Thermally and Chemically Stable Candle Soot Superhydrophobic

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Thermally and Chemically Stable Candle Soot Superhydrophobic Surface with Excellent Self-Cleaning Properties in Air and Oil Liji Xiao, Weiguo Zeng, Guangfu Liao, Changfeng Yi, and Zushun Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00363 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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ACS Applied Nano Materials

Thermally

and

Chemically

Stable

Candle

Soot

Superhydrophobic Surface with Excellent Self-Cleaning Properties in Air and Oil Liji Xiao,a,1 Weiguo Zeng,a,1 Guangfu Liao,a,b,* Changfeng Yia, and Zushun Xu,a,* a

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials,

Ministry of Education Key Laboratory for The Green Preparation and Application of Functional Materials, Hubei University, Wuhan, Hubei, 430062, PR China b

School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University,

Guangzhou 510275, China

ABSTRACT:

In this report, a people-considered waste-candle soot-generated from

incomplete combustion of the middle zone of candle was used to coat the glass slide for fabricating the superhydrophobic surface. The candle soot coating surface followed by deposition of methyltrichlorosilane (MTCS) was characterized through field emission scanning electron microscope (FESEM), contact angle measurement in air and oil, self-cleaning test, high temperature and corrosive liquid resistance test and water drop impact experiment, respectively. Interestingly, the candle soot layer after deposition MTCS presented a coral-like structure and exhibited high water contact angle of 161° and a low sliding angle of 3° in air, which demonstrated that it had excellent superhydrophobicity in air. Of course, the coating also exhibited high water contact angle and low sliding angle in oil. Apart from that, results of high temperature and corrosive liquid resistance test implied that the superhydrophobic coating can keep stability after treated by high temperature between 100 °C and 300 °C and retain high contact angle when encountered the strong acid and basic water, which indicated that it exhibited excellent thermal and chemical stability. Moreover, the prepared thermally and chemically stable superhydrophobic coating not only displayed wonderful self-cleaning properties in air and oil but also resisted to water drop impaction after deposition of MTCS, which interconnected the carbon particles via the hydrolysis of MTCS in air. In summary, we provided a fast and cost-efficient method to prepare the superhydrophobic coating with excellent thermal and chemical stability, which showed great potential in application of antifouling materials under the high temperature and corrosive condition. Keywords: Candle soot, Superhydrophobicity, Self-cleaning, Air and Oil, Thermal stability, Chemical stability 1

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1. INTRODUCTION Surfaces with self-cleaning properties in air and oil have become a popular research topic in coating research because they have the ability to make the water roll off automatically when the water encounters with them and takes up the contaminants on their surface.1-3 Researchers called such surface as superhydrophobic surface defined by a water contact angle larger than 150° and the sliding angle lower than 10°.4-6 There are many methods to prepare such superhydrophobic surface, such as, electro-spinning7-8 and plasma etching,9-10 which required expansive equipment, complicated fabricating process and time consuming, as a consequence, it limited the manufacture on large scale of the superhydrophobic surface. Therefore, a facile, low cost, time saving and universalized method is essential for the fabrication of superhydrophobic surfaces. Meanwhile, the inexpensive and readily available raw materials are also important and necessary for constructing the superhydrophobic surface. As well known, candle soot particles produced from the incompleted combustion of the middle zone of the flame formed by easy available candle are economical and low cost.11-12 Additionally, they have the advantages of convenient preparation and production scalability over other inorganic particles, especially graphene and carbon-nanotubes.13 What’s more, the candle soot particles have received much more attention since the Vollmer’s group reported a superamphiphobic surface coated with a candle soot layer followed by the deposition of tetraethoxysilane and semi-fluorinated silane.14 Up to now, there are more and more researchers to investigate the application of candle soot particles in various fields.15-21 For example, Huang et al.15 modified the candle soot with 1H, 1H, 2H, 2H - perfluorooctanol and researched the application of the fluorinated candle soot as lubricant additive in the perfluoropolyether. Also, Barra’s group looked into the electrical properties of the flame-soot nanoparticle thin film in order to explore the novel application in electrics and nanosensors.16 Zhang et al.21 modified candle soot with FeP nanoparticles for electrocatalyzing the hydrogen evolution reaction. However, the most researched field was the coating surface. Sen’s group22 deposited a candle soot layer on the polydimethylsiloxane coated glass with water contact angle of 161 ± 1° and sliding angle of 2°, which exhibited mechanical stability and chemical resistance. Zhao et al.23 prepared candle soot copper foam solidified by polydimethylsiloxane which showed very good superhydrophobicity and superoleophlicity for oil/water separation. And Li et al.24 fabricated a superhydrophobic stainless steel mesh coated by a candle soot layer following spaying the hydrophobic silicon dioxide nanaoparticles dispersion for oil/water separation. Additionally, Lars Schmüser and coworkers25 synthesized a super-amphiphobic substrate through coating a candle soot layer followed by chemical vapor deposition of tetraethyl orthosilicate and combusting carbon in furnace following chemical vapor deposition of trichloro (1H, 1H, 2H, 2H - perfluorooctyl) silane for resistance to the protein adsorption. Qahtan et al.26 prepared a superhydrophobic coating by spaying the candle soot dispersion which showed good water jet resistance and thermally stable. 2


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However, the works reported above included some drawbacks, such as, complicated experiment steps, fine equipments, expensive cost, and toxic reagents and fluorinated materials will be harmful to environment and people’s health. These drawbacks severely impeded the large scale production of superhydrophobic coating based on candle soot. Consequently, utilizing inexpensive materials and non-fluorinated materials for construction of candle soot particles superhydrophobic coating without toxic reagents and exquisite equipment is dispensable. In our work, we utilized inexpensive materials and non-fluorinated materials, methyltrichlorosilane (MTCS), to fabricate a superhydrophobic coating based on cheap and easily generated candle soot through using a simple, time-saving and cost-efficient method, which showed great potential in production by large scale. The prepared thermally and chemically stable superhydrophobic coating not only displayed wonderful self-cleaning properties in air and oil but also resisted to water drop impaction after deposition of MTCS, which interconnected the carbon particles via the hydrolysis of MTCS in air. Therefore, the prepared supehydrophobic coating had the potential application in anti-fouling and water-repellency under high temperature or even abominable environment. Compared to that of the work of groups of Jiang, Lin, Lai etc based on easy, environment friendly and water-rich system through dipping coating method, for example, Lin's group27 reported superhydrophobic fabrics prepared by dipping the fabrics into the dopamine/hexadecyl trimethoxysilane mixture, the superhydrophobic fabrics fabricated in this article showed excellent washing resistance. Besides, the group of Lai28 prepared a superhydrophobic fabric via immersing in the methyltrimethoxysilane methanol solution and then immersed in polydimethylsiloxane tetramethylene oxide solution. The resulted fabrics exhibited good self-cleaning and anti-fogging properties. Meanwhile, the group of Jiang29 prepared a flexible superhydrophobic surface by using the natural lotus leaf as the template and modified the polydimethylsiloxane surface with ZnO crystal seeds by dip-coating and heat treatment. The flexible superhydrophobic surface could resist ice and showed robustness. All of them done a lot of great works and fabricated excellent superhydrophobic surface, in which included dip coating method and other methods. However, there still existed some problems that the preparation process was so complicated and the equipment included was special. Meanwhile, the materials using in preparing superhydrophobic coating was not environment friendly. Comparing with their works, our preparation process was relatively simple and time-saving and our materials using in fabricating superhydrophobic surface was relatively environmental friendly. Meanwhile, there was no expensive and special equipments.

2. EXPERIMENT 2.1. Materials

3



Methyltrichlorosilane (98%, MTCS) was analytical pure and provided by Macklin. Absolute alcohol was analytical pure and provided by the National Medicine Group Chemical Reagent Co., LTD, China. The glass slide was also supplied by the National Medicine Group Chemical Reagent Co., LTD, China. The glass surface dish with a diameter of 12 cm was purchased from the National Medicine Group Chemical Reagent Co., LTD, China. And the sulfuric acid was analytical pure and provided by Chemical Plant in Xinyang. Potassium hydroxide was purchased from the National Medicine Group Chemical Reagent Co., LTD, China. Candle was bought from local supermarket. The deionized water was made by our laboratory. The milk powders were bought from Nestle on the internet.

2.2 Preparation of superhydrophobic coating Put the glass slide/iron sheet/sponge into the absolute alcohol liquid for ultrasonic treatment for about 10 min. And then hold the dried glass slide above the upper half of the flame of candle until the glass slide turned black almost completely for 4 min (the influence of the candle soot particles depositing time on the wettability of the coating surface was showed in Figure S2a). Finally, the black glass slide was placed into the surface dish followed by 0.4 ml of methyltrichlorosilane dripping beside the glass slide, and sealed the surface dish up to deposite in oven at 80 °C for 1 h (the influence of the depositing time of MTCS on the wettability of the coating surface was presented in Figure S2b). After that, the superhydrophobic coating surface was gained. Besides, the iron sheet was also used to prepare the superhydrophobic coating surface by this method. And the sponge was adhered on the bottom of the beaker, which was upside down on the iron ring and then the candle was fired on the bottom until the sponge turn black. The candle soot coated sponge was prepared. Scheme 1 presented the preparation procedure and mechanism schematics of the superhydrophobic candle soot coating. The formation mechanism of the superhydrophobic coating surface after deposition of MTCS was presented in Scheme S1. The chemical formula is shown as following: “CH3Si(Cl)3 + 3 H2O→CH3Si(OH)3 + x candle sootOH→CH3Si(O-candle soot)x (x=1, 2, or 3)”.

Scheme 1. The preparation procedure and mechanism schematics of the superhydrophobic candle soot coating. 4


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2.3 Characterization The surface morphologies of the coatings without and with methyltrichlorosilane deposition were recorded by field electron scanning electron microscope (FESEM, JSM7100F, Japan). The SEM measurement was conducted on a JSM7100F instrument at 15 kV. The samples were sprayed on a gold film for 100s. Moreover, the candle soot particles without methyltrichlorosilane deposition were characterized by Transmission Electron Microscope (TEM, JEM42012-HT). Also, the static contact angle and sliding angle of the coated glass surface without and with methyltrichlorosilane deposition were measured by the contact angle meter (OCA20, Dataphysics, Germany) through the sessile drop method with a water droplet of 5.0 µL at room temperature. The listed angles were average of three measurements at least on the coatings surfaces. In addition, the waterproof and bounce tests were conducted by observing the coated glass surface immersed into the water and recording the bouncing process of a 5.0µL deionized water droplet onto the coated glass with methyltrichlorosilane deposition. We performed the self-cleaning test of coated glass surface with methyltrichlorosilane deposition in air and oil by using the dirt as contamination. The durable ability was accessed by imitating the raining attack through putting the coated glass without or with methyltrichlorosilane deposition below the funnel with a 45° of tilt angle and the distance between the funnel and glass was 20 cm. The 20 drops scouring on the coating surface is defined as one cycle of durability test22. Beyond that, the resistance to high temperature was tested by placing the coated glass in the oven at different temperature from 100 °C to 400 °C for 1h14, and the resistance to corrosive liquid for the coated glass were tested through immersing the coated glass for 1h in liquids of different PH ranging from 2 to 13 and measuring the contact angles of the treatment coating surfaces30.

3. RESULTS AND DISCUSSION 3.1 Surface morphology The surface morphologies of coatings with candle soot layer before and after deposition were analyzed by FESEM. As we can see from the Figure 1a-b, the candle soot layer before deposition was composed of about 40nm carbon particle, and the boundary line between the compacted particles was clear, which demonstrated that the interaction between them was physical function. Therefore, the robustness of the candle soot layer before deposition was weak. However, the carbon particles after deposition become larger and the boundary line between the compacted particles was vague. From the Figure 1c-d, we can see that the carbon particles disappeared and interconnected together to structure like coral. All of which declared that the methyltrichlorosilane deposition was favorable to the interconnection of the carbon particles and improved the robustness of the candle soot superhydrophobic coating. The reasons for the improvement of 5



robustness might be the hydrolysis of methyltrichlorosilane deposited on the carbon particles, which made the particles become an organic whole. We also conducted the TEM characterization for the confirmation the morphologies of the candle soot particles. And from the Figure 1e and 1f, we can see that the candle soot particles was formed from approximately spherical shaped nanoparticles with the size of 40nm, which interconnected by Vander-Waals force. What’s more, from the TEM images with high magnification, we can know that the candle soot particles were almost amorphous. And the morphorlogies of candle soot particles were the similarity to the results of the SEM images.

Figure 1. SEM images of candle soot layer before deposition with low (a) and high magnification (b); SEM images of candle soot layer after deposition with low (c) and high magnification (d); TEM images of candle soot particles before deposition with low (e) and high magnification (f).

3.2 Wettability in air The water contact angles of coatings before and after deposition in air and oil were measured by the contact angle meter. The results were presented in Figure 2a-b, and their water contact 6


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angles were 160° and 161°, respectively, which showed that the methyltrichlorosilane deposition just had little influence on the water contact angle of coating. In Figure 2c, the water droplets dyed different colors stand on the coating with a spherical shape and Figure 2d declared that the water droplet could roll off on the candle soot coating easily by a low tilt angle of 3°, all of which demonstrated that the candle soot coating after deposition was superhydrophobic that might be attributed to the hydrophobic carbon nanoparticles formed carol like structure after deposition and provided abundant rough structures to construct superhydrophobic surface. Meanwhile, in order to further prove the superhydrophobicity of the coating, the bouncing property of the candle soot coating after deposition was accessed, too. In Figure 3, we can see that the water droplet of 5.0 µL could bounce back and forth for several times, which demonstrated that the coating was low adhesion to water and also proved that the prepared candle soot coating has excellent superhydrophobicity. In addition, we can see that the mirror-like phenomenon existed when the glass coated with candle soot layer was immersed in the water from Figure 3a, and there was no water droplet on the glass after it was extracted out from the water in Figure 3b. All the phenomena proved that the candle soot coating after deposition possessed superhydrophobic properties, which was attributed to the coral-like structure and hydrophobic carbon nanoparticles.

Figure 2. The water contact angles on candle soot layer before (a) and after deposition in air (b); (c) Water droplets dyed different colors formed sphere shape on candle soot layer after deposition; (d)The sliding behavior of water droplet on candle soot layer after deposition.

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Figure 3. (a) Mirror-like phenomenon can be observed on the candle soot layer after deposition in water; (b) There was no water droplet on the surface after removing from water; (c) Images of the bouncing of a water droplet on candle soot layer after deposition.

3.3 Self-cleaning property in air The self-cleaning property is one of most important application of the superhydrophobic surface. For the sake of testing the self-cleaning property of the coating after deposition in air, the dirt was used as contamination covering the slant glass slide surface coated with candle soot layer and the deionized water dyed by yellow dripped and rolled down along the glass slide surface. The dirt was removed by the droplet and the path cleaned by the water droplet returned the same as the uncontaminated place (Figure 4a). In Figure 4b and 4c, the candle soot glass contaminated by dirt was impacted by dropped water from high place and the water gathering the dirt formed sphere on the glass, which also demonstrated that the superhydrophobic coating exhibited good self-cleaning property in air. Apart from the stated testing method above, there was another method to check the self-cleaning property which was showed in Figure 4d-f. Using a sucker to guide a water droplet to move from right side to left side on the coated glass after deposition, and the water droplet could gather the dirt together along the traveling road so that the glass turned clean as the uncontaminated place, which confirmed that the excellent self-cleaning property. Furthermore, the water cleaned the contaminated glass by milk powders before and after deposition in air was present in Movie S1 and Movie S2. We can see that the carbon particles were also rolling down along with the water droplet except milk powders before deposition, while there were almost no carbon particles rolling down along with the water droplet after deposition, which could also confirmed that the deposition of MTCS was beneficial of the stability of carbon nanoparticles.

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Figure 4. (a) Self-cleaning property on a slant candle soot layer after deposition in air; (b-c) Water droplets impacted the heavy polluted candle soot layer after deposition; (d-f) Using a sucker to guide a water droplet travelling on the candle soot layer after deposition.

3.4 Wettability and self-cleaning property in oil Not only the wettability and self-cleaning property in air were characterized, but also were the wettability and self-cleaning property in oil. The wettability in oil was accessed through measuring the water contact angle and sliding angle in oil by immersing the needle into the organic solvent and dripped a water droplet onto the candle soot glass surface. From the Figure 5a, we can see that the water contact angles in different solvents were above 150°. What’s more, the sliding angle of the coating after deposition in hexane was measured by tilting the sample stage with 1° and squeezing a water droplet of 5.0 µL from the needle on the tilted glass slide. Then the water droplet rolled off along the glass with candle soot layer immediately, which declared that the candle soot layer had lower adhesion due to the hydrophobic carbon nanoparticles and the oil also played an important role in resisting to the water owing to the repulsive interaction between the oil and water. The self-cleaning test in oil was also conducted by utilizing the dirt as the contamination and cleaned by water dyed by blue color. In Figure 5c-f, the slant candle soot glass after deposition immersed in hexane was polluted by dirt, and then the contamination was all cleaned by water, which was attributed to the hexane was absorbed by the coating and replaced the air in the hole compacted by carbon nanoparticles. Consequently, the water droplet could roll down the candle soot coating and remove the contamination, which proved that the superhydrophobic coating possessed good self-cleaning property in oil. What’s more, the milk powders were also used as contaminates to test the self-cleaning properties in oil on the coating surface before and after 9



deposition. The results were revealed in Movie S3 and Movie S4. From them, we could make the conclusion that the deposition of MTCS helped the adhesion between the carbon particles and the candle soot layer coatings have the self-cleaning properties in oil.

Figure 5. (a) The water contact angles of candle soot layer after deposition under oils (hexane, octane, benzene, hexadecane and diesel); (b) the sliding behavior of water droplet on candle soot layer after deposition under hexane, (c-f) self-cleaning test of candle soot layer under hexane.

3.5 Resistance to high temperature and corrosive liquid Because the superhydrophobic coating might be used in high temperature and corrosive environment, it’s imperative to investigate the resistance to high temperature and corrosive liquid of our prepared superhydrophobic coating. For investigating the resistance to high temperature, the coating after deposition was placed in the oven at the temperature between 100 °C and 300 °C by a interval of 50 °C for 1h. The result was showed in Figure 6a, from which we can see that the water contact angle fluctuated up and down at 150° from 100°C to 300°C. This curve revealed that the high temperature did not inflect the superhydrophobicity of the prepared coating at all and demonstrated that the coating could resist to high temperature even to 300°C. What’s more, in order to test the resistance to corrosive liquid, the H2SO4 solution (PH=2) and KOH solution (PH=12) were prepared. We measured the water contact angle changes with the staying time on the coating after deposition of a 10.0µL water droplet from the prepared H2SO4 solution and KOH solution. As we can see in Figure 6b, the water contact angles of the coating surfaces after immersing in both basic liquid and acid liquid were almost more than 150° except in strong basic liquid for 1h, which demonstrated that the resistance to the corrosion of acid and weak basic liquid was excellent but strong basic liquid. Furthermore, the reason of resistance to acid liquid corrosion of the candle soot coating might be owing to the good superhydrophobicity and the unreaction of MTCS in acid. Therefore, from the results above, we can conclude that the candle soot superhydrophobic coating has the ability to resist high temperature and corrosive liquid. 10


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Figure 6. (a) Water contact angle changes with different temperature treatment for 1h, (b) water contact angles of coating surface with deposition of MTCS after immersing in different liquids of PH ranging from 1 to 13.

3.6 Durability Durability is an important quota for superhydrophobic surfaces to be applied in practice because the rough micro/nano structure required in constructing superhydrophobic surface is too fragile to resist any kind of mechanical damages. Therefore, it’s necessary to test the durability of the prepared candle soot coating after deposition. Firstly, the durability test was conducted following the Figure 7a, in which the coated glass was below the funnel with a 45° of titling angle and the distance between them was 20 cm. Then the funnel was full with 50 ml deionized water to scour the coated glass surface. The SEM images of coating glass before deposition after one cycle of durability test and after deposition after 30 cycles of durability test were showed in Figure 7b and Figure 7c, respectively. Seen from the Figure 7b, there were many bumps and hollows on the impacted coating surface before deposition. Some particles appeared after impacted, while some particles were compacted together to form lumps, which showed that the coating before deposition exhibited poor durability because there was no chemical interaction between the carbon nanoparticles just physical compacted. However, the durability of the coating after deposition was better than that of coating before deposition. In Figure 7b, the coral-like structure still kept intact after water impacted for 30 cycles, which confirmed that the coating after deposition could keep superhydrophobicity during the scouring of raining. There might be a reason that the hydrolyzed methyltrichlorosilane depositing on the carbon nanoparticles made the carbon particles interconnect together forming coral-like structure. The interconnection improved the durability of the candle soot coating after deposition. Besides, the coating after deposition still showed superhydrophobicity following by cutting the coating surface presented in Movie S5, which might be the reason that the carbon particles in the scratch redistributed on the surface retaining the superhydrophobicity. 11



Additionally, the self-cleaning property before and after scratching by knife was also tested in Figure S1. From the Figure S1a and S1c, we can see that the self-cleaning property of coatings before and after scratching changed a little. And what’s more, we also measured the water contact angle of the coatings before scratching and after scratching showed in Figure S1d and S1e. The water contact angle after scratching decreased a little compared with that of before scratching, but still reached more than 150°. All of which could confirm that the coating could resist to the scratching with knife to some extent. And the process also could apply for the substrates that resistant to the heat and flame retardant. What’s more, the soft and flammable materials such as sponge could also apply in preparation of candle soot layer deposition. The preparation process was presented in the part of “Preparation of Superhydrophobic Coating”. The results of the coated iron sheet and sponge were showed in Figure S3, from which we could see that the coated substrates exhibited excellent superhydrophobicity.

Figure 7 (a) Diagrammatic drawing of water impact test on the candle soot layer before and after deposition. (b) The SEM image of candle soot layer before modified after a cycle of water impact test. (c) The SEM image of candle soot layer after modified after 30 cycles of water impact test.

4. CONCLUSION In summary, a thermally and chemically stable superhydrophobic coating was fabricated by coating a candle soot layer on the glass slide followed by deposition of MTCS. We can conclude that the superhydrophobic coating showed a high water contact angle of 161° and a low sliding angle of 3° in air and water contact angle more than 150° in different oil and a low sliding angle of 1°, which also accounted for the good self-cleaning properties in air and oil. All of them were 12


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attributed to the hydrophobic carbon nanoparticles and coral-like structure compacted by carbon nanoparticles and interconnected by the hydrolysis of MTCS. Meanwhile, it could maintain high contact angle above 150° after treated at high temperature and corrosive liquid. And the coral-like structure can stay intact after deposition of MTCS, which declared that the addition of MTCS improved the stability of the candle soot superhydrophobic coating. Therefore, the prepared superhydrophobic coating showed great potential in application of antifouling materials under the high temperature and corrosive condition.

ASSOCIATED CONTTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]（Guangfu Liao） E-mail: [email protected] (Zushun Xu)

Author Contributions 1

These two authors contributed equally to this project.

Note The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51573039, 8571734, and 81372369).

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Thermally and Chemically Stable Candle Soot Superhydrophobic

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