Multiphase Media Antiadhesive Coatings: Hierarchical Self-Assembled Porous Materials Generated Using Breath Figure Patterns Keyu Han, Liping Heng,* and Lei Jiang Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing 100191, China S Supporting Information *
ABSTRACT: The cleaning of interface pollutants typically consumes a large amount of energy. Therefore, the development of multiphase media antiadhesive materials is urgently required to meet the demand of energy savings and environmental protection. In this study, the antiadhesive properties toward several liquid droplets and bubbles in multiple media are demonstrated on a porous Fe2O3 coating, which is prepared via a facile spincoating-assisted breath figure approach and a phase separation strategy. The prominent antiadhesive characteristic of these porous surfaces lies in their high-surface-energy hierarchical micro/nanoscale structure, which easily entraps one medium (oil or water) in the pore and repels other unmixable liquids and air bubbles. In addition, we successfully demonstrate an antifouling application of the coating, which shows excellent antiadhesive and super-antiwetting characteristics under multiple liquids. Our work extends relevant antiadhesion research from a single medium to multiple media and promises to broaden the applications of antiadhesive materials in sophisticated activities performed under complicated liquid environments, such as marine antifouling or pipeline transportation. KEYWORDS: antiadhesion, multiple media, breath figure, porous structure, hierarchical self-assembly bubble-repellent surfaces under water,22−25 water-repellent surfaces under oil,26−28 and air-bubble-repellent surfaces under oil.29 However, each of the above-mentioned selfcleaning materials presents a single antiadhesion function in one medium for the other medium; that is, each material is only oil-repellent under water, air-bubble-repellent under water, water-repellent under oil, or air-bubble-repellent under oil. In fact, liquid mediums are always complicated systems. For example, industrial wastewater usually contains numerous gas bubbles and oils, and in long-distance liquid transportation, the presence of an air gap or water/oil adhesion causes serious corrosion of pipelines,30,31 resulting in reduced equipment lifespan and wasted resources. To solve these problems, antiadhesive surfaces for applications in multiphasic media must be creatively designed. Recently, our group systematically studied the impact of surface morphologies (smooth, micro, nano, and micro/nanostructures) on the wettability behavior of silicon surfaces for different liquids (water and oil) and gas under various media (air, water, and oil).32 The results showed
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ith the increasingly serious environmental pollution levels and the energy shortage that accompanied the development of industry and society, energy savings and environmental protection have become a worldwide problem. The cleaning of interface pollutants typically consumes a large amount of energy. Therefore, the development of self-cleaning materials with excellent antiadhesive properties has drawn considerable attention and has emerged as a popular topic in the field of fundamental interface science1−3 and in various practical application areas, such as antifouling coatings,4−7 oil/water separation,8,9 anticorrosion coatings, space exploration, military, industrial and agricultural production, biomedical engineering, and daily life.10,11 As typical examples of self-cleaning materials, lotus leaf, rice leaf, and butterfly wings have inspired the study of superhydrophobic surfaces with water contact angles (CAs) greater than 150° and slide angles (SAs) less than 5° in air.12−14 Moreover, our group and other researchers have extended the studies on self-cleaning materials from the traditional air− liquid−solid systems to liquid−liquid−solid triple-phase systems, for example, the nacre,15 mussel, and fish16 scaleinspired low-adhesion underwater super-oleophobic surfaces, which exhibit amazing properties for oil−water separation.17−21 In addition, various approaches have been utilized to obtain air© 2016 American Chemical Society
Received: September 3, 2016 Accepted: December 2, 2016 Published: December 2, 2016 11087
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Figure 1. Schematic illustration of the spin-coating-assisted BF strategy used for generating the porous iron oxide coating. Owing to its hydrophilic and poor solubility in toluene, PEO/Fe3+ is in the interior region of the micelles. Polystyrene is in the exterior region of the micelles.
antiadhesion for air bubbles under multiple media. The antiadhesive characteristic of these surfaces lies in their highenergy hierarchical micro/nanoscale porous structure, which easily traps one medium (oil or water) in the pore and repels other unmixable liquids and air bubbles. In addition, we successfully utilized the coating in an antifouling application to demonstrate its excellent antiadhesive and antiwetting characteristics in multiple media. Oil droplets, water droplets, or air bubbles can quickly slip away from the tilted coating surface without leaving any residue on the sample, whether the samples are under water or under oil. This work not only aids the further understanding of the principle for the wettability behavior under various media but also provides a good strategy for designing antiadhesive materials under multiple media.
that the surface structure greatly influenced the antiadhesive properties of liquids and gas in different media. Although this work is of great significance to exploit antiadhesive material under complicated liquid environments, the mechanism illustration of the antiadhesive properties under diverse media is not clear. Moreover, the micro/nanostructured wafers used in this work need to be fabricated through complex photoetching technique, which is expensive and nonuniversal. Hence, considering the practical application, strategies using a simple and universal fabrication method and economic and common material should be developed to achieve multiphase system antiadhesive materials, and the interaction between these surfaces and the liquid droplets/air bubbles in the multiple liquid mediums should be completely understood. Constructing such multiphase media antiadhesive materials is important for engine and machine parts, oil pipelines, and chemical reactors or storage tanks. For the last 20 years, breath figures (BFs) have been extensively researched for generating porous structures in polymer films from the micrometer to sub-micrometer size scales.33,34 The formation mechanism of the honeycomb structure has been suggested by François et al.33 and was described in detail by Shimomura and co-workers.35 Over the past 20 years, studies have mainly focused on altering the polymers and solvents to prepare ordered porous films,36,37 fabricating various structures, such as patterned structures and three-dimensional structures,38,39 developing new functions, such as photoelectric conversion,40 photocatalysis,41 antireflection,42 hydrophobicity,43 high mechanical strength,44 and cell adhesion,45 and exploring new applications, such as sizeselective separation46 and liquid reprography.47 However, the above-mentioned porous materials generated by BFs usually have pore structures with uniform microscale or sub-micrometer scale size, resulting in insufficient roughness to achieve super-hydrophobic and antiadhesive characteristic,48 hence limiting their application in the antiadhesive self-cleaning field. Currently, as a result of the traditional mindset, no attention has focused on improving the traditional BF methods to obtain rougher multiscale porous materials, for example, utilizing spin-coating-assisted BF methods and phase separation of block copolymers (BCPs) for the construction of hierarchical self-assembled porous materials. Herein, we constructed a porous Fe2O3 coating via a facile spin-coating-assisted BF method and a phase separation strategy. The resulting coating exhibits excellent antiadhesive properties under multiple liquid media for several other liquid droplets. More impressively, this type of coating also shows
RESULTS AND DISCUSSION Fabrication of the Hierarchical Porous Surface Structure. To achieve excellent antiadhesive coatings in multiple media for several liquid droplets and air bubbles, the following two criteria must be satisfied: (1) the materials must have a high surface energy that allows easy spreading of various liquid droplets; the materials must also be insoluble in organic solvents; (2) hierarchical roughness structures must be present on the surface to trap oil or water molecules, which can block the unmixable liquids and air bubbles. The first requirement can be satisfied by using inorganic metal oxides, which have high surface energy and are insoluble in organic solvents. Thus, Fe2O3 was chosen as a candidate material. To meet the second criterion, a hierarchical Fe2O3 porous structure prepared using a spin-coating-assisted hierarchical self-assembly method (Figure 1) was selected as the model surface; Fe2O3 provided the rough porous structure while offering the advantages of simple fabrication, large-area capability, and flexibility. From these principles, hierarchical Fe2O3 porous structures with different topographies were fabricated via a spin-coatingassisted hierarchical self-assembly method. The polystyreneblock-poly(ethylene oxide) (PS-b-PEO) molecule (with structural formula shown in Figure S1) in toluene solution can form micelles (Figure 1) with Fe3+ via electrostatic interaction forces;49,50 this process is the first stage of the self-assembly. The PEO block associates with iron ions in the core of the micelles, and the PS block is in the shell (Figure 1). The formation of the micelles is further confirmed in Figure S2. Composite films composed of PS-b-PEO/Fe3+ self-assembled micelles were fabricated by spin-coating the solution at a high speed of 4000 rpm under an ambient humidity below 40%. After calcination was complete, the composite micelles were 11088
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Figure 2. SEM and AFM images of the three types of coatings (as-prepared) under different magnifications: (a−c) sample 1; (d−f) sample 2; and (g−i) sample 3. The images clearly show that the three types of samples present different surface topographies and structures. The mean roughness (Ra) values of sample 1, sample 2, and sample 3 are 59.1, 102.3, and 93.2 nm, respectively. The left sections of the AFM micrographs are the three-dimensional height-mode images, and the right sections are the two-dimensional height-mode images.
low surface roughness because, in the static BF method, the liquid film was thick and contained a large amount of micelles, which transformed into a large amount of nanoparticles and further sintered into aggregates under calcination. Therefore, the static BF method and the low-speed spin-coating approach must be combined to fabricate porous iron oxide coatings with hierarchical structures. Morphology of the Hierarchical Porous Surface Structures. According to the theory and method discussed above, we designed and constructed hierarchical Fe2O3 porous structures. The coatings were prepared using various solutions with an Fe/O molar ratio (R) of 0.7 (sample 1), 1.5 (sample 2), and 3.0 (sample 3). The typical field-emission scanning electron microscopy (SEM) images (Figure 2) clearly indicate that the samples have different structures. Sample 1 has a microscale porous structure with a pore size of approximately 1.20 ± 0.35 μm, whereas the inner and interstice parts of the pores are relatively smooth. This region is composed of closely packed nanoparticles with extremely small sizes, which can be attributed to the small R. Sample 2 has a typical micro/ nanoscale composite structure with pores of approximately 1.10 ± 0.27 μm and nanoparticles of approximately 23.6 ± 11.3 nm, consistent with the SEM image of Figure S2. Sample 3 also has a micro/nanoscale composite strucaqture with pores of approximately 0.95 ± 0.21 μm and nanoparticles of approximately 64.2 ± 25.7 nm. Compared with sample 3, the pore size of sample 2 is slightly larger, whereas its nanoparticle size is relatively small and uniform; this result can be attributed to the utilization of an appropriate R value of 1.5 in sample 2 and an excess R value (3.0) in sample 3. These results clearly show that the mean pore size of the coating decreased while the size of the nanoparticles increased with an increasing Fe/O molar ratio (R). This trend occurs because enhancing the FeCl3 content can reduce the water condensation time and prevent the aggregation of water droplets, leading to a smaller pore size of the coating. When more Fe3+ ions combine into one PEO micelle, larger iron oxide particles can be achieved due to the sintering agglomeration of the nanoparticles during the calcination process. The SEM images do not reflect the
converted into unified iron oxide nanoparticles with a mean size of 23.7 ± 4.9 nm (Figure S2). The second stage of the self-assembly is the utilization of BFs. The schematic representation of the fabrication process (Figure 1) for the hierarchical self-assembly of the porous iron oxide coatings is briefly outlined here. Under high humidity, the toluene solution with PS-b-PEO/Fe3+ micelles initially formed a thin liquid film due to the effect of the centrifugal force induced by spin-coating; then, the organic solvent evaporated, and the surface temperature of the solution decreased. Water from the atmosphere condensed and grew on the cooling surface.35 The interface of the condensed water was absorbed and was stabilized by the micelles. After the solvent and water droplets were completely evaporated, a porous structure was formed on the film. In contrast to traditional BFs, the order of the pore was disrupted by the centrifugal force. After calcination of the film, the organic PS-b-PEO molecules were removed and an inorganic coating with a microscale porous structure and nanoscale particles was achieved. This composite hierarchical structure was formed via the self-assembly of water droplets33,37,47 and the self-assembly of block polymers/ inorganic salt through microphase separation.51,52 The nanoparticles were derived from the simple inclusion of Fe3+ ions in the PEO micelle component of the PS-b-PEO due to the good affinity of the PEO block for the cations.49,50 Unlike the traditional BF method, we tactfully introduce the spin-coating technology into the self-assembly process because of its significant effect on promoting volatilization of the high boiling solvent, which is a key factor in the formation of the water droplet template and the final porous film. For comparison, we also fabricated composite films through the traditional static BF method using a high boiling toluene solution and a low boiling chloroform solution under a high humidity environment. After calcination was complete, we achieved iron oxide coatings: a coating that has no obvious porous structures (the toluene solution) and a coating that just possesses primary porous structure (the chloroform solution) with a pore diameter of approximately 0.98 ± 0.22 μm (Figure S3). The coating with the primary porous structure was compact and had a relatively 11089
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ACS Nano stereomorphology of the sample surface. Therefore, we measured the surface structure of the three types of asprepared coatings using atomic force microscopy (AFM). The mean roughness (Ra) of sample 1, sample 2, and sample 3 is 59.1, 102.3, and 93.2 nm, respectively. These results clearly demonstrate that the surface roughness of sample 2 with its micro/nanoscale hierarchical structures was much larger than that of sample 1 and sample 3. To verify the chemical composition and crystal phase of the coatings fabricated by calcination of the films, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies were performed. XPS studies (Figure S4) of the Fe 2p core level spectrum composed of Fe 2p3/2 and Fe 2p1/2 signal peaks at 711.3 and 725.1 eV, respectively, and the O 1s spectrum with characteristic peaks at 529.8/531.8 eV indicate that the samples consist of Fe(III) oxide (Fe2O3).53 The X-ray results (Figure S5) further demonstrate that the coating has an α-Fe2O3 crystal structure, whose characteristic peaks match those of a standard α-Fe2O3 sample (JCPDS 33-0664).54 Surface Wettability Characteristics of the Porous Surface under Multiple Media. The surface wettability in multiple media is an important topic that has been overlooked by scientists to date. To evaluate the wettability of the coatings with different surface topographies under multiple media, we selected water, the polar oil 1 (1,2-dichloroethane, with a higher density than water), and the nonpolar oil 2 (cyclohexane, with a lower density than water) as the representative liquid models55,56 and systematically studied the wettability behaviors of water, oil, and air bubbles on the coatings under different liquid systems. First, the CAs of water and two types of different oils in air were measured to quantify the static wettability of the surfaces. Figure S6 clearly shows that the water CAs of sample 1, sample 2, and sample 3 are 38.3 ± 2.7, 18.2 ± 3.1, and 27.8 ± 3.9°, respectively, indicating that the iron oxide has a high surface energy and that the surface roughness can significantly affect the sample wettability. A micro/nanoscale composite structure topography can enhance the hydrophilicity of the samples. Meanwhile, these three types of samples displayed super-oleophilicity with CAs of nearly 0° regardless of polar oil (1,2-dichloroethane) or nonpolar oil (cyclohexane). These hydrophilic and super-oleophilic properties of the coatings are the foundation for achieving antiadhesion for different media in multiphase systems. Then, we investigated the surface wettability of these coatings under water by measuring the CAs of two types of oils and air bubbles (Figure 3). Sample 1 with a microscale structure surface showed oleophobic and aerophobic properties with CAs of 137.3 ± 4.2, 135.7 ± 3.6, and 134.3 ± 2.5° for 1,2dichloroethane, cyclohexane, and air bubbles, respectively. Meanwhile, sample 2 and sample 3 with their micro/nanoscale hierarchical structures displayed underwater super-oleophobicity and super-aerophobicity with CAs >150°. These results arise from the preferable micro/nanoscale hierarchical rough porous structures, which can reserve the water molecules and prevent the unmixable oil and air bubbles from infiltrating the surface structure. Similar to the results measured in air, the surface roughness can also significantly enhance the sample wettability from oleophobicity/aerophobicity to super-oleophobicity/ super-aerophobicity. Furthermore, we studied the surface wettability of these samples under two types of oils by measuring the CAs (Figure 3) of the water/air bubble on the sample surface. A study on this aspect is of great importance for expanding material
Figure 3. Wettability of three types of coating samples under multiple media: CAs of oil 1 under water are 137.3 ± 4.2° for sample 1 (a), 157.6 ± 3.4° for sample 2 (b), and 152.6 ± 2.6° for sample 3 (c). CAs of oil 2 under water are 135.7 ± 3.6° for sample 1 (d), 155.4 ± 2.9° for sample 2 (e), and 151.7 ± 2.4° for sample 3 (f). CAs of air bubble under water are 134.3 ± 2.5° for sample 1 (g), 156.2 ± 3.7° for sample 2 (h), and 152.6 ± 2.9° for sample 3 (i). When under oil 1, CAs of water are 141.5 ± 2.3° for sample 1 (j), 156.5 ± 3.9° for sample 2 (k), and 150.2 ± 2.7° for sample 3 (l). CAs of air bubble are 150.5 ± 2.3° for sample 1 (m), 157.4 ± 2.5° for sample 2 (n), and 155.3 ± 2.9° for sample 3 (o). When under oil 2, CAs of water are 139.4 ± 3.2° for sample 1 (p), 159.1 ± 3.4° for sample 2 (q), and 152.7 ± 2.9° for sample 3 (r). CAs of air bubble are 152.6 ± 2.3° for sample 1 (s), 158.3 ± 2.8° for sample 2 (t), and 154.9 ± 2.5° for sample 3 (u). Oil 1 is 1,2dichloroethane, and oil 2 is cyclohexane.
applications, but this topic has not been the focus of a study until now. Similar to the case under water, sample 1 with a microstructured surface displays hydrophobicity under oil 1 (1,2-dichloroethane) with CAs of 141.5 ± 2.3°, whereas samples 2 and 3, with micro/nanoscale hierarchical structures, show under oil super-hydrophobicity with CAs >150°. Unlike the results obtained under water, the three types of samples show super-aerophobicity under oil 1 with CAs >150°. Analogously, when measured under oil 2 (cyclohexane), sample 1 with a microscale structured surface also displays hydrophobicity with CAs of 139.4 ± 3.2°, while samples 2 and 3 with micro/nanoscale hierarchical structures show super-hydro11090
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ACS Nano phobicity with CAs >150°. Likewise, the three types of samples display super-aerophobicity under oil 2 with CAs >150°. These results can be attributed to the microscale or micro/nanoscale hierarchical rough porous structures covered by oil molecules, which prevent the unmixable water and air bubble from contacting the sample surface. Robust Antiadhesion Characteristics of the Porous Surface under Multiple Media. Aside from their superantiwetting characteristics, self-cleaning materials for multiphase systems must show no adhesion to various media besides their super-antiwetting characteristics. For a surface with similar static CAs that may have a distinct adhesion, the underwater adhesive property of the oil/air bubble on the sample surface was further investigated quantitatively using a high-sensitivity micro-electromechanical balance system. Figure 4a−c and Figure S7a−c show the typical underwater force−distance curves of oils/air bubbles on the three types of sample surfaces.
The results in Table S1 clearly indicate that sample 1 possesses much larger adhesion values of 52.3 ± 4.9, 57.6 ± 4.3, and 49.2 ± 5.6 μN for 1,2-dichloroethane, cyclohexane, and air bubbles, respectively. Sample 2 has an adhesion that is too low to be detected (0 μN) for both oils and air bubbles. Sample 3 has adhesion values of 7.8 ± 3.5, 5.1 ± 2.4, and 6.3 ± 1.7 μN for dichloromethane, cyclohexane, and air bubbles, respectively. This comprehensive investigation clearly demonstrates that only sample 2 possesses an underwater antirepellence property for multiple media. Sample 1 has strong adhesion, and sample 3 has minor adhesion, although it has CAs similar to those of sample 2. The results can be attributed to the different surface structures of the coatings, which can remarkably influence the contact status of the oil/air bubbles on their surfaces. Then, we examined the surface adhesion of the samples under two types of oils (1,2-dichloroethane and cyclohexane). Figure 4d−g and Figure S7d−g display the typical force− distance curves of the water/air bubble on the sample surfaces under oil. Analogous to the status under water, the values in Table S1 clearly demonstrate that sample 1 has an adhesion to the water droplet of 36.1 ± 4.7 μN under oil 1 (1,2dichloroethane) that is much larger than that of sample 3 with an adhesion of 5.9 ± 2.1 μN, while sample 2 has no adhesion (0 μN). Unlike the case under water, the adhesion values between the three types of samples and the air bubbles were too low to be detected (0 μN) under 1,2-dichloroethane. Similarly, when under oil 2 (cyclohexane), sample 1 also possesses an adhesion of 42.7 ± 5.1 μN to the water droplet that is much larger than that of sample 3 with an adhesion of 9.1 ± 2.4 μN, and sample 2 still shows no adhesion (0 μN). In addition, the adhesion between the three types of samples and the air bubbles under cyclohexane was too low to be detected (0 μN). These results showed that, under oil, the adhesion characteristics of water droplets on the sample surfaces were substantially different from those of air bubbles. Sample 1 with a microscale structure only showed hydrophobicity. Samples 2 and 3 with micro/ nanoscale hierarchical structures displayed super-hydrophobicity, and the adhesion between water droplets and sample 1 was larger than that observed for samples 2 and 3. However, the three types of sample surfaces showed super-aerophobicity and low adhesion for air bubbles, indicating that the surface morphology has almost no influence on the adhesion of air under oil. In addition, we further studied the antiadhesive behavior of sample 2 toward water droplets under mixed oils (1,2-dichloroethane and cyclohexane with the volume ratio of 1:1) and mixed oils under water by measuring their CAs and adhesion forces. The results (Figure S8) clearly showed that the coating displayed super-oleophobicity (mixed oils, Figure S8a) under water and super-hydrophobicity (Figure S8b) under mixed oils, and the adhesion forces are all too low to be detected (Figure S8c,d). These results clearly demonstrate that the antiadhesion coating of sample 2 with micro/nanoscale hierarchical structures can be potentially applied to many complicated under-liquid environments. In practical liquid environments, the solid surfaces are generally fouled by other unmixable liquids and air bubbles under the application of fluid pressure, including flow pressure and hydrostatic pressure. To further confirm the robust underliquid antiadhesion capability of sample 2, the influence of the external pressure on the oil droplet, water droplet, and bubble adhesion under different liquids was examined. In the measurement process, a liquid droplet or an air bubble suspended with a metal cap under liquid was controlled to
Figure 4. Adhesion measurements of the as-prepared sample 2 coating under different media: (a) oil 1 under water, (b) oil 2 under water, (c) bubble under water, (d) water under oil 1, (e) bubble under oil 1, (f) water under oil 2, (g) bubble under oil 2, (h) adhesive force versus preload of the liquid droplet and air bubble, showing a stable ultralow adhesion until the preload reaches 100 μN. The insets are representative photographs of the liquid droplet shapes (bottom) and the air bubble (top) during contact, preload, and detachment from sample 2, revealing no stretching of the liquid droplet and air bubble during separation. The detecting medium includes oil droplets/bubbles under water and water droplets/bubbles under oils. Oil 1 is 1,2-dichloroethane, and oil 2 is cyclohexane. The results show that sample 2 has robust antiadhesion properties under different media. 11091
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g cm−3, dyed by oil red) or cyclohexane (ρ = 0.779 g cm−3, also dyed by oil red) was extruded onto the sample surface with an inclination angle of approximately 15°, the oil droplet slid down quickly along the surface without leaving any remnants on the sample (Figure 5a,b), demonstrating the underwater antiadhesive ability of the sample surface to different oils. In order to clearly demonstrate the antiadhesive behavior, easily extrude the liquid droplet and air bubble onto the face-down sample surface, and conveniently take a photograph, we selected a relatively large tilt angle of 15° to make the demonstration shot. In fact, sample 2 showed prominent self-cleaning properties even at a small tilt angle of about 5° (Figure S10). Second, the air bubble slipped quickly along the sample surface with a tilted angle of approximately 15° without leaving any residue on the sample surface under water (Figure 5c). Finally, when a water droplet (dyed by methylene blue) was squeezed onto the sample surface under two types of oils, it also slipped rapidly along the surface (Figure 5d,f). The air bubble also slid rapidly along the sample surface under two types of oils (1,2dichloroethane in Figure 5e and cyclohexane in Figure 5g) without leaving any residue, indicating that the surface was antiadhesive for air bubbles under various media. The application demonstration above clearly shows that the coating of sample 2 possessed antiadhesion self-cleaning characteristics under various liquid media. Mechanism. Studies have shown that the wettability and adhesion of material surfaces are governed by both the geometrical microstructure and the chemical composition of the materials.58,59 Given that each of these sample surfaces contain the same chemical componentspecifically highsurface-energy Fe2O 3the surface adhesion differences among the three types of samples can be attributed to their different surface roughness. A simple model was proposed (as shown in Figure 6) to explain this difference and analyze the antiadhesive characteristics of the samples under various media. Solid surface wettability with different rough structures have been extensively researched. Three classic wetting models, including the Wenzel model,60 Cassie model,61 and Wenzel− Cassie transition model,62 have been proposed. A relatively high oil droplet, air bubble, and water droplet adhesion was achieved on the surface of sample 1 (the Wenzel state in Figure 6a,d,g) because of its low surface roughness, large liquid/gas− solid contact area, and the continuous three-phase contact line (TCL) between the oil droplet, air bubble, and water droplet and the coating surface. On the contrary, owing to their high roughness, water or oil was easily retained in the gaps of the micro/nanoscale hierarchical structures of samples 2 and 3; the entrapped water or oil molecules in these surface structures significantly hindered the oil droplet, air bubble, or water droplet from contacting the solid surfaces,32,63 resulting in a small liquid/gas−solid contact area and a discrete or semicontinuous TCL. Eventually, sample 2 (the Cassie state in Figure 6b,e,h) and sample 3 (the Wenzel−Cassie transition state in Figure 6c,f,i) displayed a super-antiwetting state with an undetectable or a low oil, air bubble, or water adhesion under multiple media. Hence, when an amphiphilic coating possessing hierarchical micro/nanoscale structures is immersed in water or oil, its surface displays super-amphiphobicity (such as underwater super-oleophobicity/super-aerophobicity and under-oil super-hydrophobicity/super-aerophobicity) without adhesion characteristics for immiscible liquids and air bubbles. Interestingly, the surface structures had little impact on the adhesive behaviors of gas bubbles, and the three types of
squeeze the surface of the sample to a certain preload and then was detached from the sample surface. The change of force was monitored using a high-sensitivity balance system, and the shape of the oil droplet was recorded using a high-speed charge-coupled device (CCD). The effect of the preload on the adhesive force is shown in Figure 4h. As the preload increases from 0 to 100 μN on the water droplet, oil droplet, and air bubble under different liquids, the ultralow adhesive force is almost constant at 0 μN. This is a striking result compared with the reported results from robust clay/polymer underwater−oilrepellent materials, which have low-loading clay with a random arrangement in the interior and can only tolerate a preload of 20 μN.57 Moreover, the sample coating still maintains its original topography (Figure S9) after numerous under-liquid antiadhesive measurements with a preload of 100 μN, indicating potential application toward complicated liquid environments. Antiadhesion Application of the Porous Surface under Multiple Media. One of the most important motives for researchers to design these antiadhesive surfaces is the prospect of achieving self-cleaning properties under multiple media. We demonstrated the antiadhesive behavior of sample 2 via a simple application demonstration, as shown in Figure 5. First, when a 100 μL oil droplet of 1,2-dichloroethane (ρ = 1.26
Figure 5. Demonstration showing the self-cleaning effect of the asprepared sample 2 under different media: (a) oil 1 under water, (b) oil 2 under water, (c) bubble under water, (d) water under oil 1, (e) bubble under oil 1, (f) water under oil 2, and (g) bubble under oil 2. The results show that sample 2 presents excellent self-cleaning functions for different media in multiphase systems. Oil 1 and oil 2 are 1,2-dichloroethane and cyclohexane, respectively. The tilt angle is 15° for all samples. 11092
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on high-surface-energy and hierarchical micro/nanoscale porous structures. The prepared antiadhesive coatings possess excellent antiadhesion properties to unmixable liquids and air bubbles under various liquids (water and oil). This feature can be attributed to the high-surface-energy chemical compositions and the hierarchical micro/nanoscale topographies of the porous coating, which enables the coating to be easily covered by one medium and allows it to repel other unmixable liquids and gas bubbles. This multiscale self-assembled porous antiadhesive coating exhibits potential use for chemical microreactors, gas-borne electrodes, oil/water separation, antifouling surfaces, and antidrag materials under complex liquid-phase environments because the fabrication strategy presented can be extended to other high-energy materials such as CuO, ZnO, and Al2O3.
METHODS Preparation of Porous Fe2O3 Coatings. The samples were fabricated via a typical spin-coating-assisted dynamic BF approach, which is a hierarchical self-assembling technique that includes phase separation and the use of BFs. Silicon wafers (100-oriented, n-type, Tianjin Semiconductor Research Institute, China), each with a dimension of 1.5 × 2.5 cm 2, were sequentially cleaned by ultrasonication in acetone, ethanol, and deionized water for 15 min in each solvent to eliminate inorganic and organic surface contaminants. A precalculated amount of polystyrene-block-poly(ethylene oxide) (PS-b-PEO, MPS = 19 kg mol−1, MPEO = 6.5 kg mol−1, polydispersity index = 1.09, Polymer Source, Canada) was weighed, added to toluene (99.9%, Beijing Chemical Factory), and stirred to obtain a fully dissolved homogeneous solution (10 mg mL−1). Then, different amounts of FeCl3 powder (97%, Sinopharm Chemical Reagent, China) were added to the PS-b-PEO solution to prepare three types of precursor mixture solutions with different Fe/O molar ratios: 0.7 (sample 1), 1.5 (sample 2), and 3 (sample 3). The three types of mixed solutions were placed in an ultrasonic bath for 1 h and then aged for 24 h to obtain a homogeneous composite micellar solution. Then, 100 μL of the mixed solution was cast onto the silicon wafers under a relative humidity of 85% at 20 ± 2 °C. Spin-coating was conducted at a low speed of 400 rpm for approximately 20 s. Porous composite films composed of PS-b-PEO and FeCl3 were obtained after complete evaporation of the solvent and water. The Fe2O3 coatings were obtained after calcination treatment of the films in air at 550 °C for 3 h. Characterization. The surface morphology of the samples was characterized using a field-emission scanning electron microscope (JSM-7500F, Japan) at an accelerating voltage of 5 kV. XPS and XRD data were collected to facilitate the component analysis of the coating. XPS data were obtained by an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Mg Kα radiation. The base pressure was approximately 3 × 10−9 mbar. XRD patterns were collected by a D/max 2500 X-ray diffractometer from Rigaku with Cu Kα radiation. CAs were measured on an OCA20 (Data-Physics, Germany) contact angle system at ambient temperature in air, under water, and under oil. In each measurement, the probe drops (about 3 μL) of water, oil (1,2-dichloroethane and cyclohexane), or air bubble were dropped carefully onto the coating sample surfaces under different media (air, water, and oil). The average contact angle value was obtained at five different positions of the same sample with a final summarized result. The adhesion forces of the air bubble, water, and oil on the coating surface were measured by a high-sensitivity, microelectromechanical balance system (Data-Physics DCAT 11, Germany) under different media.64,65 A droplet of approximately 5 μL was suspended on a copper cap or a copper loop connected to the microbalance, and the sample surface was maintained on the balance stage. The stage was controlled to move upward at a constant speed of 0.04 mm/s to facilitate contact between the sample surface and probe droplet. Then, the stage was moved downward at the same speed until the probe droplet left the sample surface. The influence of external
Figure 6. Wettability models of the three types of iron oxide coatings under different media: (a,d,g) sample 1 is the Wenzel state; (b,e,h,k) sample 2 is the Cassie state; (c,f,i,l) sample 3 is the Wenzel−Cassie transition state. When an air bubble contacts the sample 1 surface under oil, the wettability model (j) is still the Cassie state.
samples displayed no super-antiwetting adhesion feature for air bubbles under oil. The wettability model (Figure 6j−l) of the three samples under this condition was all the Cassie model. This occurs because, after immersing the sample in oil, the coating surface was easily covered by oil molecules, which possess surface tension lower than that of water, and the trapped oil molecules markedly hindered contact between the air bubble and the sample surface. In addition, the air bubble possessed a relatively small gravity and low affinity for oil and the hydrophilic surface, eventually rendering adherence of the bubble to the sample surface difficult.
CONCLUSIONS In summary, we demonstrated a fabrication strategy for achieving antiadhesive coatings under multiple media based 11093
DOI: 10.1021/acsnano.6b05961 ACS Nano 2016, 10, 11087−11095
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pressure on the underwater−oil/air bubble adhesion and the under oil−water/bubble adhesion was examined by preloading different forces on the oil droplet, water droplet, and air bubble and then detaching the preloaded forces from the sample surface.64 During measurements, adhesion was monitored by a high-sensitivity balance system, and the shape of the oil droplet was recorded by a high-speed CCD. The obtained average force with standard error was calculated from five measured results at different positions of the same sample under constant conditions. The antiadhesion application demonstration was conducted with a Canon EOS 60D camera. AFM images and surface roughness values of the coatings were obtained with a LEXT OLS4500 (Japan) nano search microscope operating in tapping mode.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05961. Structural formula of PS-b-PEO; SEM image of iron oxide nanoparticles; SEM images of the nonporous and porous iron oxide coating; XPS spectra of the antiadhesive coating; XRD pattern of the antiadhesive coating; contact angles of water and two types of oils in air on the three types of samples; adhesion curves of sample 1 and sample 3 under different media; antiadhesive characteristic of sample 2 toward water under mixed oils and mixed oils under water; SEM image of sample 2 after numerous under-liquid antiadhesion measurements; underwater self-cleaning demonstration of sample 2 for 1,2-dichloroethane using a small tilt angle of about 5°; adhesive forces of the three types of samples measured under different media (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Research Fund for Fun d am e n t al K e y P r o j e c t s ( 20 14 CB 93 18 02 a n d 2013CB834705), the National Natural Science Foundation of China (51541301, 51673010), and the Fundamental Research Funds for the Central Universities (YWF-16-BJ-Y-72). REFERENCES (1) Liu, M. J.; Wang, S. T.; Wei, Z. X.; Song, Y. L.; Jiang, L. Bioinspired Design of a Superoleophobic and Low Adhesive Water/ Solid Interface. Adv. Mater. 2009, 21, 665−669. (2) Zhang, F.; Zhang, W. B.; Shi, Z.; Wang, D.; Jin, J.; Jiang, L. Nanowire-Haired Inorganic Membranes with Superhydrophilicity and Underwater Ultralow Adhesive Superoleophobicity for High-Efficiency Oil/Water Separation. Adv. Mater. 2013, 25, 4192−4198. (3) Manna, U.; Lynn, D. M. Synthetic Surfaces with Robust and Tunable Underwater Superoleophobicity. Adv. Funct. Mater. 2015, 25, 1672−1681. (4) Li, F.; Du, M.; Zheng, Q. Dopamine/Silica Nanoparticle Assembled, Microscale Porous Structure for Versatile Superamphiphobic Coating. ACS Nano 2016, 10, 2910−2921. (5) Yu, L.; Chen, G. Y.; Xu, H.; Liu, X. Substrate-Independent, Transparent Oil-Repellent Coatings with Self-Healing and Persistent Easy-Sliding Oil Repellency. ACS Nano 2016, 10, 1076−1085. (6) Wang, Z.; Zhu, L.; Li, W.; Liu, H. Bioinspired In Situ Growth of Conversion Films With Underwater Superoleophobicity and Excellent 11094
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