that self-recovered the damaged sites of the surface. Followed by this work, Zhang et al.36 developed a novel damage-tolerant superhydrophobic and superoleophilic bulk material which was made from TiO2 nanorods, hydrophobic SiO2 nanoparticles, polypropylene (PP) and a small amount of poly(dimethylsiloxane) (PDMS) that acted as a binder to improve the adhesion of the inorganic TiO2 and SiO2 particle fillers to the PP matrix. The as-fabricated nanocomposite bulk materials could sustain the rough surface textures after mechanical abrasion or oil fouling, thus ensuring continued superhydrophobicity. Inspired by these works, here we present a facile and simple method to fabricate surfaces with sustainable superhydrophobicity upon mechanical abrasion. Firstly, hollow mesoporous silica nanoparticles (HMSNs) were prepared by a sacrificial hard template

of

polystyrene

particles,

and

used

as

reservoirs

to

load

dodecyltrimethoxysilane (DDTMS). Then superhydrophobic surfaces were prepared by successively spraying coating of HMSNs loaded with DDTMS as particle stacking structure and PDMS as hydrophobic interconnection, as shown in Figure 1. The surfaces can sustain superhydrophobicity even after 1000 cycles of sand abrasion. It is expected that this simple and effective coating strategy will open a new avenue to 3



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improve the mechanical durability of superhydrophobic surfaces for practical applications.

Figure 1. Schematic illustration for fabrication of the HMSNs-based superhydrophobic coating.

2. EXPERIMENTAL SECTION 2.1. Materials Styrene (S) was purchased from Tianli Chemical Reagent Co., Ltd. Tetraethoxysilane

(TEOS),

cetyltrimethylammonium

bromide

(CTAB),

and

polyvinylpyrrolidone (PVP) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. 2,2’-Azobis (2-methylpropionamidine) dihydrochloride (AIBA) was purchased from Qingdao Kexin New Materials Science and Technology Co., Ltd. Dodecyltrimethoxysilane (DDTMS, 95%) was purchased from Jingzhou Jianghan Fine Chemical Co., Ltd. Polydimethylsiloxane (PDMS, Sylgard 184 Silicone Elastomer Kit with components of PDMS base and curing agent) was purchased from Dow Corning. Diglycidyl ether of bisphenol A (DGEBA, trade name as E-44) was purchased from FeiCheng DeYuan Chemicals Co., Ltd. Polyimide resin was purchased from ZhongDe Chemicals Co., Ltd. Tetrahydrofuran (THF), ethanol, 4


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ammonium hydroxide (NH4OH, 25% by weight) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Distilled water was commercially obtained. Glass slides (25.4×76.2 mm, Sail Brand, China) were used as the main substrates for spray coating. All chemicals were used as received without further purification. 2.2. Synthesis of polystyrene (PS) Spheres Monodisperse PS spheres were prepared with minor modification according to our previous reported work.37 In a typical process, 5 g of S, 1 g of PVP, and 90 mL of deionized water were charged into a 250 mL three-necked flask with stirring at 120 rpm (revolutions per minute) for about 10 min at room temperature. Then, 0.4 g of AIBA dissolved in 10 mL deionized water was added to the above mixture. After that, the mixture was degassed under nitrogen purge for about 20 min and then the temperature was increased to 75℃ to start the polymerization process. The reaction was allowed to process for another 24 h under a constant stirring rate of 120 rpm to complete the polymerization. 2.3. Fabrication of Hollow Mesoporous Silica Nanoparticles (HMSNs) The PS spheres were used as template to fabricate HMSNs. In a typical procedure, 40 g of PS emulsion, 1 g of CTAB, 60 mL deionized water and 40 mL ethanol were charged into a 250 mL three-necked flask with stirring at 250 rpm for about 30 min at 40℃. Then 3 mL NH4OH was added to the above mixture. After that, 3.5 g TEOS was added dropwise to the suspension at a volume rate of 0.1248 mL/min using a peristaltic pump. The reaction mixture was stirred at 40℃ for an additional 6 h after dripping. The obtained PS/SiO2 core/shell composite particles were separated 5



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from the reaction medium by centrifuging, washed 2 times with copious deionized water and ethanol, respectively. Finally, the HMSNs were obtained by removing PS core through calcination in air at 550℃ for 2 h. 2.4. Fabrication of Superhydrophobic Surfaces A 0.40 g of HMSNs were added to 40 g of ethanol and sheared at a rate of 1.2 r/min using a high-shear emulsifying machine for 30 s to make homogeneous particle dispersion. Then 2.0 g of DDTMS was added to the above dispersion, and stirred for 24 h under a vacuum condition at room temperature to allow the DDTMS molecules to diffuse into the cavity of the HMSNs. After which it was set in equilibrium at atmosphere for another 24 h under magnetic stirring to make sure that DDTMS reacted thoroughly with inner and outer walls of the HMSNs. The as-formed DDTMS/HMSNs mixture dispersion was ready for coating on glass. A base coat solution of epoxy resin was prepared by dissolving 5 g of DGEBA and 1.7 g polyimide resin into 50 g of THF. And a PDMS solution was prepared by dissolving 0.9 g of PDMS and 0.09 g of curing agent into 29.010 g of THF. Prior to coating, all glass slides were thoroughly cleaned with ethanol, followed by 5 min oxygen plasma etch at 40 W to further clean and hydroxylate the surface. After that, glass substrates (25.4×76.2 mm) were first spray-coated with 2 mL base coat solution of epoxy resin, and cured at 80℃ for about 1 min. Then the epoxy coated substrates were spray-coated with 10 mL of DDTMS/HMSNs mixture dispersion, and successively spray-coated with 3 mL of PDMS solution to form a connection bridge between the HMSNs particles. Finally, the above-fabricated 6


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coatings were cured at 135℃ for 30 min to obtain stable superhydrophobic surfaces. 2.5. Characterization Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 field emission scanning electron microscope operated at an acceleration voltage of 3 kV. Samples were sputter-coated with gold prior to examination. Transmission electron microscopy (TEM) images were obtained by using a FEI Tecnai G2 F20 S-TWIN instrument operated at 200 kV. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, FEI-Tecnai G2 F30 S-TWIN) were used to capture the STEM image and EDX mapping of the sample, which was operated at an acceleration voltage of 300 kV. Samples were prepared by placing a few drops of the dispersion onto a carbon coated copper microgrid and air dried. The N2 adsorption-desorption isotherms were obtained using an ASAP 2460 surface area and porosity analyzer by static adsorption procedures at 77 K. The specific surface areas of the samples were calculated by applying the Brunauer-Emmett-Teller (BET) equation. The pore size and pore volume were calculated from the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model. Thermogravimetric analysis (TGA) was performed with a Q500 (TA Instruments, America) instrument under a stream of nitrogen, and the sample was heated at a rate of 10 °C/min over a temperature range from 30°C to 600°C. Optical profilometer images were taken on a commercial instrument (Contour GT-X3, Bruker). Water contact angles (CA) of the coating were measured with a deionized water droplet of 5 µL on a video optical contact angle system (OCA 20, Dataphysics, Germany) at room temperature, and 7



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images were captured with a digital camera. Drop size of 10 uL water droplet was used to measure the sliding angle (SA) of the surface. The reported values of CA and SA were determined by averaging values measured at five different points on each sample surface. The abrasion resistance of the coating was tested by the cyclic sand-abrasion. And each cycle includes a downward followed by an upward thrust motion of the coating in the sand. The diameter of the sand used here is about 200-1000 µm. Sand impingement test was also performed with minor modification according to our previous reported work.37 Commercial sand (210 g) with a diameter of 200 µm-1000 µm was continuously used to impact the coating from a falling height of 40 cm, while the substrates was held at 45° to the horizontal surface. The duration of the sand flow is about 8 min. 3. RESULTS AND DISSCUSSION 3.1. Characterization of HMSNs In order to synthesize HMSNs using PS particles as sacrificial hard templates for silica coating, PS spheres were firstly fabricated through emulsion polymerization with PVP as stabilizer and AIBA as initiator. Figure 2(a) and (b) show the SEM and TEM images of the obtained PS particles. The spheres are monodisperse with very smooth surface and an average diameter of approximately 350 nm. Then, the hydrolysis-condensation of TEOS on the template of PS particles under alkaline condition was performed in the presence of CTAB surfactant as mesopore directing agent. During the process of reaction, CTAB could preferentially adsorb onto the 8


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surface of PS, thus forming a positively charged layer on PS particles. In this context, negatively charged silica species would be easier to coat on the surfaces of PS through electrostatic attraction.38 This phenomenon was similar to some previous reported works39-41 that the interaction between polymer and silica species is pre-requisite to the nanocomposite formation. As displayed in Figure 2(c) and (d), after coating with TEOS, the diameter of the spheres increased to about 425 nm with a good particle uniformity and the surface became a little rougher than that of PS spheres. Finally, HMSNs were obtained by removing CTAB and PS template through calcination at 550℃ for 2 h, as shown in Figure 2(e) and (f). The diameter of the HMSNs spheres decreased approximately to 375 nm and the cavity can be clearly observed with wall thickness of about 30 nm. Figure 3 displays the nitrogen adsorption-desorption isotherms and pore size distribution of HMSNs. The isotherms of HMSNs show a typical IV adsorption behavior and exhibit steep capillary condensation step which appeared at relative pressure ranges of 0.95-1, reflecting the presence of uniform channel-like mesopores with a mean diameter of 3.5 nm. The BET specific surface area and pore volume are calculated to be 1515 m2/g and 0.838 cm3/g, suggesting that HMSNs can provide sufficient volume for accommodating hydrophobic molecules.

9



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Figure 2. SEM images of (a) PS, (c) PS/SiO2, and (e) HMSNs; TEM images of (b) PS, (d) PS/SiO2, and (f) HMSNs.

Figure 3. Nitrogen adsorption-desorption isotherms and pore size distribution of HMSNs. 10


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Figure 4. (a) HAADF-STEM of DDTMS/HMSNs; distribution of elements (b) C, (c) O, and (d) Si; (e) EDX spectrum of (a).

The incorporation of the DDTMS inside the framework of HMSNs was confirmed by HAADF-STEM imaging and EDX spectrometry analysis,42-43 as shown in Figure 4. It can be clearly seen that the morphology of the HMSNs is almost unchanged after loading of DDTMS (Figure 4a). As shown in Figure 4(b) and Figure 4(e), the corresponding elemental mapping images and EDX spectrum indicate that the C elements appeared after DDTMS loading, which is attributed to the carbon 11



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chain on DDTMS. At the same time, the distribution of the C elements in the particle center and the wall is a little uniform than O and Si elements, suggesting that some DDTMS molecules may have been incorporated into the HMSNs (Figure 4b, 4c and 4d). TGA results revealed that the loading amount of DDTMS was calculated to be about 31.69% on the weight of DDTMS/HMSNs according to Figure S1. To further prove this loading concept, (3-aminopropyl)triethoxysilane with N element was also encapsulated with the same procedure, and the HAADF-STEM and elemental mapping images were characterized (Figure S2). The similar distribution was also found in N elements, thus indicating the effectiveness of this loading approach of hydrophobic silane agents. 3.2. Wettability of the DDTMS/HMSNs-based surfaces coated with PDMS Superhydrophobic surfaces were fabricated by a continuous spraying process, as depicted in Figure 1. In order to improve the adhesive force between particle stacking coating and the substrate, the epoxy resin was firstly spray-coated onto the substrates before DDTMS/HMSNs spraying. Epoxy resin is one of the important thermosetting plastics. The existence of polar hydroxyl and ether bond in the molecular chain of epoxy resin has high adhesion to all kinds of substances.44 After that, PDMS was then sprayed above the DDTMS/HMSNs coating. PDMS solution would penetrate into the spaces between particles and acted as an interconnection between them as well as low surface materials to hydrophobize the coating. As shown in Figure 5(a), water drops would spread on the pristine glass slide. In contrast, the glass after hydrophobically modification has a CA of 164.7±1.5° and SA of 1.5±0.5°, making water droplets 12


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spherical and easily roll off, as shown in Figure 5(b). Meanwhile, when immersing the coating into water, an obvious bright plastron layer could be clearly observed due to the total reflectance of light at the air layer trapped at the surface (Figure 5c). This trapped air can effectively prevent the surface from wetting by water. Thus, the phenomenon indicated that the

coating possessed

typical Cassie45 mode

superhydrophobicity. And because of this, a jet of dye water can also bounce off the coating without leaving a trace, as shown in Figure 5(d).

Figure 5. The picture of water drops on (a) untreated glass slide and (b) as-obtained coating; Images of the coating (c) in water, (d) with a jet of dye water bouncing off.

3.3. Sustainability of the superhydrophobic surfaces In practical applications, superhydrophobic coatings are unavoidably rubbed or scratched by sand in the wind, which would lead to destruction of the surface robustness. Thus, it is necessary to investigate the mechanical durability of the 13



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fabricated superhydrophobic coating. In this work, the mechanical stability of the superhydrophobic coating was evaluated by the cyclic sand-abrasion (see Figure 6a), which was adapted from Li et al.46 The sample was thrust into the sand and cyclically swept in the vertical direction within the container of 6 cm diameter. Each cycle includes a downward followed by an upward thrust motion of the coating in the sand. And the SEM image in Figure 6(b) shows that the sands used are irregular with diameters in millimeters which would cause damages to the coatings easily. Figure 6(c) shows the change in CAs and SAs as a function of abrasion cycles for the superhydrophobic surfaces. The results showed that the CAs and SAs of the coating have a little change with increasing cycles of sand abrasion. After initial 100 cycles of sand abrasion, the CA of the coating decreased to 162±2°, and the SA drastically increased to 6.1±0.7°. After that, the CA and SA of the coating changed little even when the sand abrasion cycles increased to 800, indicating excellent mechanical stability of the fabricated superhydrophobic surfaces. To explain this phenomenon, we propose the sustainable process of the superhydrophobicity as follows. With the help of vacuum treatment, DDTMS molecules can easily diffuse into the cavity of the HMSNs, thus the inside wall of HMSNs became hydrophobic after DDTMS loading. Meanwhile, the outside wall of the HMSNs is also hydrophobic due to the existence of DDTMS and PDMS. This combination of particle stacking roughness and lowsurface-energy material thus resulted in the original superhydrophobicity of the coating. Under the action of the abrasion, new roughening structures which caused by the cavity of the HMSNs would expose, thus maintaining suitable hierarchical 14


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roughness, favouring sustainable superhydrphobicity of the coating in combination with the low surface energy of DDTMS in the exposed walls. This concept that maintaining surface properties during removal of outermost material under multiple mechanical abrasion cycles was also realized by other researchers,33-34 although the obtained coatings possessed different microstructures due to different fabrication strategies. As for the as-obtained coating in this work, when continuously extending the cycles of sand abrasion to 1000, the CA of the coating obviously declined to 154.4±1.5° with the SA of 9.6±1°. From Figure 6(d), it can be clearly observed that some part of the coating has been worn off and the epoxy resin layer may become exposed, thus resulting in a little decrease in CAs and a little increase in SAs.

Figure 6. (a) Snapshot of the superhydrophobic coating thrust into the sand followed by an upward thrust motion of the coating in the vertical direction within the beaker. (b) SEM image of the sand used for the sand-abrasion test. (c) CA and SA change of superhydrophobic surfaces with abrasion cycles, (d) photograph of water drops on the coating after 1000 cycles of abrasion test. 15



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To further explore the sustainability of the superhydrophobicity upon cyclic sand abrasion, the morphology of the coating was also investigated. As illustrated in Figure 7(a) and (b), it can be found that the original coating is a homogeneous particle packing structures with PDMS as particle interconnection. And the PDMS also covered on the DDTMS/HMSNs coating, acting as the main hydrophobic material to lower the surface energy of the coating. While, after 200 cycles of sand abrasion, most of the particles were broken and the cavities have been exposed, as shown in Figure 7(d). In this case, the roughness of the coating was mainly provided by the cavities of the particles, and also low surface energy materials may come from the PDMS between particle intervals and DDTMS within the particles. Nevertheless, it can also be found that the coating is smoother at the micrometer scale (Figure 7c), indicating that the coating would have been abraded flatly during the initial abrasion process, thus the CAs decreased a little and the SAs increased to some extent compared with the original superhydrophobic surfaces. In contrast, when extending the sand-abrasion cycles to 600, the coating has been abraded more roughly (Figure 7e and f). As shown in Figure 7(e), chunks of aggregates have been formed on the coating. The possible reason for that is some particles of the coating has been ground into fragments during the severe abrasion cycles, thus the fragments may further aggregate into blocks with the increasing of abrasion cycles. In order to further prove this, the high magnification SEM image of the coating was taken (Figure 7f). It was found that the fragments of the particles indeed exist in the coating, and some intact and broken particles can also be simultaneously observed in the coating. In this context, the sustainable 16


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superhydrophobicity of coating may mainly come from the as-formed roughness structure and DDTMS. Based on this, increasing in sand abrasion cycles to 1000 would result in a smoother morphology (Figure 7g). However, higher magnification image showed that the surfaces were still rough enough to sustain the superhydrophobicity, as shown in Figure 7(h). This continuation of the low surface energy hydrophobic layer, when combined with the newly emerging rough surface textures, may give rise to the sustainability of the superhydrophobicity. Optical profilometer images are shown in Fig S3. After 200 cycles of abrasion, the roughness of the coating reduced from 16.107 µm to 12.902 µm, thus resulted in a little decrease of superhydrophobicity. While, when the abrasion cycles reached to 600, the roughness of the coating sharply increased to 15.143 µm due to covering with chunks of particle fragment aggregates. After that, increasing sand abrasion cycles may result in a gradual decrease in surface roughness, with Ra of 12.747 µm after 1000 cycles of abrasion, which led to the decrease of superhydrophobicity.

17



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Figure 7. SEM images of the superhydrophobic coating (a, b) before abrasion; (c, d) after 200 cyclic sand-abrasion; (e, f) after 600 cyclic sand-abrasion; (g, h) after 1000 cyclic sand-abrasion. 18


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The

sand

impingement

test

showed

that

the

surfaces

still

sustain

superhydrophobicity with CA of 159.8±2° and water drops can slide off easily with SA of 5±1° after 8 min of 210 g sand impingement. In addition, adhesive tape peeling test was conducted to evaluate the mechanical durability of the coatings. The double sided adhesive tape is pressed with approximately 12 kPa to the surfaces for about 1 min. After peeling the tape off, only a few of the coating shed. And the wettability of the remaining coating is almost unchanged with CA of 163.9±1°. However, increasing the cycles of adhesive tape peeling would result in a gradually falling of the coating, thus causing the decline of the superhydrophobicity. When letting running water impacts the coating for 30 min from a falling height of 15 cm, water drops can still easily drain off the surface (Movie S1). In the meantime, when the surface is cut off with a sharp object such as a knife, the cavities of the HMSNs in the cross section of the coating can still be observed, as shown in Figure 8(b) and (d). It can also be found that the thickness of the original superhydrophobic coating is about 370 µm (Figure 8a). However, after 1000 cycles of sand abrasion, the thickness of the coating decreased to approximately 270 µm, indicating that 100 µm of the coating has been removed during the abrasion process (Figure 8c). More importantly, the freshly exposed surface created by abrasion can still maintain superhydrophobicity. However, it can also be observed clearly that some part of the coating has been severely worn off, especially in the forefront side of the coating, as shown in Figure 6(d). Undoubtedly, further extending the abrasion cycles will result in loss of superhydrophobicity of the coating, thus indicating that the 19



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mechanical property of the superhydrophobic coating should be further improved in the future work.

Figure 8. Cross-sectional SEM images of (a) original superhydrophobic coating, (b) is the high magnification image of (a); (c) the coating after 1000 cycles of sand-abrasion, (d) is the high magnification image of (c).

4. CONCLUSIONS In summary, surfaces with sustainable superhydrophobicity were prepared by spray-coating of DDTMS/HMSNs and PDMS. The surfaces could maintain superhydrophobicity upon cycles of abrasion due to the fact that new roughening structures which caused by the cavity of the HMSNs would expose, thus maintaining suitable hierarchical roughness, favouring superhydrphobic property of the coating. And

also

no

expensive

fluorochemicals

are

required

20


to

realize

the

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superhydrophobicity, which may have great potentials in applications.

ASSOCIATED CONTENT Supporting Information HAADF-STEM

of

(3-aminopropyl)triethoxysilane/HMSNs,

TG

curves

of

DDTMS/HMSNs and optical profilometer images of the coating. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51372146, 51572161), Research Fund for the Doctoral Program of Higher Education of China (20136125110003), Key Scientific Research Group of Shaanxi province (2013KCT-08), and Scientific Research Group of Shaanxi University of Science and Technology (TD12-03).

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Surfaces with Sustainable Superhydrophobicity upon Mechanical

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