Fabrication of Tunable, Stable, and Predictable Superhydrophobic

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Fabrication of Tunable, Stable and Predictable Superhydrophobic Coatings on Foam Ceramic material Xingang Li, Peng Yan, Hong Li, and Xin Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02541 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Industrial & Engineering Chemistry Research

Fabrication of Tunable, Stable and Predictable Superhydrophobic Coatings on Foam Ceramic material Xingang Lia, b, c, Peng Yana, Hong Lia, b, c, Xin Gaoa, b, c, * a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b

National Engineering Research Center of Distillation Technology, Tianjin 300072, China

c

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

KEYWORDS:

superhydrophobic; foam ceramic material; dip-coating; tunable; stability;

predictable.

ABSTRACT: Superhydrophobic foam ceramic material can be used as distillation column internals and surface wettability has remarkable influence on column trays. There are no existing methods for industrial fabrication of superhydrophobic surfaces on foam ceramic material. This paper presents a facile method to fabricate stable superhydrophobic/hydrophobic coatings on the outside and inside of foam SiC material with submicron silica particles, for dip-coating, and alkylchlorosilane, for surface reaction. SEM, AFM, Nano-ZS90 and FTIR were employed to

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o o study the physical and chemical details. The contact angle of coatings can be tuned to 155 , 140 o o , 125 and 95 by adapting coating times and particle size. Superhydrophobic surfaces presented

excellent stability under various conditions. In addition, a theoretical prediction strategy by surface microscopic morphology and surface chemical composition based on Cassie-Baxter model was also presented. Briefly, this paper provides possibility for large-scale preparation of superhydrophobic coatings on foam ceramic material.

1 INTRODUCTION Foam materials are widely used in many application fields, such as heat transfer1, distillation2-6, membrane technology7, 8, resources recovery9,

10

and heterogeneous catalysis11. Specifically,

various column trays and structured packings have been developed for distillation based on foam silicon carbide (SiC) ceramics in our previous works2-4. Their performance is mainly affected by these factors as follows: material types, surface properties and geometric structures. And surface wettability is one of the most important factors for equipment efficiency in industrial processes. To be exact, wettability affects liquid flow patterns, spreading of liquid film, bubble formation and permselectivity etc.12-15. Effect of wettability on multiphase flow in microfluidic devices has been investigated extensively16. Meanwhile, experimental investigation and theoretical analysis indicated the importance of wettability in traditional gas-liquid two-phase flow. According to previous research, surface contact angles affect capillary pressure and apparent pore size in the process of bubble formation on foam materials, and research results also showed that transformation of bubble shape and size is extremely influenced by wettability17-19. What’s more, research results pointed out that larger contact angle can produce better gas distribution performance and less chance of pore blocking by the liquid, which are beneficial to reduce


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pressure drop and boost mass transfer efficiency. Therefore, hydrophobic and superhydrophobic modification of foam ceramic material is significant. But due to structural specialty of foam ceramic material, it is not easy to perform surface modification. In the last decade, superhydrophobic surfaces, with the water contact angle greater than 150

o

o and the sliding angle less than 5 , have acquired considerable attention in both fundamental and

applied research owing to their superior performance, including self-cleaning, anti-freezing, antibacterial activity, corrosion resistance and oil-water separation9,

20-23

. Many methods have

already been developed to produce superhydrophobic surfaces by adjusting chemical composition and constructing rough geometrical structure simultaneously23-25. Fabrication methods can be divided into two categories: bottom-up strategy including CVD, powder spraying and sol-gel method etc.23-28; and top-down strategy including lithographic processes, chemical etching24, 29. But existing methods which are used to fabricating superhydrophobic surfaces on plain substrates or fabrics, are rarely developed for foam ceramic materials9, 10. In addition, it is not facile to fabricate controllable, uniform and stable superhydrophobic coatings rapidly. Sol-gel method is a popular way to prepare homodisperse particles30, and can be used to construct rough morphology on the substrates by dip-coating31, 32. And it is good enough with simple equipment, facile reaction condition, nontoxic reagents and the method is flexible for control of particle size. So sol-gel method may be a facile method for surface modification on foam materials. Other than the development of fabrication methods and application exploration, relevant research of wettability theory has never stopped, from pioneering Young’ equation to Wenzel model, Cassie-Baxter model33, 34. Theoretical models are helpful to understand liquid behaviors on superhydrophobic surfaces and get deep insight into intrinsic mechanism. In addition,


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accurate prediction strategies coupled with theoretical models are fantastic tools for wettability research. This is beneficial for controllable design of surfaces with specific wettability and replacing experimental measurement under special condition35. In the present work, we firstly develop a facile method to fabricate superhydrophobic surfaces on foam materials through a bottom-up strategy, which is divided into three steps: dip-coating, coating gelatization and chemical modification. Secondly, considering the demand for different contact angles, which is of importance to explore effects of contact angles on hydrodynamics and mass transfer efficiency of distillation column tray, we try to obtain surfaces with tunable contact angles through adjusting key influential factors. Except surface energy, surface morphology is the other key factor affecting contact angle, so particle size and coating times are choosed as influential factor for investigation. And then stability tests were conducted in various situations in order to tackle possible rugged surroundings in the fundamental research and industrial applications. Finally, we explore the possibility to predict contact angle based on the morphology and theoretical model.

2 MATERIALS AND METHODS 2.1 Materials Foam silicon carbide (SiC) material were supplied by the Institute of Metal Research, Chinese Academy of Science, and they were prepared based on the impregnation of the polyurethane foam with a homogeneous mixture of silicon, charcoal and phenolic resin. Foam SiC materials used in the experiment were cylindrical with the diameter of 44 mm, the thickness of 6 mm, and the mean pore size of 3 mm. Tetraethyl orthosilicate (TEOS), ethanol (AR.


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Degree), 25wt% ammonium hydroxide, 98wt% sulfuric acid, 30wt% hydrogen peroxide, deionized water and 37wt% hydrochloric acid were supplied by Tianjin Jiangtian Chemical Technology Co., Ltd. in China. Toluene (AR. Degree), n-butanol (AR. Degree), gasoline, nhexane (AR. Degree) and 96.0wt% solid-state sodium hydroxide were purchased from Tianjin Guangfu

Chemical

Technology

Co.,

Ltd.

in

China.

Furthermore,

97wt%

n-

octadecyltrichlorosilane was purchased from J&K Chemicals Co., Ltd. in China.

2.2 Fabrication process Foam SiC material was cleaned with the mixture (98wt% sulfuric acid to 30wt% hydrogen peroxide equals 7:3 in liquid volume) at 90℃ for 30min, deionized water 3 times, and then ethanol 2 times to remove surface contaminants. Then material was dried at 60℃ for 30min and cooled down naturally. In order to obtain superhydrophobic surfaces on foam SiC materials, several steps are carried out successively. And fabrication procedures are illustrated in Figure 1 and described as follows. Firstly, SiO2 sol was prepared by titrating 3ml TEOS dropwise into the mixture of 3ml/2.5ml/2ml ammonium hydroxide and 50ml ethanol at 25℃ about 25~30 min, followed by constant stirring for 2 hours at 25℃. The as-prepared sol solution was sealed in flask to prevent ethanol from volatilizing and preserved at room temperature for aging 24 hours. After that, the sol dispersed uniformly with sub-micron scale SiO2 particles was acquired. Subsequently, the sol solution was used to dip-coat foam SiC materials. Foam SiC materials were processed step by step as follows: dip-coated into the prepared sol solution for 30 seconds and pulled out at a velocity of 1~3mm/s. Foam SiC materials have special structure of outside flat surfaces caused by cutting in the fabrication process and inside structure with small curved surfaces which may


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face any direction in cubic space. For the purpose of forming uniform coating distribution on any surfaces of foam SiC materials inside and outside, the dip-coated foam SiC materials must be rotated constantly when they just pulled out from sol solution waiting for solvent’s volatility. After the solvent volatilized by nature and heat sufficiently, foam SiC materials were dip-coated repeatedly according to above procedures.

Figure 1 Schematic diagram of experimental procedures After dip-coating process, gelatinization was done by thermal treatment at 60~100℃ for 2 hours in order to acquire relatively stable coatings. So far, micro-nano dual structure was built successfully. Finally, surface chemical modification was performed to acquire low-surfaceenergy surfaces. The process mechanism of chemical reaction and self-assembly is shown in Figure 232, 36, and the process was described as follows: the as-prepared coatings was immersed in 5mmol/L n-octadecyltrichlorosilane/n-hexane solution for 2 hours at 25℃.Eventually, the asprepared samples were heated at least 2 hours in drying oven at 60℃ and annealed.


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Figure 2. Schematic diagram of formation mechanism of silicone network on SiO2 particles

2.3 Characterization analysis methods 2.3.1 Contact angle measurement Contact angles on coating surfaces were measured using the sessile drop method37 with video optical contact angle measuring instrument (OCA20, DataPhysics Instruments GmbH, Filderstadt, Germany) at 293 ± 1 K. Drop images were recorded using a high-speed CCD camera with a resolution of 768×576 pixels. Standard liquid volume was 3 µL water (doubly distilled). Multi-points measurement was performed on each sample and at least 3 parallel samples were measured for each kind of fabrication condition. Eventually, the average value of contact angles on parallel samples was adapted. Because the diameter of 3 µL water droplet is smaller than 3 mm, the mean pore size, flat places on the foam frame are select as the measured points. 2.3.2 Micromorphology analysis Surface morphology was visualized by FE-SEM instrument (S-4800, Hitachi Limited, Japan). All specimens were sprayed with conductive gold powders for better electrical conductivity that is necessary for sample characterization, then a 2D pictures showing stacking rules of particles were obtained. Furthermore, coatings’ 3D topology was analyzed by atomic force microscope


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(AFM) instrument (nanoscope multimode 8, Bruker Coporation, USA) with tapping mode, then topology image and depth distribution data were obtained. 2.3.3 Particle size distribution The size and size distribution were determined by dynamic light scattering method using Nano-ZS90 (Malvern, UK). 2.3.4 Analysis of surface composition Surface chemistry compositions were analyzed by fourier transform infrared spectroscopy, namely FTIR, (MAGNA-560, Nicolet company, USA). Every sample was scanned for 200 times o with a resolution of 4 cm-1, and the mid-infrared 30 reflection spectral was obtained at the

wavelength range of 4000~400 cm-1.

2.4 Performance test Performance tests consist of acid-base resistance tests, thermal stability tests, stability tests exposed in air and saturated organic gas, and heat-healing ability tests impacted by organic solvents’ contamination. 2.4.1 Acid-base resistance The samples were immersed in solution with various pH values (1, 3, 5, 7, 8, 10, 12, 14) for a series of stepping time (1, 3, 5, 12, 24, 48, 96 hours). The acid and base solutions were buffer solutions, and the neutral solution was doubly distilled water. The immersed samples were then rinsed with water and dried at room temperature for 30 min. 2.4.2 Thermal stability Thermal stability test were performed in the drying oven (DZF, Beijing Yongming Medical Instruments Co., Ltd., Beijing, China) for 2 hours each time at the temperature series of 60~170 ℃. The samples were then cooled down at room temperature.


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2.4.3 Exposure stability The samples were exposed in different gas environments, including natural air and saturated toluene steam, for 100 days to test the exposure stability. 2.4.4 Heat-healing ability In order to identify if samples contaminated by organic solvents can recover original superhydrophobic property, the samples were immersed in various organic solvents for 24 hours and then taken out to measure instantaneous contact angles and contact angles after treated by heating in drying oven for 30 min and the process was circled by 10 times. Toluene, n-butanol and gasoline were selected as model pollutants.

3 RESULTS & DISCUSSION 3.1 Superhydrophobicity o The initial foam SiC material is hydrophilic, whose average water contact angle is 51 . When

the sol was prepared by titrating 3 ml TEOS, the substrates coated by one time presented the o

contact angle of 125 , which means the coated substrates become hydrophobic. And the contact o angle of the substrates coated by 3 times was larger than 150 , which indicates superhydrophobic

coatings were prepared successfully. Therefore, hydrophobicity and superhydrophobicity are obtained successfully on foam SiC materials by adopting above bottom-up strategy. As shown in Fig. 3(a), 3µl water droplet is almost spherical and presents a small contact area only owing to gravity and surface tension. The optical image about superhydrophobic phenomena of large water drop dyed by red ink on foam SiC material is shown in Fig. 3(b). And in video S1, we can observe that liquid droplets roll on superhydrophobic surface without


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permeation on foam SiC materials, even with the pore size of 3 mm. However, an opposing phenomenon, rapid spreading, on hydrophilic foam SiC materials can be seen in video S2. As shown in Figure 4, FE-SEM images indicate that roughness increases with more dipcoating times and the increased roughness is beneficial to superhydrophobicity, and Figure 4(b) shows that there exist discrete particles and stacked particles, which exactly form micro-nano double structure. Furthermore, in order to show the cubic topology formed by SiO2 particles, the coated substrates were characterized by AFM. As shown in Figure 5(a), the cubic topology of the coatings is similar to pillar array. The deep valleys and pillars are arranged regularly. The valley between adjacent pillars is occupied by air, and the needle-like pillars make up rest percentage of the surface. The valley-pillar special microstructure is the foundation of superhydrophobicty. In addition, Figure 5(b) shows that the height of pillars is mostly ranged from 80nm to 130nm. The difference of the height contributes to the sliding property of water droplet on superhydrophobic coatings, as shown in Video S1. The surface chemistry composition, which generates low-surface-energy surface, was characterized by FTIR and shown in Figure 6. There are obvious peaks at the wavenumber of 2917.2 cm-1, 2849.2 cm-1 and 930 cm-1, which represent -CH3, -CH2- and -Si-O-, respectively. The FTIR result confirms the successful grafting of n-octadecyltrichlorosilane to the SiO2 particle surface. More exactly, the chemical reaction process generates cross-linked organic long-chain through -Si-O-Si- bonds and process mechanism is demonstrated in Figure 6.


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Figure 3. Optical images of superhydrophobic surfaces: (a) image of 3µL water droplet on superhydrophobic surface obtained by Contact Angle Measurement Instrument; (b) image of distilled water dyed by red ink on superhydrophobic foam SiC materials.

(a)


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(b)

Figure 4. FE-SEM images of SiO2 coatings with (a) 1 dip-coating time and (b) 3 dip-coating times

(a)


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(b) Figure 5. AFM images of superhydrophobic surfaces: (a) 3D surface profile, and (b) valleys’ depth probability distribution.

Figure 6. FTIR spectra of the chemically unmodified sample and the chemically modified sample.


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3.2 Effects of coating times and particle size on contact angle

Figure 7. Effects of SiO2 coating times on contact angles.

(a)


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(b)

(c)


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(d)

Figure 8. (a-c) Particle size distribution based on 3ml, 2.5ml, 2.0ml ammonium hydroxide， respectively; (d) Effects of coating times and average particle sizes on contact angles of silanized particulate surfaces From previous section, we can preliminarily find that dip-coating parameters had significant effect on contact angles of coatings. Figure 7 shows the relationship between coating times and contact angles in which situation the SiO2 sol is obtained by titrating 3 ml TEOS in ethanol solution containing 3ml ammonium hydroxide. Roughness of surface morphology increased with the increase of coating times, so the contact angle increases gradually. But surface morphology is self-similar with the former one when the coating times continue to increase after 3 coating times, the contact angle tends to maintain stable. As shown in Figure 8(a-c), multiple statistical particle size distribution can be obtained by adding different amount of ammonium hydroxide, and dynamics light scattering measurement showed that the mean diameters of particles were 220 nm, 145 nm, 95 nm. The effect of the


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number of coating layers on contact angle at various particle sizes is shown in Figure 8(d). In fact, the particle shape also has influence on the morphology then the contact angle, the shape of single particle and aggregation can been seen in Figure 4. All the samples become hydrophobic with one coating time, but the change laws are very different when they get more coating times before becoming superhydrophobic. This feature results in a strategy to get coatings with tunable contact angles by adjusting coating times together with particle size. 155o , 140o , 125o can be easily obtained by these three kinds of particle size. Furthermore, the contact angles of

90o ~ 100o can also easily be obtained by dip-coating the solution of n-octadecyltrichlorosilane oligomer. Table 1. The needed coating times under the condition of different particle sizes when surface become superhydrophobic Particle size(nm)

95

145

220

500

1000

1500

Layers

7

2

3

>5(NG)

>5(NG)

>5(NG)

154.0 ± 2.5

158.6 ± 1.4

158.9 ± 1.8

-

-

-

Contact angle

Notes: (1) NG represents that the coating doesn’t become superhydrophobic at the largest number given in the literature. (2) “-” represents that there is no specific values.

Combined with Tsai’s results38, the change law of needed least coating times is shown in Table 1 when surface become superhydrophobic. There is some distinct difference at coating times to be superhydrophobic under the condition of different particle sizes. It is because smaller and nonspherical particles with proper size distribution can produce more random stacking style mathematically, which is helpful for producing rough morphology. Therefore, it’s recommended to use proper particle size and shape for preparing superhydrophobic surfaces. It is also recommended to get more stable superhydrophobic surfaces with less coating times because


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coatings prepared by sol-gel method have the intrinsic weakness of more possible cracking with the increase of coating thickness. Therefore, it is helpful for guiding us to formulate better strategy to quickly fabricate more stable hydrophobic or superhydrophobic surfaces with tunable contact angle.

3.3 Stability of coatings

Figure 9. Effects of pH on surface contact angle. Stability is very important for functional materials during use, especially acid and base solution surroundings, which decide if we can use the material under these conditions. As is shown in Figure 9, the as-prepared superhydrophobic coatings show acid stability in the pH range of 1~7 without obvious decrease of contact angles, even immersed in the acid solution for 96 hours. However, as-prepared coatings are immersed in basic solution with pH of 8~12, the contact angle of coatings will decrease. And the coating presented partial shattering with pH of 14. The reason is that the OH- in the basic solution still have a chance to reacts with bare –OH on the


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surface of SiO2 particles, but the number of bare –OH is limited which can be illustrated by FTIR wavenumber of 3500~3000 cm-1 in Figure 6. The morphology is damaged over time.

Figure 10. Thermal stability of superhydrophobic coatings. In addition to acid-base stability of superhydrophobic surfaces, thermal stability is another important property. Because it provides guidance for the range of application and storage surroundings. We measured the contact angle of superhydrophobic surfaces after the samples were heated for 2 hours with the temperature range from 60 to 170℃. Figure 10 shows the change trend of the as-prepared surfaces with the temperature, and it reveals that the superhydrophobic surfaces can maintain excellent stability when temperature is below 160℃. In order to confirm the reasons why the contact angle decrease sharply at 170℃, we designed an experiment of baking coatings without chemical modification at 170~200℃ and then conducted chemical modification. Consequently, the samples show excellent superhydrophobicity. Therefore, the reason of the loss of superhydrophobicity is partial degradation of organic longchain instead of morphology change.


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Figure 11. Stability of superhydrophobic surfaces exposed in air and toluene saturated steam at room temperature, respectively Other stability experiments of superhydrophobic surfaces were performed in two different surroundings: a container full of natural air and a container full of model organic gas respectively, which are commonly encountered in laboratory or industry. As is shown in Figure 11, the results reveal that superhydrophobic surfaces still maintain superhydrophobic performance even when the experiments were performed continuously for 100 days. It tells us that the as-prepared superhydrophobic surfaces presented excellent stability under various external gas surroundings. This can be explained by good chemical and structural stability of stacked SiO2 particles and noctadecyltrichlorosilane.


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Figure 12. Heat-healing ability of superhydrophobic surfaces upon treatment with various solvents Superhydrophobic surfaces do not necessarily repel oily organic contaminants and may be polluted accidentally. Therefore, we further examined the resistance of the superhydrophobic coating to organic contaminants using toluene, n-butanol and gasoline as the model pollutants and investigated that if superhydrophobicity can be restored by using a certain method. As shown in Figure 12, the contact angles of superhydrophobic coatings decrease from above 150° to less than 90°for all model pollutants. However, after heat treatment in drying oven, the coating surface recovered superhydrophobic. After 10 cycles of contamination and heat treatment, the coatings still remained superhydrophobic. This indicates the coatings owned heathealing ability after polluted by organics. And the heat-healing phenomena can be explained as follows: when model pullutants contaminate the samples, organics fill the valleys of rough surfaces to generate smooth surfaces whose interface with air is consisted of model pollutants’


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molecules, so the measured contact angles are balanced contact angles of water resting on organics. But when the contaminated samples are heated, organic pollutants are removed from valleys of rough surfaces to recover original morphology and chemical compositions without any damage.

3.4 Theoretical prediction strategy Wenzel model and Cassie-Baxter model are popular theories for wettability of rough surface. Wenzel model assumes that surface hydrophobicity can be increased by increasing its microscopic roughness when the original smooth surface is hydrophobic33. As shown in equations (1, 2), roughness factor ( r ) is the key parameter to calculate apparent contact angle (

θc ) of rough surface by the contact angle of original smooth surface ( θ ). However, Cassie-Baxter model assumes that hydrophobicity is produced by trapped air in the valleys because of trapped air’s blockage for water’s invasion. The apparent phenomenon is that water droplet is partially suspended by air pockets34, 39. As shown in Equations (3, 4), apparent contact angle is calculated by two parameters of area fraction of liquid-solid contact ( f1 ) and liquid-air contact ( f 2 ) coupled with the contact angle of smooth surface ( θ ). These two theoretical models correspond to two different states of hydrophobicity, called Wenzel state and Cassie-Baxter state. And their dynamic behaviors are very different39, 40.

cos θc = r cos θ r=

the area of actual surface the area of geometric surface

cos θc = f1 cosθ − f 2

(1) (2) (3)


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As shown in Video S1 and S2, as-prepared superhydrophobic surface shows more like CassieBaxter dynamic behavior of slipping rather than Wenzel state of pinning. So we propose that Cassie-Baxter model is more proper to predict contact angles35. As shown in Figure 4(a), the area percentage occupied by SiO2 particles is calculated to be 37% by Image-Pro Plus software. The intrinsic contact angle of smooth surface modified by n-octadecyltrichlorosilane is 105o 38, 41, then the predicted contact angle value is 136.5o by Cassie-Baxter model. Compared with the average measurement value of 127.2o , the error of theoretical prediction is just 7.3%. But when the substrates are coated time by time, the stacked SiO2 particles will occupy almost the whole area in 2D view, it will be hard to statistic the area fraction. So, it needs a new strategy to get adequate and accurate data of area fraction. Obtaining 3D scanning images by AFM is an alternative method. As shown in Figure 4(a), experimental value of contact angle is 158.9o . The value of f 2 is 0.072 by AFM bearing analysis. Then 161.1o can be calculated by Cassie-Baxter model. Comparing experimental value with theoretical prediction value, we can get a small error, 0.77%. However, according to Lei Jiang et al.42, six different Contact Angle states are possible for superhydrophobic surfaces, then it will produce some errors that we hypothesize the state of as-prepared samples as strict Cassie state. In addition, other aspects, including the imperfection and scope of application of Cassie-Baxter model34 and ignoring the effect of pattern and geometry43, also bring some errors. But it is reasonable, important and acceptable to capture the key characteristics and neglect some terms in modelling and prediction44. In short, this theoretical prediction strategy shows good match between the experimental value and the calculated value. Meanwhile, validity of Cassie-Baxter model is also proven to some extent.

4 CONCLUSIONS ACS Paragon Plus Environment

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In summary, we have demonstrated a bottom-up strategy for the fabrication of surperhydrophobic coatings on foam SiC material using dip-coating and surface chemical modification. The contact angle of coated foam SiC material can be tuned to 155o , 140o , 125o and 95o by adjusting coating times and particle size. This provides possibilities for fundamental research of foam material related to wettability, specifically gas-liquid two-phase flow in distillation. And then we point out that particle size is a critical parameter for fast fabrication of stable superhydrophobic surfaces. Mean particle diameters of 220 nm, 145 nm, 95 nm and coating times from 0 to 25 are investigated. Performance tests indicate coatings presented good stability, still owning contact angle larger than 150o , with acid-neutral solutions, thermal stability below 160℃, exposure stability under various ambience and excellent heat-healing ability after polluted by organics. Finally, a theoretical prediction strategy by surface microscopic morphology and surface chemical composition based on Cassie-Baxter model is put forward and the error is less than 8%. This opens a path to produce stable hydrophobic or superhydrophobic on foam ceramic material and subsequent fundamental research and industrial application.

SUPPORTING INFORMATION Video S1: water droplet behavior on superhydrophobic foam SiC material Video S2: liquid behavior on hydrophilic foam SiC material This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: +86-022-27404701; Fax: +86-022-27404705

ACKNOWLEDGEMENTS The authors acknowledge financial support from National Nature Science Foundation of China (Nos. 21306128 and 21336007), National High Technology Research and Development Program of China (No. 2015AA03A602) and Key Technology R&D Program of Tianjin (No. 15ZCZDGX00330), and gratefully acknowledge the Institute of Metal Research, Chinese Academy of Science, for providing foam SiC materials.

REFERENCES (1) Kim, S. J.; Kim, D., Discussion: Heat transfer measurement and analysis for sintered porous channels" ( Hwang, G. J., and Chao, C. H., 1994, ASME J. Heat Transfer, 116, pp. 456-464). J. Heat Transfer 2000, 122, 632-633. (2) Li, X. G.; Gao, G. H.; Zhang, L. H.; Sui, H.; Li, H.; Gao, X.; Yang, Z. M.; Tian, C.; Zhang, J. S., Multiscale Simulation and Experimental Study of Novel SiC Structured Packings. Ind. Eng. Chem. Res. 2012, 51, 915-924. (3) Zhang, L. H.; Liu, X. K.; Li, H.; Sui, H.; Li, X. G.; Zhang, J. S.; Yang, Z. M.; Tian, C.; Gao, G. H., Hydrodynamic and Mass Transfer Performances of a New SiC Foam Column Tray. Chem. Eng. Technol. 2012, 35, 2075-2083.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

(4) Gao, X.; Li, X. G.; Liu, X.; Li, H.; Yang, Z. M.; Zhang, J. S., A novel potential application of SiC ceramic foam material to distillation: foam monolithic tray. Chem. Eng. Sci. 2015, 135, 489500. (5) Li, H.; Wang, F.; Wang, C.; Gao, X.; Li, X., Liquid flow behavior study in SiC foam corrugated sheet using a novel ultraviolet fluorescence technique coupled with CFD simulation. Chem. Eng. Sci. 2015, 123, 341-349. (6) Lei, Z.; Chen, B.; Ding, Z., Special Distillation Processes; Elesevier : Amsterdam, 2005. (7) Castricum, H. L.; Sah, A.; Mittelmeijer-Hazeleger, M. C.; ten Elshof, J. E., Hydrophobisation of mesoporous gamma-Al2O3 with organochlorosilanes - efficiency and structure. Microporous Mesoporous Mater. 2005, 83, 1-9. (8) Zhao, J.; Zhao, X. T.; Jiang, Z. Y.; Li, Z.; Fan, X. C.; Zhu, J. N.; Wu, H.; Su, Y. L.; Yang, D.; Pan, F. S.; Shi, J. F., Biomimetic and bioinspired membranes: Preparation and application. Prog. Polym. Sci. 2014, 39,1668-1720. (9) Zhu, H. G.; Chen, D. Y.; Li, N. J.; Xu, Q. F.; Li, H.; He, J. H.; Lu, J. M., Graphene Foam with Switchable Oil Wettability for Oil and Organic Solvents Recovery. Adv. Funct. Mater. 2015, 25, 597-605. (10) Zhang, X. Y.; Li, Z.; Liu, K. S.; Jiang, L., Bioinspired Multifunctional Foam with SelfCleaning and Oil/Water Separation. Adv. Funct. Mater. 2013, 23, 2881-2886. (11) Dai, C.; Lei, Z.; Zhang, J.; Li, Y.; Chen, B., Monolith catalysts for the alkylation of benzene with propylene. Chem. Eng. Sci. 2013, 100, 342-351.


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Subramanian, K.; Wozny, G., Analysis of Hydrodynamics of Fluid Flow on Corrugated Sheets of Packing. Int. J. Chem. Eng. 2012, 2012, 269-278. (13) Szulczewska, B.; Zbicinski, I.; Górak, A., Liquid Flow on Structured Packing: CFD Simulation and Experimental Study. Chem. Eng. Technol. 2003, 26, 580-584. (14) Li, X.; Fan, H.; Fan, X., Effect of chemical structure of organics on pore wetting. Chem. Eng. Sci. 2015, 137, 458-465. (15) Dai, C.; Lei, Z.; Zhang, J.; Li, Q.; Chen, B., Simulation of structured catalytic packings in a bubble-point reactor. Chem. Eng. Sci. 2013, 100, 373-383. (16) Li, W.; Nie, Z. H.; Zhang, H.; Paquet, C.; Seo, M.; Garstecki, P.; Kumacheva, E., Screening of the effect of surface energy of microchannels on microfluidic emulsification. Langmuir 2007, 23, 8010-8014. (17) Byakova, A. V.; Gnyloskurenko, S. V.; Nakamura, T.; Raychenko, O. I., Influence of wetting conditions on bubble formation at orifice in an inviscid liquid - Mechanism of bubble evolution. Colloids Surf., A 2003, 229, 19-32. (18) Gnyloskurenko, S. V.; Byakova, A. V.; Raychenko, O. I.; Nakamura, T., Influence of wetting conditions on bubble formation at orifice in an inviscid liquid. Transformation of bubble shape and size. Colloids Surf., A 2003, 218, 73-87. (19) Li, H.; Fu, L.; Li, X.; Gao, X., Mechanism and analytical models for the gas distribution on the SiC foam monolithic tray. AIChE J. 2015, 61, 4509-4516. (20) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q., Superhydrophobic surfaces: from structural control to functional application. J. Mater. Chem. 2008, 18, 621-633.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

(21) Wang, B.; Liang, W. X.; Guo, Z. G.; Liu, W. M., Biomimetic super-lyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature. Chem. Soc. Rev. 2015, 44, 336-361. (22) Ueda, E.; Levkin, P. A., Emerging Applications of Superhydrophilic-Superhydrophobic Micropatterns. Adv. Mater. 2013, 25, 1234-1247. (23) Lu, Y.; Sathasivam, S.; Song, J. L.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P., Robust selfcleaning surfaces that function when exposed to either air or oil. Science 2015, 347, 1132-1135. (24) Bellanger, H.; Darmanin, T.; de Givenchy, E. T.; Guittard, F., Chemical and Physical Pathways for the Preparation of Superoleophobic Surfaces and Related Wetting Theories. Chem. Rev. 2014, 114, 2694-2716. (25) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D., Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67-70. (26) Gu, G. T.; Tian, Y. P.; Li, Z. T.; Lu, D. F., Electrostatic powder spraying process for the fabrication of stable superhydrophobic surfaces. Appl. Surf. Sci. 2011, 257, 4586-4588. (27) Latthe, S. S.; Imai, H.; Ganesan, V.; Rao, A. V., Superhydrophobic silica films by sol–gel co-precursor method. Appl. Surf. Sci. 2009, 256, 217–222. (28) Olin, P.; Hyll, C.; Ovaskainen, L.; Ruda, M.; Schmidt, O.; Turner, C.; Wågberg, L., Development of a Semicontinuous Spray Process for the Production of Superhydrophobic Coatings from Supercritical Carbon Dioxide Solutions. Ind. Eng. Chem.Res. 2015, 54, 10591067.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Xu, W. G.; Shi, X. F.; Lu, S. X., Controlled growth of superhydrophobic films without any low-surface-energy modification by chemical displacement on zinc substrates. Mater. Chem. Phys. 2011, 129, 1042-1046. (30) Bohn, W. S. A. F. E., Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62-69. (31) Tsai, P. S.; Yang, Y. M.; Lee, Y. L., Hierarchically structured superhydrophobic coatings fabricated by successive Langmuir-Blodgett deposition of micro-/nano-sized particles and surface silanization. Nanotech. 2007, 18, 465-604. (32) Oliveira, N. M.; Reis, R. L.; Mano, J. F., Superhydrophobic Surfaces Engineered Using Diatomaceous Earth. ACS Appl. Mater. Interfaces 2013, 5, 4202-4208. (33) Wenzel, R. N., Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 7. (34) A. B. Cassie; S, B., Wettability of porous surfaces. Trans. Faraday. Soc. 1944, 40, 6. (35) Farsinezhad, S.; Waghmare, P. R.; Wiltshire, B. D.; Sharma, H.; Amiri, S.; Mitra, S. K.; Shankar, K., Amphiphobic surfaces from functionalized TiO 2 nanotube arrays. Rsc Adv. 2014, 4, 33587-33598. (36) Gao, L. C.; McCarthy, T. J., A perfectly hydrophobic surface (theta(A)/theta(R)=180 degrees/180 degrees). J. Am. Chem. Soc. 2006, 128, 9052-9053. (37) Zhang, C.; Zhou, W.; Wang, Q.; Wang, H.; Tang, Y.; Hui, K. S., Comparison of static contact angle of various metal foams and porous copper fiber sintered sheet. Appl. Surf. Sci. 2013, 276, 377-382.


29


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Page 30 of 31

(38) Tsai, P. S.; Yang, Y. M.; Lee, Y. L., Fabrication of hydrophobic surfaces by coupling of Langmuir-Blodgett deposition and a self-assembled monolayer. Langmuir 2006, 22, 5660-5665. (39) Wang, S.; Jiang, L., Definition of superhydrophobic states. Adv. Mater. 2007, 19, 34233424. (40) Tian, Y.; Su, B.; Jiang, L., Interfacial Material System Exhibiting Superwettability. Adv. Mater. 2014, 26, 6872-6897. (41) Fadeev, A. Y.; McCarthy, T. J., Self-assembly is not the only reaction possible between alkyltrichlorosilanes and surfaces: monomolecular and oligomeric covalently attached layers of dichloro-and trichloroalkylsilanes on silicon. Langmuir 2000, 16, 7268-7274. (42) Xia, F.; Jiang, L., Bio-inspired, smart, multiscale interfacial materials. Adv. Mater. 2008, 20, 2842-2858. (43) Cansoy, C. E.; Erbil, H. Y.; Akar, O.; Akin, T., Effect of pattern size and geometry on the use of Cassie–Baxter equation for superhydrophobic surfaces. Colloids Surf., A. 2011, 386, 116124. (44) Koch, B. M. L.; Amirfazli, A.; Elliott, J. A. W., Modeling and Measurement of Contact Angle Hysteresis on Textured High-Contact-Angle Surfaces. J. Phys. Chem. C 2014, 118, 18554-18563.


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Fabrication of Tunable, Stable, and Predictable Superhydrophobic

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