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Facile Design and Fabrication of Superwetting Surfaces with Excellent Wear-Resistance Wenbo Zhang, Tianhao Xiang, Feng Liu, Ming Zhang, Wentao Gan, Xianglin Zhai, Xin Di, Yazhou Wang, Guoxiang Liu, and Chengyu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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ACS Applied Materials & Interfaces

Facile Design and Fabrication of Superwetting Surfaces with Excellent Wear-Resistance Wenbo Zhang,† Tianhao Xiang,† Feng Liu,† Ming Zhang, †, ‡ Wentao Gan,† Xianglin Zhai,§ Xin Di,† Yazhou Wang,† Guoxiang Liu† and Chengyu Wang*, † †

Key Laboratory of Bio-Based Material Science and Technology of Ministry of Education,

Northeast Forestry University, Harbin, 150040, P. R. China ‡

Department of Materials Science and Engineering, University of Pennsylvania, 216 LRSM

Building, 3231 Walnut Street, Philadelphia, PA 19104, USA §

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA

KEYWORDS: superhydrophilic, superhydrophobic, self-cleaning, wear-resistant, self-healing

ABSTRACT Preparation of mechanically durable superwetting surfaces is imperative, yet challenging for the wide range of real applications where high durability is required. Mechanical wear on superwetting surfaces usually degrades weak roughness, leading to loss of functions. In this study, wear-resistant superhydrophilic/underwater superoleophobic and superhydrophobic surfaces are prepared by anchoring reinforced coatings via adhesive-swelling and adhesivebonding processes, respectively. The results of the sandpaper abrasion (grit no. 600, 24 kPa) show that superhydrophilic nylon/SiO2 coatings and superhydrophobic polyurethane/TiO2 coatings retain their functions after suffering the abrasion distances of 70 cm and more than 1000

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cm, respectively. Reinforced coatings formed by consecutive roughness and improved adhesion between coatings and substrates are responsible for repeatedly generated superwettability after exposure to mechanical stresses and demonstrated to be feasible for designing wear-resistant superwetting surfaces. Furthermore, this novel architecture of “reinforced coating with consecutive roughness + high adhesion” may demand desired coating materials and reliable coating-fixing techniques for sustaining sufficient roughness, and is superior to currently existing technologies in advancing wear-resistance of superwetting surfaces.

INTRODUCTION Superwetting surfaces attract tremendous interest from chemistry, physics, engineering, and biological sciences owing to their special surface properties. Thereinto, artificially engineered superhydrophilic and superhydrophobic surfaces with the inspiration from nature, have been demonstrated to be capable of solving large-scale practical problems (e.g., self-cleaning,1–7 oilwater separation,8,9 anti-fogging,1–3,10,11 and enhanced heat transfer12,13). Based on various techniques and practical purposes, these surfaces have been fabricated on different materials (e.g., stainless-steel meshes,8,9 glass fibers,14,15 sponges16,17 and glass surfaces1–3,11) and their features (e.g., wettability, topography, usage, application fields and real performance) were characterized comprehensively. However, compared to those arresting features of superwetting surfaces, their mechanical durability—a critical factor affecting the lifetime—appears to be less, but also challenging, to be improved and investigated accurately. The microscopic roughness features make it hard to obtain robust superwetting surfaces; even such surfaces can be fabricated, it is still difficult to test and present the durability from the viewpoint of standardization, thus making comparisons of different superwetting surfaces impossible.


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Recently, an increasing number of researches have focused on fabricating robust superwetting surfaces and their durability was characterized through a variety of physical methods. For superhydrophilic surfaces, compression/tensile measurements,18,19 approach force,18 nanoindentation,20 sand grain impact,19,20 finger wipe,19 tape peeling19 and linear abrasion21 were adopted in a few literatures. More methods in tribology have been concerned and introduced into the durability assessment of superhydrophobic surfaces, including tangential abrasion,22–29 blade/knife scratching,22,24,25,27–29 pencil hardness tests,30,31 finger wiping,22,24,25 sand grain impact,30,32 water jet tests,33,34 laundering tests,29,35 etc. The linear abrasion test is always recommended as the most principal means for evaluating the mechanical durability of superhydrophobic surfaces against wear.36,37 Although linear abrasion employing commercial standardized abradants offers a comparable basis for numerous superwetting materials, it still cannot apply to all surfaces (e.g., irregularly shaped materials, for which accurate abraded areas cannot be calculated). Once it becomes unavailable, other mechanical tests will seem to be insufficient to distinguish between varied superwetting surfaces according to their real performances. Prior to the durability testing for such surfaces, there is a necessity for better estimation of their durability basing on some theoretical models, which is favorable for understanding the surface durability from a macro sense. In this work, superhydrophilic/underwater superoleophobic nylon/SiO2 coatings and superhydrophobic polyurethane/TiO2 coatings, which have improved strength and thickness, were successfully prepared and further anchored firmly on glass slides through adhesiveswelling and adhesive-bonding processes, respectively. Both the coatings have consecutive roughness and invariable chemical compositions, endowing the surfaces with excellent wear resistance and durable superwettability. These surfaces can maintain superwettability even after


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the long-distance abrasion with grit no. 600 sandpaper under a pressure of 24 kPa (for superhydrophilic surfaces, moved for 70 cm; for superhydrophobic surfaces, moved for more than 1000 cm). When exposed to environmental wear, the outer layers of the coatings peel off and new rough surfaces emerge, which exhibit a wettability-regenerating capability. Excellent mechanical durability makes the surfaces satisfy the real-world needs in applications (e.g., constant superwettability, ease of regenerating or self-healing, facile removal of contamination). Further, a wear model is proposed to compare typical superwetting surfaces and estimate their durability, which may yield new ideas for fabricating mechanically durable superwetting materials. EXPERIMENTAL SECTION Materials. Nylon 6,6 powder (T-255) was provided by Shanghai Zhenwei Composite Materials Co., Ltd. (China). Polyurethane (PU) foam was provided by Chengdu Shuxin Sponge Product Co., Ltd. (China) and cleaned in ethanol and toluene before using. Inorganic particles used in fabricating superwetting coatings were silicon dioxide (~10 µm in diameter, hydrophilic) (Aladdin), TiO2 P25 (~21 nm in diameter) (Degussa) and titanium oxide (anatase) nanoparticles (~100 nm in diameter, hydrophilic) (Aladdin). The epoxy resin used for coating immobilization was Ausbond 92 conformal coating with curing agent and thinner, offered by Ausbond (China) Co., Ltd. 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (C10F17H4SiCl3, ≥ 98%) and 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (C8F13H4Si(OCH2CH3)3, ≥ 98%) were purchased from Shanghai Sinofluoro Chemicals Co., Ltd. (China). Hydraulic oil #68 was purchased from Yingkou Petrochemical Co., Ltd. (China). Corn oil was purchased from Yihai Kerry Food Sales Co., Ltd. (China). Methylene blue and Sudan III used for dye were purchased from Sinopharm


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Chemical Reagent Co., Ltd. (China). Sudan Blue II (Aladdin) was used for dyeing carbon tetrachloride. Fe3O4 nanoparticles (100–300 nm in diameter) (Aladdin) was used to tint the corn oil. The petroleum sample was offered by Shengli oilfield in Shandong (China). Formic acid (HCOOH, ≥ 88%), dimethicone, methylbenzene, n-hexane, anhydrous ethanol, carbon tetrachloride and petrolatum were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (China). All chemicals were used as received without any purification. Fabrication of Superhydrophilic/Underwater Superoleophobic Surfaces with and Without Resin-Immobilization. A mixture of Ausbond 92, curing agent and thinner (weight ratio = 5:1:20) was first sprayed on cleaned glass slides evenly. Afterwards, the samples were put into an oven at 120 °C for 2 h to make total solidification of the adhesive. Next, the resincovered and pristine glass slides were separately immersed in a thoroughly mixed solution (100 mL of HCOOH, 44 g of SiO2 particles and 10 g of nylon 6,6) for 10 min, then drawn out vertically and slowly with the sticky solution adhering on the surfaces of glass slides uniformly. The samples were dried in an oven at 25 °C for 12h, and the superhydrophilic/underwater superoleophobic surfaces with (for the resin-covered glass slides) and wihout (for the pristine glass slides) resin-immobilization were obtained respectively. Fabrication of Hydrophobic Unfilled Cushions and Nanoparticles-Filled ContinuouslyPorous Cushions. Cleaned PU foams with a thickness of 2 cm were soaked in a mixture of Ausbond 92, curing agent and thinner (weight ratio = 5:1:20) for 5 min. After being withdrawn and squeezed manually to remove the redundant adhesive, the samples were immediately compressed into compact cushions through a compressing machine under a pressure of 20 MPa. After the complete rigidity, the samples were immersed in an n-hexane solution of 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (2.0 mg mL-1) for 10 min. Next, the samples were taken out


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and washed with n-hexane several times. After drying, the hydrophobic unfilled cushions were obtained. Then the hydrophobic unfilled cushions were put into a thoroughly mixed solution (20 mL of ethanol, 4 g of TiO2 P25 and 0.8 g of 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane) and kept for 10 min at 0.08 MPa (vacuum degree -0.08 MPa). After drying, the nanoparticles-filled continuously-porous cushions were obtained and directly glued to glass slides through Ausbond 92 until the resin was completely solidified for further mechanical tests. Fabrication of Superhydrophobic TiO2 Coatings on Glass Surfaces. A mixture of Ausbond 92, curing agent and thinner (weight ratio = 5:1:20) was first sprayed on cleaned glass slides evenly. Afterwards, a thoroughly mixed solution (99 g of anhydrous ethanol, 6 g of TiO2 P25, 6 g of titanium oxide (anatase) nanoparticles and 1 g of 1H, 1H, 2H, 2Hperfluorooctyltriethoxysilane) was sprayed onto the adhesive. The samples were put into an oven at 120 °C for 2 h. Characterizations. SEM images were collected on a HITACHI TM3030 tabletop microscope (accelerated voltage: 15.0 kV). Static contact angles and sliding angles (5 µL for both oils and water) were collected on a JC2000C contact angle system offered by Shanghai Zhongchen Digital Technic Apparatus Co., Ltd (Shanghai, China) at room temperature. At least three locations were tested in order to get the average value. Dynamic wettability of the superhydrophilic surface toward water was captured by an OCA20 system (Data-physics, Germany). The chemical stability and oil repellency of the superhydrophilic surfaces were evaluated by completely immersing the superhydrophilic surfaces in aqueous solutions in different circumstances, and then underwater oil contact angles (OCA) and oil sliding angles (OSA) were measured after rinsing the surfaces with water. Linear abrasion tests on all surfaces were carefully conducted in the manner as proposed in the references.23,36 Grit no. 600 sandpaper,


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an abraded area of 1 cm × 0.9 cm and a pressure of 24 kPa were adopted for following wettability and mass loss measurements. The apparatus used for abrasion tests and the details of the related experiments are shown in Figure S1 in the Supporting Information. Surface profile testing was done using a D-120 (KLA-Tencor Corporation) surface profiler and a ContourGT-K 3D optical microscope (Bruker Corporation). RESULTS AND DISCUSSION The superhydrophilic and underwater superoleophobic surfaces were fabricated by solution immersion, wherein the superhydrophilic coatings were anchored on the glass slides through an adhesive-swelling process. Figure 1a shows a schematic of the process for the coating immobilization. A layer of the acid-resistant epoxy resin (Ausbond 92) on the glass slide is immersed in a formic acid solution, where contains high concentrations of SiO2 particles (~10 µm in diameter) and nylon 6,6. The swelling of the resin occurs slowly over time. The formic acid (a common component in traditional paint strippers) causes the coating swelling by interfering with inter-chain hydrogen bonding of the crosslinked polymer network and maintains a continuing diffusion of the solvent molecules that creates spaces between the resin chains,38–40 thus permitting the penetration of nylon molecules. As the swelling develops, the nylon molecules fit into the spaces and tend to interlace with the resin chains. As a result of complete evaporation of formic acid, the superhydrophilic coating forms and is firmly immobilized on the glass slide. Top-view scanning electron microscopy (SEM) images show that the resin surface on the glass slide is completely different from the superhydrophilic surface (Figure 1b). The resin coating is smooth almost without any observable roughness, while the superhydrophilic coating is composed of two types of granular matter: nylon-coated SiO2 particles and nylon micro particles. The high-magnification SEM image in Figure 1b reveals randomly distributed


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aggregates of nylon micro particles at the micro- and nanometer scales around the spherical SiO2 particles, thus giving rise to a rather rough surface. The formation of these nylon particles can be attributed to the phase separation in the transition from an aqueous solution to a solid state, during which the nylon crystalline spherulites grew up radially by continuously eliminating the solvent.41,42 The micro- and nanoscale symmict structure is beneficial to obtain superwetting behavior according to the Cassie’s mode derived from Young’s equation.43 In addition, micronano hierarchically structured nylon 6,6 surfaces have been proved to be superhydrophilic,42 which is expected to be superoleophobic underwater. When a water droplet (5 µL) contacts with the superhydrophilic surface, it immediately spreads out and permeates into the gaps in the coating within 1s, and a contact angle (CA) of nearly 0° is obtained (Figure S2, Supporting Information); an underwater oil (1, 2-dichloroethane) CA of 160.2 ± 1.3° is also observed, suggesting a superhydrophilic and underwater superoleophobic property.


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Figure 1. Fabrication and applications of the superhydrophilic/underwater superoleophobic surfaces. a) Schematic of the adhesive-swelling process for the coating immobilization. b) SEM images of the epoxy resin coating and the upper nylon/SiO2 coating on a glass slide. c) Removal of tinted corn oil on a water-wetted superhydrophilic surface via moving water. Note the oil attachment to the surface before water rinsing. d) Stable oil repellency of the WBORS in oily environments. Note the clean surfaces after water rinsing, indicating the robust function in the wide ranges of pressures, temperatures and oil types.


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The as-prepared superhydrophilic and underwater superoleophobic surfaces possess prominent ability against oils contamination after the absorption of water. Figure 1c shows the side view of oil droplets (red: tinted corn oil; black: Fe3O4-tinted corn oil) with a low vapor pressure on a water-wetted superhydrophilic surface. Although the oils didn’t move as freely as the fluids on ‘slippery liquid-infused porous surfaces’ (SLIPS),44 they can be readily removed by pouring water, indicating self-cleaning was achieved in the presence of water (Movie S1, Supporting Information). The oil-repellent ability is attributed to the hierarchically rough structure constituted by closely connected nylon-coated SiO2 particles with the attachment of smaller nylon grains and the hydrophilicity of nylon 6,6.42,45 When the superhydrophilic surface is wetted by water, water could be trapped into the seamed structure to form a stable water-barrier layer, which prevents the contact between oils and the solid surface.8,46–48 By contrast, the oils on the unwetted surface cannot be entirely removed by water, leaving a contaminated surface (Figure S3 and Movie S1, Supporting Information). Inspired by above phenomena, we propose a synthetic oil-repellent surface—‘water-based oil-repellent surface (s)’ (WBORS)—that isolates substrates from oils depending on a continuous film of water. However, this capability could be compromised over time if water evaporates in air, so reducing water loss enables prolonged operation. The under-oil tests on WBORSs performed at different pressures and temperatures show that the WBORSs are capable of isolating immiscible oils and these sticky oils can be washed away by water without residual, even after a prolonged immersion of 7 days (Figure 1d and Movie S2, Supporting Information). Completely encased by the oils down to the molecular scale, the water on the WBORSs is preserved with no detectable evaporation. However, the oil repellency of WBORSs can be utterly broken in high-temperature/low-pressure oily environments, as illustrated in Figure S4 (Supporting Information). When a WBORS was heated


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to 80 °C at a pressure of 0.2 atm (the boiling point of water in this condition is ~60 °C) underoil, the water-barrier layer on the WBORS boiled violently, producing a large number of bubbles. After the boil stopped, the water-barrier layer disappeared entirely and the oil filled the spaces within the texture, forming a continuous oil film. This oil film was extremely stable and cannot be washed away by water. In spite of this, WBORS is particularly applicable for the need of oil repellency in general oily environments, which is more competitive in cost, safety and environmental benefit in contrast with other oleophobic surfaces.44,49–53 Textured surfaces are highly susceptible to mechanical wear,36,37,54 thus leading to the loss of superwettability. Searching for wear-resistant materials may improve coating robustness. Nylon 6,6, a type of hydrophilic polyamide, which is frequently used when high mechanical strength, rigidity, good dimensional stability are required,45 serves as both agglomerant and antifriction in the coatings to empower the superhydrophilic surfaces to resist moderate wear. The finger-wipe test shows that the water-wetted coating directly attached to the glass slide was removed in the forms of fragments after being wiped; whereas the resin-immobilized coating remained intact (Figure 2a and Movie S3, Supporting Information). Although nylon 6,6 improves the coating strength, the adhesion between the coating and the no-resin-covered glass slide is still weak, from which the cracks initiated and expanded. Further sandpaper abrasion tests affirm the retained superoleophobicity of the resin-immobilized coatings after each abrasion cycle. The surfaces both wetted and unwetted by water show the consistent wear performance: the underwater OCAs remained above 160° and the OSAs remained below 5° after the 70 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa (Figure 2b and Figure S5 in the Supporting Information). The SEM image of the unwetted coating after the 10 cm abrasion reveals the same morphology compared to the pristine coating (Figure S6a, Supporting


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Information). However, the surface height profiles offer distinguishing results: the more intensive fluctuations in the worn area verify that the abrasion process was itself inducing the coarse texture into the coating (Figure S6b and S6c, Supporting Information), even if it seemed not to change the wettability of the coating. Note that the coating mass loss increased linearly with the total abrasion distances and only 1.9~2.8 mg of the coating was removed in each abrasion cycle (Figure S7, Supporting Information). To our knowledge very few reports have focused on the abrasion resistance of superhydrophilic and underwater superoleophobic surfaces because these surfaces are often synthesized via deposition of brittle matter or soft crosslinked hydrogels,55,56 most of which have only single-stage textured topography. The formation of our coatings is different from that: the viscous formic acid solution containing concentrated particles induces abundant adsorption of the solution on the glass slide, forming an approximately 200 µm-thick dense coating (Figure 2c), which shows a disorderly particles-stacked structure with the same height as the thickness of the coating. There are numerous cavities sized at the micro- and nanoscale formed among the multilayer assemblies of SiO2 particles, thereby constituting continuous channels from all quarters of the coating. It should be noted that the aggregates of nylon micro particles also show a random distribution in the open cavities, which resulted from the precipitation of the polymer chains induced by the solvent evaporation. In this process, nylon plays a pivotal role in improving the durability of the coating: it offers stable interfacial strength among SiO2 particles through strong hydrogen bonding interaction between the massive Si–OH groups on the SiO2 surfaces and the nylon chains at molecular level. From the SEM image with a higher magnification in Figure 2c, a layer of resin underneath the coating can be recognized, indicating the resin survived after the swelling proceeded from appearance to inside of it. Combining the good mechanical property of the layer-by-layer deposited rough structure and the


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reliable fixation of the resin, the self-repairing topography of the surface enables good resistance against mechanical wear: when exposed to external wear, the outer layers of the coating peel off and new rough surfaces emerge, thus sustaining its superoleophobicity, as illustrated in Figure 2d.

Figure 2. Mechanical stability tests of the superhydrophilic/underwater superoleophobic surfaces. a) Finger-wipe test on the water-wetted coatings on glass slides without and with resinimmobilization (tagged with A and B, respectively). b) Variation of underwater OCA and OSA of the water-wetted resin-immobilized coating with increasing abrasion distances of grit no. 600 sandpaper under a pressure of 24 kPa. c) SEM images for cross section of the resin-immobilized coating on a glass slide (the area highlighted in red has a higher magnification). d) Wear pattern of the resin-immobilized coating with layer-by-layer deposited rough structure. e) Schematic of the water jet scouring tests on the resin-immobilized coatings. f) Removal of tinted corn oil on a scratched WBORS via moving water. g) Photographs of the resin-immobilized coatings after the water jet scouring tests (the impacted areas are highlighted in red). Water jet scouring tests were performed on the resin-immobilized coating as well as the boundary between the coating and the resin (Figure 2e). Water columns with a diameter of 5 mm


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impinged the surface from a height of ~30 cm, corresponding to an impact velocity of 3.5 m s-1 and an impact time of 1 h. There was no observable damage in the tested regions (Figure 2g) and no difference was detected between the SEM images, surface height profiles and 3-D surface profiles of the coating before and after the vertical test (Figure S8, S9 and S10, Supporting Information), confirming sufficient robustness to completely resist water impact. The WBORS after the knife-scratch retained oil repellency, leaving clearly discernible scratches by water rinsing, as shown in Figure 2f (also see Figure S11 and Movie S4, Supporting Information). The chemical stability of the resin-immobilized coatings was further characterized in terms of oil droplets mobility (OSAs) and OCAs underwater. There is no significant degradation of the oil repellency after exposure to air (-30 °C for 72 h, OCA = 164°, OSA = 3.4°; 100 °C for 72 h, OCA = 164°, OSA = 8.1°), water (80 °C for 12 h, OCA = 166°, OSA = 2.3°), acid (pH 1 H2SO4 for 72 h, OCA = 162°, OSA = 1.5°), saline solution (3.5 wt% NaCl for 72 h, OCA = 162°, OSA = 1.1°), toluene (72 h, OCA = 164°, OSA = 2.7°), or ethanol (72 h, OCA = 165°, OSA = 6.4°) (Figure S12, Supporting Information). None of these demanding conditions aroused blistering, cracking or peeling of the coatings, which is highly needed in fairly complex water environments. Large-scale production of superhydrophobic surfaces is restricted for the demand for microscopic roughness (generally in nanoscale), always leading to decreased surface strength and loss of non-wettability after mechanical contacts. Currently, superhydrophobicity can be sustained mainly relying on increased wear resistance or self-healing features. From reported superhydrophobic materials, improved wear resistance has been demonstrated by employing hierarchical roughness54 or/and commercial adhesives.22 While these alternatives permit detention of nanoscale roughness, non-wettability will fail once nanostructures are worn to the degree that stable Cassie state no longer remains—both of them provide only single-stage


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protection for fragile nanoroughness, which can be limited in advancing robustness. Self-repair with the aid of external stimuluses allows periodically renewable non-wettability even if repeated wear occurs on topography, but suffers from low regeneration rates or laborious procedures.27,29,35,57–60 Wear-induced wettability regeneration as previously described is available to the superhydrophilic surfaces, but it has been little resultful for the renovation of superhydrophobic surfaces, on which more rigid patterns are required for the water/gas/solid three-phase system. Some studies reveal that superimposed roughness can be renewed through wear without degeneration of superhydrophobicity, nevertheless not accessible at significant wear.25,60 As yet no superhydrophobic materials are identified to display consecutive roughness with desired robustness to resist long-term wear—although such structures are fabricated increasingly. For the first time, highly wear-tolerant non-wettability was obtained from a ‘nanoparticlesfilled continuously-porous cushion’ (NFCPC). The ‘cushion’, in this case, compacted polyurethane (PU) foam, was used as both a shelter and an accommodation to protect and preserve intrinsically weak nanoparticles in its interconnected microchannels. The pristine PU foam with smooth fiber surfaces on the scale of dozens of micrometers is composed of threedimensional (3-D) multi-hierarchical porous structures, and the dimensions of the pores are on the order of a few hundreds of micrometers (Figure S13a–S13c, Supporting Information). Compared with the pristine PU, there is a distinct lack of deformation on the dimensions of the fibers of the compacted one, but with dramatic height reduction and roughness improvement due to the tangles of PU fibers generating from the high mechanical pressure (Figure S13d and S13e, Supporting Information). Additionally, the physical compression resulted in the widespread jagged cracks on the edges of PU fibers (Figure S13f, Supporting Information), while not


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compromising the structural integrity of PU: the compacted PU foam (without adhesive processing) exhibited the impermanent deformation after unloading, and recovered its original appearance after the immersion in ethanol (Figure S14, Supporting Information). To give long lasting hold in geometry, the adhesive conformally covered the twisted PU fibers and extra submicrometer-sized protrusions can be found (Figure S13g, Supporting Information), further enhancing the surface roughness. Following the fluoridation and the particles filling, the compressed cushion was glued to a glass slide to create a robust non-wetting surface that thoroughly eliminates adhesion of water, withstands severe mechanical stresses, and instantaneously self-heals and regains its function simply by mechanical wear. The severe sandpaper abrasion (grit no. 320 and a pressure of 32 kPa) partially peeled and curled the coating (Figure 3a); as a result of the more moderate wear (grit no. 600 and a pressure of 24 kPa), the water CA of the coating remained above 155° after the 1000 cm abrasion (Figure 3b, also see Figure S15 and Movie S5, Supporting Information), far superior to any existing superhydrophobic materials. Only 44.2 mg of the coating had peeled off after the 1000 cm abrasion, showing the limited damage by sandpaper abrasion (Figure S16, Supporting Information). Three dominant roles of the compacted PU foam in sustaining superhydrophobicity were concluded as follows: First, the coating is reinforced through the tangles of PU fibers and the conformal adhesive to ensure that shear forces caused by friction can largely dissipate into the entire coating instead of the local abraded area. Second, intrinsically weak nanoroughness is well protected by 3-D PU microfibers: hydrophobe-encapsulated TiO2 nanoparticles occupy the open-cell space in the cushion so as to improve hydrophobicity (Figure S17, Supporting Information) and significantly reduce their loss, maintaining adequate roughness for superhydrophobicity. Third, the continuously-porous structure of the compacted PU and the


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considerable stockpile of TiO2 nanoparticles allow repeated healing of roughness: abrading the coating with hard abradants can lead to emergence of the subjacent material but with the similar roughness as the original surface, confirmed by Figure 3c, 3d and S18 in the Supporting Information. This composite construction, similar to ferroconcrete, confers durable superhydrophobicity while also helps to protect the surface from water impalement into the cushion. Figure S19a shows the impinging behavior of water onto the severely worn NFCPC. Water was repelled in all directions on the surface without any impalement, but it wetted and impaled an unfilled cushion (Figure S19b and Movie S6, Supporting Information), where water tended to distort to twist through its open pores. Similar considerations as for the case of particulate contaminants (TiO2 P25, moist soil and waterlogged Fe3O4 nanoparticles) can be made: the closed-cell structure of the NFCPC guarded against the pinning of the particulates by allowing self-cleaning via moving water that gathered the particles and rolled off the surface, whereas the unfilled cushion was plugged by the dirt and even wetted after the same cleaning (Figure S20 and Movie S7, Supporting Information).


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Figure 3. The as-prepared superhydrophobic surfaces show durable non-wettability towards manifold mechanical damage. a) Photograph of the NFCPC before (left) and after (right) the 10 cm abrasion with grit no. 320 sandpaper under a pressure of 32 kPa (the abraded area is 2.5 cm × 1 cm, as highlighted in red in left. b) Variation of water CA of the NFCPC with increasing abrasion distances of grit no. 600 sandpaper under a pressure of 24 kPa. SEM images of the NFCPC before (c) and after (d) the severe sandpaper abrasion. e) Wear pattern of the NFCPC, where multilayer nanoroughness is well protected by PU fibers, allowing repeated healing of non-wettability. f) A series of sequential physical stresses exerted against the NFCPC: manual sandpaper abrasion (grit no. 320), knife-scratch, finger-wipe and knife-wipe. Tiny and large


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water droplets as the detecting liquids (blue and transparent, respectively) were used to show the restoration of water repellency after each of the stresses. Consequently, superhydrophobicity survived and dyed water droplets on the resulting NFCPC are shown in (g). In view of the regeneration of superhydrophobicity guided by the consecutive roughness (Figure 3e) and the closed-cell feature of the NFCPC, it is expected that the NFCPC recovers its non-wettability through wear after contamination by viscous grease, which is seldom concerned but lethal to superhydrophobicity. Spreading semisolid crude oil and petrolatum on an NFCPC failed the non-wettability, which could be regained by a two-step wipe: commercially available degreasing cotton was first used to remove as much superficial grease as possible, and then the surface was rubbed against sandpaper so that the residual grease could be cleared with the stripping of the coating (Figure S21 and Movie S8, Supporting Information). This facile, costeffective and matter-independent process can effectively overcome the difficulty in rapidly selfhealing superhydrophobic surfaces with no need for complex stimuluses. Given manifold mechanical damage to superhydrophobic surfaces in the real world, a series of sequential physical stresses were exerted against an NFCPC (Figure 3f). The effect of these hard conditions was not accessible by recording the changes in contact angle hysteresis or sliding angles because water droplets tended to be impeded to move by the protruding fibers of the PU. Alternatively, the water extruded from a syringe needle (25 G, to produce tiny drops) and a wash bottle (to produce larger drops) can be used as the tracer. The NFCPC suffered the serious disruption after the stresses of sandpaper abrasion, knife-scratch, finger-wipe and knife-wipe, but it still remained constant in water repellency that no wetting or contamination was detected for the varisized water droplets (Figure 3g and Movie S9, Supporting Information). The solvent immersion test was implemented by soaking an NFCPC into toluene for 5 days. After the immersion, no exposed voids generated at the interfaces between the adhesive and the foam fibers and only a small number of TiO2 nanoparticles were found to be removed out of the


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cushion’s microchannels due to being dispersed into toluene (Figure S22a and S22b, Supporting Information). In spite of this, the immersion process had no effect on the water repellency of the NFCPC, as demonstrated in Figure S22c and S22d (Supporting Information). The results drawn above emphasize that the idea of “reinforced coating with consecutive roughness + high adhesion” can be effective to extend the mechanical life of superwettability, as the pattern shown in Figure 4a: frictional action exerted on the surface tends to gradually wear away the coating or to be totally relieved by the coating rather than invalidate the coating, which determines sustainable wettability. For the roughness at two length scales (Figure 4b), the microscale bumps protect the fragile nanoscale roughness to stabilize the Cassie state,54 despite the overdependence on the mechanical performance of the microscale structures. Accordingly, superwettability will fail if the single layer of the microscale structures markedly degrades. Another popular construction, “weak coating + high adhesion” (e.g., paint + adhesives), on which wear can remove most of the paint, exposing the coating adjacent to adhesives— considered to be robust, but not to be overload-resistant—impetuous wear can rapidly break the single-stage protection of adhesives and make the coating fail (Figure 4c and Figure S23 in the Supporting Information), unlike the situation in Figure 3: mechanical stresses are prior moderated by the coating instead of the adhesive to establish long-term function. Poor adhesion between coatings and substrates is always not expected for practical demands—even slight disturbance yields fatal damage (Figure 4d).


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Figure 4. Wear patterns of superwetting surfaces with different structural compositions. a) Reinforced coatings with consecutive roughness have high adhesion to substrates. b) Coatings consisting of microbumps with nanoroughness on them have high adhesion to substrates. c) Weak coatings have high adhesion to substrates. d) Coatings have poor adhesion to substrates. These wear patterns raise the possibility of roughly predicting or evaluating the durability of superwetting materials, particularly when rigorous comparisons between them cannot be made. Moreover, through the identification of the wear pattern of superwetting surfaces, pertinent measures can be adopted to strengthen coatings or/and adhesion between coatings and substrates for improving mechanical durability. CONCLUSION In summary, superhydrophilic/underwater superoleophobic and superhydrophobic coatings were firmly anchored on glass slides through an adhesive-swelling process and an adhesive-bonding process, respectively. The reinforced multilayer roughness and insusceptible chemical compositions of the coatings support durable functions even in the aftermath of diversified


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mechanical stresses. The coatings are especially resistant to wear: while peeling off the upperlayer roughness, wear creates new rough surfaces with unaltered superwettability. This feature suggests that the solution of “reinforced coating with consecutive roughness + high adhesion” can be feasible for designing wear-resistant superwetting surfaces. With this inspiration, appropriate coating materials and reliable coating-fixing techniques are expected to be used in constructing large-size superwetting surfaces with high mechanical durability. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The apparatus used for abrasion tests, water contact angle and underwater oil contact angle of the superhydrophilic coating, photographs showing the contamination of a superhydrophilic surface by tinted corn oil, photographs showing the contamination of a WBORS in a hightemperature/low-pressure oily environment, variation of underwater OCA and OSA of the unwetted resin-immobilized coating with increasing abrasion distances of grit no. 600 sandpaper under a pressure of 24 kPa, SEM image of the unwetted resin-immobilized coating after the 10 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa, surface height profiles of the resin-immobilized coating before and after the 10 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa, variation of total mass loss of the unwetted resin-immobilized coating with increasing abrasion distances of grit no. 600 sandpaper under a pressure of 24 kPa, SEM images, surface height profiles and 3-D roughness profiles of the resin-immobilized coating before and after the vertical water jet scouring test, photographs showing the ability to repel tiny oil droplets underwater by the knife-scratched WBORS, the underwater OCAs and OSAs of the resinimmobilized coatings after the tests in various circumstances, a photograph of raw PU foam,


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SEM images of raw PU foam and an unfilled cushion, photographs showing the appearance restoration of compacted PU foam (without adhesive processing) in ethanol, a photograph of the NFCPC after the 1000 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa and corresponding SEM images, photographs showing water repellency for tiny and large droplets of the NFCPC worn to 1000 cm, variation of total mass loss of the NFCPC with increasing abrasion distances of grit no. 600 sandpaper under a pressure of 24 kPa, water contact angles of raw PU foam, an unfilled cushion and an NFCPC, surface height profiles of the NFCPC before and after the 100 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa, photographs showing water impact tests on the severely worn NFCPC and an unfilled cushion, photographs showing self-cleaning tests on an NFCPC and an unfilled cushion, photographs showing regeneration of non-wettability for an grease-contaminated NFCPC, SEM images of an NFCPC before and after the immersion in toluene for 5 days, photographs showing the effect of toluene contamination on the NFCPC, photographs, water contact angles and SEM images of a superhydrophobic glass surface before and after the 10 cm abrasion with grit no. 600 sandpaper under a pressure of 24 kPa (PDF) Removal of tinted corn oil on a WBORS, contamination of a superhydrophilic surface by tinted corn oil, oil repellency of the WBORS in oily environments, finger-wipe test on the water-wetted superhydrophilic coatings without and with resin-immobilization, removal of tinted corn oil on a scratched WBORS, oil repellency underwater by the knife-scratched WBORS, water repellency of the NFCPC worn to 1000 cm, water impact tests on the severely worn NFCPC and an unfilled cushion, self-cleaning tests on an NFCPC and an unfilled cushion, regeneration of nonwettability for an grease-contaminated NFCPC, physical stresses exerted against an NFCPC (Movies)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the Fundamental Research Funds for Central Universities (2572015EB01) and the National Natural Science Foundation of China (31470584). Notes The authors declare no competing financial interest. ABBREVIATIONS PU, polyurethane; SEM, scanning electron microscopy; CA, contact angle; SLIPS, slippery liquid-infused porous surfaces; WBORS, water-based oil-repellent surface; OSA, oil sliding angle; OCA, oil contact angle; NFCPC, nanoparticles-filled continuously-porous cushion; 3-D, three-dimensional. REFERENCES (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Light-induced amphiphilic surfaces. Nature 1997, 388, 431– 432.


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