Study of Oil Dewetting Ability of Superhydrophilic ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 18865−18875

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Study of Oil Dewetting Ability of Superhydrophilic and Underwater Superoleophobic Surfaces from Air to Water for High-Effective SelfCleaning Surface Designing Lei Tang,†,‡ Zhixiang Zeng,*,† Gang Wang,*,† Luli Shen,† Lijing Zhu,† Yingxin Zhang,§ and Qunji Xue†

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†

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science, Ningbo 315201, P. R. China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § School of Materials Science & Engineering, Ningbo University of Technology, Ningbo 315211, People’s Republic of China S Supporting Information *

ABSTRACT: The superhydrophilic self-cleaning surface can perfectly deal with oil pollution, which cannot be realized by the superhydrophobic surface. This research is designed to study the mechanism of wetting behavior of superhydrophilic coating with different function groups and guide to design a stable self-cleaning surface. We prepare several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. The results indicate that the wettability of the coating to water and oil is the key factor to determine the self-cleaning performance. The smooth 3-sulfopropyl methacrylate potassium salt (SA) anionic coating shows the best self-cleaning performance even when polluted by heavy crude oil in the dry state in air. It is also found that in the dry state, the rough hydrophilic anionic surface will lock up the oil in the structures and then lose its self-cleaning ability underwater, whereas the oil droplet can detach from the smooth coating surface quickly. Meanwhile, the superhydrophilic and underwater superoleophobic SA anionic surfaces also exhibit excellent anti-fogging and oil−water separation performance. KEYWORDS: self-cleaning, superhydrophilic, underwater superoleophobic, ionic, zwitterionic

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take away the dusts without leaving any trace,10,11 whereas water can completely spread out on some surfaces such as Calathea zebrina and Ruellia devosiana leaves and wash away the pollutants.12 In nature, wettability is a unique method for some species used to realize self-cleaning.13 According to wettability, self-cleaning surfaces can be briefly divided into two categories: superhydrophobicity and superhydrophilicity.14 The former is defined as a surface with the water contact angle (WCA) greater than 150° and the sliding angle lower than 10°.15 For a superhydrophobic surface, the water droplet can roll on the surface freely and take away the contaminated particles and liquids, keeping the surface clean.16 However, the superhydrophobic surface is powerless for the oil pollution because the superhydrophobic surface is always oleophilic because of its high surface energy. Therefore, water-

INTRODUCTION

A self-cleaning surface is defined as a surface on which pollutants or dust can automatically detached or be degraded under the actions of wettability, light, electricity, and heat.1−3 The self-cleaning surface can not only save people’s financial and material resources but also improve the performance and service life of materials.4−7 This demand is particularly prominent in some high-tech fields. Dirt will seriously affect the photoelectric conversion efficiency of solar panels and cause much power loss, whereas manual cleaning is difficult and cleaning agents will pollute the environment.8 Marine biofouling is more serious, which not only increases the resistance and energy consumption of ships but also accelerates the corrosion of the ship surface and even causes the invasion of alien species.9 Inspired by some organisms in nature, a selfcleaning phenomenon has been gradually recognized by human beings. In addition, the manifestations of the selfcleaning phenomenon are different. For lotus leaves and butterfly wings, the water droplet can completely roll off and © 2019 American Chemical Society

Received: March 20, 2019 Accepted: April 30, 2019 Published: April 30, 2019 18865

DOI: 10.1021/acsami.9b04948 ACS Appl. Mater. Interfaces 2019, 11, 18865−18875

Research Article

ACS Applied Materials & Interfaces

chemistry, wettability, and surface morphology.33 According to our previous findings, the α-cellulose-based surface had a unique oil detachment ability even when it was wetted by oil in air in 2015.34 We considered that the excellent competitive wettability of α-cellulose molecules played major role to selfcleaning. In recent days, Zhou and co-workers further studied this wetting behavior of α-cellulose.35 There are many types of hydrophilic molecules, but their oil detachment abilities are different, and their oil detachment mechanisms are still unclear. Therefore, systematic studies are necessary to figure out the function of different hydrophilic functional groups on the oil dewetting process. In order to further study the mechanism of the wetting behavior of various hydrophilic molecules with different function groups and guide to design stable self-cleaning surface, in this work, several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings are prepared to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state and three hydrophobic coatings are chosen as contrast. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. Experiments show that the wettability of the coating surface to water and oil is the core factor determining its performance. Anionic coating shows the best self-cleaning performance because of its extremely strong water absorption and surface hydration ability. It is also found that the surface roughness would be a hinder to self-cleaning because of the interlocking effect of the rough structure, whereas the smooth surface shows an excellent self-cleaning property. In addition, the anionic surface also shows excellent performance in antifogging and oil−water separation. Meanwhile, the environmental and temporal stabilities of the coating are also satisfactory. This research can help us to have a better understanding of the superhydrophilic and underwater superoleophobic self-cleaning system and provide a simple preparation method to construct a self-cleaning coating.

insoluble organic substances such as oil contamination cannot leave the surface with the water droplet. It is worth noting that the superamphiphobic surface is both superhydrophobic and superoleophobic; thus, it is the most ideal self-cleaning material. However, it requires a specific micro−nanocomposite structure such as concave angle and overhang.17,18 Overdependence on the structure greatly limits its applications. In addition, the stability is also poor, which will turn the Cassie state (low liquid adhesive surface) to the Wenzel state (high liquid adhesive surface) and then lose superamphiphobicity.19 Therefore, a stable superoleophobic surface is desired to realize self-cleaning.20,21 As we all know, it is still a world challenge to fabricate a stable superoleophobic surface. Fortunately, it is found that a hydrophilic surface in air (liquid−gas−solid threephase interface) will be oleophobic underwater (liquid− liquid−solid three-phase interface).22 With the wettability amplification of roughness, a superhydrophilic and underwater superoleophobic surface with WCA lower than 10° and underwater oil contact angle higher than 150° is easy to implement and be found in nature, such as fish surface and lower side of the lotus surface. It is proved that water spreads rapidly on the surface to form a water film and penetrates between pollutants and surface so as to achieve the selfcleaning.23 In addition, by using photoinduced superhydrophilicity with photocatalytic degradation to clean contamination of some semiconductor materials such as TiO2 or ZnO is an effective approach to realize self-cleaning.24,25 However, this photochemical method is still being studied because of the low degradation rate and ability of photocatalysts and the toxicity of the degradation products. The superhydrophilic self-cleaning system is favored and widely used because it has a wide range of sources (extensive hydrophilic monomers), simple preparation methods, and various applications such as oil−water separation, anti-fogging, and anti-fouling.26−28 However, as mentioned above, a superhydrophilic surface is oleophilic in air which also called amphiphilic. This requires that the surface must be wetted by water in advance to form a hydration layer before use. Otherwise, once the surface is wetted by oil and forms the stable Wenzel state in air, the oil cannot be replaced by water when it is immersed in water.29 In recent years, with the development of superhydrophilic coatings, only a few types of hydrophilic coatings have been reported to have excellent desorption properties even after they are wetted by oil in air and then exposed to water. He and co-workers reported a zwitterionic surface which not only showed complete oil repellency in a water-wetted state but also realized the cleaning of oil fouling in the dry state by water.30 In addition to coating, membranes and other materials can also achieve satisfactory results. Gao and co-workers have prepared a robust hydrogel membrane composed of sodium polyacrylate-grafted poly(vinylidene fluoride) (PVDF) that possessed an unparalleled anti-oil-adhesion property.31 Deng and co-workers fabricated a series of PVDF membranes via phase separation in combination with in situ cross-linking copolymerization of poly(methyl methacrylate-co-glycidyl methacrylate) and polyethyleneimine which exhibited superhydrophilicity and underwater superoleophobicity as well as underwater anti-oil-fouling performance.32 Membrane materials can also realize other functions through surface manipulation. Yang and co-workers designed a liquid/liquid interface-confined surface engineering strategy to fabricate a Janus membrane which owning a liquidunidirectional transportation feature due to its asymmetric

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EXPERIMENTAL SECTION

Materials. 2-Hydroxyethyl methacrylate (HEMA 99%), acrylamide (AM 99%), 2-acrylamido-2-methylpropane sulfonic acid (AMPS 98%), 3-sulfopropyl methacrylate potassium salt (SA, 96%), methacryloxyethyltrimethyl ammonium chloride (TMA 75%), undecylenic acid (98%), 1-decene (95%), benzoin dimethyl ether (DMPA 99%), α-ketoglutaric acid (98%), methylene-bis-acrylamide (BIS 99%), cyclohexane (99.5%), octane (>99%), hexadecane (98%), xylene (99%), oil red O, and methylene blue (≥70%) were purchased from Aladdin Industrial Corporation (Shanghai, China). 1H,1H,2H,2H-Heptadecafluorodecyl acrylate (Viscoat 17F), acrylic acid (≥98%), castor oil, petroleum ether, dichloromethane (≥99.5%), anhydrous alcohol (≥99.7%), acetone (≥99.5%), sodium hydroxide (NaOH), sodium chloride (NaCl ≥99.8%), hydrogen peroxide (H2O2 30%), sulfuric acid (H2SO4 95.0−98.0%), and hydrochloric acid (HCl 36.0−38.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). Sulfobetaine methacrylate (SBMA 97%) was purchased from Sigma-Aldrich (Shanghai, China). All reagents mentioned above were used without further purification. Sandpapers (1200 grit) were purchased from Suisun Co., Ltd (Hong Kong, China). Glass slides were purchased from Leigu Instrument Co., Ltd (Shanghai, China). Crude oil was obtained from Zhenhai Oil Refining and Chemical Company, Sinopec. Copper mesh, aluminum sheet, copper−zinc alloy, and nonwoven and polycarbonate (PC) plastic were all purchased from local stores. All of the water used is deionized water which is home-made in our laboratory. 18866


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ACS Applied Materials & Interfaces Preparation of Coatings with Different End Groups. All coating samples were prepared by free-radical polymerization. Glass slides with dimensions of 75 × 25 × 1 mm3 and sandpapers (1200 grit) were used as substrates. Before use, they were ultrasonically cleaned with acetone, ethanol, and deionized water for 10 min. The uncoated Si−OH surfaces were prepared by immersing glass slides into a newly configured Piranha Solution [a mixture of 3:1 (v/v) 98% sulfuric acid and 30% hydrogen peroxide] for 2 h at 85° and used immediately after cleaning with ethanol. For oil soluble monomers, monomer (0.04 mol TMA, HEMA, AM, AA, undecylenic acid, 1-decene, and viscoat 17F), cross-linking agent BIS (3 mmol), and initiator DMPA (0.4 mmol) were dissolved in anhydrous alcohol (0.2 mol). After stirring magnetically for 20 min, the solutes were fully dissolved. Then, the solution was coated evenly on the substrate by a Spin Processor. After spin coating, the substrates were cured under an UV Curing Machine (4000 W) for 3 min. For water-soluble monomers, monomer [14 mmol SA, SBMA, AMPS, 5/5 (n/n) TMA−SA mixture], cross-linking agent BIS (1.4 mmol), and initiator α-ketoglutaric acid (0.14 mmol) were dissolved in 5/5 (w/w) water−ethanol mixtures (10 g). The follow-up steps are the same as oil soluble monomers. The effect of ethanol is to reduce the surface tension of the solution for spin coating. The structural formulas of all of the polymers used are illustrated in Scheme 1.

laboratory air). The oil−water separation experiment was conducted using a home-made device. The coated copper mesh was fixed between two glass tubes, and then, the oil−water mixture was poured in and the separation process was driven by gravity.

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RESULTS AND DISCUSSION Chemical composition and surface micro-/nanostructures are the two main factors for superhydrophilic and underwater superoleophobic surfaces.36 We first study the effect of chemical composition on the self-cleaning ability. Eleven kinds of coatings with different end groups, including 4 kinds of ionic groups (anion, cation, zwitterions, and an equivalent mixture of anion and cation), 4 kinds of nonionic hydrophilic groups (hydroxyl, acylamino, sulfonate, and short-chain carboxyl), and 3 kinds of nonionic hydrophobic groups (fluoroalkyl, alkyl, and long-chain carboxyl), are prepared and tested the oil desorption property using two kinds of high viscosity oil, castor oil and crude oil. The chemical composition of the coatings is characterized by EDS and Micro-FTIR. The spectra of all coatings are shown in Figures S2−S4, and Table S1 contains the peak-group correspondence in all spectra. According to these spectra, all of the coatings are successfully prepared on the substrates. The self-cleaning test can be described as follows: we first contaminate the surface with an oil droplet in the dry state in air, and then we put the surface into water to test the oil desorption performance. There are three main phenomena in the process (Figures 1a−c and S5). When the SA coating surface is immersed into the water, the oil layer on the coating shrinks into a droplet rapidly and detaches in 2.2 s (Figures 1a and S5a). For the uncoated Si−OH surface, the oil layer can also shrink into a sphere but without detaching, so the droplet is adhered on the surface (Figures 1b and S5b). In contrast, the oil layer on the 1-decene coating surface can only shrink into a hemisphere with a contact angle about 45° (Figures 1c and S5c). Figure 1d,e shows the underwater oil contact angles of different coatings wetted by castor oil and light crude oil in the dry state and the detachment time (because of the detachment of the oil droplet from some surfaces, the under WCAs of these coatings are 180°. The whole process of the oil drop from deformation to desorption is shown in Figure S6). The trends of Figure 1d,e are roughly the same (crude oil is more difficult to desorb because of its high viscosity). From the two uncoated surfaces and the surface with hydroxyl, we can draw the conclusion that hydrophilic group and coating can improve the oil desorption performance. Overall, the oil desorption performance of ionic coatings is better, and the anionic coating is the best which may be attributed to the larger ionic radius and the stronger hydration ability caused by it,37,38 followed by cationic coating, the equivalent mixture of anionic and cationic coating, and zwitterionic coating ranks the last. Hydrophilic nonionic coatings also have a certain oil desorption ability, but the properties differ greatly. The specific order is as follows (from good to bad): sulfonate, acylamino, hydroxyl, and shortchain carboxyl coating. Although the oil desorption performance of hydrophobic nonionic coatings is unsatisfactory, these surfaces exhibit oleophilic underwater. The wetting behavior of coating to water and oil directly determines the self-cleaning ability in water and help us to understand the difference of different coatings in oil desorption performance better. Figure 2 shows the SRs of all coatings and dynamic changes of WCAs on these surfaces. After comparison, we can find that the hydrophilicity of the coating

Scheme 1. Structural Formulas of the Polymers on all Coating Surfaces

The block hydrogel materials were prepared as follows: the solutions were injected into a pre-designed transparent glass mold (two silicone-sealed glass plates), and the mold was placed under a UV lamp (38 W) for 5 h to obtain the hydrogels. Then, the hydrogels were soaked in deionized water to remove the unreacted monomers. Characterization. The chemical compositions of coatings were characterized by a Micro-Fourier transform infrared spectroscopy (Micro-FTIR, Cary 660 + 620, U.S.). The contact angles were measured by a contact angle meter (OCA 20, Germany) with a water or oil droplet of 2.0 μL. The surface morphologies were observed by a high-resolution scan electron microscopy (FEI Quanta 250 FEG, U.S.) equipped with an energydispersive spectrometer (EDS) operated at an acceleration voltage of 20.0 kV. The surface adhesion forces were detected using a dynamic contact angle measuring device and a tension meter (DCTA 21, U.S.). The detailed information is shown in Figure S1. The surface roughness was measured by an atomic force microscope (Dimension 3100V, U.S.) and a laser scanning confocal microscopy (LSM700, Germany). The swelling ratio (SR) of the coating was calculated from the following expression

SR = (Ws − Wd)/Wd

(1)

where Ws is the weight of the swollen hydrogels and Wd is the weight of the freeze-dried hydrogels. All camera photos were taken with a high-speed camera (Fastcam Mini UX100, Photron Inc.) with different frame rates of 50, 125, and 500. The anti-fogging performance was tested by steam test (samples were placed above the beaker containing boiled water) and freeze test (samples were placed in the freezer at −18 °C for 1 h and then taken out to humid 18867


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Figure 1. (a−c) High-speed camera photographs of the oil desorption processes of (a) SA coating; (b) uncoated Si−OH; and (c) 1-decene coating surfaces. The oil is castor oil dyed by oil red O. The frame rate used here is 50 fps. (d,e) Underwater oil contact angles and desorption time of different coatings wetted by (d) castor oil and (e) light crude oil in the dry state (the initial surface refers to the ultrasonically cleaned uncoated surface; the (N)hydroxyl surface refers to the uncoated Si−OH surface; the (L-c)carboxyl surface refers to the hydrophobic long-chain carboxyl coating surface; and the (S-c)carboxyl surface refers to the hydrophilic short-chain carboxyl coating surface. The same below).

Figure 2. (a−c) Dynamic change of WCA on (a) ionic; (b) nonionic hydrophilic; and (c) nonionic hydrophobic coating surfaces. The interval between the run numbers is 0.05 s. (d) SRs and contact angles of all coatings.

and being more oleophilic relatively though the WCAs are less than 90°). It is noteworthy that for AA coating and HEMA coating, the former has a greater degree of SR and it is generally believed that each carboxyl group can form two intermolecular hydrogen bonds, so its hydrophilicity should be better than the latter. However, in fact, the situation is just the opposite. This may be attributed to the larger roughness of the AA coating surface. As Figure S7 shows, the roughness of the AA coating surface is much higher than others (Figure S7a). There are many pits with different sizes on the surface, which is not as smooth as other coating surfaces (Figure S7b,c). Similarly, the wetting behavior of the water droplet on these surfaces under oil circumstance can further confirm this point. Figure 3 shows the WCAs of all coatings under hexane (the

is basically the same as the oil desorption performance described above. Although the SRs of ionic coatings are limited, the strong hydration between ions and water molecules still ensures their excellent oil desorption performance. Correspondingly, hydrophilic nonionic coatings have a greater degree of SR, but the surface groups rely on the hydrogen bond to bind to water molecules. Because the binding strength is much smaller than that of ionic bonds produced by hydration, so their oil desorption performance is slightly inferior. Although for hydrophobic nonionic coatings, their SRs are very small. In addition, there is no interaction between surface groups and water molecules, so they show oleophilic underwater (1-decene and undecylenic acid coatings are defined as hydrophobic because of their extremely low SR 18868


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illustrate the interaction between the oil droplet and the coating surface more accurately, the contact processes underwater were recorded by a tension meter and a highspeed camera. Figure 5 reveals the adhesion force curves and adhesive behaviors of three typical coating surfaces. Because the oil droplet is affected by buoyancy in water, the force is not constant when moving in the vertical direction. However, we can judge the difference in adhesive behaviors from the inflection point in the curves. As Figure 5a shows, the curve has only two distinct inflection points. Point A is the point at which the machine begins to adjust the initial position, and point B is the point at which the contact begins to detach. These two points are the same for each curve. There is no other inflection point, indicating that there is no obvious force on the oil droplet in the detaching process. That is to say, the adhesion force of this surface to the oil droplet underwater is minimal (Figure 5b). The bounding test of an oil droplet can further prove this conclusion. As Figure S9 shows, when an oil droplet falls from a certain height underwater, it bounces on the surface and then rolls off spontaneously, whereas another inflection point C appears in Figure 5c. It is the separating point, suggesting that the oil droplet has been subject to a distinct adhesion force before detaching, and part of the oil droplet remains on the surface after detachment (Figure 5d). For an oleophilic surface, the oil droplet spreads out instantly at the contacting moment (Figure 5f), which is characterized as the inflection point D in Figure 5e. However, the oil droplet is less stressed when detaching from the surface. The adhesion force curves of other surfaces are shown in Figure S10, suggesting that the change rule of adhesion force coincides with the change rule of hydrophilicity. Consequently, we can tentatively draw the conclusion that the wettability of the coating is the most important factor affecting its self-cleaning performance. The better the hydrophilicity, the stronger the interaction force between water molecules and surface groups, and the easier it is to absorb water and desorb oil. In the meanwhile, the oil adhesion force underwater would decrease, which further promotes the self-cleaning performance. In Figure 5g, we further jet a trickle of hexane on the SA coated and uncoated surfaces. All of the oil droplets can bounce off over the coated surface without leaving any trace, demonstrating excellent anti-oil-adhesion performance underwater, where-

Figure 3. WCAs of all of the coating surfaces under hexane.

fluoroalkyl coating surface is not included in the subsequent experiments because of its strong hydrophobicity and underwater oleophilicity). The results are basically the same as those in Figure 1. The better the hydrophilicity of the coating, the stronger the oil desorption performance. Water droplets can completely wet those superhydrophilic surfaces and be absorbed by the coatings, but only partially wet or even not wet the surfaces with poor hydrophilicity. Three typical processes of a water droplet contacting the surface immersed in hexane are shown in Figure S8. When the surface was taken out, the water droplet has been absorbed and could not move a bit on the SA hydrogel coating (Figure S8a). However, on the HEMA hydrogel coating surface, part of the water could slip off the surface (Figure S8b), whereas the whole droplet can roll down from the 1-decene coating surface (Figure S8c). In addition to the wettability to water, the difference in the wettability to oil of the coatings also affects the oil desorption performance. Figure 4 shows the dynamic oil contact angles in air and static oil contact angle underwater of all coating surfaces. Because the surface tension of oil is much smaller than that of water, all surfaces are oleophilic in air. However, no matter in air or underwater, the hydrophilicity and oleophilicity of the coatings are antagonistic. In other words, the stronger the hydrophilicity is, the weaker the oleophilicity is. Only AA hydrogel coating shows a difference because of the roughness and surface morphology (Figure S7). In order to

Figure 4. (a−c) Dynamic change of oil contact angles on (a) ionic; (b) nonionic hydrophilic; and (c) nonionic hydrophobic coating surfaces. The interval between the run numbers is 0.02 s. (d) Underwater oil contact angles of different coating surfaces. The oil used here is hexane. 18869


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Figure 5. (a−f) Adhesion force as a function of position and high-speed camera photographs of adhesion behavior for (a,b) SA, (c,d) HEMA, and (e,f) 1-decene coating surfaces. The frame rate used here is 125 fps. (g) Photographs of a trickle of hexane (dyed by oil red O) jetted on the SA hydrogel coated surface (top) and uncoated surface (bottom) to reveal the underwater anti-oil-adhesion performance.

Figure 6. High-speed camera photographs and diagrammatic sketches of the process of immersing the hexane fouled surface into the water. (a,c) Rough surface; (b,d) smooth surface. The frame rate used here is 500 fps.

as oil droplets would gather on the uncoated surface even that hexane is a kind of low viscosity and low density oil. We further investigated the effect of surface structure on selfcleaning performance. Sandpaper is a kind of commonly used abrasive material with small sand grains densely lined on the surface; thus, it can also be used to prepare a simple superhydrophobic or underwater superoleophobic surface.39,40 Here, we chose 1200 grit sandpaper (particle size is small enough to produce underwater superoleophobicity) as the rough surface and glass slide as the smooth surface with the same SA hydrogel (best oil desorption performance showed in Figure 1d,e) coating for comparison to test the oil desorption performance when the surface has been already wetted by oil in the dry state. Figure 6 reveals the high-speed camera

photographs and diagrammatic sketches of the processes of immersing the hexane-fouled rough surface and smooth surface into the water, respectively. As shown in Figure 6a and Movie S1, before immersion, oil could spread completely on the rough surface in air and there is no change when putting the sample into the water for a period of time. However, Figure 6b and Movie S2 indicate that the smooth surface shows the similar oil wettability in air, but when the surface is immersed into the water, the oil layer spontaneously shrinks into a sphere and quickly detaches from the surface. According to this difference, we can draw two diagrammatic sketches to show more vividly. As shown in Figure 6c,d (black, gray, orange, and blue areas represent substrate, coating, oil phase, and water phase, respectively. The same below), the oil layer can move 18870


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Figure 7. (a) Contact angle WCA-A (square), OCA-W (circle), and WCA-O (triangle) of smooth surface and rough surface, the oil used here is hexane; (b) surface morphologies of the smooth surface before and after coating; (c) surface morphologies of the rough surface before and after; and (d,e) processes of a water droplet contacting (d) rough surface and (e) smooth surface immersed in hexane (water was dyed by methylene blue).

Figure 8. (a−f) Oil desorption performance of light crude oil-fouled SA hydrogel coating on (a) glass slide, (b) metal sheet, (c) alloy, (d) metal mesh, (e) nonwoven, and (f) PC plastic. (g) The self-cleaning performance of large area heavy crude oil-fouled coating surface and uncoated surface.

freely on the smooth surface, but oil molecules are bound by structures on the rough surface. In order to explore this phenomenon more deeply, we studied the water wetting behavior on the surface immersed in oil. As shown in Figure 7a, the rough surface shows the superhydrophicility and underwater superoleophobicity similar to the smooth surface. However, there is a marked difference on the WCA under oil. The WCA-O of rough surface is larger than 30°, whereas the other is nearly 0°. The surface morphology is the only difference between the rough surface and the smooth surface. As shown in Figure 7b,c, there is hardly any morphology on the smooth surface both before and after coating, so free oil molecules can form a continuous oil film. Although the rough surface is not very compact with many abrasives on it, the interaction between volume shrinkage effect of the coating and abrasive grains can produce stress in the course of polymerization, which leads to coating cracking. Therefore, oil molecules might be trapped in these defects. As

shown in Figure S11, the roughness of the smooth surface increases slightly after coating, but it is still only about 10 nm. Although the roughness of the rough surface decreases significantly after coating, but it is still in the micron level. The process of a water droplet falling in oil and touching the surface may prove this assumption. As shown in Figure 7d and Movie S3, a water droplet cannot spread thoroughly in limited time when contacting the rough surface wetted by oil. Water can cross the outer oil to reach the coating surface because of gravity, but it cannot totally penetrate into the microstructure of the surface. In addition, when we take the sample out, part of the water could still slide on the surface, indicating that water has not been absorbed completely by the coating. Whereas the situation of the smooth surface is completely different, as shown in Figure 7e and Movie S4; the water droplet can completely spread on the surface in a short time. In addition, water cannot move a bit on the surface after the sample is taken out. 18871


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Figure 9. (a) Oil−water separation process of SA hydrogel-coated copper mesh using hexane (dyed by oil red O)−water (dyed by methylene blue) mixture. (b) Schematic illustration of the water and oil wetting model of SA hydrogel coating mesh. (c) Fogging test of SA hydrogel coating glass slide including steam test (top) and freeze test (bottom).

Figure 10. (a) OCA-W of SA hydrogel coating surface after immersing in acid, alkali, and salt environment for 1, 4, 7, and 10 days. The oil used here is hexane. (b) OCA-W and WCA-O of the SA hydrogel coating surface to various oils.

On the basis of all experiments mentioned above, a mechanism can be summarized to explain this phenomenon. Hydrophilic coating interacts with oil and water molecules through van der Waals force and ion bonds generated by hydration, respectively.41,42 As we all know, ionic bond is much stronger than the van der Waals force.43 Therefore, when the oil-wetted smooth surface approaches the water, strong hydration will occur where the coating is in contact with water molecules. As a result, oil molecules begin to desorb from the edge of the oil layer and gradually extend to the center, then forming a spherical droplet and eventually detaching from the surface (Figure S6). On the contrary, if the oil molecules are completely trapped in the microstructures of the rough surface, the surface defects would become a mechanical barrier to oil molecules. Therefore, for both underwater superoleophilic surfaces, the smooth surface is easier to achieve the automatic desorption of oil droplets. An anionic smooth coating surface exhibits the best performance among all of the surfaces. Because of the simplicity and rapidly of this one-step polymerization method, we prepared several SA hydrogel coatings on different substrates and validated their self-cleaning properties. As shown in Figures 8a−f and S12, the SA hydrogel is proved to

be easily coated onto diverse substrates, including glass slide, metal sheet, alloy, metal mesh, nonwoven and PC plastic, and all coating surfaces show good self-cleaning properties. Crude oil could be easily desorbed from these surfaces, whereas the images in red frames suggest that uncoated surfaces show no self-cleaning capability underwater. Even for a large area of heavy crude oil pollution, surface self-cleaning can also be achieved by applying mechanical sloshing (Figure 8g). Likewise, this coating surface also performs well in the field of oil−water separation and anti-fogging, as shown in Figure 9 and Movies S5 and S6. When the oil−water mixture is poured into the glass tube fixed with a SA hydrogel-coated copper mesh, the oil phase first touches the sample, but it is blocked above the sample and cannot pass through the copper mesh. Moreover, the coating can withstand a certain oil pressure without penetration. Subsequently, the aqueous phase enters the tube. Water can pass through the oil phase, and the sample in sequence driven by gravity. Finally, water is collected in the beaker below, whereas oil remains above the sample. Therefore, the oil−water separation process is realized (Figure 9a). In addition, no oil droplets reside on the surface of the tested sample. Figure 9c reveals that the smooth SA hydrogel coating surface exhibits excellent anti-fogging property in both 18872


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bear a certain pressure. We do believe that this work can help us to have a better understanding of superhydrophilic and underwater superoleophobic self-cleaning system and provide a simple preparation method.

steam test and freeze test. Water droplets would spread on the surface completely and form an almost continuous water film which can help light to propagate in a straight line and reduce scattering.44 To further study the mechanism of the oil−water separation process, the wetting processes are modeled in Figure 9b. Previous studies have shown that liquid must wet the bottom of mesh pore before passing through. In other words, the energy barrier caused by intrusion pressure (Δp) must be overcome. Δp can be formulated as follows Δp = −lγOW(cos θa)/A

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04948. Photograph of the DCAT 21 device and diagrammatic sketch of the process of measuring the surface adhesive force; EDS energy spectra of all different coatings and FTIR spectra of all different coatings corresponding to the energy spectra; high-resolution photographs of the oil desorption processes; photographs of the whole process from deformation to desorption; root-meansquare roughness and arithmetic average roughness of all hydrogel coatings, surface morphologies of AA and SA hydrogel coating surface; three typical processes of a water droplet contacting the hydrogel surface immersed in hexane; the bouncing and rolling off processes of an oil droplet; adhesion force as a function of position for other hydrogel coating surfaces; the surface morphologies and roughness values of the rough surface and smooth surface before and after coating; the surface morphologies of other substrates before and after hydrogel coating modification; the anti-oil-adhesion performance of the SA hydrogel coating surface after immersing; high-speed camera photographs of the oil droplet recovery process after extrusion by two identical hydrogel coated copper meshes under water; and peakgroup correspondence table in Figures S2−S4 (PDF) Process of immersing the hexane-fouled rough surface into the water (AVI) Process of immersing the hexane-fouled smooth surface into the water (AVI) Process of a water droplet contacting the rough surface immersed in hexane (AVI) Process of a water droplet contacting the smooth surface immersed in hexane (AVI) Oil−water separation process of SA hydrogel-coated copper mesh using hexane (AVI) Oil−water separation process of SA hydrogel-coated copper mesh using hexane (AVI)

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where γOW is the oil−water interfacial tension, l is the pore’s perimeter, θa is the advancing contact angle of water or oil on the coating, and A is the pore’s area.45−47 In this work, θa is larger than 90° because of underwater superoleophobicity when the oil phase touches the coating, so the oil phase is blocked because Δp is positive. For the water phase, the situation is the opposite. Therefore, the water phase and the oil phase are separated on both sides of the coating. The stability and universality of the coatings are important for their long-term use. To evaluate the temporal stability and environmental stability of the SA hydrogel coating surface, the underwater oil contact angles are measured after being immersed in 0.1 M HCl solution, NaOH solution, and NaCl solution for 1, 4, 7, and 10 days. As Figure 10a shows, with the prolongation of immersion time, the contact angle of samples in each solution decreases slightly, but the states of underwater superoleophobicity are basically maintained. In addition, the samples still exhibit excellent underwater anti-oil-adhesion performance (Figure S13). Meanwhile, Figure 10b reveals that this coating has no selectivity for oil and maintains consistent wettability for various oils. Previously, we have experimentally proved that the SA hydrogel-coated copper mesh can withstand a certain amount of oil pressure without damage. Similarly, it can also withstand a certain physical pressure. As Figure S14 shows, the sample exhibits superoleophobic underwater; then, we squeeze the oil droplet with another identical coated copper mesh. The oil droplet can recover after being deformed and cannot pass through the mesh, indicating its stable underwater superoleophobicity under pressure.

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CONCLUSIONS In summary, we prepare several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. The results show that the anionic coating surface shows the best selfcleaning performance because of its extremely strong hydration ability. The higher the hydrophilicity of the coating, the weaker the oleophilicity, the lower the adhesion force to oil droplet, and the better the self-cleaning performance. It is also found that the rough surface will reduce its self-cleaning ability because of the binding effect of the microstructures to oil, whereas the smooth surface exhibits excellent self-cleaning property. The superhydrophilic and underwater superoleophobic SA anionic surface also shows terrific oil−water separation and anti-fogging performance. In addition, this coating shows good temporal and environmental stabilities, and it can also

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (G.W.). ORCID

Lei Tang: 0000-0001-7096-2457 Gang Wang: 0000-0003-3923-3234 Lijing Zhu: 0000-0003-2819-5615 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (grant no. 51703235) and Project U1809213 and U1809214 supported by National Natural Science Foundation of China. 18873


Research Article


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(22) Liu, M.; Wang, S.; Wei, Z.; Song, Y.; Jiang, L. Superoleophobic Surfaces: Bioinspired Design of a Superoleophobic and Low Adhesive Water/Solid Interface. Adv. Mater. 2010, 21, 665−669. (23) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Light-Induced Amphiphilic Surfaces. Nature 1997, 388, 431−432. (24) de Jesus, M. A. M. L.; da Silva Neto, J. T.; Timò, G.; Paiva, P. R. P.; Dantas, M. S. S.; de Mello Ferreira, A. Superhydrophilic SelfCleaning Surfaces Based on TiO2 and TiO2/SiO2 Composite Films for Photovoltaic Module Cover Glass. Appl. Adhes. Sci. 2015, 3, 5. (25) Lizama, C.; Freer, J.; Baeza, J.; Mansilla, H. D. Optimized Photodegradation of Reactive Blue 19 on TiO2 and ZnO Suspensions. Catal. Today 2002, 76, 235−246. (26) Son, J.; Kundu, S.; Verma, L. K.; Sakhuja, M.; Danner, A. J.; Bhatia, C. S.; Yang, H. A Practical Superhydrophilic Self Cleaning and Antireflective Surface for Outdoor Photovoltaic Applications. Sol. Energy Mater. Sol. Cells 2012, 98, 46−51. (27) Guo, H.; Ma, Y.; Sun, P.; Cui, S.; Qin, Z.; Liang, Y. Selfcleaning and antifouling nanofiltration membranes-superhydrophilic multilayered polyelectrolyte/CSH composite films towards rejection of dyes. RSC Adv. 2015, 5, 63429−63438. (28) Zhang, J.; Severtson, S. J. Fabrication and Use of Artificial Superhydrophilic Surfaces. J. Adhes. Sci. Technol. 2014, 28, 751−768. (29) Lu, Y.; Sathasivam, S.; Song, J.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P. Robust Self-Cleaning Surfaces that Function when Exposed to either Air or Oil. Science 2015, 347, 1132−1135. (30) He, K.; Duan, H.; Chen, G. Y.; Liu, X.; Yang, W.; Wang, D. Cleaning of Oil Fouling with Water Enabled by Zwitterionic Polyelectrolyte Coatings: Overcoming the Imperative Challenge of Oil-Water Separation Membranes. ACS Nano 2015, 9, 9188−9198. (31) Gao, S.; Sun, J.; Liu, P.; Zhang, F.; Zhang, W.; Yuan, S.; Li, J.; Jin, J. A Robust Polyionized Hydrogel with an Unprecedented Underwater Anti-Crude-Oil-Adhesion Property. Adv. Mater. 2016, 28, 5307−5314. (32) Deng, Y.; Zhang, G.; Bai, R.; Shen, S.; Zhou, X.; Wyman, I. Fabrication of Superhydrophilic and Underwater Superoleophobic Membranes via an in situ Crosslinking Blend Strategy for Highly Efficient Oil/Water Emulsion Separation. J. Membr. Sci. 2019, 569, 60−70. (33) Yang, X.; Yan, L.; Ran, F.; Pal, A.; Long, J.; Shao, L. InterfaceConfined Surface Engineering Constructing Water-Unidirectional Janus Membrane. J. Membr. Sci. 2019, 576, 9−16. (34) Wang, G.; He, Y.; Wang, H.; Zhang, L.; Yu, Q.; Peng, S.; Wu, X.; Ren, T.; Zeng, Z.; Xue, Q. A cellulose sponge with robust superhydrophilicity and under-water superoleophobicity for highly effective oil/water separation. Green Chem. 2015, 17, 3093−3099. (35) Zhou, X.; Koh, J. J.; He, C. Robust Oil-Fouling Resistance of Amorphous Cellulose Surface Underwater: A Wetting Study and Application. Langmuir 2019, 35, 839−847. (36) Fu, Y.; Maguire, R.; Liu, H.; Zhong, W.-H. Special Wetting Behavior of a Graphitic Nanofiber-Modified Epoxy Generalized for Rough Textured Fabric Surfaces. Colloid Polym. Sci. 2010, 289, 141− 148. (37) Miao, R.; Wang, L.; Zhu, M.; Deng, D.; Li, S.; Wang, J.; Liu, T.; Lv, Y. Effect of Hydration Forces on Protein Fouling of Ultrafiltration Membranes: The Role of Protein Charge, Hydrated Ion Species, and Membrane Hydrophilicity. Environ. Sci. Technol. 2017, 51, 167−174. (38) Tansel, B. Significance of Thermodynamic and Physical Characteristics on Permeation of Ions during Membrane Separation: Hydrated Radius, Hydration Free Energy and Viscous Effects. Sep. Purif. Technol. 2012, 86, 119−126. (39) Nilsson, M. A.; Daniello, R. J.; Rothstein, J. P. A Novel and Inexpensive Technique for Creating Superhydrophobic Surfaces Using Teflon and Sandpaper. J. Phys. D: Appl. Phys. 2010, 43, 045301. (40) Li, F.; Geng, X. G.; Chen, Z.; Zhao, L. The Relationship between Substrate Roughness and Wettability of Superhydrophobic Metal Carboxylate Surface. Adv. Mater. Res. 2011, 337, 95−97. (41) Israelachvili, J. N. Intermolecular and Surface Forces. Q. Rev. Biol. 2011, 2, 59−65.

REFERENCES

(1) Ye, Y.; Huang, J.; Wang, X. Fabrication of a Self-Cleaning Surface via the Thermosensitive Copolymer Brush of P(NIPAAmPEGMA). ACS Appl. Mater. Interfaces 2015, 7, 22128−22136. (2) Xue, C.-H.; Zhang, Z.-D.; Zhang, J.; Jia, S.-T. Lasting and Selfhealing Superhydrophobic Surfaces by Coating of Polystyrene/SiO2 Nanoparticles and Polydimethylsiloxane. J. Mater. Chem. A 2014, 2, 15001−15007. (3) Ragesh, P.; Anand Ganesh, V.; Nair, S. V.; Nair, A. S. A review on “self-cleaning and multifunctional materials”. J. Mater. Chem. A 2014, 2, 14773−14797. (4) Tian, X.; Verho, T.; Ras, R. H. A. Moving Superhydrophobic Surfaces toward Real-World Applications. Science 2016, 352, 142− 143. (5) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and Self-Cleaning Properties of Artificial Superhydrophobic Surfaces. Langmuir 2005, 21, 956−961. (6) Zhang, X.-T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Self-Cleaning Particle Coating with Antireflection Properties. Chem. Mater. 2005, 17, 696−700. (7) Liu, K.; Jiang, L. Bio-Inspired Self-Cleaning Surfaces. Annu. Rev. Mater. Res. 2012, 42, 231−263. (8) Hee, J. Y.; Kumar, L. V.; Danner, A. J.; Yang, H.; Bhatia, C. S. The Effect of Dust on Transmission and Self-Cleaning Property of Solar Panels. Energy Procedia 2012, 15, 421−427. (9) Xue, L.; Lu, X.; Wei, H.; Long, P.; Xu, J.; Zheng, Y. Bio-Inspired Self-Cleaning PAAS Hydrogel Released Coating for Marine Antifouling. J. Colloid Interface Sci. 2014, 421, 178−183. (10) Cheng, Y.-T.; Rodak, D. E. Is the Lotus Leaf Superhydrophobic? Appl. Phys. Lett. 2005, 86, 144101. (11) Yong, J.; Yang, Q.; Chen, F.; Zhang, D.; Du, G.; Bian, H.; Si, J.; Hou, X. Bioinspired Superhydrophobic Surfaces with Directional Adhesion. RSC Adv. 2014, 4, 8138−8143. (12) Koch, K.; Barthlott, W. Superhydrophobic and Superhydrophilic Plant Surfaces: an Inspiration for Biomimetic Materials. Philos. Trans. R. Soc., A 2009, 367, 1487−1509. (13) Ganesh, V. A.; Raut, H. K.; Nair, A. S.; Ramakrishna, S. A Review on Self-Cleaning Coatings. J. Mater. Chem. 2011, 21, 16304− 16322. (14) Zheng, B.; Jiang, G.; Wang, W.; Mei, X. Fabrication of Superhydrophilic or Superhydrophobic Self-Cleaning Metal Surfaces Using Picosecond Laser Pulses and Chemical Fluorination. Radiat. Eff. Defects Solids 2016, 171, 461−473. (15) Xi, J.; Jiang, L. Biomimic Superhydrophobic Surface with High Adhesive Forces. Ind. Eng. Chem. Res. 2008, 47, 6354−6357. (16) Yang, C.; Hao, P.; He, F. Effect of Upper Contact Line on Sliding Behavior of Water Droplet on Superhydrophobic Surface. Chin. Sci. Bull. 2009, 54, 727−731. (17) Wang, X.; Hu, H.; Ye, Q.; Gao, T.; Zhou, F.; Xue, Q. Superamphiphobic Coatings with Coralline-Like Structure Enabled by One-Step Spray of Polyurethane/Carbon Nanotube Composites. J. Mater. Chem. 2012, 22, 9624−9631. (18) Peng, S.; Yang, X.; Tian, D.; Deng, W. Chemically Stable and Mechanically Durable Superamphiphobic Aluminum Surface with a Micro/Nanoscale Binary Structure. ACS Appl. Mater. Interfaces 2014, 6, 15188−15197. (19) Li, L.; Breedveld, V.; Hess, D. W. Design and Fabrication of Superamphiphobic Paper Surfaces. ACS Appl. Mater. Interfaces 2013, 5, 5381−5386. (20) Xu, Z.; Wang, Z.; Kung, J.; Woods, J.; Wu, X.; Kotlyar, L.; Sparks, B.; Chung, K. Separation and Characterization of Foulant Material in Coker Gas Oils from Athabasca Bitumen. Fuel 2005, 84, 661−668. (21) Chouparova, E.; Lanzirotti, A.; Feng, H.; Jones, K. W.; Marinkovic, N.; Whitson, C. Characterization of Petroleum Deposits Formed in a Producing Well by Synchrotron Radiation-Based Microanalyses. Energy Fuels 2005, 18, 1199−1212. 18874


Research Article

ACS Applied Materials & Interfaces (42) He, Y.; Hower, J.; Chen, S.; Bernards, M. T.; Chang, Y.; Jiang, S. Molecular Simulation Studies of Protein Interactions with Zwitterionic Phosphorylcholine Self-Assembled Monolayers in the Presence of Water. Langmuir 2008, 24, 10358−10364. (43) Margenau, H.; Kestner, N. R. Theory of Intermolecular Forces. Pure Appl. Chem. 1969, 51, 1627−1636. (44) Cebeci, F. Ç .; Wu, Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Nanoporosity-Driven Superhydrophilicity: a Means to Create Multifunctional Antifogging Coatings. Langmuir 2006, 22, 2856−2862. (45) Wang, G.; Zeng, Z.; Wang, H.; Zhang, L.; Sun, X.; He, Y.; Li, L.; Wu, X.; Ren, T.; Xue, Q. Low Drag Porous Ship with Superhydrophobic and Superoleophilic Surface for Oil Spills Cleanup. ACS Appl. Mater. Interfaces 2015, 7, 26184−26194. (46) Tian, D.; Guo, Z.; Wang, Y.; Li, W.; Zhang, X.; Zhai, J.; Jiang, L. Phototunable Underwater Oil Adhesion of Micro/Nanoscale Hierarchical-Structured ZnO Mesh Films with Switchable Contact Mode. Adv. Funct. Mater. 2014, 24, 536−542. (47) Wang, F.; Lei, S.; Xue, M.; Ou, J.; Li, C.; Li, W. Superhydrophobic and Superoleophilic Miniature Device for the Collection of Oils from Water Surfaces. J. Phys. Chem. C 2014, 118, 6344−6351.

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