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Bioinspired Omniphobic Coatings with Thermal Self-Repair Function on Industrial Materials Jing Wang, Keiko Kato, Alexandre P. Blois, and Tak-Sing Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00194 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

Bioinspired Omniphobic Coatings with Thermal Self-Repair Function on Industrial Materials Jing Wang,a Keiko Kato,b Alexandre P. Blois,a and Tak-Sing Wong a,* a

Department of Mechanical and Nuclear Engineering, and Material Research Institute, The

Pennsylvania State University, University Park, PA, 16802, USA. b

Department of Materials Science, University of Illinois at Urbana-Champaign, Urbana, IL,

61820, USA KEYWORDS: Self-healing, slippery surfaces, cross-species bio-inspiration, omniphobic coatings, robustness, industrial materials.

ACS Paragon Plus Environment

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ABSTRACT

Inspired by the wax regeneration ability of plant leaves and the slippery surfaces of the Nepenthes pitcher plants, we have developed a new form of cross-species bio-inspired slippery liquid-infused porous surfaces (X-SLIPS) that can self-repair under thermal stimulation even under large area physical and chemical damage. The performance and underlying mechanism of the thermal-healing property has been studied and characterized in details. These thermally selfhealing omniphobic coatings can be applied onto a broad range of metals, plastics, glass, and ceramics of various shapes, and show excellent repellency towards aqueous and organic liquids.

1. Introduction The recent development of Nepenthes pitcher plant inspired slippery liquid-infused porous surfaces (SLIPS) has shown great promise in medical and industrial applications with functions ranging from self-cleaning,1-2 anti-icing,3 anti-fouling and anti-coagulation,4-7 anti-staining,8 and drag reduction,9-10 to enhanced condensation heat transfer.11-12 Applying SLIPS onto a substrate requires a micro/nano-textured surface with proper surface chemical functionalization so that a liquid lubricant with lower surface energy will completely wet the substrate. The lubricant layer is dynamically reconfigurable due to its liquid state, and lends SLIPS the ability to self-heal after damage.1 However, such self-healing requires the presence of a molecularly continuous or nearly continuous underlying chemically functionalized layer; if this layer is damaged, the lubricating liquid can no longer form a continuous layer that adheres to the substrate, permanently damaging the overall liquid repellency of the material. One method nature uses to address issues of selfrepairing is exhibited in plant leaves, which have a hydrophobic wax layer to protect them from


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biological and environmental stresses, such as excessive water transpiration and irradiation from sunlight. If the protective layer is damaged, the wax layer can be repaired by the plant to restore its function.13-18 Inspired by the concepts of plant leaf wax repair14-16, 18 and the slippery surfaces of the Nepenthes pitcher plant19 (Figure 1a), we report a new form of cross-species bioinspired slippery liquid-infused porous surfaces (X-SLIPS) with a chemically functionalized layer that can be selfrepaired by heating, even after significant physical and chemical damage over large areas (see Supporting Video 1). Our approach distinguishes from other state-of-the-art self-healing SLIPS coatings, which are only capable to repair damages of physical size on the order of 1 mm.1, 20-21 We further demonstrate that the thermally self-healing slippery coatings can be applied onto a broad range of metals, plastics, ceramics, and glass of various shapes, and exhibit excellent repellency towards various aqueous and organic liquids. To create an X-SLIPS coating, we first created micro/nanoscale textures on a surface (see Experimental Section and Figure S1).22 The textured surfaces are coated with perfluorinated silane through liquid-phase silanization, making the surfaces highly hydrophobic (Figure 1b). The silane layer serves as an intermediate layer between the textured solid and the lubricant to induce complete wetting of the liquid lubricant. While a monolayer of silane is sufficient to create this intermediate layer,23-25 the important step here is that we prolong the silanization process to ensure excess silane molecules are deposited onto the textured surfaces (see Experimental Section). Part of the excess silane molecules can survive in physical/chemical damages, and serve as a source for the thermal self-repairing process (Figure 1c and Figure S2). After the silanization, we apply perfluorinated lubricant onto the chemically functionalized surface by a spray coating or spin coating process. Since perfluorinated lubricant is immiscible to


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both aqueous and organic phases, the resulting surface can then repel a broad range of aqueous and organic fluids. 2. Experimental Section 2.1 Materials. Stainless steel 304, stainless steel 316, carbon steel, titanium, aluminum, copper, glass, ceramics (high-temperature nonporous high-alumina ceramic), polyethylene, polypropylene, polystyrene, and ABS were purchased from McMaster-Carr Inc. 4-inch silicon wafers were purchased from Semiconductor Solutions, LLC. Perfluorinated silane used in the experiment was (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane from Gelest Inc. Other types of silanes used in the experiments included 1H,1H,2H,2H-perfluorodecyltriethoxysilane, trimethoxy(octyl)silane, and trimethoxy(propyl)silane, all purchased from Sigma Aldrich Corporation. The chemical etchants used for surface roughening were hydrochloric acid and ferric chloride acid both from Sigma Aldrich Corporation. Sandpaper (grit 700) was purchased from 3M Company. The fluorinated lubricant was DuPont Krytox oils (100 to 107) purchased from TMC Industries, Inc. 2.2 Surface Roughness Formation. Nanostructures can be formed on titanium surface through etching using concentrated hydrochloric acid for 1 hour at 40 ℃. Stainless steel (304 and 316), carbon steel, and copper surfaces were etched with ferric chloride acid for 1 hour at 40 ℃ to produce micro/nano structures. Aluminum surfaces were reacted with water vapor at 100 ℃ for 15 to 30 min, forming boehmite nanostructures.26 We formed micro-structures on plastics, including polyethylene, polypropylene, polystyrene, and ABS, through physical abrasion using sandpaper. The glass surfaces were etched using a slow etching process where the sample was immersed in 0.5 mol/mL of boiling sodium bicarbonate solution for 48 hours. The etching


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process can form nano-porous structures on the surface.27 Micro/nanostructures on silicon were created by photo-lithography and deep reactive ion etch (DRIE). The ceramic was as-fired, which was intrinsically rough as received. We have measured the roughness and waviness of our samples using Zygo optical profilometer (see Table S1). 2.3 Chemical Functionalization and Lubrication. Before functionalizing the surface, the roughened industrial metals, glass, ceramic, plastic, and silicon were cleaned and surfaceactivated using oxygen plasma for 10 min. The materials were immersed in a silane solution consisting of 1 – 4 mM of (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane in ethanol. During the silanization process, the solution was exposed to air. The surface chemical functionalization process was completed upon the full evaporation of the solution which typically took less than 24 hours. By heating the solution at 70 ℃, the evaporation can be accelerated to less than 5 hours. The average thickness of the silane coating is ~ 2.40±0.01 µm on flat titanium samples based on a 4 mM (heptadecafluoro-1,1,2,2tetrahydrodecyl)trichlorosilane in ethanol. The silane layer thickness was determined by weighing the samples before and after the deposition process, where the density of 1H,1H,2H,2H-perfluorodecyltriethoxysilane is 1389 kg/m3 (Sigma Aldrich). The silane deposited onto the substrates is the byproduct of the reaction between (heptadecafluoro-1,1,2,2tetrahydrodecyl)trichlorosilane (C10H4Cl3F17Si) and ethanol (C2H5OH):1H,1H,2H,2Hperfluorodecyltriethoxysilane (C16H19F17O3Si).28 The chemical functional intermediate layers created by these types of silane (e.g., 1H,1H,2H,2H-perfluorodecyltriethoxysilane, trimethoxy(octyl)silane, and trimethoxy(propyl)silane) can all self-repair under thermal stimulation (Figure S2). Note that the silane molecules that are covalently coated onto the surface are the product from the reaction of (heptadecafluoro-1,1,2,2-


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tetrahydrodecyl)trichlorosilane (C10H4Cl3F17Si), as well as hydroxyl groups on the surfaces that were created using oxygen plasma treatment. After chemically functionalizing the surface, the surface was lubricated with perfluorinated oils by spraying or spin coating. 2.4 Thermal-Repairing Characterizations. X-SLIPS coating was considered to be damaged if a liquid droplet was pinned onto the surface (i.e. sliding angle as 90o). In comparison, if a liquid droplet was highly mobile on the thermally repaired X-SLIPS without leaving any residues, the coating is considered to be fully recovered. To characterize the surface, we first damaged the intermediate chemical layer of X-SLIPS using oxygen plasma for 3 min. A plasma cleaner from Harrick Plasma was used for oxygen plasma treatment in all of the experiments. The pressure in the plasma chamber is ~700 mTorr, and the maximum power is 30 W. The frequency is in the 10 MHz range, and the maximum voltage is 750 V. After the surface damage, we lubricated the surface with Krytox 101, and then measured the sliding angle of a liquid droplet (i.e., 15 µL of octane) using a tilting stage (Fowler adjustable v-block set). To repair the chemical layer, we heated up the substrate at elevated temperature (e.g., from 60 ℃ to 200 ℃) for 5 to 15 min, and repeated the aforementioned sliding angle measurement. A 20 µL octane droplet was used for measuring the sliding angle on a tilting stage. The thermally repaired surface with lubricant has a very small sliding angle of ~10°; while the surface after damage has a 90° sliding angle. 2.5 Physical Abrasion Test. The abrasion test was performed on lubricated (Krytox 100) and non-lubricated aluminum surfaces, which were boehmitized and silanized. Each abrasion cycle involved dragging a sandpaper (solid fraction of ~0.5) of average particle size of 470 µm along with a pressure of 15.6 kPa across the surface with a velocity of ~0.02 m/s. After each 5 cycles,


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we measured the sliding angle of a 20 µL droplet with our tilting stage. The liquid droplets used for the tests were decane for lubricated surfaces and deionized water for the non-lubricated surfaces. 3. Results and Discussion 3.1 Thermal-Repairing Characterizations 3.1.1 Surface Chemistry Analysis To characterize the thermal self-repairing process, we chemically damaged the silane layer on textured titanium surfaces using oxygen plasma prior to lubrication. Applying oxygen plasma across the whole surface (surface area: 4 cm2) critically damages the silane layer, as is evident from the dramatic change in static contact angle (Figure 2a and Figure S2). This is further confirmed by the X-ray photoelectron spectroscopy (XPS) analysis (Figure 2b), which showed that the atomic percentage of the fluorine and the carbon element drop significantly, indicating that a significant portion of the silane coating (C-F bond) has been destroyed. The superhydrophilic nature of the damaged sample is attributed to the generation of hydrophilic SiO-chain network at the silane surface layer, produced during a chemical reaction between the silane and oxygen plasma.29 The undamaged perfluorinated silane underneath the network of SiO chains could serve as a source to replenish the perfluorinated functional groups at elevated temperatures. To demonstrate that the excess silane residues could facilitate functional layer repairing, we heated the substrate at 150 ℃ for 5 min. After this heating process we observed that the contact angle returned to more than 150° (Figure 2a), indicating the regeneration of the surface


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hydrophobic property. Furthermore, the fluorine and the carbon concentration increased and returned to the initial level after the heating process. Specifically, XPS analysis demonstrated that the atomic percentage of fluorine returned to 54.4% after the heating process. If this increase in fluorine and carbon is a result of fluorinated silane re-functionalizing the damaged regions of the substrate, we would expect that it will cause a decrease in non-functionalized substrate (here, titanium). We observed that the change in atomic percentage of titanium is converse to the change of fluorine and carbon, confirming that the fluorine reattached to the damaged substrate areas during the thermal process. The absence of titanium after self-repairing suggests that the silane has become uniformly distributed again on the textured titanium surface with a thickness larger than 10 nm–the penetration depth of the X-ray in the X-ray photoelectron spectroscopy. This observation indicates recovery of perfluorinated silane on the titanium surface. The repaired silane layer is fully functional as the perfluorinated lubricant can stably wet the repaired layer without being displaced by external fluids, such as octane (surface tension ~21.6 mN/m). 3.1.2 Self-Repairing Repeatability To explore the thermal self-repairing repeatability, we firstly investigated the healing repeatability at a given temperature (Figure 3a, b). For example, the titanium pieces can last for at least seven damage-healing cycles with a healing temperature of 200 ℃. To further explore the maximum number of damage-healing cycles on silanized titanium surfaces at a given temperature, we repeated the damage-and-healing cycles, and counted the number of successful healing until the samples failed to repair after 2 hours of continuous heating. At a healing temperature of 150 ℃ and 180 ℃, the coating can fully repair for 18±1 and 16±2 times, respectively. When the applied temperature is reduced to 120 ℃, the number of re-healing cycles


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is 25±2. The maximum number of self-healing cycles reduces as the applied temperature increases. This reduction is attributed to the vaporization of the silane molecules such that some of these molecules do not redeposit onto the substrate (see Figure S3a). In addition, the maximum number of thermal healing is linearly dependent on the average silane layer thickness (Figure S3b). Based on the linear fitting in Figure S3b, we have shown that the silane layer was removed by approximately ~0.28 µm per damage-healing cycle at a healing temperature of 200 ℃.

3.1.3 Self-Repairing Mechanisms We investigated the relationship between the repairing temperature and the time for the complete healing of the chemical functionalized layer (see Figure 3c) to explore the self-healing mechanism. To investigate this relationship, we first treated the silanized titanium surface with oxygen plasma (3 min) to damage the surface functional group, and then heated the substrate at different temperatures for specific time intervals until the surface hydrophobicity was recovered. Thermal-healing time is highly dependent on the heating temperature. For example, at temperature higher than 150 ℃, it takes less than 5 min for complete healing; whereas the healing would take ~300 min at temperature of 60 ℃. The temperature dependence of healing time follows the Arrhenius equation (Figure 3c), which can be expressed as,

ln(k ) =

− Ea 1 + ln( A) R T

(Eq. 1)

where k is the self-healing rate at the absolute temperature T (in kelvins) – here we consider the reciprocal of time (s-1) to be the self-healing rate, A is the pre-exponential factor, Ea is the activation energy, and R is the universal gas constant (8.31 kJ/kmol·T). This equation allows us


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not only to predict the time required for the self-healing to occur at a specific temperature, but also to understand the thermal-healing mechanism. The activation energy of self-healing (Ea = 53.3 kJ/mol) is in line with the vaporization enthalpy (of the perfluorinated silane on the surface (i.e., on the order of 50 – 65 kJ/mol),30 which suggests that vaporization of the silane molecules is another important driving force for the thermal-healing process in addition to the thermal molecular migration.31-39 We further confirmed the vapor phase transfer of silane experimentally, and showed that the process can occur for temperatures from 200 ℃ down to 60 ℃. (see Figure S4) 3.2 Robustness Characterizations One of the primary silanes used in the study was (heptadecafluoro-1,1,2,2tetrahydrodecyl)trichlorosilane which forms longer carbon chains28 under the presence of ethanol and water. These long-carbon-chain silanes have a higher molecular weight, which leads to a much higher viscosity and appear to be in a solid form. Also, the fluorine-fluorine interactions40 between the silane molecules help to keep these silane molecules together. Therefore, the excess silane could stably stay on the surface. To investigate the robustness of the coating, we firstly studied the coating adhesion of the silanized textured surfaces as compared to that of a commercially available spray-coated superhydrophobic surface. With adhesion experiment (see Figure S5), we demonstrated that the silane layer has a strong adhesion with the base textured substrate. In addition, we have rinsed the silanized surfaces with organic solvents to test the silane coating longevity. Specifically, we rinsed a silanized titanium surface with acetone for 100 times, and measured the weight of the sample before and after the rinse. From our experimental measurements, the sample losses ~2.79×10-9 g/ mm2 per rinse (i.e., a weight loss of 0.0019 g


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over a surface area of ~6800 mm2 after 100 times rinse). The estimated thickness of the silane layer is ~2.4 µm, which is equivalent to 3.33×10-6 g/mm2 of silane. Therefore, it takes ~1200 times of rinse by acetone to remove all the excess silane on the surface. The excess silane can be washed away by organic liquids (e.g. acetone), but the removal rate is so small that it will take many rinses in order to completely remove the excess silane layer from the surface. Furthermore, we investigated the scratch resistance of the X-SLIPS coating. We used a sandpaper to create mechanical abrasions on a lubricated and non-lubricated nano-textured aluminum substrate (see Experimental Section). Specifically, the lubricated coatings can survive over 200 physical damage cycles than the non-lubricated coatings (~ 5 cycles) due to the greatly reduced friction (Figure 4a). In addition, with harder materials, for example titanium, the surface textures have higher strength and therefore higher scratch resistance. By heating at 150 ℃ for 10 min, the X-SLIPS on aluminum substrate restore its liquid repellency shown in Figure 4a. Note that the thermal-repairing process can occur with or without the presence of lubricant. However, the process for lubricated aluminum surfaces generally takes longer time than those of nonlubricated surfaces because the silane is limited to thermal migration (i.e., the lubricant layer protects the silane from vaporizing). Unlike oxygen plasma damage where the silane layer is uniformly reacted and removed across the whole substrate, mechanical abrasion removes the silane by destroying the surface texture of the substrate. The undamaged silane coatings present near the mechanically damaged areas then serve as the healing source to replenish the silane coating upon heating. We have further demonstrated that the physically damaged substrate can be thermally self-healed on different materials, such as metals, glass, ceramic and polyethylene (Figure 4b, Figure S6).


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3.3 Liquid Repellency of X-SLIPS on Industrial Materials Our coating method can be extended on most industrial materials, such as metals (stainless steel, aluminum, copper, titanium, etc.), glass, ceramics, and plastics (e.g., polyethylene) and a variety of surface geometries (Figure 5). In Figure 5b, commonly used industrial materials, stainless steel, aluminum, titanium, glass, copper, carbon steel, ceramic, and polyethylene, are coated with a perfluorinated silane and lubricated with perfluorinated oil (Dupont Krytox®). These surfaces can repel aqueous fluids, hydrocarbons and other organic fluids with a broad range of surface tensions ranging from ~21.6 mN/m (octane) to ~72.8 mN/m (water). (see Figure 5, Figure S7, and Supporting Videos 2 and 3). The contact angle hysteresis of these liquids on the lubricated surfaces is typically less than 5 degrees (see Experimental Section, Figure 5a and Table S2), which indicates that X-SLIPS coatings on various substrates are super slippery. In supporting videos 4 and 5, a coated ceramic can repel viscous complex liquids, such as ketchup (aqueous-based) and mustard (oil-based). We have shown that all of these coated materials can be thermally healed under large area chemical or physical damage. 4. Summary Drawing inspirations from the wax repair of plant leaves and the slippery surfaces of the Nepenthes pitcher plants, we have developed a cross-species bioinspired slippery coating that can perform self-repair function upon thermal stimulation. We stress the importance of healing the intermediate chemical functional layer as it is crucial for the proper function and longevity of the slippery surfaces. The resulting surfaces can be applied onto a range of metals, glass, and plastics with different surface geometries, and can self-repair even with critical large area physical damage. Since X-SLIPS can be customized depending on the application requirements,


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one can choose bio-compatiable or environmental friendly lubricants for specific applications. The robustness and self-repairing nature of our surfaces will provide a robust solution that could address the “sticky” problems in a number of industrial, medical and daily life applications.


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Figure 1.Cross-species bio-inspired slippery coating: X-SLIPS. a. Optical images showing a plant leaf and a Nepenthes pitcher plant. b. Schematic showing the concept of slippery coating fabrication process inspired by the slippery rim of the Nepenthes pitcher plant. c. Schematic showing the concept of self-repairing inspired by the wax repair of plant leaves. The excess silane layer (hydrophobic wax) on a substrate serves a source to repair the surface hydrophobicity, and to prevent the lubricant from being displaced by the foreign liquids. Insets showing the liquid repellency comparison of octane droplets on lubricated substrates with undamaged (left) and damaged (right) silane coating.


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Figure 2. Mechanism of thermal self-repairing. a. The contact angle change before and after oxygen plasma damage, as well as after the thermal self-repairing. Before the plasma treatment, the advancing angle is 133.6±1.0°, and the receding angle is 63.5±0.5°. After plasma treatment, the surface becomes superhydrophilic with a contact angle less than 10°. b. XPS data of silanized surface before (red) and after oxygen plasma (green), as well as after thermal treatment (blue). The substrate is titanium, and the silane is perfluorinated silane. The atomic percentage of fluorine, titanium, and carbon at each condition is shown in the inset chart, and is the average of three different measuring areas. XPS measurements detected atoms within the depth of 10 nm within a detection area of 1 mm in diameter on the surface. We repeated these measurements on three different locations on the titanium surface, and on three different titanium samples, yielding


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9 independent measurements. c. Schematics of self-repairing mechanism, which involves the vaporization and thermal migration of the silane molecules.


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Figure 3. Robustness and thermal self-healing performance on the chemical intermediate layer of X-SLIPS. a. Optical images showing the water repellency comparison between a damaged and a self-healed chemical intermediate layer on titanium. b. A plot showing the thermal self-healing repeatability of the intermediate layer of X-SLIPS. The intact or self-repaired surface with lubricant has a very small sliding angle of around 10°; while the surface after damage has a 90° sliding angle. c. A plot showing the dependence of thermal-healing time on applied temperature. Damaged intermediate layer can be healed around 280 min at 60 ℃, while less 1 min at more than 180 ℃. The semi-log plot shows that the self-healing rate and the heating temperature fits the Arrhenius relationship. In the inset, t is the thermal healing time in second, and T is heating temperature in Kelvin. The sample for this experiment is silanized titanium. Error bars indicate standard deviations for three independent measurements.


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Figure 4. Self-repair of X-SLIPS coating. a. A plot showing the physical abrasion performance comparison between lubricated and non-lubricated samples. The lubricated samples can be thermally repaired with or without a lubricant under the temperature of 150 ℃. The insets demonstrate the self-repairing of X-SLIPS. b. Optical images showing the thermal repair of XSLIPS on glass, polyethylene, and ceramic. The coating was damaged by physical abrasion of sandpaper. The colored droplets are dyed octane in all optical images.


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Figure 5. Liquid repellency of X-SLIPS coating on various materials. a. A plot showing the contact angle hysteresis of various liquids on the X-SLIPS surfaces. Inset shows a dodecane droplet sliding with a very small sliding angle (less than 3°) on a coated carbon steel substrate, indicating the super slippery nature of the surfaces. Error bars represent the standard deviations for at least four independent measurements (see Table S2 for the detailed wetting characterizations data). b. The coating method can be applied to various substrates such as stainless steel, aluminum, titanium, glass, copper, carbon steel, ceramic, and polyethylene. The colored droplets are 10 µL dyed octane. c. Comparison of untreated and X-SLIPS-coated stainless steels of different surface geometries (flat plate, pipe section, and sphere) for the repellency of dyed octane (in yellow).


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ASSOCIATED CONTENT Supporting Information. Supplementary videos on the thermal healing and liquid repellency performances of X-SLIPS, as well as supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author To whom correspondence should be addressed. Electronic address: [email protected].

Conflict of Interest: A United States provisional patent has been filed for this work.

ACKNOWLEDGMENT We thank Vincent Bojan for the help with XPS measurement and data processing, and Birgitt Boschitsch Stogin for the help with manuscript preparation. We acknowledge funding support by the Office of Naval Research MURI Award#: N00014-12-1-0875 (materials fabrication), Advanced Research Projects Agency-Energy Award#: DE-AR0000326 (energy efficient coatings), the National Science Foundation CAREER Award#: 1351462 (cross-species materials & wetting science), and PPG Industries Foundation. Part of the work was conducted at the Penn State node of the NSF-funded National Nanotechnology of Infrastructure Network.


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Inspired by the wax-repair of plant leaves and the slippery surfaces of the Nepenthes pitcher plants, we have developed robust cross-species omniphobic coatings (X-SLIPS) that can selfrepair through thermal stimulation under large area physical and chemical damage. Our coatings can be applied onto a broad range of materials of various shapes, and show excellent repellency towards aqueous and organic liquids.


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Bioinspired Omniphobic Coatings with a Thermal ... - ACS Publications

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