Sustainable Repellent Coatings Based on Renewable Drying and Nondrying Oils

Contamination of surfaces can cause loss of performance in a variety of applications. Bioinspired coatings based on the lotus or pitcher plants provide surface topographies that create superhydrophobic or slippery features with self‐cleaning properties. However, typical fabrication procedures often involve potentially toxic chemicals, perfluorinated compounds, nondegradable polymers, and energy‐intensive methods, with negative consequences for the environment. Here, a sustainable coating process based on renewable materials to prepare superhydrophobic and liquid‐infused coatings with minimal environmental impact is presented. A scalable spray coating protocol is used. Synthetic liquid and polymeric materials are substituted with natural drying oils, i.e., oils that react with ambient oxygen and cure to solid materials, as polymeric binder in which silica particles are partially embedded. The self‐cleaning characteristics against aqueous contaminations are investigated as a function of the drying oil used as binder. The assessment of the mechanical stability reveals the advantage of an underlying “primer layer” of the pure oil. Furthermore, it is demonstrated that oils from renewable sources can act as lubricants for the creation of slippery surfaces. The efficiency of such sustainable slippery coatings in reducing concrete adhesion points toward their applicability in real world scenarios.

chemicals were found to accumulate in blood, kidneys, and liver and show long half-life times in humans. [47][48][49][50] In addition, binder materials are often composed of nondegradable, synthetic polymers [51][52][53] and fabrication processes can involve energy intensive treatments and annealing steps. [53][54][55][56] Efforts to produce more environmentally benign coatings are made [57] by, for example, using waterborne methods that can be conducted at ambient conditions to chemically modify materials, [29] employing natural lubricants from renewable resources, [58,59] or utilizing biodegradable and recyclable matrices as the basis. [60] However, there is still the need for methods that combine the scalable fabrication, efficient performance and sustainability of both, the process, and the coating.
We recently developed a scalable fabrication method aimed at a reduced environmental impact. [61] Our method is based on aqueous dispersions of a nontoxic polymeric binder and commercially available, hydrophobic fumed silica, which are processed from aqueous dispersions by spray coating at room temperature. Upon drying, the coating self-organizes into a hierarchical surface structure that shows superhydrophobic behavior without the need of any further surface functionalization. Additionally, the textured surface can be converted to a SLIPS coating by infiltration of a lubricant in a second spray coating step. Despite the environmentally friendly process conditions, this approach relies on synthetic polymethacrylates as binder and silicone oil as lubricant.
Here, we aim to provide a coating with improved sustainability by replacing all synthetic materials with renewable and abundant resources. Natural drying oils are promising candidates as substitutes for the synthetic polymeric binder. Such oils are characterized by a high content of (poly-)unsaturated fatty acids and can cure into a solidified network at ambient conditions in an oxidative polymerization process called autoxidation. [62,63] Natural nondrying oils, in contrast, provide a renewable alternative as lubricant to the commonly used synthetic silicone oil. Together, these replacements enable the formation of superhydrophobic and liquid-infused repellent coatings based on sustainable materials, providing a versatile coating system to be used with small environmental impact.

Preparation of the Coating System Based on Drying Oils
Our coating system employs commercially available hydrophobic fumed silica particles to create the required combination of topography and low surface energy needed to form Cassie-Baxter wetting states with water. [61] A polymeric binder is necessary to provide stable adhesion of these silica particles to the surface. In addition, this binder must be hydrophobic to ensure compatibility with the hydrophobic particles but should exhibit a higher surface energy than the silica particles (γ = 15.5 mJ m −2 ) [64] to promote the formation of the hierarchical micro-and nanostructures where the silica particles protrude out of the polymer film. [61] For our sustainable coating, we use natural drying oils as the binder component. In general, a higher degree of unsaturated bonds and the presence of conjugated bonds reduces the curing time. [63,65] Nevertheless, autoxidation at ambient conditions is a rather slow process taking several days. [62,66,67] In general, there are three possibilities to shorten the curing time of drying oils: chemical catalysis by addition of drying agents (i.e., siccatives), [63] physically increasing the reaction rate using elevated temperatures or UV light, [62] or pre-polymerizing the oil. [68] Most commercially available siccatives contain cobalt carboxylates, which are under toxicological investigation and suspected to be carcinogenic and thus not suitable for an environmentally friendly approach. [69] A natural alternative used as siccative is colophony, a solid resin obtained from coniferous trees. It can be combined with oils upon heating, leading to the formation of a chemically crosslinked hybrid compound. [70] The heat treatment additionally pre-polymerizes the oil molecules and hence leads to enhanced and faster crosslinking during the curing process. [70][71][72][73] Here, we focus on three drying oils differing in their molecular composition, degree of unsaturated bonds and hence their crosslinking abilities; namely tung oil (having a surface tension of γ = 33.1 mJ m −2 ), [74] linseed oil (γ = 31.3 mJ m −2 ), [74] and a mixture of linseed oil and colophony. The latter is well known to lower the surface energy of water [75] and increase the hydrophobicity of blends with polymers. [76] Therefore, we assume a decrease of the surface energy for our mixture compared to pure linseed oil. Tung oil has a high content of conjugated triple unsaturated α-eleostearic acids and therefore exhibits shorter curing times than linseed oil, which mainly consists of nonconjugated di-unsaturated linoleic and triple unsaturated linolenic fatty acids. [63] Due to the long curing times, up to several days, at ambient conditions, we chose to accelerate the curing by an UV light treatment with a wavelength of λ = 365 nm (curing times of the different oils used as binders at ambient conditions and under UV light can be found in Table S1, Supporting Information). However, we note that the curing at ambient conditions is generally also possible to further reduce energy consumption in the coating process. Figure 1 schematically illustrates the coating process. First, an aqueous dispersion containing emulsion droplets of binder and silica particles is formed (Figure 1a). To obtain the desired surface structure with a rough topography formed by the protruding silica particles, we tested weight ratios between 1:1 and to 1:2 of binder and silica particles. Lower silica content prevents the formation of a surface topography while higher contents compromise the adhesion to the substrate. [61] To disperse the silica particles in the binder, i.e., the drying oil, in the required high concentrations, the addition of an auxiliary solvent was needed. We chose tert-butyl acetate, an environmentally benign [77,78] solvent that shows a similar evaporation behavior as the continuous phase water. Therefore, the homogenous drying and distribution of the surface coating and the formation of a porous network created by the silica particles and the polymeric binder at ambient conditions is promoted. It is important to ensure there is no solvent or water left before curing the drying oils to produce a sufficiently crosslinked polymeric network. [79] The formed aqueous emulsion is subsequently spray-coated in an automated setup, providing a scalable process to homogeneously coat large area substrates (Figure 1b). After the deposition onto glass substrates in one or www.advmatinterfaces.de multiple spray coating steps, the auxiliary solvent and the continuous water phase evaporate, producing the targeted surface morphology. The UV irradiation enhances the curing of the natural drying oil, thus consolidating the structure to form a SHS. In an additional spray coating step, the structures can be infiltrated with a lubricant (i.e., nondrying oils) and SLIPSs are generated.

Preparation of Superhydrophobic Surfaces
We first evaluated the formation and properties of SHSs prepared with the three natural drying oils as binder materials, using different mixtures of silica and drying oil ( Figure S1, Supporting Information). The best results were formed with an oil to silica ratio of 1:2 ( Figure 2).
The spray coating process ( Figure 2a) formed a homogeneous surface coating on the glass substrate ( Figure 2b). We used water contact angle (CA) and contact angle hysteresis (CAH) measurements to assess the repellency properties of the formed coatings as a function of the applied oil and the number of spray cycles. High CAs around 150° formed immediately upon the first spray coating cycle, regardless of the used oil ( Figure 2c). The low standard deviations of the CAs indicate a uniform hydrophobic behavior across the entire sample. Coatings with linseed oil and tung oil also showed low CAHs below 5° directly from the first spray cycle, indicative of superhydrophobic properties. The coatings prepared Figure 1. Schematic illustration of the sustainable fabrication process of SHSs and SLIPSs. a) Preparation of the dispersion-based coating system with water as the continuous phase, containing binder, i.e., drying oils, structuring components, i.e., hydrophobic fumed silica, and an environmentally benign auxiliary solvent, i.e., tert-butyl acetate. First, the drying oil is dissolved in the solvent. Afterward, fumed silica particles are added, and the mixture is dispersed in water. b) Formation of the coating by spray coating. After the spray coating step, water and solvent evaporate, and the coating is cured by an UV-light treatment to form SHSs. An additional spraying step with a lubricant, i.e., silicone oil or nondrying oils, prepares SLIPSs.

Figure 2.
Fabrication of SHSs. a) Schematic illustration of the SHS deposited via spray coating. b) Exemplary macroscopic image of a coating derived from a system containing tung oil as polymeric binder and an oil to silica ratio of 1:2 with 3 spray cycles, showing that the surface of the glass substrate is homogenously covered by the coating. c) Wetting properties of the coatings based on the different drying oils as polymeric binder, exemplary shown for an oil to silica ratio of 1:2.

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with the linseed oil-colophony mixture showed the desired SHS properties with low CAH only after the third spray cycle. Noteworthily, after the first and second spray cycle the coatings already exhibited very high CAs but still retained a large CAH, pointing to the presence of defects in the coatings (as shown in Figure S2, Supporting Information). Such "sticky hydrophobic surfaces" resemble the natural example of the rose petal [80] and are less efficient for self-cleaning but can be used for example as bio scaffolds promoting cell proliferation and increasing biocompatibility. [81] In general, concerning the SHSs based on the lotus effect, the lowest CAH values could be achieved by conducting three or four spray cycles for all systems. Further, we analyzed the morphology of the coatings in detail. Scanning electron microscopy (SEM) images (top views and cross section images) as well as atomic force microscopy (AFM) height images (Figure 3a-c) revealed the rough and porous surface structures and confirm the presence of a hierarchical texture, enabling Cassie-Baxter wetting and therefore efficient water-repellency. Nevertheless, as stated above a hierarchical topography does not always enable Cassie-Baxter wetting, which should only be attributed to coatings exhibiting low CAHs. As Figure 3d shows, the layer thickness linearly depended on the number of spray cycles. This linear trend derived from SEM cross section measurements was further confirmed with profilometry measurements. Hence, an increasing number of spray cycles leads to more uniformly coated areas without exposed substrate areas and thus less potential pinning points, explaining the reduced CAH values. Note, however, that also within these more uniform coatings the required hierarchical topography consisting Figure 3. Microscopic analysis of the coatings. a) Representative optical microscope and SEM images of the coatings using tung oil as a natural binder, after one, three, and six spray cycles. All scale bars correspond to 50 µm or 1 µm, respectively. b) Exemplary AFM height profile showing the rough surface structure. c) Representative side view SEM image of a tung oil containing coating derived in three spray cycles showing the hierarchical topography and porous structure of the coating. d) Thickness of the prepared coatings as a function of the number of spray cycles measured for the entire range for the tung oil coating and for three spray cycles for the linseed oil and linseed oil colophony mixture coatings.

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of micro-and nanoscale features prevails, leading to a stable Cassie-Baxter wetting state characterized by low CAH. On the other hand, larger numbers of spray cycles can result in cracks (Figure 3a), which exposes the underlying substrate and may thus serve as potential pinning points. The representative sideview image in Figure 3c reveals even more clearly the just described morphology and porosity of a coating.
The microscopic analysis also revealed why coatings formed with the mixture of linseed oil and colophony required more cycles to form SHSs. Due to the pre-polymerization of the oil mixture and the increased viscosity, the silica particles cannot be uniformly dispersed and form larger particle agglomerates which are deposited on the surface ( Figure S2, Supporting Information). Therefore, the coatings exhibited less uniform areas with insufficient coverage coexisting with regions of large amounts of solid material. Based on these results and their reproducible nature ( Figure S3, Supporting Information), we chose a number of three spray cycles for all further experiments.

Stability of the Coatings
The nanostructures enabling superhydrophobicity are rather fragile and can be easily damaged, leading to a compromised performance. We investigated the mechanical properties of the coatings formed with the different drying oils using two stability tests, a scotch tape test to evaluate surface adhesion and a linear abrasion test to assess the overall integrity of the coatings (Figures 4 and 5). Additionally, we evaluated the influence of a tung oil primer layer, i.e., an underlying layer of pure tung oil that was not fully cured upon application of the coating, which we assumed would promote the substrate adhesion of the formed coating.
Over three consecutive tape tests the CAs decreased for all samples while the CAH increased (Figure 4a,b). All tested samples retained their repellent properties after the first tape test. However, after the second test, low CAH only persisted for the sample with a primer layer, indicating enhanced stability compared to the other coatings. The CAs of the linseed oil coating showed the strongest decrease. The SHSs with the linseed oil-colophony mixture and with a tung oil primer layer retained comparably high CA values above 120° for all tape test iterations. Figure 4c shows representative optical microscopy images of the tung-oil containing coatings with and without the tung oil primer layer. After three consecutive tape tests, the coating without primer layer was mostly removed, while the coating with a primer layer remained largely intact. These microscopic images further underline the enhanced stability of the coating containing a tung oil primer layer. Nevertheless, even this optimized coating adheres less well to the substrate compared to a similar coating formed with poly(butyl methacrylate) (pBMA) as a synthetic binder material (which maintained repellence up to three tape tests). [61] The results of the linear abrasion test ( Figure 5) corroborate the results of the tape peel test. While all samples can still be classified as superhydrophobic after the first abrasion cycle (showing large CAs and low CAHs), the CAHs and their standard deviation increase significantly with further abrasion cycles. Again, the coating with linseed oil shows the weakest performance while the coating with the linseed oilcolophony mixture and the tung oil one with a primer layer still show comparably high CAs and low CAHs after five abrasion cycles.
From the results of the mechanical tests, we conclude that colophony used as a natural siccative as well as the introduction of a primer layer efficiently enhances the substrate adhesion and overall integrity of the fabricated coatings. Generally, the tung oil-based coating with a primer layer seems superior to the linseed oil-colophony mixture as the tung oil-based coating shows a better reproducibility ( Figure S3, Supporting Information) and a significantly shorter curing time (Table S1, Supporting Information). In addition, the necessary heat treatment to produce the linseed oil/colophony blend and the longer UV exposure upon curing increases energy consumption, further disfavoring this binder. In the direct comparison of microscopic images of the tung oil coating with and without primer layer during the stability tests (Figures 4c and 5c) we see that significantly more material remains on the surface of coatings with a primer layer. We assume that the superior mechanical stability is due to the silica particles being embedded in the not yet cured oil layer during deposition. Furthermore, the attachment Figure 4. Tape peel test with the different drying oil binders. a) Mean CAs and b) mean CAHs before and after one, two, or three tape tests. c) Representative microscopic images of the tung oil-containing coating with and without primer layer before stability testing and after three tape tests. All scale bars correspond to 50 µm. Note that the darker spots in the images are the dried oil droplets from the primer layer.

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and presumably crosslinking of the oil contained in the coating system to the primer oil layer is facilitated, enhancing substrate adhesion.

Preparation of Slippery Liquid-Infused Porous Surfaces
Taking advantage of the porous topography of the SHSs, we produced SLIPSs coatings by infusion with a fluid lubricant in a second spray coating step (Figure 6a). We investigated the repellency properties for different natural nondrying oils used as sustainable alternatives to synthetic lubricants and compare their performance to the synthetic, state of the art lubricant silicone oil. All investigated SHSs could be successfully infiltrated and produced fully transparent SLIPS coatings (Figure 6b). The time-lapse images of Figure 6c, taken from Movie S1 (Supporting Information) show a 30 µL water droplet sliding down a SLIPS coating prepared with sunflower oil as lubricant. We  www.advmatinterfaces.de characterized the coatings by means of CA and CAH measurements ( Figure 6d). All prepared coatings exhibited low CAHs below 10°. The higher CAs observed for the reference SLIPS coating with synthetic silicone oil are due to the lower surface energy of silicone oil [82] (γ = 18.9 mJ m −2 ) compared to the natural oils [83] (γ ≈ 33 − 35 mJ m −2 ). However, the coconut and olive oil mixture bares the risk of solidifying at lower temperatures due to the melting point of virgin coconut oil of ≈25 °C, [84] making it less suitable for outdoor applications. Comparing the nondrying oils in terms of their hydrophobicity, sunflower oil-infiltrated coatings show the highest water CA (Figure 6d). Thus, we hypothesize that sunflower oil can be retained in the porous structure most efficiently and should show the best long-term stability when subjected to impinging water droplets. Hence, we chose silicone oil and sunflower oil as natural alternative for further experiments.

Accelerated Aging Test
In addition to the mechanical properties, the long-term stability over time and upon exposure to environmental conditions is important for any real-life applications. Following a cyclical climate change test specification PV 1200 by Volkswagen AG, [85] we exposed the SLIPS coatings infiltrated with silicone and sunflower oil based on tung oil with an oil primer layer to rapidly changing environmental conditions (Figure 7). These conditions mimic an accelerated aging in an outdoor environment, with each cycle simulating summer (+80 °C) and winter periods (−40 °C). The SLIPSs infiltrated with silicone oil remained slippery even after six aging cycles. The CAHs increased slightly over time due to depletion of the lubricant but sliding angles of a 10 µL water droplet below 10° persisted, confirming the retained repellent properties. The coatings infused with sunflower oil remained slippery only during the first two aging cycles. While this value indicates substantial stability, it is much lower compared to the synthetic silicone oil-based analogue. We attribute this difference to the reactivity of the natural oils. Similar to the drying oils, sunflower oil can also undergo a thermal oxidation process due to the double bonds present in the molecular structure, albeit at a much lower rate. [86,87] In the course of the oxidation process, the oil becomes sticky and solidifies over time. Thus, the fluid nature of the coating is lost, causing pinning of water droplets. However, it should be considered that the accelerated aging process uses prolonged and reoccurring heating phases with temperatures up to 80 °C, which promote this thermal degradation. In everyday applications, such temperatures are unlikely to occur. This may therefore lead to a better performance compared to the prediction by the accelerated aging test. Compared to a similar coating system based on pBMA infiltrated with silicone oil, the long-term stability of both investigated renewable SLIPS systems is reduced by four or eight aging cycles. [61]

Application as Antiadhesion Coating
To evaluate the applicability of the produced coatings in selfcleaning applications, we performed a cement adhesion study. Cement soiling of shoe soles leads to risks for employees at construction sites. Additionally, cement attacks the used materials leading to brittleness. Therefore, the equipment in contact with the aggressive cement needs to be replaced regularly. Preventing the adhesion on rubber materials would reduce material consumption but is challenging due to the rubber material of shoe soles and the strong adhesion and complex drying mechanisms of cement. We measured the adhesion strength of cured cement on glass slides coated with the different repellent coatings (Figure 8a). The presence of an SHS coating reduced cement adhesion from 25.3 ± 3.7 to 9.7 ± 1.7 kPa. On SLIPSs, however, only negligible adhesion within the measurement error was found. While cement seemingly, or at least partially, penetrates the structures to adhere to the surface, the presence of the fluid lubricant efficiently prevents contact of the cement to the underlying surface.
Additionally, we investigated the cement adhesion and removal from shoe soles. We coated shoe soles with a 1:2 tung oil coating infiltrated with either silicone oil or sunflower oil and covered the entire sole with cement. Upon hitting the coated soles against an uncoated reference, we qualitatively monitored the amount of cement being removed after each hit. Figure 8b shows time-lapsed images taken from Movies S2 and S3, which can be found in the Supporting Information. The coating infiltrated with silicone oil was free of cement after only three hits while a significant amount of cement remained on the uncoated reference. For the shoe sole infiltrated with sunflower oil a large amount of cement was removed after three hits. After five hits the shoe sole was essentially free of cement, whereas the reference remained covered in large areas. We assume that the differences between silicone oil and sunflower oil infiltrated SLIPSs is due to small defects in the coating where the sunflower oil partly de-wetted. Nevertheless, these results highlight not only the performance of our coating system against complex contaminations but also its versatility since the shoe soles exhibit a complex surface geometry and a rubber material that is difficult to coat with many conventional processes. Furthermore, the efficient removal of cement also indicates stability against high pH value (cement during curing has a pH > 12). The reduced adhesion of the cured cement indicates that the natural oils used as lubricant are not decomposed by the basic environment. The coatings were also able to repel highly acidic liquids (HCl, 1 n, pH = 0) upon short contacts and could Figure 7. Repellency performance upon simulated aging according to test specification PV 1200 by the Volkswagen AG, [85] carried out for SLIPSs based on a tung oil coating with primer layer using silicone oil as a synthetic lubricant and sunflower oil as the renewable alternative.

Conclusion
We developed environmentally friendly water-repellent coatings based on renewable materials, which are fabricated in a simple and scalable spray deposition process. We successfully implemented drying oils, i.e., oils that cure to solid polymeric networks in an oxidation process with ambient oxygen, as a polymeric binder component to replace previously used synthetic poly methacrylates. Surface coatings with hierarchical topo graphy at the micro-and nanoscale with superhydrophobic properties were formed from the dispersions containing the drying oil and fumed silica particles.
The infiltration with silicone oil efficiently converted the SHSs into SLIPSs. We identified sunflower oil as a renewable alternative for the synthetic lubricant. In all cases, SHSs and SLIPSs with comparable water-repellency were obtained with our sustainable approach. Nevertheless, the mechanical robustness and long-term stability were more pronounced for the synthetic materials.
By replacing all involved materials with renewable alternatives, our approach provides an important step in the realization of fully sustainable coatings. Cement adhesion tests underline that sustainable coatings can provide efficient performance in the repellency of adhesive contaminations on complex materials and topographies, opening pathways toward the replacement of synthetic materials with renewable alternatives in the design of self-cleaning surfaces.

Experimental Section
All chemicals were purchased from commercial suppliers and used as received without further purification if not stated differently. Experiments were performed at ambient conditions unless stated otherwise.
Oils and Mixture: Tung oil (100% pure tung oil, Uulki) and linseed oil (virgin linseed oil, cold pressed, Carl Roth) were used as purchased. The mixture of linseed oil and colophony (gum rosin, natural resin, Sigma-Aldrich) was prepared following a publication of Tirat, et al. [72] for a compound with 20wt% colophony.
Coating System: The coating systems with linseed oil, tung oil, and the linseed oil compound with 20wt% colophony serving as polymeric binder, were prepared adapting a protocol by Walter et al. [61] 6 g TBAc (tert-butyl acetate, ≥99%, Sigma-Aldrich) was added to the respective amount of oil (0.30 g, 0.24 g, or 0.20 g, depending on the oil:silica ratio) and the mixture was gently stirred (150 rpm) for a few minutes. Subsequently, the desired amount of silica particles (0.20 g, 0.36 g, or 0.40 g, for the 1:1, 1:1.5, and 1:2 oil:silica mixture, respectively) was added, and the mixture forming the hydrophobic phase was stirred again at slow speed (75 rpm) until the particles were dispersed homogeneously. Next, 24 g of a 3.2 g L −1 sodium dodecyl sulfate (SDS, ≥99%, Carl Roth) solution was added to the as-prepared hydrophobic phase. The mixture was ultrasonicated six times for 30 s with a pause of 10 s at an amplitude of 90% under ice-cooling to form the aqueous dispersion used for the coating process.
Superhydrophobic Surfaces: To produce SHSs, the coating systems were deposited as prepared onto soda-lime glass substrates (microscope slides, Menzel-Gläser, Thermo Scientific) via spray coating with an airbrush (Evolution Airbrush, 0.4 mm nozzle, Harder & Steenbeck) in a custom-built automated setup. The glass substrates were cleaned prior to coating by immersing them consecutively in acetone, ethanol, and ultrapure water under sonication for 5 min each and activated by O 2 plasma treatment (Femto, Diener electric, 0.2 mbar, 4 sccm O 2 , 100 W) for 5 min. The number of spray cycles was varied with 30 s pause in-between for repeatedly sprayed samples. For any spray coating step the instrument was placed 15 cm from the sample. For samples with a tung oil primer layer one layer of pure tung oil was applied with the automated spray coating setup. The oil layer was cured under UV light (λ = 365 nm) for 1 h. Afterward, the coating system containing tung oil was applied in three spray cycles. Following the last spray cycle, all samples were put in a horizontal position to let the water and TBAc evaporate. Samples were cured under UV light directly after spray coating for at least the duration determined for pure oil films in preliminary studies (see Table S1, Supporting Information). Afterward, surfactants were removed by immersing the samples in ultrapure water in up to twelve washing steps.
Slippery Liquid-Infused Porous Surfaces: SHSs were prepared as stated above and infiltrated with silicone oil (silicone oil M10 low viscous, www.advmatinterfaces.de 10 cSt, Carl Roth), sunflower oil (100% sunflower oil, EDEKA), peanut oil (cooking oil, Mazola), olive oil (virgin olive oil, REWE), or a 1:1 mixture of olive oil and coconut oil (virgin coconut oil, REWE Bio) using the automated spray coating setup. Coatings derived from spraying the coating systems three times, as well as samples with the tung oil primer layer, were chosen for preparing SLIPSs. The nondrying oils were applied in three spray cycles.
Characterization: The static CA of a 5 µL water droplet was measured by sessile drop method (DSA100, Krüss) and analyzed using tangent 1 fitting with Drop Shape Analysis [DSA4] 2.0(a). For measuring the CAH, an initial water drop volume of 3 µL was increased by 5 µL at a constant dosing rate of 50 µL min −1 and then decreased at the same rate until the droplet was completely withdrawn from the surface by the syringe. Images were recorded at a rate of 5 fps. All plotted values are the mean of at least five independent measurements in different locations on a sample's surface.
The morphology of the fabricated samples was characterized by optical microscopy (Ergolux, Leitz equipped with a DCC3260C microscope camera from Thorlabs), AFM (NanoWizard 3, JPK), and SEM (GeminiSEM 500, ZEISS). AFM was operated in tapping mode with a scan rate of 0.2 Hz. The used soft cantilever (HQ:NSC18/AI BS, Mikromasch) had a resonance frequency of 75 kHz and a force constant of 2.8 N m −1 . The SEM images were taken using the SE2 detector, an aperture of 15 µm, and an acceleration voltage of 0.5 keV.
Layer thicknesses was determined by cross section SEM images at an angle of 90° using the same settings mentioned above and confirmed by means of profilometry (DektakXT controlled with Vision64, Bruker). The profilometer was operated in standard scan mode with a range of 6.5 µm, the stylus force was set to 3 mg, and a scan length of 4000 µm was chosen. The layer thickness was averaged over a representative area.
Stability Assessment: For the tape test Scotch tape (Scotch Magic Tape 810, 3m, synthetic acrylic adhesive, adhesion strength to steel 2.5 N cm −1 ) was applied to the SHS samples, carefully pressed onto the substrate to ensure no air is trapped and the tape is in contact with the coating, and then peeled off at a shallow angle. A total of three tape tests were performed on each sample.
In the linear abrasion test, samples were placed on a table and a lintfree tissue (precision wipes 7552, KIMTECH Science) with a constant weight of 30 g on top was pulled across the surface in one direction. Pulling the tissue over the surface once is considered one abrasion cycle.
The test of the long-term stability was carried out in a climate test chamber (VCL 7006, Haereus Vötsch) according to test specification PV 1200 from the Volkswagen AG. [80] A total of 6 test cycles were carried out. Each cycle simulates summer (+80 °C) and winter conditions (−40 °C). Three samples of the tested species were prepared and taken out after 2, 4, or 6 aging cycles, respectively.
Cement Adhesion: The cement (Universalzement, SAKRET) was mixed with water in a ratio of 3:1. The wet cement was filled in cylindrical molds with an inner diameter and height of 2 cm on the sample surface to keep the contact area constant. The cement was cured for 24 h. Subsequently the adhesion strength was measured by recording the force needed to laterally push the cement from the surface at a constant speed of 5 mm s −1 with a force gauge (digital, FK 500, Kern and Sohn GmbH). The adhesion force was averaged from testing three samples for each substrate. The SLIPSs and SHSs tested were based on the tung oil 1:2 coating system comprising a primer layer. Note that for the uncoated glass the experiment had to be repeated several times until the cement could be removed and the shown values are those measured in the first attempt. Mean adhesion strengths were recorded in multiple test repetition.
Furthermore, shoe soles kindly provided by the Uvex Safety Group were coated with the tung oil 1:2 coating system and were infiltrated with either silicone or sunflower oil. Beforehand the soles were cleaned with ethanol and ultrapure water respectively to remove any dust residues. The cement was directly applied to the coated soles and the soles were covered completely. The cement was left to cure for 24 h. The shoe soles were hit together with an uncoated reference and images were taken after each hit.
Acid Resistance: The repellence of SLIPS coatings against hydrochloric acid (HCl, 1 N, Carl Roth) was evaluated by dropping 30 µL droplets on the samples and checking the slipperiness. Additionally, the coatings were submersed into 1 n HCl for a prolonged time and evaluated after 48 and 96 h.
Statistical Analysis: All shown data were used without any preprocessing. The results of the contact angle and contact angle hysteresis measurements as well as the layer thickness and adhesion strength evaluation are presented as mean values ± standard deviation. For each sample type 5 independent CA and CAH measurements were evaluated, and, in case of the abrasion test 7 individual measurements. For the tape peel test, three individual measurements were used. The results of the aging study (exposure to extreme climate) were evaluated with 10 individual measurements. Layer thickness was evaluated using five SEM images at different locations and measuring the thickness at five spots in each picture using ImageJ. Adhesion strength measurements were performed with three samples of each type. The data analysis was done using the software OriginPro and ImageJ.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.