Enhancing the Robustness of Superhydrophobic Coatings via the

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Enhancing robustness of superhydrophobic coatings via addition of sulfide Zhen Xiao, Qiaoling Wang, Daozhou Yao, Xinquan Yu, and Youfa Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00690 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Enhancing robustness of superhydrophobic coatings via addition of sulfide Zhen Xiao, Qiaoling Wang, Daozhou Yao, Xinquan Yu, Youfa Zhang* Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China *corresponding author: E-mail: [email protected] (Y. Z.)

ABSTRACT Micro/nano hierarchical structures with special wettability impart a wide range spectrum of unique properties to the superhydrophobic surfaces that applicable in different potential fields. Therefore, it is necessary to develop advanced superhydrophobic materials with excellent wear resistance property. In this study, the PDMS-base robust superhydrophobic coatings, which used MoS2 or WS2 as solid lubricant, PDMS as binder and SiO2 as filler, were prepared on glass substrate by the one-step air spaying method. Lamellar MoS2 and WS2 with high crystallinity had intrinsic hydrophobic properties. The MoS2@SiO2-PDMS (MSP) and WS2@SiO2-PDMS (WSP) coatings with highly rough textures showed good water-repellent behavior with water contact angle of 167.8º and 166.2º, respectively. The results demonstrated that the addition of micro-sized MoS2 or WS2 could easily format micro/nano second-level hierarchical structures, thus realizing the superhydrophobic properties. The friction coefficient decreased gradually with the increasing of MoS2 or WS2. A 4:1 ratio of SiO2 to MoS2/WS2 could make the samples preserve the superhydrophobic properties even after 100 cycles on the abraser. As a result, the superhydrophobic coatings with excellent wear resistance will be good candidate for water-repellent surfaces to meet practical emerging needs in industry applications. KEYWORDS: Superhydrophobicity, Spray-coating, Wear-resistance, PDMS, Molybdenum disulfide 1

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INTRODUCTION Up to now, superhydrophobic coatings have attracted significant attention mainly due to their unique properties such as water-repelling1, self-cleaning2, anti-icing3,

4

and anti-corrosive

properties5, 6. Any material with high hydrophobic surface exhibits a contact angle for water greater than 150°, and a small sliding angle less than 10° can be assigned with the superhydrophobicity. Thus, highly intrinsic hydrophobic materials with hierarchical micro/nano binary structures in nature are highly warranted for superhydrophobic applications. Various strategies have been developed to fabricate the special surfaces, such as electro spinning technique7-9, layer-by-layer assembly10-12, electrochemical deposition13-15, spray-coating16-20, and sol-gel method21. The above methods basically include two strategies: i) constructing micro/nanoscale hierarchical texture and ii) making the prefabricated rough surface modified with low surface free energy materials. There are few practical products that apply this superhydrophobic surface until now, although many methods of making superhydrophobic surfaces have been frequently reported over the past few decades. This is primarily resulted from the fact that mechanical contact generated under actual environmental conditions tends to damage the surface textures resulting in dramatic loss of superhydrophobic properties22-24. As typical lamellar solid lubricants, molybdenum disulfide (MoS2) and Tungsten sulfide (WS2) have been used since the middle of last century. Unlike the well-known graphite, the strong covalent bonding within the S-Mo-S and S-W-S sandwich layers and the weak interlayer interaction endow MoS2 and WS2 with intrinsic excellent lubrication effectiveness25-28. More interestingly, the aged MoS2 sheets display a stabilized hydrophobicity when hydrocarbons in the air are adsorbed onto the clean surface29, 30. Therefore, the wear resistance of superhydrophobic coatings would be greatly improved if the lubricants were used in the application on interface materials. Unfortunately, 2


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there have been few studies on the preparation of durable superhydrophobic properties by adding solid lubricant still now. Tang et al31 fabricated a superhydrophobic coating via spraying the MoS2 particles hybrid in polyurethane (PU) composite suspension on various substrates. The feasibility of sulfide application in wear-resistant and hydrophobic materials was verified. Gao and Wang32 fabricated a superoleophilic but superhydrophobic sponge with the MoS2 hybridization for oil-water separation application. The hydrophobic property of MoS2 has been indirectly proved in the paper. Zhang et al33 prepared a multifunctional superhydrophobic coating with composited PES/POTS/MoS2-PDA-TiO2 via spraying method, in which the preparation was somewhat complicated. Obviously, these approaches are difficult to mass produce because of the expensive materials, preparation complexities or repuired strict conditions, which is limited in practical applications. Therefore, it is highly desirable to develop a simple and scalable method to prepare superhydrophobic surfaces with enhanced robustness. This study illustrates the creation of the superhydrophobic surfaces comprised of MoS2/WS2 as solid lubricant, PDMS as binder and SiO2 as filler, which were prepared by the one-step air spaying method on glass substrate via spraying the mixed emulsion. The MSP and WSP coatings with hierarchical structure shows superhydrophobic behavior with the high water contact angle of 167.8º and 166.2º, respectively. With good film-forming and hydrophobic properties, PDMS resin acts as bonding stuff which can providing hydrophobicity in the coating system. Adding hard particles to soft PDMS resin is a common method to increase wear resistance. The ratios of resin to particles have a great influence on the final morphology, roughness, wettability and wear resistance of hydrophobic coatings34-36. In this paper, the ratio is fixed at 3:1. Although the exsisted superhydrophobic surfaces have been fabricated, the tribological properties of these superhydrophobic coatings have rarely been 3


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investigated37, 38. As commonly used solid lubricants, MoS2 and WS2 can improve the tribological properties of coatings due to the excellent properties for reducing adhesion and friction. The wear properties of coatings were tested by the wear tester and friction-abrasion tester. With the addition of the sulfide, the friction coefficient of the coatings decreases rapidly, thus improving the wear resistance of materials remarkably. EXPERIMENTAL SECTION Materials. Polydimethysiloxane (PDMS) (Sylgard-184) used with curing agent at 10:1 mass ratio was obtained from DowCorning Corporation. MoS2 and WS2 micro-particles were purchased from Shanghai Zaibang Chemical Co., Ltd., China. Both the deionized water and anhydrous ethanol were purchased from Nanjing WanQing Chemical ClassWare & Instrument Co., Ltd., China. Butylacetate and other laboratory chemical reagents were obtained from Sinopharm Chemical Regent Co. Ltd., China. Fabrication of hydrophobic non-fluorine SiO2 nanoparticles. The hydrophobic SiO2 nanoparticles (HSNs) were synthesized via hydrolysis reaction in our lab. Briefly, moderate amount of NaSiO3·5H2O was dissolved in deionized water to obtain a 300 ml solution. And then added with 100 ml of a 0.24 M NH4Cl solution, followed by stirred at 80 °C for 3 h. After reaction, the resulting product and 0.05 mol of chlorotrimethylsilane (TMCS) were mixed in ethanol. The solution followed by filtered and vacuum-dried to obtain TMCS-functionalized SiO2 nanoparticles. The morphology and energy spectrum analysis are shown in Fig. S1. Preparation of the robust superhydrophobic coatings. Firstly, 0.5 g particles, both the HSNs and MoS2/WS2 microparticles were added to 50 ml butyl acetate solution with a certain proportion of weight (HSNs: MoS2/WS2:SiO2=5:0, 4:1, 3:2, 2:3, 1:4, 0:5, respectively). Then ultrasonic 4


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dispersion was conducted by an ultrasonic cell breakage instrument to ensure good dispersion of HSNs and MoS2/WS2 microparticles. In a typical synthesis, 1.5 g PDMS and a curing agent were added at mass ratio of 10:1. Followed by ultrasonic cell breakage and the superhydrophobic colloidal solution was prepared completely. Subsequently, the superhydrophobic solutions were spayed evenly with the pressure controlled above 0.2 MPa on the bases. And then put the coated substrate into the oven for 1 h at 120 °C to obtain the required coating. The fabrication process schematic illustration of the robust superhydrophobic coatings with MoS2/WS2-SiO2 and PDMS hybrid is shown in Fig. 1. Chemical characterizations. The morphology and microstructure of the sample were examined by a field-emission scanning electron microscope (FESEM, FEI, United States) and high resolution scanning transmission electron microscopy (HR-STEM, FEI, United States). An energy dispersive X-ray analysis (EDAX) attached to the FESEM was used to analyze the chemical compositions of coatings. The crystal structures were studied by X-ray diffraction (XRD, Bruker, Germany) and Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific, United States) using Nicolet 5700. Static contact angles (SCAs) and sliding angles (SAs) for water were tested by an OCA 15Pro machine (DataPhysics, Germany) at ambient temperature. The SCAs values were measured using 5 μL DI water droplets. And 10 μL water droplets were used in the SAs measurement with a platform tilting speed of 2 °/s. Friction and wear tests. The wear resistance test was using an abraser (BGD 523, Biuged Laboratory Instruments (Guangzhou) Co. Ltd.) at a speed of 60 rad/min and a pressure of 250 g with a wheel of CF-10 (TABER, America). The friction and wear behaviors were tested via a friction and wear micro-tester (UMT-3, Lanzhou Zhongke Kaihua Technology Development Co., Ltd., China). The AISI 52100 steel ball (diameter 4 mm) was subjected to a rotational sliding test 5


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with sliding speed and sliding diameter of 35 mm/s and 3 mm, respectively. And a weight of 10 g was selected to be the applied load. The friction coefficient was measured by a friction and wear tester which attached with computer automatically records. And the surface profiler (Dektak 150) produced by Veeco Company of the United States was applied to measure the roughness value of the coating surface. RESULTS AND CONCLUSION Morphology analysis of the MoS2/WS2 particles. Fig. 2a and 2d show the morphology of MoS2 and WS2 multilayer micro-particles, respectively. It is obvious that the sizes of the particles range from 1 to10 μm, and both the two sulfides are lamellar with a thickness about 150 nm. Both of the MoS2 and WS2 micro-particles show the hexagonal sheet structures, which means they belong to 2H hexagonal phase. Diffraction patterns of the atomic layers and typical TEM observations of MoS2 micro-particles are presented in Fig. 2b and 2c, and WS2 micro-particles are presented in Fig. 2e and 2f. In Fig. 2b and 2e, the HR-STEM images are taken on standing hexagonal shaped micro sheets, where the layers of MoS2 and WS2 with exposed edges are separated by 0.62 and 0.67 nm, respectively. The interlayer spacing of the MoS2 is same to the d-spacing of (002) planes of a hexagonal MoS2 bulk lattice (0.62 nm). And the interlayer spacing is very close to the hexagonal WS2 bulk lattice ((002) planes, 0.65 nm). Furthermore, the bright spots as it can be observed in the fast fourier transform (FFT) pattern displayed in Fig. 2c and 2f indicating good crystalline quality of the MoS2 and WS2 micro-particles. The high crystallinity could also be observed by Fig. S2. The XRD peaks tested with powders are indexed to the character of 2H-MoS2, and almost every diffraction peak of crystal plane is sharply present, which suggests high crystallinity of MoS2 and WS2 micro-particles. Wettability of surfaces. Superhydrophobicity was obtained via a spraying coating of SiO2 6


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nanoparticles with (MoS2/WS2)@PDMS hybrid on the glass substrates. With the addition of the MoS2, the SCAs for water first increased and then decreased gradually, while the SAs were on the contrary. Benefitted from the hydrophobicity SiO2 nanoparticles and PDMS, the coating could reach superhydrophobic state with the contact angle (CA) of 156.8±1.8° since the sulfides have not been added (Fig. 3a). Nevertheless, when glass was coated with MSP surface that the ratio of SiO2 to MoS2 is 4:1, the SCA for water was 167.8 ± 1.9° and the SA was 1.2±0.6° on glass, which showed higher water repellency. Interestingly, it still maintains semi-translucency for the MSP coating that the ratio of SiO2 to MoS2 is 4:1 in the Fig. 3b. From the SEM images when the ratio of SiO2 to MoS2 is 4:1, it is obvious that micro/nano second-level structures distributed on the coating surface uniformly. It is clear from the cross section image that the coating is completely covered on the surface of glass sheet in Fig. 3d. The micron bulge structure on the surface of coating is quite obvious. The highest peak point is 40.4 μm, while the lowest valley point is only 10.6 μm. This considerable roughness of the surface is an important reason which leads to excellent hydrophobicity. Wear resistance of surfaces. The level of superhydrophobicity at different abrasion cycles was evaluated by the values of SCA (Fig. 4a) and SA (Fig. 4b) of the MSP surfaces. The results show that the SCA decreased and the SA built up over the abrasion cycles increased for all coatings. After 40 cycles, the changing trend of the SCA and SA were tended to be stable. For the coatings of the ratio of SiO2 to MoS2 are 4:1, 3:2 and 2:3, the value of SCAs are stabled at about 150° after 100 cycles. Especially when the ratio of SiO2 to MoS2 is 4:1, the SCA and SA of coating are stabled at 154.6° and 27.9° after 100 cycles, which exhibit excellent hydrophobic properties. Obviously, the intrinsic hydrophobicity of sulfides contributes to the excellent water repellency of coatings to some extent (Fig. S7). For the MSP coatings of ratio of SiO2 to MoS2 is 4:1, 3:2, 2:3 after 100 cycles, the 7


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water droplets on the surfaces could also roll off at steeper angles although the sliding angles are greater than 10°. In comparison, for the MSP coatings of the ratio of SiO2 to MoS2 are 5:0, 1:4 and 0:5, the sliding angles of coating are greater than 90° even at a few cycles. And the good adhesion of the coatings to different substrates such as glass and aluminum are shown in Fig. S8. The results above have demonstrated the highly durable superhydrophobicity of the MPS coatings. And the similar experimental results of WSP superhydrophobic coatings are shown in Fig. S4. But the superhydrophobicity of the coatings with WS2 additions is little worse than the coatings with MoS2 additions. The main manifestations are the decrease of SCAs and the increase of SAs, which are mainly due to the increase in relative molecular mass of WS2 relative to MoS2 leads to decrease in the volume fraction of WS2. The roughness values tested by a surface profiler before and after wear for 50 cycles are shown in Fig. 4c. Surprisingly, the roughness data became extraordinarily large in the experimental results, benefiting from the addition of sulfides. The roughness of the coating surface increases rapidly with the small amount addition of MoS2. When the ratio of SiO2 to MoS2 is 3:2, the roughness of MSP is maximized with a value of 28.19 μm. But as the sulfide continues to be added, its roughness begins to decrease, which is consistent with the decrease in hydrophobic properties of the samples. After wearing for 50 cycles, the coatings’ roughness had a different degree of reduction except for the sample that the ratio of SiO2 to MoS2 was 4:1.This result was similar to that of WS2 addition (Fig. S4c). Though the roughness of the samples with added sulfides reduced after wear, they were able to maintain a large roughness values which helped their ability to repel water. In the Cassie-Boxter model39, the CA is the average between air and solid, which can be calculated for a simple geometric textures. Denoting φs as the fraction of solid in contact with the liquid, there is the following formula: 8


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cosθ* = −1 + φs (1 + cosθ)

(1)

Where θ* is the intrinsic contact angle (CA on ﬂat surface). The equation 1 applies to substrates that are very hydrophobic when it has a large θ or very rough when it has a large φs. Obviously, based on formula (1), when the surface of a superhydrophobic coating has large roughness, lots of air tend to be stored to support its own gravity with small contact area (large φs) between the liquid droplet and surface. And when the roughness is small, more surfaces need to be touched (small φs) in order to store equal amounts of air to balance their gravity. This may only be applicable to the coatings that account for a certain range of roughness, which needs to be further research. That’s may be the reason of the ability of the coatings to repel water after the addition of hydrophobic sulfides was enhanced. The preparation of such the large-roughness hydrophobic surface is also an important idea for designing super-repellent liquids. Surface morphology. The coatings’ surface topographies of prepared MSP were characterized by SEM. As exhibited in Fig. 5a1, the SiO2-PDMS coating surface was relatively flat when no MoS2 was added. For the coatings of the ratio of SiO2 to MoS2 are 4:1 and 3:2 shown in Fig. 5a2 and 5a3, the micro/nano hierarchical structures are becoming more and more obvious (the nano-structures are shown in Fig. 2c). And the micron structures are becoming larger and larger, which is probably caused by the local aggregation of MoS2 due to the large amount of sulfide added. The diameters of the nipple-shaped protuberances were distributed randomly and evenly, ranging from 10 to 50 μm on the coatings when the ratio of SiO2 to MoS2 is 4:1. But the diameters of the papillae have a larger size range from 50 to 200 μm on the coatings when the ratio of SiO2 to MoS2 is 3:2. In this case, some protrusions are formed on the surface of coating. With MoS2 microparticles inserted in the PDMS to form a composite surface, both the micron and nano-structures are decreasing and the relatively smooth surfaces were formed on the coatings (Fig. 9


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5a4, 5a5 and 5a6). Fig. 4b shows the SEM images of MSP superhydrophobic coatings after wear of 50 cycles in the wear resistance test. The surface of the SiO2-PDMS coating has severe wear marks, and the rough textures were almost worn away (Fig. 4b1). Nevertheless, the nipple-shaped protuberances on the coating were not broken during the abrasion test, but the deformation occurred when additive amount of MoS2 was small (shown in Fig. 4b2 and Fig. 4b3). Such phenomenon is closely related to the inhomogeneous hardness because of the addition of rigid silica particles in the elastic polymer resins. As the increase of MoS2 addition, the surfaces of the coatings become more slippery after wear, which could be attributed to the self-lubricating properties of MoS2. And the similar morphological characteristics of WSP superhydrophobic coatings are shown in Fig. S5. The EDS maps for the coating of the ratio of SiO2 to MoS2 is 1:4 are shown in Fig. S6a. The uniformly distributed elements detected on the surface include C, O, Si, S and Mo, especially the S and Mo elements show very dispersive uniformity. It also proves that the MoS2 particles added in the composite coatings are well dispersed. Friction

and

wear

behavior

of

coatings.

Furthermore,

the

durability

of

the

(MoS2/WS2)@SiO2-PDMS coatings during the process of wear were also investigated. The friction coefficients (FCs) of MSP coatings on the glass slide are shown in Fig. 6a. The values of FC are 1.18, 0.92, 0.75, 0.60, 0.51, 0.24, which corresponding to the proportion of SiO2 and MoS2 are 5:0, 4:1, 3:2, 2:3, 1:4, 0:5, respectively. With the addition of the MoS2, the FC of the coating decreases rapidly. The FC of the PDMS/SiO2 coating showed a large value of 1.18, yet the PDMS/MoS2 coating exhibited tribological property with the average FC value of 0.24. The coating on the glass slide did not wear out after the end of the experiment (10 min), which means the wear life of MSP coating was recorded more than 2000 cycles. And the values are similar to the WSP coatings which 10


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are shown in Fig. 6b. The FCs of the WSP coatings maintained are 1.18, 0.98, 0.80, 0.63, 0.50, 0.25, which are corresponding to the proportion of SiO2 and WS2 are 5:0, 4:1, 3:2, 2:3, 1:4, 0:5, respectively. Compared with coated samples with MoS2, the FC values are slightly above that of with WS2, which is mainly due to the constant mass fraction and less volume fraction of WS2. In Fig. 6, it is obvious that line lengths differ greatly in color areas which means the stabilization time of coating structures is different in the process of friction. With the addition of MoS2, the time required for the FC to stabilize first increases and then decreases, which is a positive proportional relationship to the surface roughness of coating material. In addition, the vibration deviation of the FC of sulfide-added samples is larger than that of SiO2-only samples. Although the additions of tungsten sulfide have beneficial effect on reducing the FC of the materials, the vibration deviation in the friction process will be greater because of the larger structures caused by the micron particles. Above all, the mechanism of the enhancement of robustness via MoS2/WS2 can be mentioned as follows: ⅰ) since hydrophobic components penetrate the surface and interior of the coatings, the exposed components are still hydrophobic after wear; ⅱ) the addition of sulfides is micron level, and the addition of such large size particles can easily form a larger rough structure after spraying, thus increasing the hydrophobicity of coatings; and ⅲ) hydrophobic sulfides are commonly used as solid lubricants, which can reduce friction coefficient and wear loss during wearing. Moreover, due to the inhomogeneous hardness results of the addition of rigid silicon oxide at the same time, the coatings can maintain a certain rough structure after wear, thus retaining superhydrophobic characteristics. CONCLUSIONS In summary, one-step air spaying method was demonstrated for the preparation of 11


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(MoS2/WS2)@SiO2-PDMS superhydrophobic coatings with hierarchical highly rough structures. It was ascertained that the lamellar MoS2 and WS2 with high crystallinity have intrinsic hydrophobic properties. Benefitted from the different particle sizes and morphologies of sulfides and the modification of SiO2, the coatings are easy to form the second-order rough structures of micro and nanometer. Benefitted from the intrinsic hydrophobicity of sulfides and the non-fluorine modified nano-SiO2, the coatings could be obtained the excellent superhydrophobic state easily. And the friction coefficient decreased gradually with the increasing of MoS2 or WS2. A 4:1 ratio of SiO2 to MoS2/WS2 could make the samples preserve the superhydrophobic properties even after 100 cycles on the abraser. As a result, the superhydrophobic coatings with excellent wear resistance will be good candidate for water-repellent surfaces to the practical industry applications in the future.

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Fig. 1 Fabrication process schematic illustration of the robust superhydrophobic coatings with MoS2/WS2-SiO2 and PDMS hybrid.

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Fig. 2 SEM and TEM characterizations of MoS2/WS2 micro-sheets. a, d, Low and high magnification SEM images of MoS2 and WS2, respectively. b, e, TEM of MoS2 and WS2, respectively. The distance between two layers of MoS2 is about 0.62 nm, and the distance between two layers of WS2 is about 0.67 nm. c, f, Diffraction patterns of the MoS2 and WS2 atomic layers.

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Fig. 3 Wettability of the MSP superhydrophobic coatings and SEM images for the ratio of SiO2to MoS2 is 4:1. a, SCAs and SAs of the MSP superhydrophobic coatings. b, Water droplets on the coating surface for the ratio of SiO2 to MoS2 is 4:1. c, Low and high magnification SEM images, and d, Cross-sectional SEM image.

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Fig. 4 Wetting state of different wear test cycles of MSP superhydrophobic coatings and their roughness value. a, SCAs. b, SAs. c, Roughness of MSP coatings before and after wear for 50 cycles.

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Fig. 5 SEM images of MSP superhydrophobic coatings before and after were of 50 cycles. a, before wear. b, after wear for 50 cycles and 1, 2, 3, 4, 5, 6 are the ratio of SiO2 to MoS2 of 5:0, 4:1, 3:2, 2:3, 1:4, 0:5, respectively.

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Fig. 6 The friction coefficients of superhydrophobic coatings on glass slides. a, MSP. b, WSP.

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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

■ AUTHOR INFORMATION Corresponding Author Email: [email protected] Notes The authors declare no competing financial interest

■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51671055, 51676033), the China National Key R&D Program (2016YFC0700304).

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Enhancing the Robustness of Superhydrophobic Coatings via the

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