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Article Cite This: ACS Omega 2018, 3, 1395−1402

Green, Biodegradable, Underwater Superoleophobic Wood Sheet for Efficient Oil/Water Separation Jiale Yong,† Feng Chen,*,† Jinglan Huo,† Yao Fang,† Qing Yang,*,‡ Hao Bian,† Wentao Li,‡ Yang Wei,† Yanzhu Dai,† and Xun Hou† †

State Key Laboratory for Manufacturing System Engineering and Key Laboratory of Photonics Technology for Information of Shaanxi Province, School of Electronics & Information Engineering, and ‡School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China S Supporting Information *

ABSTRACT: Superwettable (by water or oil) materials have been used in oil/water separation to cope with the growing oily industrial sewage discharge and oil spill accidents. The artificial superwetting materials for oil/water separation that have been previously reported are expensive, and using them usually causes secondary pollution, so practical, large-scale uses of those materials are limited. Here, we find that wood sheet shows underwater superoleophobicity and low oil adhesion in water, resulting from its strong capacity of absorbing water. A throughmicrohole array was created on the wood sheet surface by a simple mechanical drilling process. The prewetted porous sheet had great ability to separate the mixtures of water and oil with high separation efficiency. Wood is a low cost, green, and natural eco-friendly material; therefore, we believe that such a simple, low-cost, efficient, and green route of large-scale oil/water separation has great potential to practically solve the pollution problems caused by oil spill and oily industrial wastewater.

1. INTRODUCTION Oil spill accidents occur frequently, and the amount of industrial oily sewage is increasing year by year.1−4 Those accidents result in not only huge economic losses but also serious environment pollution.5,6 To address the abovementioned problems, developing effective oil/water separating technologies and materials is the critical path and is emerging as a hot research field nowadays.2−4 Recently, microporous materials with special wettability (i.e., superhydrophobicity or superoleophobicity) have been successfully applied in oil/water separation devices.2,3,7−10 In 2004, Feng et al. prepared a rough polytetrafluoroethylene-coated metal mesh which showed both superhydrophobicity and superoleophilicity in air and used it to separate the mixture of water and oil.11 Superhydrophobicity resulted in water staying on the mesh, whereas superoleophilicity allowed oil to fully wet and quickly penetrate through such a mesh, achieving the separating function.12−17 However, this kind of “oil removing” material can be easily contaminated and blocked by oil adhesion, resulting in an apparent decline of separation efficiency.18−22 But since then, the research on wettability-dependent oil/water separation still developed very slowly. Until 2011, Xue et al. successfully separated an oil/water mixture by using a nanostructured hydrogel-coated stainless steel mesh.18 The mesh was superhydrophilic in air but became superoleophobic after immersing in water. The underwater superoleophobicity prevented the oil © 2018 American Chemical Society

from really touching the mesh surface; therefore, the rough mesh could not be blocked even though the separation process was repeated many times. Such “water removing” materials which can remove water from the mixture have a better application prospect in oil/water separation.2 Following this strategy, various mesh or porous films with underwater superoleophobicity were developed and applied in oil/water separation.23−32 For example, Zhang et al. prepared a superhydrophilic and underwater ultralow oil-adhesive superoleophobic Cu(OH)2 nanorods-structured copper mesh by a chemical-based oxidation method.23 Such an all-inorganic membrane could effectively separate the mixtures of immiscible water and oil, as well as the oil-in-water emulsions. Wen et al. successfully achieved oil/water separation by using a zeolitecoated mesh.24 The zeolite coating had characteristic pores and endowed the coated mesh with superhydrophilicity and underwater superoleophobicity. Sun et al. coated a layer of graphene oxide (GO) on commercial wire meshes.33 After plasma treatment, it was demonstrated that the GO-coated meshes were superoleophobic in water and could separate the mixture of bean oil and water. Gao et al. fabricated an underwater superoleophobic porous nitrocellulose film with Received: December 26, 2017 Accepted: January 22, 2018 Published: February 2, 2018 1395

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Figure 1. Underwater superoleophobicity and antioil ability of the carp skin. (a−c) Scanning electron microscopy (SEM) images of the fish scale surface. Inset of (a): Photograph of a carp. Inset of (c): Shape of an oil (1,2-dichloroethane) droplet on a fish scale surface in a water medium. (d−f) Snapshots of an oil droplet dripping onto the skin of a horizontal carp.

micro/nano dual-scaled pores.19 The dual-scaled porosity allowed the film to rapidly separate water from various oil/ water mixtures with a separation efficiency of above 99%. Tao et al. prepared a superamphiphilic poly(vinylidene fluoride) membrane that showed superoleophobicity in water and superhydrophobicity in oil.25 Such a membrane exhibited remarkable separation efficiency, permeability, recyclability, and fouling resistance in dealing with both oil-in-water and waterin-oil emulsions. Sometimes, the oil in the mixture is heavy oil whose density is higher than that of water, and a horizontal “water removing” membrane cannot easily separate such a mixture by gravity. Alternatively, a vertical underwater superoleophobic membrane or a Janus porous membrane with distinct wettability is usually used to remove water from the mixture of water and heavy oils. Zhang et al. fabricated a Janus mesh that shows superhydrophobicity and superoleophilicity on one side and shows superhydrophilicity and underwater superoleophobicity on the other side by laser ablating the mesh followed by selective modification with fluorosilane and GO, respectively.34 This Janus mesh can separate the mixture of water with not only light oil but also heavy oil. The abovementioned underwater superoleophobic materials have been proven effective in separating the mixture of water and oil, but some inherent disadvantages restrict those separating technologies to just the laboratory. First and foremost, the cost of the adopted materials, the expensive equipment, and the human labor for constructuring rough microstructures become a tremendous burden for large-scale applications. Another problem is secondary pollution. The most widely used metal meshes and porous polymers need to be treated by chemical corrosion or other chemical treatments to form underwater superoleophobic rough microstructures. The waste produced to prepare the separating materials will bring a new environmental problem, although the oil pollution problem is solved. Therefore, taking into account the large-scale practical application, a simple, low-cost, eco-friendly, green route that can efficiently separate large amounts of the mixtures of water and oil is highly desired. Here, we presented that wood sheet has strong waterabsorbing ability and retention of water, endowing wood sheet with superoleophobicity and very low oil adhesion in water. After generating a through-microhole array by a simple drilling process, the resultant underwater superoleophobic thin wood sheet was successfully applied in oil/water separation with excellent separating ability. The good separation effect allows

this low-cost, large-scale, and green strategy for oil/water separation to have great potential for practical application.

2. RESULTS AND DISCUSSION The underwater superoleophobic phenomenon was first observed from fish scales by Jiang’s group.33,35 Carp (inset of Figure 1a) has tenacious survival capability even in an adverse natural environment. Its skin is fully covered by fan-shaped scales which are composed of hydrophilic calcium phosphate skeleton and protein and a thin layer of mucus. The scale surface is not smooth but shows a kind of a micro-/nanoscale hierarchical structure. There are many micropapillae with the size of several hundred micrometers fanning out on the scale surface (Figure 1a,b). The size of the micropapillae decreases from the central area to the edge of the scales. The top and the side wall of every micropapilla are further decorated with abundant nanoscale “small pimples” (Figure 1c). Underwater superoleophobicity is presented by fish scales. In water, when an oil (1,2-dichloroethane) droplet is put on a scale surface, the oil droplet will keep as a ball shape with an oil contact angle (OCA) of 151.5° ± 2° (inset of Figure 1c). Figure 1d−f shows the result of dripping an oil droplet onto the skin of a horizontal carp. The oil droplet can easily roll away without leaving any stain on the fish skin (Movie S1, Supporting Information). Such excellent antioil ability allows fish to maintain its very high swimming speed in turbid or even oilpolluted waters. If some scales peeled from a fresh fish are exposed to the air, they will curl after being dried, demonstrating that fresh fish scales are high in water content. It is the high water content (16.4−17.8%) as well as the inherent hydrophilicity that results in underwater superoleophobicity of fish scales and further endows fish skin with an oil-repellent function. Wood coming from trees is a low-cost, green, and natural eco-friendly material. Balsa, as a kind of natural wood, can be purchased directly from the market or through internet shopping and is very cheap. Its density is very low because of its rich porous inner structure but its strength is quite high; therefore, it is widely applied in aviation, shipping, transportation, building, wind energy, tables, chairs, and so on. It is well-known that wood is one of the superabsorbent materials. A 1.005 g dry wooden block (44 × 37 × 5 mm3) will become 2.021 g if it fully absorbs water (Figure S1, Supporting Information). The absorbed water (1.016 g) has almost the same weight as the dry wooden block. The water content of the wetted balsa wood in water can be estimated at about 50.3%; 1396

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Figure 2. Microstructure and underwater superoleophobicity of the wood sheet surface. (a−c) SEM images of the wood sheet surface. Inset of (a): Shape of an underwater oil droplet (1,2-dichloroethane) on the wood surface. Inset of (c): Corresponding high-magnification image of the wood texture. (d) Snapshots of a water droplet spreading on a wood sheet in air. (e) Snapshots of an oil droplet rolling off a wood sheet tilted 6.5° in water.

this value is much greater than that of fresh fish scales. The high water content of wetted balsa wood reveals that it may be underwater superoleophobic and could possibly be used to separate water/oil mixtures. There are abundant micropipes in the tree trunk, allowing water to flow from the base to the top of the tree. When the tree was sawn into sheets, those micropipes would break, resulting in many microgrooves on the surface of the wood sheets. Figure 2a−c shows the SEM images of the surface of a wood sheet. The surface is fully covered by ordered microgrooves. The grooves are about several tens microns in width. The wall of the grooves is not smooth. It was found that a very large number of protrusions with a size of several hundred nanometers are randomly distributed on the wall of the microgrooves (Figure 2c). When a water droplet was dripped onto the wood surface, the droplet would spread out quickly and be absorbed by the wood sheet (Figure 2d and Movie S2 in the Supporting Information). The water contact angle was close to zero, revealing inherent superhydrophilicity of the wood surface. It has been demonstrated that most in-air superhydrophilic materials will show superoleophobicity in water.19,23,35−41 When the wood sheet was immersed in water and some oil droplets (1,2-dichloroethane) were placed on its surface, the oil droplets maintained a spherical shape rather than spreading out (inset of the Figure 2a). The OCA was as high as 162.5° ± 2°, so the wood sheet exhibited underwater superoleophobicity. If the surface was tilted more than 6.5°, the underwater oil droplet would roll off the wood surface easily (Figure 2e and Movie S3 in the Supporting Information). Such a low oil sliding angle (OSA) value (6.5°) indicated a very small adhesive force between the underwater wood surface and the oil droplet. The adhesive force was measured to be only 3.7 μN. In addition to 1,2-dichloroethane, the wood sheet also showed underwater superoleophobicity and very low oil adhesion to a wide range of other oils, regardless of whether they were heavy oils or light oils. Figure 3 depicts the static shapes and dynamic rolling snapshots of some common oils droplets on the wood sheet in water. The measured OCAs and OSAs are 151.5° and 6° for hexadecane, 161.5° and 8.5° for dodecane, 152° and 8° for decane, 153.5° and 7° for petroleum ether, 150.5° and 4° for paraffin liquid, 151.5° and 5° for

Figure 3. Static shape and the dynamic rolling behavior of different oil droplets on a wood sheet in a water medium.

sesame oil, 153.5° and 5.5° for kerosene, and 154.5° and 3.5° for crude oil, respectively. Surface wettability is mainly governed by the surface chemistry and microstructures.42−47 As soon as the wood sheet was immersed in water, the wood would absorb water quickly, and water would fully wet the surface microstructures (Figure S2a,b, Supporting Information). A thin water layer was trapped in the microstructures. When an oil droplet was put on the wood sheet immersed in water, the trapped water like a water cushion would prevent the oil droplet from contacting with the wood substrate effectively because the polar water molecule generally repels the nonpolar oil molecules.24 In fact, the oil droplet could only touch the tips of the rough microstructures, which was at the underwater Cassie wetting state, as shown in Figure S2c (Supporting Information).1,35,38 The contact area between the wood 1397

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surface microstructures but also the through microholes. The underwater oil droplet on the MHWS surface was also at the underwater Cassie state, as shown in Figure 4g. Taking advantage of the in-air superhydrophilicity and inwater superoleophobicity of the MHWS, a simple oil/water separating device was designed, as shown in Figure 5a. The

substrate and the oil droplet is so small that the wood sheet is endowed with underwater superoleophobicity and low oil adhesion. A through-microhole array with a period of 1 mm was easily formed on a wood sheet (thickness of 1 mm) by a mechanical drilling process, as shown in Figure 4a,b. These microholes with

Figure 5. Separation of the mixture of oil (petroleum ether, red color) and water by the MHWS. (a−d) A whole cycle of oil/water separation: (a) before, (b) prewetting the wood sheet with water, (c) pouring the oil/water mixture into the upper tube, (d) after separation, and (e−h) restarting the separation process by adding new water into the device.

Figure 4. Microstructures and wettability of the MHWS. (a) Optical microscopy image of the MHWS in water. (b) SEM image of the through-microhole on the wood sheet. (c,d) Process of water droplets penetrating through the MHWS (in air) with increasing water volume continuously. (e) Static shape and (f) rolling snapshot of an underwater oil droplet on the MHWS surface. (g) Schematic diagram of the wetting state of an underwater oil droplet on the MHWS.

MHWS was used as the separating membrane and was sandwiched between two glass tubes. A small amount of water was first poured into the separating device to prewet the wood sheet (Figure 5b). Then, the oil/water separation would start. When the mixture of oil (petroleum ether, red color, dyed with oil red O) and water was poured into this device (Figure 5c), it was observed clearly that only water quickly passed through the MHWS and dripped into the beaker below, whereas oil was stopped by the prewetted wood and stayed in the upper tube (Figure 5d, Movie S6, Supporting Information). The separation process was just driven by the gravity. As a result, the mixture was divided into two parts: separated oil and separated water. Such a separating process could be finished in a very short time, and the separated oil and water can be used in our daily life again. The great separating ability of the MHWS is mainly the result of its remarkable water absorption and underwater superoleophobicity. The superhydrophilicity allows water to wet and pass through the microholes quickly and endows the prewetted wood with underwater superoleophobicity as well, whereas the underwater superoleophobicity prevents oil from touching and wetting the prewetted wood sheet surface. It is worth nothing that prewetting is a critical step to successfully separate the mixtures of water and oil. If the mixture was directly poured onto a dry MHWS, oil would also pass through the sheet with water. Most traditional materials for separating oil and water are disposable.2,3 Those materials have many limitations for practically solving pollution problems such as oil spills and

the diameter of ∼340 μm go right through the sheet. The diameter of the through microholes is larger than that of the used drill bit (300 μm) because of the drilling-process-induced extrusion and stretch effects. The rest area maintains its inherent micro-/nanoscale hierarchical structures except for the microholes (Figure 4b). When the microholes-through wood sheet (MHWS) was immersed in water, the wood sheet would absorb a large amount of water. Although the diameter of the microholes would decrease slightly as well, the drill-induced microholes were still open after some time. This could be observed by an optical microscope. As shown in Figure 4a, the microhole region showed white color because the backlight generated from the optical microscope could pass through the microholes. In contrast, the backlight was blocked by the rest region and such a region looked very dark. If a water droplet was dripped onto the MHWS, the water would spread out quickly within only 0.02 s because of the strong superhydrophilicity of wood (Figure 4c). With more and more water droplets being dripped onto the resultant surface, the water penetrated through the sheet and dripped down because there are a lot of perforated micropores distributing on the wood sheet (Figure 4c,d, Movie S4 in the Supporting Information). The MHWS still had superoleophobicity and low oil adhesion with an OCA of 155.5° ± 2° (Figure 4e) and an OSA of 7° ± 2° (Figure 4f, Movie S5 in the Supporting Information) in a water medium. In this case, water was trapped in not only the 1398

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fabrics.24,27 The influence of the pore size on the water flux, intrusion pressure, and separation efficiency was also investigated. For comparison, a bigger drill bit with a diameter of 500 μm was used to generate larger microscale through-holes on a wood sheet. Figure S5 (Supporting Information) shows the variation of the flux and intrusion pressure with the period for the larger-holes-structured wood sheet. A similar changing trend with the abovementioned results can be obtained, but there are some small differences. The flux values for the latter case are much larger than that for the former case, whereas the intrusion pressure values for the latter case are much less than those for the former case. Such low intrusion pressure sometimes leads to a failed separation. Therefore, using the drill bit with a diameter of 300 μm to treat the wood sheet is optimal. In addition, we found that increasing the thickness of the wood sheet is bad for achieving oil/water separation. The through-microholes of the thicker wood sheet are more inclined to be blocked after immersing in water because of the expansion of the wood. Dry wood is a well-known water-absorption material. The wood mainly consists of hydrophilic cellulose nanofibrils, lignin, and hemicelluloses. Such a hydrophilic nature and an inherent porous inner structure endow the wood surface with excellent underwater superoleophobicity. Because the oil/water separating process is more involved in the underwater superoleophobicity of the wood surface but nothing to do with the wood species, we believe that most types of wood sheets can potentially be used to separate the oil/water mixture after the generation of through-microholes and prewetting treatment. For the practical application, Balsa wood has many advantages such as low density and low cost. The property of light wood enables the separating device to be easily transported, and the low price can save cost for oil/water separation. Wood is a natural material and can be directly biodegraded by microbes in soil. The degradation of lignin is the critical process and step of utilizing plant cellulose materials effectively. Once the MHWS is damaged or contaminated after a long-term work, it will be easily degraded and run off in soil by a biodegradable treatment. Therefore, our fabricated MHWS is a green and eco-friendly separating membrane. In addition, wood sheets generally have high structure strength, so the separating devices with various shapes (such as wooden box, bucket, basket, etc.) can be prepared. The prewetted microhole-structured wooden box, bucket, and basket can be used to separate the mixture of water and heavy oils. Such good strength also ensures the large-scale oil/water separation by the MHWS.

oily industrial wastewater. As far as our designed device, it stopped and quieted down at last after finishing a cycle of oil/ water separation. Interestingly, when we further poured some water into the upper tube, the separation process would be reactivated (Figure 5e−h, Movie S7 in the Supporting Information). The new added water passed through the MHWS again, and the oil remained in the upper tube all the time, demonstrating that the separation process could be restarted instantly as soon as a new mixture was “fed”. After 10 cycles of separation, the wood sheet still retained its color without any part being stained by red oil which was predyed by oil red O (Figure S3, Supporting Information). This result reveals that the MHWS was not contaminated by oil during the whole separation process because the oil was repelled by the trapped water cushion in the microstructures of the wood sheet surface and could not effectively come in contact with the wood surface. So, the separation process can be cycled a large number of times, and the separation device can work continuously. In addition, the separation efficiency of the prewetted MHWS was investigated by an optical microscope, as shown in Figure S4 (Supporting Information). The result indicates that the prewetted MHWS showed very high separation efficiency for the mixture of oil and water. The period of the microhole array can be easily controlled by a computer program during the drilling process, which has a major impact on the separating flux and intrusion pressure of the designed separation system. The water fluxes of the MHWSs with different periods were assessed by measuring the time of a water column with the height of 15 cm passing through the resultant sheets, as shown in Figure 6. With

Figure 6. Variation of flux and intrusion pressure with the period of the MHWS which was drilled with a thin drill bit 300 μm in diameter.

increasing the period, the water flux decreases quickly because the density of the microholes that allow water to pass through reduces as well. The larger flux value is obtained by the microhole-structured sheet with a smaller period. For example, the water flux reaches up to 1.77 × 104 L m−2 h−1 when the period is 1 mm (density of the microholes = 100 cm−2). As another important performance of the separation device, the intrusion pressure of oil was simply calculated by measuring the maximum supporting height of the oil column. It can be seen from Figure 6 that the intrusion pressure changes little as the period increases from 1 to 3.5 mm. The intrusion pressure is almost a constant value in the range of experimental errors, regardless of whether the microholes are close or not. In fact, the drilled microholes worked independently and had no interference with each other, so the intrusion pressure did not depend on the density as well as the period of the microhole array. Such a result is very different from the rough meshes and

3. CONCLUSIONS In conclusion, we found that the wood sheet shows underwater superoleophobicity and ultralow oil adhesion to a wide range of oils, such as 1,2-dichloroethane, hexadecane, dodecane, decane, petroleum ether, paraffin liquid, sesame oil, kerosene, and crude oil. The measured OCA and OSA were 162.5° ± 2° and 6.5°, respectively, for a 1,2-dichloroethane droplet on the wood sheet in a water medium. The underwater superoleophobicity of the wood sheet was caused by its strong water absorbing ability. After generating a through-microhole array on the wood sheet surface by a simple mechanical drilling process, such a resultant sheet was successfully applied to separate the mixture of water and oil and showed excellent separating ability. This is a pure green separating method: green materials, green fabrication, and green results. Wood is a low-cost and very common material; therefore, we believe such a simple, low-cost, efficient, 1399

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4. EXPERIMENTAL SECTION 4.1. Materials. The wood sheet with a thickness of 1 mm that was used in this experiment is balsa, which was purchased directly through internet shopping. Carp was bought from the market. 4.2. Generating Through-Microhole Arrays. A manmade drilling mechanical system was used to generate microholes that fully across from one side to another side of the wood sheet. The wood sheet was fixed on a platform in advance. A mini drill was used. The diameter of the drill bit was 0.3 mm. Then, the mobile platform was controlled to move up and down and the drill bit was allowed to pass through the wood sheet with the speed of 0.3 mm/s. A uniform throughmicrohole array could be further fabricated by controlling the drilling points. 4.3. Oil/Water Separation. The wood sheet with a microhole array was applied as a separating membrane. It was sandwiched between two glass tubes 30 mm in diameter. During a whole separating process, the wood sheet was prewetted by a small amount of water in advance, and then, the mixture of oil (petroleum ether) and water (Voil/Vwater = 1:1) was quickly poured into the designed separating device. To distinguish oil and water, the used oil was dyed by oil red O and showed red color. 4.4. Characterization. The surface microstructures of the wood sheet and the through-microhole-structured wood sheet were observed by a scanning electron microscope (Quanta 250 FEG, FEI, America). The wettability, including contact and sliding angles, was investigated via a contact-angle system (JC2000D, Powereach, China). Deionized water and petroleum ether were used as the main probe liquids. The underwater oil wettabilities of hexadecane, dodecane, decane, petroleum ether, paraffin liquid, sesame oil, kerosene, and crude oil droplets were also tested. The adhesive force was measured by a highsensitivity micro-electromechanical balance system (DCAT 11, DataPhysics, Germany).



Process of oil/water separation based on the microholestructured wood sheet (AVI) Restarting the oil/water separation process by added new water into the designed device (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.C.). *E-mail: [email protected] (Q.Y.). ORCID

Feng Chen: 0000-0002-7031-7404 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China under the grant no. 2017YFB1104700, the National Science Foundation of China under the grant nos. 51335008, 61475124, the NSAF grant no. U1630111, China Postdoctoral Science Foundation under the grant no. 2016M600786, the Collaborative Innovation Center of Suzhou Nano Science and Technology and the International Joint Research Center for Micro/Nano Manufacturing and Measurement Technologies. The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University.



REFERENCES

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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/acsomega.7b02064. Weight of a dry wooden block before and after absorbing water; schematic diagram of the wetting state of an underwater oil droplet on a wood sheet in a water medium; photograph of the MHWS after several cycles of oil/water separation; separation efficiency of the prewetted MHWS; variation of the flux and intrusion pressure with the period of the MHWS which was drilled with a thick drill bit 500 μm in diameter (PDF) Dripping some oil droplets onto a carp skin (AVI) Water droplet being absorbed by a wood sheet in air (AVI) Oil droplet rolling on a wood sheet in a water medium (AVI) Water penetrating through the microhole-structured wood sheet and dripping down (AVI) Underwater oil droplet rolling on the microholestructured wood sheet (AVI) 1400

DOI: 10.1021/acsomega.7b02064 ACS Omega 2018, 3, 1395−1402

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DOI: 10.1021/acsomega.7b02064 ACS Omega 2018, 3, 1395−1402

Article

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DOI: 10.1021/acsomega.7b02064 ACS Omega 2018, 3, 1395−1402

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