Low Drag Porous Ship with Superhydrophobic and Superoleophilic

Research Article www.acsami.org

Low Drag Porous Ship with Superhydrophobic and Superoleophilic Surface for Oil Spills Cleanup Gang Wang,§,†,‡ Zhixiang Zeng,§,‡ He Wang,‡ Lin Zhang,‡ Xiaodong Sun,‡ Yi He,‡ Longyang Li,‡ Xuedong Wu,‡ Tianhui Ren,*,† and Qunji Xue‡ †

Key Laboratory for Thin Film and Microfabrication, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ‡ 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 Sciences, Ningbo 315201, P. R. China S Supporting Information *

ABSTRACT: To efficiently remove and recycle oil spills, we construct aligned ZnO nanorod arrays on the surface of the porous stainless steel wire mesh to fabricate a porous unmanned ship (PUS) with properties of superhydrophobicity, superoleophilicity, and low drag by imitating the structure of nonwetting leg of water strider. The superhydrophobicity of the PUS is stable, which can support 16.5 cm water column with pore size of 100 μm. Water droplet can rebound without adhesion. In the process of oil/water separation, when the PUS contacts with oil, the oil is quickly pulled toward and penetrates into the PUS automatically. The superhydrophobicity and low water adhesion force of the PUS surface endow the PUS with high oil recovery capacity (above 94%) and drag-reducing property (31% at flowing velocity of 0.38m/s). In addition, the PUS has good corrosion resistance and reusability. We further investigate the wetting behavior of water and oil, oil recovery capacity, drag-reducing property, and corrosion resistance of the PUS after oil absorbed. The PUS surface changes significantly from superhydrophobic to hydrophobic after absorbing oil. However, the oil absorbed PUS possesses better drag-reducing property and corrosion resistance due to the changes of the motion state of the water droplets. KEYWORDS: superhydrophobic, superoleophilic, pours metal mesh, oil/water separation, biomimetic, unmanned ship, drag-reducing, anticorrosion

■

tion, and so on.7,8 For oil removing and recycling from water surface, the flexible three-dimensional (3D) porous adsorbing materials with superhydrophobicity and superoleophilicity are supposed to the promising materials. The 3D porous structure of the absorbents can concentrate and transform liquid oil into a semisolid phase that facilitates the removal of the oils by removal of the absorbent materials.9 Simple squeeze process can achieve the oil spills recovery. In recent studies, lots of superhydrophobic and superoleophilic sponges and gels with high oil wettability, absorption capacity, oil retention, and good recyclability are fabricated for oil contaminations cleanup.10−15 However, the process of oil transportation and recovery are heavy, complicated, time-consuming, and easily cause secondary pollution. The repeated squeeze process will damage the superhydrophobicity and mechanical strength of the sponge.

INTRODUCTION

Crude oil plays an important role in modern industry and energy. However, oil spills resulting from the frequent accidents of oil exploration, transportation, and storage not only bring great loss of energy resource but also cause adverse impacts to ecosystems.1−3 The traditional techniques and materials used for oil spills cleanup such as oil collection machines, oil dispersing agents, oleiphilus, and oil adsorbing materials cannot meet the demands of marine environment protection and resource reuse due to its low sorption capacity, inefficiency, oil unrecyclable, poor reusability, and secondary pollution.4,5 Especially, the recovered oil with high water content presents a state of “chocolate mousses,” which is difficulty to postprocess.6 How to efficiently remove and recycle oil spills has significant meaning to marine environment protection and resource reuse, which is still a world challenge. The superwetting surfaces with liquid contact angle higher than 150° and the sliding angle lower than 10° have attracted increased attention due to their wide applications, including self-cleaning, antifogging, drag-reducing and oil/water separa© XXXX American Chemical Society

Received: September 1, 2015 Accepted: November 12, 2015

A

DOI: 10.1021/acsami.5b08185 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. A Sketch of the Oil Spill Cleanup and Recovery by Porous Unmanned Ship

contaminated by nuclear, biological, or chemical agents to avoid casualties.23 There is no doubt that oil spill removal and recycling without artificial operation is necessary due to the harsh marine environment and toxic oil spills. If a USV has the function of oil spill removal and recycling, the oil spill cleanup will be more simple, efficient, and without endangering maritime personnel life safety. In this work, inspired by water striders, we fabricated a porous unmanned ship (PUS) with properties of superhydrophobicity and superoleophilicity for oil spill removal and recycling without artificial operation. First, to imitate the structure of nonwetting leg of water strider, we constructed aligned ZnO nanorod arrays on the surface of the porous stainless steel wire mesh via hydrothermal method, which is simple and easy scaleup. Then, the mesh was coated by a layer of cross-linked polydimethylsiloxane (PDMS) to lower the surface energy to achieve the superhydrophobic and superoleophilic property. The superhydrophobicity and low water adhesion force of the mesh surface endowed the as-prepared PUS with high oil/water separation capacity and drag-reducing property. The porous structure and superoleophilicity of the mesh surface ensure the oil infiltrated through the mesh and gathered into the as-prepared PUS automatically. In addition, with the change of the water wetting behavior (from superhydrophobic to hydrophobic), the motion state of the water droplets on the PUS turns from rolling to sliding, and the PUS has better drag-reducing, corrosion resistance, and reusability before and after absorbing oil, which ensures its practical application. The process of oil spill cleanup and recycle by PUS is shown in Scheme 1.

Above all, these processes must be done manually, which is harmful to the health of the operators. Similar to the superwetting sponge, metal mesh with special wettability is widely used in oil/water separation, because of its high oil/water separation efficiency, low cost, and superior mechanical properties.16−19 However, the separation of oil/ water mixtures by superwetting mesh needs to collect oil/water mixture in bulk first and then pour onto the mesh, which is complex and energy-consuming due to its 2D structure. So, Shi and co-workers fabricated a superhydrophobic and superoleophilic nickel foam box by combining electroless metal deposition with self-assembled monolayers for highly efficient and inexpensive oil-spill cleanup.20 Song and co-workers designed a superhydrophobic and superoleophilic stainless steel mesh capped container, which can simultaneously filter and collect floating oil from oil−water mixtures.21 Li and coworkers prepared a porous, superoleophilic, and superhydrophobic miniature oil containment boom by a one-step electrodepositing of Cu2O film on Cu mesh surface without low surface energy chemical modification for the in situ separation and collection of oils from the surface of water.22 Compared to the superwetting sponge, these novel devices can collect and recycle oil without extrusion process. Nevertheless, first, the fabrication of superhydrophobic and superoleophilic mesh is complicated and relies on the equipment such as direct-current power or high-temperature heater. These processes are difficult to achieve and scale up, and these devices also need manual operation. Second, under certain water pressure, some superhydrophobic surface of Cassie state is easy to transform into Wenzel state, which is unfavorable in oil collection on water surface. A stable superhydrophobic surface must be designed to avoid this transition. And last, the performance of the oil-absorbed device (oil filled in the micro/nanotopography) has not been studied, which is important to the reusability. So, more effective, stable, durable, scalable, and lower-cost methods and materials are required to remove and recycle large-scale oil spills in the harsh marine environment, and further research work is needed. With the development of remote sensing technology, more and more unmanned surface vehicles (USVs) conduct a wide range of missions in highly dangerous environments or areas

■

RESULTS AND DISCUSSION In previous study, Gao and Jiang have found that water strider can stand effortlessly and move quickly on water surface due to its remarkable superhydrophobic legs.24 The hierarchical structures of many oriented tiny hairs with fine nanogrooves and secreted wax on legs of the water strider were believed to be responsible for the water striders’ floating.25 Moreover, the superhydrophobic legs can not only provide a larger supporting force but also help to decrease the insect’s total density beneath B


Research Article

ACS Applied Materials & Interfaces

Scheme 2. Schematic Illustration of Fabrication Process for Superhydrophobic and Superoleophilic Porous Unmanned Ship

Figure 1. Structure characterization of ZnO-coated mesh. (a) Large-area view of ZnO-coated mesh. (b) SEM image of single wire of ZnO-coated mesh. (c, d) The higher magnification SEM images of the ZnO-coated mesh surface. (e) The SEM image of a single ZnO nanorod. (inset) Nanosheet presents regular hexagon.

Figure 2. Chemical composition of stainless steel meshes with various coatings. (a−c) EDS spectra of Blank Mesh, Mesh/ZnO, Mesh/ZnO/PDMS film, respectively. (d) XRD patterns of Blank Mesh, Mesh/ZnO film.

porous stainless steel wire mesh via a simple one-step hydrothermal method to imitate the hierarchical micro- and nanostructures of water strider’s leg. The surface morphology of the ZnO-coated mesh is observed using scanning electron microscopy (SEM), shown in Figure 1a−e. As shown in Figure 1a, the ZnO-coated mesh is

the water surface and allow it to move faster on the water surface.26 To realize porous ship with properties of floating, fast moving, and loading, we fabricated a superhydrophobic surface that is similar to water striders’ leg on metal mesh (The fabrication of the PUS is illustrated in Scheme 2). First, we constructed aligned ZnO nanorod arrays on the surface of the C


Research Article

ACS Applied Materials & Interfaces Table 1. Liquid Contact Angles and Sliding Angles of Various Mesh Surface Mesh/PDMS

WCA WSA OCA

Mesh/ZnO/PDMS

Blank Mesh

Mesh/ ZnO

PDMS 0.2 g/ 100 mL

PDMS 0.6 g/ 100 mL

PDMS 1.0 g/ 100 mL

PDMS 0.2 g/ 100 mL

PDMS 0.6 g/ 100 mL

PDMS 1.0 g/ 100 mL

125° ± 2°

131° ± 3°

128° ± 3°

133° ± 4°

138° ± 1°

0°

0°

0°

0°

0°

151° ± 5° 12° ± 2° 0°

158° ± 1° 5° ± 1° 0°

156° ± 5° 9° ± 2° 0°

knitted from stainless steel wires with an average wire diameter of 52 μm and pore size of 100 μm. Unlike the smooth surface of blank stainless steel mesh (Figure S1), the ZnO-coated mesh surface is rough and white, and there is no obvious changes in the wire diameter and pore size after coated by ZnO (Figure 1a). It is seen from the magnified SEM image (Figure 1b) that the wire surface is wrapped thickly and uniformly by needleshaped crystals after hydrothermal treatment in the zincate solution for 1 h. The higher magnification image (Figure 1c) reveals the aligned ZnO nanorod arrays, and most of these nanorod are ∼2 μm in length. Interestingly, through detailed microscope study (Figure 1e), the ZnO nanorod is composed of hexagon nanosheet, and there are many nanogrooves on the surface of the ZnO nanorod. So, the structure of constructed ZnO is similar to that of water strider’s legs composed by hierarchical nanorods and nanogrooves. In addition, the onestep hydrothermal method is simple, facile, and scalable, which can construct the aligned ZnO nanorod arrays on various substrates (Figures S2 and S3). The chemical composition of the coating is characterized by energy dispersive spectrometry (EDS). Compared with the Blank Mesh film (Figure 2a), the EDS analysis shows strong Zn and O peaks for the mesh after hydrothermal treatment in the zincate solution for 1 h (Figure 2b). X-ray diffraction (XRD) reveals the structure and phase composition of the as-prepared ZnO coating. As shown in Figure 2d, diffraction peaks marked with spades match well with standard hexagonal ZnO crystallography (see inset in Figure 2d (JCPDS card No. 36− 1451)), and the four small peaks marked with diamonds can be ascribed to the stainless steel mesh substrate. The above results indicate that the hierarchical aligned ZnO nanorod arrays are successfully coated on the stainless steel mesh substrate. The wax on the surface of water strider’s legs and the lotus leaves, which have low surface energy combined with typical micro/nanoscale hierarchical structure, made them superhydrophobic.27−29 In previous studies, to achieve superhydrophobicity, the rough surface is usually modified by fluorinated molecules via chemical reaction to reduce the surface energy. So, the rough surface must have reaction activation to the fluorinated monomers.14,30 In addition, fluorinated monomers are costly and toxic. Here, we coat a layer of cross-linked PDMS on the surface of the ZnO nanorod arrays to reduce surface energy, which is inexpensive and easy to achieve. As shown in the magnified SEM image (Figure S4), there is a thin PDMS film on the ZnO surface. The EDS analysis showes evidence of very strong Si peak for the ZnO coated mesh covered with a PDMS layer (Figure 2c). In contrast, no Si peak is found for the Blank mesh and ZnO/ Mesh films (Figure 2a,b). After the PDMS coating, the surface energy of ZnO coated mesh decreases from 82.86 to 24.81 mN/m. In Table 1, although the blank mesh is coated with micro/ nanoscale hierarchical structures ZnO, the Mesh/ZnO film is hydrophobic (131°) but not superhydrophobic due to the high

surface energy of ZnO. And, the blank mesh modified with different concentrations of PDMS solution show only hydrophobicity with water contact angle (WCA) range from 128° to ∼138°. According to Young’s equation, cos θ* = r cos θy, the roughness amplifies the wetting properties. In other words, a hydrophobic substrate will be more hydrophobic, and an oleophilic substrate will be more oleophilic. Here, θ* is the apparent contact angle, and r is the roughness factor defined as the specific area/projected area.29 Combined with the low surface energy modification of cross-linked PDMS and the amplification effect of the hierarchical structure of ZnO, the WCA of Mesh/ZnO/PDMS film (151° to ∼158°) is much higher than that of Mesh/ZnO and Mesh/PDMS films. So, chemical composition and micro/nanoscale hierarchical structures are the two main factors to govern surface wettability on solid surfaces.31 The lower the surface tension is, the easier the liquid spreads on a solid surface. The oil, whose surface tension is much lower than that of water, spreads rapidly across the surface of Blank Mesh, Mesh/ZnO, and Mesh/ZnO/PDMS film, leading to an oil contact angle (OCA) of ∼0° (Table 1; oil (n-hexane) permeates into mesh quickly). To verify the amplification effect of the hierarchical structure to the oil wettability, we select high-viscosity oil (liquid paraffin, Table S1) to test the oil diffusion velocity by recording the changes of the OCAs on various mesh surfaces. Compared with the Blank Mesh and Mesh/PDMS, the Mesh/ZnO and Mesh/ZnO/PDMS with micro/nanostructures have higher oil wettability than the oil diffusion on the mesh surface quickly in 2 s (Figure 3). After 10 s, the OCAs of the Blank Mesh and Mesh/PDMS are ∼27° and 21° (inset Figure 3), respectively. Therefore, the roughness of the mesh and the special micro/nanostructures on the mesh

Figure 3. Dynamic contact angles of oil droplets (liquid paraffin, 3 μL) on various meshes. (inset) Optical images of oil contact angles after 10 s. D


Research Article


Figure 4. High-speed camera photographs of the interaction of Blank Mesh, Mesh/ZnO, and Mesh/ZnO/PDMS film with water (water dyed by methylene blue) (a−c) and oil (n-hexane dyed by oil red O) (d−f).

Figure 5. (a) Side view of the maximal dimple and plastron effect just before the superhydrophobic PUS penetrates the water surface. (b) Top view of superhydrophobic PUS floating on water surface with a loading of 14 times its weight. (c) Image of water droplets (water was dyed by methylene blue) sit on the PUS surface formed ball-like shape and oil droplets (n-hexane was dyed by Oil Red O) spread rapidly across the PUS surface. (d) Schematic illustration of the water wetting model of the as-prepared Mesh/ZnO/PDMS film. (e) Schematic illustration of the oil-wetting model of the as-prepared Mesh/ZnO/PDMS film. (f) The effect of the pore size of the mesh substrates on WCA and water-intruding pressure.

E


Research Article


Figure 6. Oil−water separation properties of the PUS from water surface (Motor oil was dyed by Oil Red O). (a) Photograph of the process of oil cleanup by PUS on water surface. (b) The separation efficiency as a function of the viscosity of oils (motor oil, paraffin liquid, vegetable oil, crude oil, and n-hexane). (c) The reusability of PUS (The oil was n-hexane). (d) The effect of pore size on the immersion time of the PUS with volume of 6 cm3 (3 cm × 2 cm × 1 cm).

camera (Figure 4a−c, Movies S1−S3). In Figure 4a, after the water droplet hit the Blank Mesh film surface, it first penetrated into the mesh, deformed, and then stuck in the film with the dissipation of the drop’s kinetic energy. To the ZnO coated mesh (Figure 4b), a part of the water droplet was crushed into small droplets after the droplet penetrated into the mesh unlike that of Blank Mesh film. To the Mesh/ZnO/PDMS film, the water droplet did not penetrate into the mesh. The drop’s kinetic energy was transformed into vibrational energy, allowing the droplet to rebound before it underwent damped oscillations and finally rested on the surface in the Cassie state.37,38 Water bouncing results, revealing the superhydrophobicity of the Mesh/ZnO/PDMS film, indicated that the porous ship could effectively prevent water intrusion. To the oil (Figure 4d−f and Movie S4−S6), after the oil droplet hit the mesh surface, they all first penetrated into the mesh, but the oil on the Mesh/ZnO and Mesh/ZnO/PDMS films expanded, retracted, and then stuck on the surface. The oil droplet on the Blank Mesh film crashed into small droplets and fell. These results illustrated that the oil can not only pass though the mesh freely but also adhere on the mesh. Superhydrophobic legs of the water strider tread tremendous dimples without piercing the water surface generate a supporting force to support the weight of its body, exhibiting a striking flotation ability.25 We test flotation ability of the PUS on water surface. Figure 5a,b shows the process of the PUS depressing the free surface of the water from the side and overhead views. There is a plastron surrounding the immersed part of the PUS in water formed by a thin layer of air trapped in the micro/nanotopography of the PUS surface, which can avoid direct contact between the PUS surface and water. Similar to the leg of water strider on the water surface, a dimple formed by the deformation of the water surface occurs under the PUS, yielding a supporting force to support the weight of PUS and loads. The PUS can float on the water surface and even hold a

not only amplifies the hydrophobic surface to be superhydrophobic but also amplifies the oleophilic surface to be superoleophilic. Water droplet deposited on the superhydrophobic surface can be sticky (Wenzel state) with high adhesive force or nonsticky (Cassie state) with low adhesive force. In Table 1, the Blank Mesh, Mesh/ZnO, and Mesh/PDMS films are sticky surface with high hysteresis even when the surface turned 90° (Figure S5). However, the Mesh/ZnO/PDMS film surface is nonsticky with water sliding angle (WSA) ranging from 5° to ∼12°. It has been demonstrated that the air trapped in the micro/nanotopography can significantly influence the adhesive force between the liquid and surface.32 The area fractions of liquid droplet makes contact with the solid surface and air can be calculated by the model presented by Cassie and Baxter: cos θr = f1 cos θ − f 2 where θr is the contact angle (CA) on a rough surface, θ is the intrinsic CA on a corresponding smooth surface, f1 and f 2 are the area fractions of liquid droplet makes contact with the solid surface and air, respectively, and f1 + f 2 = 1.33 The WCA of the Mesh/ZnO/PDMS film is 158° ± 4°, and the WCA on a smooth corresponding smooth PDMS surface is 110°. Substituting the values of θr and θ in to the equation, the value of f1 and f 2 is ∼11.1% and 88.9%, respectively, which indicates that ∼88.9% of the rough surface area is covered by the air, and only 11.1% is in contact directly with water. According to the above analysis, due to the air layer trapped in the micro/nanotopography, water can slide easily on the Mesh/ ZnO/PDMS surface. Compared to the liquid sliding angle test, the liquid bouncing test can more directly reflect adhesion effect of the surface to the porous materials, such as mesh, cotton wool, and foam.34−36 We further investigate the interaction between liquid (water and oil) and meshes. Water droplet with a radius of 1 mm impinged on the mesh surface from the height of 100 mm, and the process was recorded by using the high-speed F


Research Article


Figure 7. Water drag-reducing effect of various mesh films. (a) The motion state of water droplet (3 μL) on the SLIPS, the sliding angle is 8°. (b) The drag of Blank Mesh, Mesh/ZnO, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films under different flow velocity. (c) The drag-reducing of the Mesh/ZnO, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films compared to the Blank Mesh film under various flow velocity.

force of ∼14 times the weight of the PUS without piercing the water surface (the weight of PUS and loading are 0.54 and 7.72 g, respectively; Figure 5b). In Figure 5c, the water droplets dyed by methylene blue stand on the PUS surface formed balllike shape, and oil was dyed by Oil Red O spreads across the PUS surface, which indicates the excellent water barrier property and oil wettability. To thoroughly understand the results of superhydrophobicity and superoleophilicity of the PUS, the water and oil wetting processes are modeled in Figure 5d,e. Previous studies demonstrated that the property of preventing spontaneous transition from the Cassie to the Wenzel state for superhydrophobic mesh is attributed to the advancing angle θa that the liquid must exceed for wetting the bottom of each mesh pore; that is, the intrusion pressure, ΔP, must be overcome. And ΔP can be given as Δp = (2γ)/R = −lγ (cos θa)/A where γ is the surface tension, l is the pore’s perimeter, R is the meniscus’s radius, A is the pore’s area, and θa is the advancing contact angle of water or oil on the film.31,33,39 In this work, for water (Figure 5d), θa is larger than 90° due to the superhydrophobicity of mesh, and ΔP is apparently larger than 0. So, water underneath and on the PUS surface cannot penetrate mesh spontaneously. As to oil, the θa is nearly 0° due to the superoleophilicity of mesh. According to the equation, Δp < 0, which means that oil can spontaneously pass through the mesh. Therefore, the Mesh/ZnO/PDMS film is very suitable to be used in oil collection on the water surface. We further investigate effect of the pore size of the mesh substrates on WCA and water intruding pressure. In Figure 5f, when the pore size is higher than 180 μm, the WCA decreases to 147°,

and superoleophobicity cannot be obtained. With the increase of the pore size, the water intruding pressure drops. The water intruding pressure inclines to a low value of 0.68 kPa at the pore size of 180 μm, which only can support 7 cm water column (Figure S6). Thus, the mesh with pore size of 100 μm is suitable for the application on the sea. Because of the excellent wettability of super low and high adhesive force to water and oil, respectively, we fabricated the Mesh/ZnO/PDMS film to PUS for oil spills cleanup and recycle. To investigate the oil collection capacity of the PUS on the water surface, we pour oil on the water surface and collect it by the PUS. In Figure 6a and Movie S7, the PUS can float and move easily on the water surface due to its superhydrophobicity. When the PUS contacts the oil, due to its superoleophilicity, the oil is quickly pulled toward and spread over the PUS. Then the oil penetrates into the PUS from the pores and is gathered automatically. Water is excluded. Finally, the oil is collected and separated from water surface completely by the PUS. With the load of oil, the PUS can still float on the water. Compared to the oil absorption sponge, a vacuum pump can achieve the oil recycling without heavy and complicated squeezing process. So the oil can be in situ collected simultaneously and continuously without manual operation, which can prevent the injury to the human by the harsh marine environment and toxic oil spills. To investigate the separation efficiency of the PUS, we supervise the quality change of the PUS before and after the oil collection, and the separation efficiency (R (%)) can be calculated according to the following equation: R% = (wa − wb)/woil × 100, where wa and wb are the weight of PUS after G


Research Article


Figure 8. Corrosion resistance of the Blank Mesh, Mesh/ZnO, Mesh/PDMS, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films. (a) Photos of salt spray test of the Blank Mesh, Mesh/ZnO, Mesh/PDMS, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films at different exposure time. (b) Potentiodynamic polarization curves of the Blank Mesh, Mesh/ZnO, Mesh/PDMS, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films. (c) Schematic illustration of the water droplet rolling off and sliding away the inclined surface of Mesh/ZnO/PDMS and Mesh/ZnO/PDMS/Oil films, respectively. (d) The water contact angle of Mesh/ZnO/PDMS film after salt spray test at different exposure time.

Mesh/ZnO/PDMS/Oil film surface. The Mesh/ZnO/PDMS/ Oil film forming by the oil trapped in the hierarchical structure after absorbing oil is also at no water captured status and may be applied in the field of drag-reducing of vehicle motion on or under water surface. Flow resistance characteristics of different adhesive mesh surfaces under different flow velocity condition are researched. In Figure 7b, the results show that the drag increases, while the flow velocity increases. Compared with the Blank Mesh film, the Mesh/ZnO, Mesh/ZnO/PDMS, and Mesh/ZnO/PDMS/Oil films displayed drag-reducing effect (Figure 7c). Because of the water captured status of Mesh/ZnO film and no water captured status of the Mesh/ZnO/PDMS film, the Mesh/ZnO/PDMS film has a better drag-reducing effect than Mesh/ZnO film. Interestingly, the drag-reducing effect of hydrophobic Mesh/ZnO/PDMS/Oil film is much better than that of the superhydrophobic Mesh/ZnO/PDMS film. The difference between the Mesh/ZnO/PDMS and Mesh/ZnO/PDMS/Oil film in the water is the liquid contacting mode, liquid−air−solid contacting type, and liquid−liquid-solid contacting type, respectively. Maybe the liquid−liquid-solid contacting type is more advantageous to the reduction of resistance. The maximum drag-reducing effects of Mesh/ZnO/PDMS and Mesh/ZnO/PDMS/Oil films are as high as 30.61% and 39.04% at flowing velocity of 0.38 and 1.2 m/s, respectively (Figure 7c). Therefore, the PUS has good drag-reducing property at various flow velocity before and after oil collection, which is conducive to vehicle motion. The PUS used on the sea will suffer from the marine corrosion. The corrosion reaction between the metal and corrosive medium leads to the failure of metal materials and the superhydrophobicity of the PUS. Nonwetted surface, which can prevent the contact between liquid and subject surface, has

and before oil collection. woil is the weight of oil poured into the water surface. Figure 6b shows the separation efficiency of the PUS as a function of the viscosity of oils. It shows that the separation efficiency for different kinds of oils are all above 94%. To the low-viscosity oil, when the oil pulled into water, the oil quickly spreads on the water surface and forms a thin oil layer. The thin oil layer is difficult to collect by the PUS in a short time. The thin oil layer and the volatilization of the oil are the two main mass losses of the collected oil that affect separation efficiency. Unlike the low-viscosity oil, the crude oil with high viscosity can be collected completely with the increased weight of water, which is adhereded to oil on the bottom of PUS. Moreover, the PUS can be reused in the oil collection with the separation efficiency above 90% (Figure 6c and Figure S7). Interestingly, in the process of repeated use, the oil absorbed in the walls does not block the pores but accelerates the speed of oil absorption. In Figure 6d, with the increase of the pore size, the oil permeability through the mesh increases. To the mesh with pore size of 100 μm, the PUS with volume of 6 cm3 (3 cm × 2 cm × 1 cm) only needed 18.3 s to fully fill with oil. Previous studies revealed the superhydrophobic surface at no water captured status (Cassie mode) have drag-reducing effect due to the low water adhesive force.40 Slippery lubricantinfused porous surface, called SLIPS, is verified to be a low adhesive surface and applied in self-cleaning.41 The Mesh/ ZnO/PDMS film surface with a water contact angle of 158° and a roll-off angle of 5° is at no water captured status due to the air trapped in the hierarchical structure. In Figure 7a, when the Mesh/ZnO/PDMS film absorbed oil, the WCA decreases from 158° to 95.6° due to the oil layer. Although the WCA decreases significantly, the water droplet can slide freely on the H


Research Article


Figure 9. Mechanical durability of the as-prepared mesh. (a) Contact angle as a function of peeling times. (b) The contact angle of the as-prepared mesh after abrasion on various grid SiC sandpaper. (c) The contact angle of the as-prepared mesh after immersing in various oils and seawater for 5 d.

for the good durability of the superhydrophobic film (Mesh/ ZnO/PDMS). The mechanical durability of the as-prepared mesh is tested by tape-peeling, sandpaper abrading, and oil immersion tests. The adhesion of the coating to mesh is evaluated by tapepeeling test. The mesh surface (2 cm × 2 cm) is pressed with adhesive tape (Scotch-600) by a weight of 500 g to ensure the good contact. Then the tape is peeled off, and contact angle is measured. After 100 times peeling off, the contact angle of the mesh is still above 150° (Figure 9a), and the coating is integrated without stripping (Figure S9). The antiabrasion of the coating is evaluated by an abrasion test. The as-prepared mesh surface is subjected to a 500 g loading (ca. 15 kPa) and slid on various grid SiC sandpaper for 25 cm in one direction. After the abrasion, the contact angle and surface microstructure of the mesh are measured. As shown in Figure 9b, the asprepared mesh exhibits good antiabrasion to 800 and 1200 grid SiC sandpaper. Although the hydrophobicity of the as-prepared mesh showes a decline after the abrasion of 400 grid SiC sandpaper, it is still hydrophobic with contact angle of 145° (Figure S10). To evaluate the adhesion of the PDMS coating, the as-prepared mesh is immersed in various oils and seawater for 5 d, and the hydrophobicity and surface microstructure of the mesh were measured. After 5 d of immersion in various oils and seawater, the mesh is still superhydrophobic (Figure 9c), and there is no change on the mesh surface (Figure S11).

been shown to be effective to depress the process of corrosion.42 In this work, the superhydrophobic surface of the PUS changes to a hydrophobic surface after absorbing oil. The corrosion resistance of the superhydrophobic PUS before and after absorbing oil was investigated by salt spray test and electrochemical measurement. The visual performances of the samples after salt spray test are shown in Figure 8a. After 2 h of salt spray test, the corrosion phenomena first occurring on the Blank Mesh, Mesh/ZnO, and Mesh/PDMS films reveals that the barrier action of hydrophobic coating of ZnO or PDMS does not prevent the corrosion occurred on metal mesh effectively. And, after 24 h of salt spray test, the Blank Mesh, Mesh/ZnO, and Mesh/PDMS films are seriously corroded. In contrast, there is little rust on Mesh/ZnO/PDMS and Mesh/ ZnO/PDMS/Oil films after 24 h of salt spray test. Even after 144 h of salt spray test, there are only a few rust spots on the two surfaces, which indicates the Mesh/ZnO/PDMS and Mesh/ZnO/PDMS/Oil films have good corrosion resistance (Figure S8). Figure 8b shows the potentiodynmatic polarization curves of Blank Mesh, Mesh/ZnO, Mesh/PDMS, Mesh/ZnO/ PDMS, and Mesh/ZnO/PDMS/Oil films. The corrosion currents of Blank Mesh, Mesh/ZnO, and Mesh/PDMS films are 2.75 × 10−7, 1.892 × 10−7, and 6.259 × 10−7 A/cm2, respectively, which are similar to each other. However, the corrosion current of Mesh/ZnO/PDMS and Mesh/ZnO/ PDMS/Oil films decrease to a low value of 2.959 × 10−8 and 2.149 × 10−8 A/cm2, respectively, indicating the corrosion resistance of the two films is much better than that of the Blank Mesh, Mesh/ZnO, and Mesh/PDMS films. The results of electrochemical measurement are consistent with these of the salt spray test. To the superhydrophobic film (Mesh/ZnO/ PDMS), the corrosive medium cannot wet its surface, and corrosive droplets on its surface roll off (Figure 8c, left) easily, which can effectively restrain the contact of the corrosive solution with the substrate. After the oil absorbing process, the air trapped in the hierarchical structure is replaced by oil. The superhydrophobic surface is turned to be hydrophobic with WCA of 96°. The motion state of the corrosive droplets on the Mesh/ZnO/PDMS/Oil film surface (hydrophobic) is changed from rolling (superhydrophobic surface) to sliding. Although the Mesh/ZnO/PDMS/Oil film surface is hydrophobic, the surface is also nonsticky (corrosive droplets can slide away the surface easily (Figure 8c, right)). So, the PUS has stable corrosion resistance after absorbing oil, which will ensure its application on the sea. In Figure 8d, the consistently high water contact angle (above 150°) in the salt spray test is responsible

■

CONCLUSIONS

In summary, we fabricated a PUS with properties of superhydrophobicity and superoleophilicity by imitating superhydrophobic water strider’s leg via simple hydrothermal method and PDMS modification for oil spills cleanup and recycle without artificial operation. The superhydrophobicity and low water adhesion force of the mesh surface endow the PUS with high oil/water separation capacity (above 94%) and drag-reducing property (31% at flowing velocity of 0.38m/s). The porous structure and superoleophilicity of the mesh surface ensure the oil infiltrating through the mesh and gathering into the as-prepared PUS automatically. In addition, after absorption of oil, the motion state of the water droplets on the PUS turns from rolling to sliding, and the PUS has better drag-reducing (39.04% at flowing velocity of 1.2 m/s), corrosion resistance, and reusability. We do believe the oil spills cleanup and recycle will be more simple, efficient, and unmanned by the PUS. I


Research Article


■

EXPERIMENTAL SECTION

Materials. The 304 stainless steel meshes were purchased from Shanghai Xinan Metal Mesh Product Co., Ltd., China. PDMS prepolymer and curing agent (Sylgard 184) were purchased from Dow Corning. All chemical regents were purchased from Aladdin Reagent and used without further purification. Coating Fabrication. The stainless steel mesh was cleaned sequentially with acetone and deionized water and then dried at 80 °C.The aligned ZnO nanorod arrays coated mesh was prepared by one-step hydrothermal route at a relatively lower temperature (∼90 °C). The stainless steel mesh was immersed into a 100 mL aqueous solution, which consisted of 0.01 mol of Zn(NO3)2, 0.002 mol of NHCl4, and 5 mL of 25% ammonia, and then the system was heated to 90 °C and kept for 1 h. Subsequently, the as-prepared mesh was washed with deionized water and dried at 80 °C. The superhydrophobic and superoleophilic mesh films were obtained by a dip-coating method. The as-prepared aligned ZnO nanorod array coated mesh was dipped into a solution of PDMS, curing agent, and ethyl acetate. The thickness of the PDMS coating was controlled by repeating the dipping and drying process. Finally, the Mesh/ZnO/PDMS film was cured at 80 °C for 6 h to obtain the superhydrophobic and superoleophilic mesh film. Coating Characterization. The surface microstructures and chemical composition of the Blank Mesh, Mesh/ZnO, and Mesh/ ZnO/PDMS films were characterized by SEM (FEI Quanta 250 FEG, U.S.) equipped with EDS. The XRD patterns of the Blank Mesh and Mesh/ZnO film were measured by an X-ray diffraction (XRD, BrukerAXS: D8 Advance, Germany) with Cu Kα radiation, λ = 1.542 Å. The WCA and OCA, respectively, were detected by a contact angle meter (OCA20, Germany). Water or oil droplet with a volume of 2.0 μL was dropped carefully onto the mesh surface. The WSAs were obtained by gradually inclining the tilting stage until the water droplet began to roll. At least five locations of the same surface were tested to get the average value. Electrochemical measurements were performed using an electrochemical workstation (Modulab ECS, Solartron Analytical Ltd., U.K.) at room temperature with a traditional three-electrode system. The 3.5 wt % NaCl aqueous solution, platinum stick electrode, and saturated calomel electrode were used as the electrolyte, counter electrode, and reference electrode, respectively. The potential was taken between −400 and +800 mV versus SCE at a scan rate of 0.5 mV/s. All samples were immersed for 15 min to ensure the steady state. We recorded the behavior of droplet motion from above by using the high-speed camera at a rate of 1000 frames per second (FASTCAM Mini UX100, PHOTRON LIMITED, Japan). The corrosion resistance properties of the meshes were studied by salt spray test (Q-FOG CCT-1100, Q-Lab, U.S.; NaCl 5 wt % solution; according to ASTM B117,43 the chamber temperature was controlled at 35 °C, the salt solution used was 5 wt % NaCl, and its pH value was ∼7.0). The drag-reducing test of the meshes was performed in the water tunnel. The experimental setup and detail are shown in Figure S12.

■

■

Video of interaction mesh. (AVI) Video of interaction mesh. (AVI) Video of interaction mesh. (AVI) Video of interaction mesh. (AVI) Video of interaction mesh. (AVI)

between liquid (water and oil) and between liquid (water and oil) and between liquid (water and oil) and between liquid (water and oil) and between liquid (water and oil) and

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Project Nos. 51475450 and 51335010 were supported by the National Nature Science Foundation of China. This material is also based upon work funded by the National Basic Research Program of China (No. 2014CB643302), Zhejiang Provincial Innovation Team (Grant No. 2011R50006), Ningbo social benefiting plan by science and technology (2015C50055), and Ningbo Municipal Innovation Team (Grant No. 2011B81001). The authors are grateful to the National Natural Science Foundation of China (Grant No. 21272157) for the financial and technical support of the work reported in this article.

■

REFERENCES

(1) Schrope, M. Oil spill: Deep wounds. Nature 2011, 472, 152−154. (2) Short, J. Long-term Effects of Crude Oil on Developing Fish: Lessons from the Exxon Valdez Oil Spill. Energy Sources 2003, 25, 509−517. (3) Dubansky, B.; Whitehead, A.; Miller, J. T.; Rice, C. D.; Galvez, F. Multitissue Molecular, Genomic, and Developmental Effects of the Deepwater Horizon Oil Spill on Resident Gulf Killifish (Fundulus grandis). Environ. Sci. Technol. 2013, 47, 5074−5082. (4) Wang, B.; Liang, W.; Guo, Z.; Liu, W. Biomimetic Superlyophobic and Super-lyophilic Materials Applied for Oil/water Separation: a New Strategy beyond Nature. Chem. Soc. Rev. 2015, 44, 336−361. (5) Xue, Z.; Cao, Y.; Liu, N.; Feng, L.; Jiang, L. Special Wettable Materials for Oil/water Separation. J. Mater. Chem. A 2014, 2, 2445− 2460. (6) Fingas, M.; Fieldhouse, B.; Mullin, J. Water-in-oil Emulsions Results of Formation Studies and Applicability to Oil Spill Modelling. Spill Sci. Technol. Bull. 1999, 5, 81−91. (7) Wang, H.; Xue, Y.; Ding, J.; Feng, L.; Wang, X.; Lin, T. Durable, Self-Healing Superhydrophobic and Superoleophobic Surfaces from Fluorinated-Decyl Polyhedral Oligomeric Silsesquioxane and Hydrolyzed Fluorinated Alkyl Silane. Angew. Chem., Int. Ed. 2011, 50, 11433−11436. (8) Teisala, H.; Tuominen, M.; Aromaa, M.; Stepien, M.; Mäkelä, J. M.; Saarinen, J. J.; Toivakka, M.; Kuusipalo, J. Nanostructures Increase Water Droplet Adhesion on Hierarchically Rough Superhydrophobic Surfaces. Langmuir 2012, 28, 3138−3145. (9) Choi, H. M.; Cloud, R. M. Natural Sorbents in Oil Spill Cleanup. Environ. Sci. Technol. 1992, 26, 772−776. (10) Zhang, Z.; Sèbe, G.; Rentsch, D.; Zimmermann, T.; Tingaut, P. Ultralightweight and Flexible Silylated Nanocellulose Sponges for the Selective Removal of Oil from Water. Chem. Mater. 2014, 26, 2659− 2668.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08185. Detailed characterization data from SEM, WCA, salt spray test, and drag-reducing test. Photo of various original sheets and superhydrophobic-coated sheets. Description of experimental setup of drag-reducing test. Tabulated data of density, viscosity, and surface tension of oils. (PDF) Video of interaction between liquid (water and oil) and mesh. (AVI) Video of interaction between liquid (water and oil) and mesh. (AVI) J


Research Article

ACS Applied Materials & Interfaces (11) Wang, G.; Zeng, Z.; Wu, X.; Ren, T.; Han, J.; Xue, Q. Threedimensional Structured Sponge with High Oil Wettability for the Clean-up of Oil Contaminations and Separation of Oil-water Mixtures. Polym. Chem. 2014, 5, 5942−5948. (12) Hu, H.; Zhao, Z.; Gogotsi, Y.; Qiu, J. Compressible Carbon Nanotube−Graphene Hybrid Aerogels with Superhydrophobicity and Superoleophilicity for Oil Sorption. Environ. Sci. Technol. Lett. 2014, 1, 214−220. (13) Zhu, Q.; Pan, Q. Mussel-Inspired Direct Immobilization of Nanoparticles and Application for Oil−Water Separation. ACS Nano 2014, 8, 1402−1409. (14) Ruan, C.; Ai, K.; Li, X.; Lu, L. A Superhydrophobic Sponge with Excellent Absorbency and Flame Retardancy. Angew. Chem., Int. Ed. 2014, 53, 5556−5560. (15) Calcagnile, P.; Fragouli, D.; Bayer, I. S.; Anyfantis, G. C.; Martiradonna, L.; Cozzoli, P. D.; Cingolani, R.; Athanassiou, A. Magnetically Driven Floating Foams for the Removal of Oil Contaminants from Water. ACS Nano 2012, 6, 5413−5419. (16) Feng, L.; Zhang, Z.; Mai, Z.; Ma, Y.; Liu, B.; Jiang, L.; Zhu, D. A Super-Hydrophobic and Super-Oleophilic Coating Mesh Film for the Separation of Oil and Water. Angew. Chem., Int. Ed. 2004, 43, 2012− 2014. (17) Kwon, G.; Kota, A.; Li, Y.; Sohani, A.; Mabry, J. M.; Tuteja, A. On-Demand Separation of Oil-Water Mixtures. Adv. Mater. 2012, 24, 3666−3671. (18) Zhang, W.; Shi, Z.; Zhang, F.; Liu, X.; Jin, J.; Jiang, L. Superhydrophobic and Superoleophilic PVDF Membranes for Effective Separation of Water-in-Oil Emulsions with High Flux. Adv. Mater. 2013, 25, 2071−2076. (19) Liu, K.; Jiang, L. Metallic surfaces with special wettability. Nanoscale 2011, 3, 825−838. (20) Cheng, M.; Gao, Y.; Guo, X.; Shi, Z.; Chen, J. F.; Shi, F. A Functionally Integrated Device for Effective and Facile Oil Spill Cleanup. Langmuir 2011, 27, 7371−7375. (21) Song, J.; Huang, S.; Lu, Y.; Bu, X.; Mates, J. E.; Ghosh, A.; Ganguly, R.; Carmalt, C. J.; Parkin, I. P.; Xu, W. J.; Megaridis, C. M. Self-Driven One-Step Oil Removal from Oil Spill on Water via Selective-Wettability Steel Mesh. ACS Appl. Mater. Interfaces 2014, 6, 19858−19865. (22) Wang, F.; Lei, S.; Xue, M.; Ou, J.; Li, W. In Situ Separation and Collection of Oil from Water Surface via a Novel Superoleophilic and Superhydrophobic Oil Containment Boom. Langmuir 2014, 30, 1281−1289. (23) Yan, R. J.; Pang, S.; Sun, H. B.; Pang, Y. J. Development and Missions of Unmanned Surface Vehicle. Journal of Marine Science and Application 2010, 9, 451−457. (24) Gao, X.; Jiang, L. Biophysics: Water-repellent Legs of Water Striders. Nature 2004, 432, 36−36. (25) Feng, X. Q.; Gao, X.; Wu, Z.; Jiang, L.; Zheng, Q. S. Superior Water Repellency of Water Strider Legs with Hierarchical Structures: Experiments and Analysis. Langmuir 2007, 23, 4892−4896. (26) Shi, F.; Niu, J.; Liu, J.; Liu, F.; Wang, Z.; Feng, X. Q.; Zhang, X. Towards Understanding Why a Superhydrophobic Coating Is Needed by Water Striders. Adv. Mater. 2007, 19, 2257−2261. (27) Cheng, Q.; Li, M.; Zheng, Y.; Su, B.; Wang, S.; Jiang, L. Janus Interface Materials: Superhydrophobic Air/solid Interface and Superoleophobic Water/solid Interface Inspired by a Lotus Leaf. Soft Matter 2011, 7, 5948−5951. (28) Celia, E.; Darmanin, T.; Taffin de Givenchy, E.; Amigoni, S.; Guittard, F. Recent Advances in Designing Superhydrophobic Surfaces. J. Colloid Interface Sci. 2013, 402, 1−18. (29) Bellanger, H.; Darmanin, T.; Taffin de Givenchy, E.; Guittard, F. Chemical and Physical Pathways for the Preparation of Superoleophobic Surfaces and Related Wetting Theories. Chem. Rev. 2014, 114, 2694−2716. (30) Zhang, X.; Li, Z.; Liu, K.; Jiang, L. Bioinspired Multifunctional Foam with Self-Cleaning and Oil/Water Separation. Adv. Funct. Mater. 2013, 23, 2881−2886.

(31) Tian, D.; Zhang, X.; Wang, X.; Zhai, J.; Jiang, L. Micro/ nanoscale Hierarchical Structured ZnO Mesh Film for Separation of Water and Oil. Phys. Chem. Chem. Phys. 2011, 13, 14606−14610. (32) Liu, K.; Jiang, L. Bio-inspired Design of Multiscale Structures for Function Integration. Nano Today 2011, 6, 155−175. (33) 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. (34) 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. (35) Lu, Y.; Sathasivam, S.; Song, J.; Xu, W.; Carmalt, C. J.; Parkin, I. P. Water Droplets Bouncing on Superhydrophobic Soft Porous Materials. J. Mater. Chem. A 2014, 2, 12177−12184. (36) Crick, C. R.; Gibbins, J. A.; Parkin, I. P. Superhydrophobic Polymer-coated Popper-mesh; Membranes for Highly Efficient Oilwater Separation. J. Mater. Chem. A 2013, 1, 5943−5948. (37) Richard, D.; Clanet, C.; Quéré, D. Surface Phenomena: Contact Time of a Bouncing Drop. Nature 2002, 417, 811−811. (38) Deng, X.; Mammen, L.; Butt, H. J.; Vollmer, D. Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Science 2012, 335, 67−70. (39) 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. (40) Cheng, M.; Song, M.; Dong, H.; Shi, F. Surface Adhesive Forces: A Metric Describing the Drag-Reducing Effects of Superhydrophobic Coatings. Small 2015, 11, 1665−1671. (41) Yao, X.; Ju, J.; Yang, S.; Wang, J.; Jiang, L. Temperature-Driven Switching of Water Adhesion on Organogel Surface. Adv. Mater. 2014, 26, 1895−1900. (42) Ishizaki, T.; Masuda, Y.; Sakamoto, M. Corrosion Resistance and Durability of Superhydrophobic Surface Formed on Magnesium Alloy Coated with Nanostructured Cerium Oxide Film and Fluoroalkylsilane Molecules in Corrosive NaCl Aqueous Solution. Langmuir 2011, 27, 4780−4788. (43) ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus 2011.

K


Low Drag Porous Ship with Superhydrophobic and Superoleophilic

Recommend Documents