The study of oil dewetting ability of superhydrophilic and underwater

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Surfaces, Interfaces, and Applications

The study of oil dewetting ability of superhydrophilic and underwater superoleophobic surface from air to water for high effective self-cleaning surface designing Lei Tang, Zhixiang Zeng, Gang Wang, Luli Shen, Lijing Zhu, Yingxin Zhang, and Qunji Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

The study of oil dewetting ability of superhydrophilic and underwater superoleophobic surface from air to water for high effective self-cleaning surface designing Lei Tangab, Zhixiang Zeng*a, Gang Wang*a, Luli Shen a, Lijing Zhu a, Yingxin Zhangc, Qunji Xuea a

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 *E-mail: [email protected]; [email protected] b

Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100049, China c

School of Materials Science & Engineering, Ningbo University of Technology,

Ningbo 315211, People’s Republic of China Keywords: self-cleaning, superhydrophilic, underwater superoleophobic, ionic, zwitterionic

ABSTRACT Superhydrophilic self-cleaning surface can perfectly deal with oil pollution, which cannot be realized by superhydrophobic surface. This research is 1

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designed to study the mechanism of wetting behavior of superhydrophilic coating with different function groups and guide to design 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 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 be polluted by heavy crude oil in dry state in air. It is also found that in dry state the rough hydrophilic anionic surface will lock up the oil in the structures and then lose its self-cleaning ability underwater, while oil droplet can detach from the smooth coating surface quickly. Meanwhile, the superhydrophilic and underwater superoleophobic SA anionic surface also exhibits excellent anti-fogging and oil-water separation performance.

INTRODUCTION 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 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, while 2


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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, selfcleaning phenomenon has been gradually recognized by human beings. And the manifestations of self-cleaning phenomenon are different. For lotus leaves and butterfly wings, water droplet can completely roll off and take away the dusts without leaving any trace.10,11 While water can completely spread out on some surfaces like 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 But the superhydrophobic surface is powerless for the oil pollution because superhydrophobic surface is always oleophilic due to its high surface energy. Therefore, water-insoluble organic substances like oil contamination cannot leave the surface with water droplet. It is worth noting that the superamphiphobic surface is both superhydrophobic and superoleophobic, thus, it is the most ideal self-cleaning material. But it requires a specific micro-nano composite structure like concave angle and overhang.17,18 Overdependence on structure greatly limits its application. And the 3



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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 a hydrophilic surface in air (liquid-gas-solid three-phase 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 water contact angle 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 louts surface. It is proved water spreads rapidly on the surface to form a water film and penetrates between pollutants and surface so as to achieve the self-cleaning.23 In addition, by using photoinduced superhydrophilicity with photocatalytic degradation to clean contamination of some semicounductor materials like TiO2 or ZnO is an effective approach to realize self-cleaning.24,25 However, this photochemical method is still being studied due to 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 like 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 4


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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 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) that possessed an unparalleled anti-oil-adhesion property.31 Deng and co-workers fabricated a series of poly(vinylidene fluoride) (PVDF) membranes via phase separation in combination with in situ crosslinking copolymerization of Poly(methyl methacrylate-co-glycidyl methacrylate) P(MMA-co-GMA) and polyethyleneimine (PEI) which exhibited superhydrophilicity and underwater superoleophobicity as well as underwater anti-oilfouling performance.32 Membranes materials can also realize other functions through surface manipulation. Yang and co-workers designed a liquid/liquid interface-confined surface engineering strategy to fabricate Janus membrane (JM) which owning liquidunidirectional transportation feature due to its asymmetric chemistry, wettability and surface morphology.33 According to our previous findings, -cellulose based surface had unique oil detachment ability even when it was wetted by oil in air in 2015.34 We considered that the excellent competitive wettability of -cellulose molecules played major role to self-cleaning. In recent days, Zhou and co-workers further studied this 5



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wetting behavior of -cellulose.35 There are many types of hydrophilic molecules but their oil detachment abilities are different and their oil detachment mechanisms are still unclear. So systematic studies are necessary to figure out the function of different hydrophilic functional groups on 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 selfcleaning 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 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 due to its extremely strong water absorption and surface hydration ability. It is also found that the surface roughness would be a hider to self-cleaning due to the interlocking effect of the rough structure, while the smooth surface shows an excellent self-cleaning property. In addition, the anionic surface also shows excellent performance in anti-fogging and oilwater separation. Meanwhile, the environmental and temporal stabilities of the coating is also satisfactory. This research can help us to have a better understanding of superhydrophilic and underwater superoleophobic self-cleaning system and provide a simple preparation method to construct a self-cleaning coating. 6


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EXPERIMENTAL SECTION Materials. 2-Hydroxyethyl methacrylate (HEMA 99%), acrylamide (AM 99%), 2Acrylamido-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, methylene blue (≥70%) were purchased from Aladdin Industrial Corporation (Shanghai, China). 1H,1H,2H,2HHeptadecafluorodecyl 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%), hydrochloric acid (HCl 36.038.0%) 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, nonwoven and polycarbonate (PC) plastic were all purchased from local stores. All of the water used is deionized water which is homemade in our laboratory. 7



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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 minutes, respectively. The uncoated Si-OH surfaces were prepared by immersing glass slides into newly configured Piranha Solution (a mixture of 3:1 (v/v) 98% sulfuric acid and 30% hydrogen peroxide) for 2h 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, viscoat 17F), crosslinking 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 UV Curing Machine (4000W) for 3 minutes. For water-soluble monomers, monomer (14 mmol SA, SBMA, AMPS, 5/5 (n/n) TMA-SA mixture), crosslinking agent BIS (1.4 mmol) and initiator α-Ketoglutaric acid (0.14 mmol) were dissolved in 5/5 (w/w) water-ethanol mixtures (10g). 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 the polymers used were illustrated in Scheme 1. 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 (38W) for 5h to obtain the hydrogels. Then, the 8


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Scheme 1. Structural formulas of the polymers on all coating surfaces.

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, Cary660+620, U.S.). The contact angles were measured by a contact angle meter (OCA20, Germany) with a water or oil droplet of 2.0 μL. The surface morphologies were observed by a high resolution scan electron microscopy (Hr-SEM, FEI Quanta 250 FEG, U.S.) equipped with an energy dispersive 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 detail information was shown in Fig S1. The surface roughness was measured by an atomic force microscope (AFM, Dimension3100V, U.S.) and a laser scanning confocal microscopy (LSCM, LSM700, Germany). The swelling ratio (SR) of the coating was calculated from the following expression: 9



SR = (𝑊𝑠 ― 𝑊𝑑)/𝑊𝑑

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(1)

Where 𝑊𝑠 is the weight of the swollen hydrogels and 𝑊𝑑 is the weight of the freezedried hydrogels. All camera photos were taken with a high speed camera (FastcamMini UX100, Photron Inc.) with different frame rates of 50, 125 and 500, respectively. 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℃ for 1h, and then taken out to humid 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 oil/water mixture was poured in and the separation process was driven by gravity. 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. 11 kinds of coatings with different end groups, including 4 kinds of ionic groups (anion, cation, zwitterion 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 are characterized by EDS and Micro-FTIR. The spectra of all coatings are shown in Fig S2 to S4 and Table S1 contains the peak-group correspondence in all spectra. According to these spectra, all 10


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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 dry state in air, and then we put the surface into the water to test the oil desorption performance. There are three main phenomena in the process (Fig 1a-1c and Fig S5). When SA coating surface is immersed into the water, the oil layer on the coating shrinks into a droplet rapidly and detaches in 2.2s (Fig 1a and Fig 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 (Fig 1b and Fig S5b). In contrast, the oil layer on the 1-Decene coating surface can only shrink into a hemisphere with a contact angle about 45° (Fig 1c and Fig S5c). Figure 1d and 1e show the underwater oil contact angles of different coatings wetted by castor oil and light crude oil in dry state and the detachment time (Due to the detachment of the oil droplet from some surfaces, the underwater contact angles of these coatings are 180°. The whole process of the oil drop from deformation to desorption is shown in Fig S6). The trends of Fig 1d and Fig 1e are roughly the same (crude oil is more difficult to desorb due to 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 attribute 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 11



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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; (c) 1-Decene coating surface. 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 dry state. (Initial surface refers to the ultrasonically cleaned uncoated surface; (N)Hydroxyl surface refers to the uncoated Si-OH surface; (L-c)Carboxyl surface refers to the hydrophobic long-chain carboxyl coating surface; (S-c)Carboxyl surface refers to the hydrophilic short-chain carboxyl coating surface. The same below).

properties differ greatly. The specific order is as follows (from good to bad): sulfonate, acylamino, hydroxyl and short-chain carboxyl coating. While the oil desorption performance of hydrophobic nonionic coatings is unsatisfactory, and these surfaces exhibit oleophilic underwater. The wetting behavior of coating to water and oil directly determines the self-cleaning 12


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ability in water and help us to understand the difference of different coatings in oil desorption performance better. Fig 2 shows the swelling ratios of all coatings and dynamic changes of water contact angles on these surfaces. After comparison, we can find that the hydrophilicity of the coating is basically the same as the oil desorption performance described above. Although the swelling ratios 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 swelling ratio, but the surface groups rely on 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. While for hydrophobic nonionic coatings, their swelling ratios are very small. And there is no

Figure 2. (a-c) The dynamic change of water contact angle on (a) ionic; (b) nonionic hydrophilic; (c) nonionic hydrophobic coating surface. The interval between the run number is 0.05s. (d) The swelling ratios and contact angles of all coatings. 13



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interaction between surface groups and water molecules, so they show oleophilic underwater (1-Decene and undecylenic acid coatings are defined as hydrophobic due to their extremely low swelling ratio and being more oleophilic relatively though the water contact angles are less than 90°). It is noteworthy that for AA coating and HEMA coating, the former has a greater degree of swelling ratio, and it is generally believed that each carboxyl group can form two intermolecular hydrogen bonds, so its hydrophilicity should be better than the latter. But in fact, the situation is just the opposite. This may be attributed to the larger roughness of the AA coating surface. As Fig S7 shows, the roughness of the AA coating surface is much higher than others (Fig S7a). There are many pits with different sizes on the surface, which is not as smooth as other coating surfaces (Fig S7b and S7c). Similarly, the wetting behavior of water droplet on these surfaces under oil circumstance can further confirm this point. Fig 3 shows the water contact angles of all coatings under hexane (the fluoroalkyl coating surface is not included in the subsequent experiments due to its strong hydrophobicity and underwater oleophilicity). The results are basically the same as those in Fig 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 Fig S8. When the surface was taken out, the water droplet has been absorbed and could not move a bit on the SA hydrogel coating (Fig S8a). But on HEMA hydrogel coating surface, part of the 14


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Figure 3. The water contact angles of all the coating surfaces under hexane.

water could slip off the surface (Fig S8b), while the whole droplet can roll down from the 1-Decene coating surface (Fig S8c). In addition to the wettability to water, the difference in the wettability to oil of the coatings also affects the oil desorption performance. Fig 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. But 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 (Fig S7). In order to illustrate the interaction 15



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Figure 4. (a-c) The dynamic change of oil contact angles on (a) ionic; (b) nonionic hydrophilic; (c) nonionic hydrophobic coating surface. The interval between the run number is 0.02s. (d) The underwater oil contact angles of different coating surfaces. The oil used here is hexane.

between the oil droplet and the coating surface more accurately, the contact processes underwater were recorded by a tension meter and a high speed camera. Fig 5 reveals the adhesion force curves and adhesive behaviors of three typical coating surfaces. Since the oil droplet is affected by buoyancy in water, the force is not constant when moving in the vertical direction. But we can judge the difference in adhesive behaviors from the inflection point in the curves. As Fig 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 (Fig 5b). The bounding 16


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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 and b) SA, (c and d) HEMA, (e and f) 1-Decene coating surface. The frame rate used here is 125 fps. (g) Photographs of a trickle of hexane (dyed by oil red O) jetted on SA hydrogel coated surface (top) and uncoated surface (bottom) to reveal the underwater anti-oiladhesion performance.

test of an oil droplet can further prove this conclusion. As Fig S9 shows, when an oil droplet falls from a certain height underwater, it bounces on the surface and then rolls off spontaneously. While another inflection point C appears in Fig 5c. It’s the separating point, suggesting that the oil droplet has been subject to a distinct adhesion force before 17



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detaching, and part of the oil droplet remains on the surface after detachment (Fig 5d). For an oleophilic surface, the oil droplet spreads out instantly at the contacting moment (Fig 5f), which is characterized as the inflection point D in Fig 5e. However, the oil droplet is less stressed when detaching from the surface. The adhesion force curves of other surfaces are shown in Fig S10, suggesting 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 Fig 5g, We further jet a trickle of hexane on the SA coated and uncoated surfaces. All the oil droplets can bounce off over the coated surface without leaving any trace, demonstrating excellent anti-oiladhesion performance underwater. While 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 self-cleaning 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 Fig 1d and 1e) coating for comparison to test the oil desorption 18


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

performance when the surface has been already wetted by oil in dry state. Fig 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 Fig 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, Fig 6b and Movie S2 indicate that the smooth surface shows the similar oil wettability of in air, but when the surface is immersed into the water, the oil layer spontaneously shrinks into a sphere and quickly detachs from the surface. According to this difference, we can draw two diagrammatic sketches to show more vividly. As shown in Fig 6c and 6d (Black, grey, orange and blue areas represent substrate, coating, oil phase and water phase, respectively. The same below), the oil layer can move 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 19



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Figure 7. (a) The 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; (d-e) The processes of a water droplet contacting (d) the rough surface and (e) the smooth surface immersed in hexane (Water was dyed by methylene blue).

behavior on the surface immersed in oil. As shown in Fig 7a, rough surface shows the similar superhydrophicility and underwater superoleophobicity with smooth surface. But there is a marked difference on the water contact angle under oil. The WCA-O of rough surface is larger than 30°, while the other is nearly 0°. Surface morphology is the only difference between the rough surface and the smooth surface. As shown in Fig 7b and 7c, there is hardly any morphology on the smooth surface both before and after coating, so free oil molecules can form a continuous oil film. While the rough surface 20


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is not very compact with many abrasives on it, and 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. So oil molecules might be trapped in these defects. As show in Fig S11, the roughness of smooth surface increases slightly after coating, but it is still only about 10nm. While the roughness of rough surface decreases significantly after coating, but it is still in micron level. The process of a water droplet falling in oil and touching the surface may prove this assumption. As shown in Fig 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 due to gravity, but it cannot totally penetrate into the microstructure of the surface. And 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. While the situation of the smooth surface is completely different, as shown in Fig 7e and Movie S4. The water droplet can completely spread on the surface in a short time. And water cannot move a bit on the surface after the sample is taken out. 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 Waal force and ion bonds generated by hydration, respectively.41,42 As we all know, ionic bond is much stronger than Van der Waal 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 21



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spherical droplet and eventually detaching from the surface (Fig S6). On the contrary, if the oil molecules are completely trapped in the micro-structures of the rough surface, the surface defects would become a mechanical barrier to oil molecules. So for both underwater superoleophilic surfaces, the smooth surface is easier to achieve the automatic desorption of oil droplets. Anionic smooth coating surface exhibits the best performance among all the surfaces. Due to 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 Fig 8a-8f and Fig S12, 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 shows good self-cleaning properties. Crude oil could be easily desorbed from these surfaces, while the images in red frames suggest that uncoated surfaces show no self-cleaning capability underwater. Even for a

Figure 8. (a-f) The 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 selfcleaning performance of large area heavy crude oil fouled coating surface and uncoated surface. 22


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large area of heavy crude oil pollution, surface self-cleaning can also be achieved by applying mechanical sloshing (Fig 8g). Likewise, this coating surface also performs well in the field of oil-water separation and anti-fogging, as shown in Fig 9, Movie S5 and S6. When the oil-water mixture is poured into the glass tube fixed with SA hydrogel coated copper mesh, the oil phase first touches the sample, but it is blocked above the sample and can not 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, while oil remains above the sample. Therefore, the oilwater separation process is realized (Fig 9a). And no oil droplets reside on the surface of the tested sample. Fig 9c reveals that smooth SA hydrogel coating surface exhibits excellent anti-fogging property in both 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 Fig 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 (∆p) must be overcome. ∆p can be formulated as follows ∆p = ―l𝛾𝑂𝑊(cos 𝜃𝑎) 𝐴

(2)

where 𝛾𝑂𝑊 is the oil/water interfacial tension, l is the pore’s perimeter, 𝜃𝑎 is the

23



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Figure 9. (a) The 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).

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° due to underwater superoleophobicity when the oil phase touches the coating, so the oil phase is blocked because ∆p 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 0.1M HCl solution, NaOH solution and NaCl solution for 1, 4, 7 and 10 days, 24


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

respectively. As Fig 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. And the samples still exhibit excellent underwater anti-oil-adhesion performance (Fig S13). Meanwhile, Fig 10b reveals that this coating has no selectivity for oil, and maintains consistent wettability for various oil. Previously, we have experimentally proved that the SA hydrogel coated copper mesh can withstand a certain amount of oil pressure without damage. Similarly, it also can withstand a certain physical pressure. As Fig 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. CONCLUSIONS In summary, we prepare several hydrophilic coatings including nonionic, ionic and zwitterionic coatings to investigate their self-cleaning performance underwater when 25



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they have been polluted by oil in 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 self-cleaning 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 selfcleaning ability due to the binding effect of the microstructures to oil, while the smooth surface exhibits excellent self-cleaning property. The superhydrophilic and underwater superoleophobic SA anionic surface also shows terrific oil-water separation and antifogging performance. In addition, this coating shows good temporal and environmental stabilities, and it also can bear a certain pressure. We do believe this work can help us to have a better understanding of superhydrophilic and underwater superoleophobic self-cleaning system and provide a simple preparation method.

ASSOCIATED CONTENT Supporting Information Figure S1. Photograph of DCAT21 device, diagrammatic sketch of the process of measuring the surface adhesive force; Figure S2-S4. EDS energy spectra of all different coatings, FTIR spectra of all different coatings corresponding to the energy spectra; Figure S5. High-resolution photographs of the oil desorption processes; Figure S6. Photographs of the whole process from deformation to desorption; Figure S7. Root 26


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mean square roughness and arithmetic average roughness of all hydrogel coatings, surface morphologies of AA and SA hydrogel coating surface; Figure S8. Three typical processes of a water droplet contacting the hydrogel surface immersed in hexane; Figure S9. The bouncing and rolling off processes of an oil droplet; Figure S10. Adhesion force as a function of position for other hydrogel coating surfaces; Figure S11. The surface morphologies and roughnesses of rough surface and smooth surface before and after coating; Figure S12. The surface morphologies of other substrates before and after hydrogel coating modification; Figure S13. The anti-oil-adhesion performance of SA hydrogel coating surface after immersing; Figure S14. High speed camera photographs of the oil droplet recovery process after extrusion by two identical hydrogel coated copper meshes under water. Table S1. Peak-group correspondence table in Fig S2 to S4. Movie S1. The process of immersing the hexane fouled rough surface into the water; Movie S2. The process of immersing the hexane fouled smooth surface into the water; Movie S3. The process of a water droplet contacting the rough surface immersed in hexane; Movie S4. The process of a water droplet contacting the smooth surface immersed in hexane; Movie S5 and S6. The oil-water separation process of SA hydrogel coated copper mesh using hexane.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] 27



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*E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 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. REFERENCES [1] Ye, Y.; Huang, J.; Wang, X. Fabrication of a Self-Cleaning Surface via the Thermosensitive Copolymer Brush of P(NIPAAm-PEGMA). ACS Appl. Mater. Interfaces 2015, 7 (40), 22128-22136. [2] Xue, C. H.; Zhang, Z. D.; Zhang, J.; Jia, S. T. Lasting and Self-healing Superhydrophobic Surfaces by Coating of Polystyrene/SiO2 Nanoparticles and Polydimethylsiloxane. J. Mater. Chem. A 2014, 2 (36), 15001-15007. [3] Ragesh, P.; Anandganesh, V.; Nair, S.; Nair, A. S. A Review on `Self-Cleaning and Multifunctional Materials'. J. Mater. Chem. A 2014, 2 (36), 14773-14797. [4] Tian, X.; Verho, T.; Ras, R. H. A. Moving Superhydrophobic Surfaces toward RealWorld Applications. Science, 2016, 352 (6282):142-143. [5] Fürstner, R.; Barthlott, W.; Neinhuis, C. Wetting and Self-Cleaning Properties of Artificial Superhydrophobic Surfaces. Langmuir, 2005, 21 (3):956-961. [6] Zhang, X. T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Self28


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Comparison of self-cleaning and anti-oil-adhesion performance between coated and uncoated surfaces 221x174mm (150 x 150 DPI)


The study of oil dewetting ability of superhydrophilic and underwater

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