Extending the Lotus Effect: Repairing Superhydrophobic Surfaces

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Interface-Rich Materials and Assemblies

Extending the Lotos Effect: Repairing Superhydrophobic Surfaces after Contamination or Damage by CHic Chemistry Roland Hönes, and Jürgen Rühe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01179 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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Extending the Lotos Effect: Repairing Superhydrophobic Surfaces after Contamination or Damage by CHic Chemistry Roland Hönes†,‡ and Jürgen Rühe∗,†,‡ †Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany. ‡Freiburg Center for Interactive Materials and Bioinspired Technologies (FIT), University of Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany E-mail: [email protected] Phone: +49 761 203 7160. Fax: +49 761 203 7162

Abstract Superhydrophobic surfaces (SHS) have gained a reputation to show a self-cleaning behavior (“Lotos effect”) as drops rolling off the surface take along loosely adhering durst particles. However, this self-cleaning process reaches its limits when such surfaces are brought in contact with sticky contaminants like oils and smaller particles. Once intimate contact between the surface and a small particle is established, it will be almost impossible to remove it because of strong surface interactions. Such contaminations, however, lead to contact line pinning and destroy the superhydrophobic effect. Because the fragility of the micro- and nanostructures prohibits any mechanical cleaning, the sample is then usually doomed. Here, we report a universal method for restoring superhydrophobicity: By simple dipcoating, a conformal ultrathin layer (≈ 10 nm) of a highly hydrophobic and photoreactive fluoropolymer is deposited. Through short UV irradiation (5 min), this thin layer is crosslinked and chemically attached to the underlying surface by C,H-insertion crosslinking (CHic), thus covering the contaminant like a thin veil. We use this “cover up” strategy of masking the 1 ACS Paragon Plus Environment

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contaminants to restore superhydrophobicity. We demonstrate this principle by deliberately soiling the surface with various model contaminants, such as oily substances and particles, and study the repair process.

Introduction Superhydrophobic surfaces (SHS) are used by quite a number of living organisms, both animals and plants.1,2 Some well-known examples are water striders3 and the Stenocara beetle from the animal kingdom,4 and from the plant kingdom (Plantae) the Lady’s mantle (Alchemilla vulgaris),5 and of course, the lotus leaf (Nelumbo nucifera), the certainly most famous case.6 When water is brought on such a surface, the result are nearly spherical drops that roll freely on the surface or bounce off and thus leave the surface eventually. To obtain an SHS, the surface needs to be composed of or coated with a material with low free surface energy, and it has to be highly rough on the nanoscale.6–8 Driven by the large number of possible applications,1,8–11 a vast amount of work has been undertaken in order to understand2,9,12–15 and mimic natural superhydrophobicity,2,8 and accordingly, a great number of different SHS have been created.16– 20

Despite the strong scientific progress made in the past years, commercial applications of SHS are still rather rare.10,17,21,22 The basic problem is that they are not reliable enough yet. There are two main reasons for this: First, the nanostructures that are required for the generation of SHS are intrinsically mechanically weak, especially against shear. This high vulnerability is a fundamental issue: Due to the small cross-sectional area on the individual nanostructures, even for rather small forces, the resulting pressures and stresses are quite high.8,9,18,23,24 In practically all known examples of nanostructured surfaces, even moderate shear forces such as those caused by rubbing with a finger may result in significant surface damage and reduction of the surface roughness. In many cases, e. g., in the break-off of silicon nanograss, this will generate hydrophilic spots on the surface, leading to pinning of the contact

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line of droplets on these surfaces and a complete loss of superhydrophobicity. In recent publications,25,26 several strategies to mitigate this issue have been reported. Second, any surface in contact with a complex environment for a long time is prone to eventual contamination. In the case of SHS, the “Lotos Effect” helps to mitigate this issue, and it does so quite effectively. When the term was coined by Barthlott and Neinhuis in 1997,27 it was demonstrated impressively how the non-wettability of highly rough plant leaves is related to contamination resistivity. It was shown that irrespective of their chemical nature, dry-deposited particulate contaminations that were brought in contact with superhydrophobic plant leaves could be removed almost completely by a quick wash with water. Drops rolling on the surface pick up the dust particles due to the high surface tension of water and take them along as they eventually roll off. This effect is a most significant evolutionary advantage for these plants because it helps fight, e. g., pathogens and leaf burns due to overheating caused by attached particles in the sun.6 On non-superhydrophobic leaves on the other hand, a significant fraction of the particles remained stuck on the surface. The strategy was subsequently transferred to technical surfaces which also show an impressive self-cleaning behavior. Nevertheless, there are a number of scenarios where even SHS will become soiled.23 This has been studied extensively in long-term outdoor experiments,28 where organic contaminants were identified as especially problematic. This holds true for both oily substances and for proteins – the latter are known to denature on surfaces, rendering them completely insoluble, and then, they can no longer be washed away. For any type of particle on the surface, the strength of adhesion is the key factor – loosely adhering particles on the surface will become completely surrounded by water quickly, and they get carried off. Only when the particle– surface adhesion is increased drastically, this self-cleaning effect will break down. In such cases, as most contaminants will have a higher surface energy than the original surface material, the result will be an increased contact line pinning and a breakdown of the superhydrophobic


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Cassie–Baxter-like wetting state, as illustrated in figure 1. The drops transition into the Wenzel wetting regime where they are strongly pinned on the surface.29

Figure 1: Illustration of cases where superhydrophobic surfaces (SHS) get contaminated despite the “Lotos Effect”: Long-term contact with dispersions of (colloidal) particles and eventual sedimentation leads to close contact and strong adsorption by surface interactions. Organic contaminants like viscous oils or proteins can form films on the surface that strongly adsorb as well. In all these cases, a dissolution of the contaminants by simple washing is not likely to succeed. In the case of proteins, denaturation upon contact to the surface further increases the difficulties.

In this article, we discuss the influence of common contaminations such as oils, proteins, and particle dispersions on the wetting behavior of SHS, using nickel “nanoflowers”, surfaces covered with aggregates of acute, pentagonal nanopyramids of nickel made by electroplating, and coated with a highly fluorinated polymer as a model system. We propose a strategy to coat the contaminated surface with photogenerated ultrathin surface-attached perfluorinated networks in order to cover the contaminants and restore superhydrophobicity. The concept followed in this work is illustrated in figure 2. By dip coating, thin layers of highly hydrophobic, fluorinated polymers are deposited on the nanostructured surfaces. These polymers include a small fraction of a diazo compound as a comonomer. Upon UV irradiation or heating, a carbene intermediate is generated, and this species is capable of crosslinking the polymer chains among each other by C,H insertion (C,H insertion crosslinking, CHic). The same process also leads to a


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covalent attachment of the polymer to the substrate. Thus, the contaminant is covered completely with a thin fluorinated veil and can no longer influence the wetting behavior. We study the wetting properties of such surfaces before and after this repair process.

Figure 2: Restoring superhydrophobicity after it was lost by contamination: The original surface and the contaminations are coated with a thin layer of highly hydrophobic polymer containing a diazo compound as comonomer. This polymer layer is then converted into a surface-attached network by brief UV irradiation, using the C,H insertion crosslinking mechanism (CHic): By photochemical or thermal activation of the diazo compound, N2 is cleaved off and a carbene intermediate is formed. This carbene is capable of C,H insertion with neighboring aliphatic C,H groups, resulting in a network which is covalently attached to the substrate.

Experimental Poly-1H,1H,2H,2H-perfluorodecyl acrylate (PFA) was synthesized by free radical polymerization in 1,1,1,3,3-pentafluorobutane according to literature procedure.30–32 When needed, the photochemical crosslinker 1-(4-(methacryloyloxy)butyl)-3-methyl-2-diazomalonate (MAz) was used as a comonomer in the polymerization (5% (n/n)). Nickel “nanoflowers” were grown by electrodeposition on Cu foil substrates in a modified literature procedure:33–36 Cu substrates were cleaned by rinsing with isopropanol, additional 5 ACS Paragon Plus Environment

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cathodic, alkaline cleaning (commercial cleaner Extra AP13, pH 14, 30 s), and a short dip into a solution of (NH4)2S2O8 (ca 5 s). The electrolyte contained NiCl2 ·6 H2O (1 mol·L−1), ethylene diamine dihydrochloride (1.5 mol·L−1) and H3BO3 (0.5 mol·L−1). The pH was adjusted to about 4 by adding HCl and/or NH4OH (10%); the temperature was kept at 60 °C. Different current densities and deposition times were tested. Octadecane phosphonic acid (ODPA) layer deposition. The nanostructured nickel samples were rinsed with isopropanol, acetone, and toluene, then dried in a stream of nitrogen. To ensure a complete oxide layer was present on the surface, the samples were immersed in aqueous NaOH solution (1 mol·L−1, 10 min), rinsed clean with deionized water, and further cleaned in an air plasma chamber (Diener Electronic, Zepto; 100 W, 0.1 mbar, 60 s). Immediately afterwards, the samples were dipped into a solution of ODPA in THF (1 mmol·L−1; a mechanical testing device (BZ2.5/TN1S, Zwick Materials Testing, Germany) was used for dip coating; immersion and retraction speed 100mm·min−1 each), and annealed at 100 °C for at least 3 h to ensure chemical bonding of the ODPA to the nickel. Non-bound material was removed by sonication in THF (30 min). Coating with P(FA-co-MAz). After drying in a stream of nitrogen, the samples were dipped into a solution of P(FA-co-MAz) in 1,1,1,3,3-pentafluorobutane (10 g·L−1; immersion and retraction speed 100 mm·min−1 each). Afterwards, by UV irradiation at 254 nm (1.00 J·cm−2; Stratalinker 2400, Stratagene, United States), the MAz comonomer within the PFA was activated, resulting in polymer network formation and surface attachment of the forming PFA network to the ODPA layer below. Recoating of the samples after deliberate contamination or scratching was performed in exactly the same fashion. Contact angles were measured using an OCA20 device (Dataphysics GmbH, Germany). Water drops of 10 µL were soft-landed on the sample by raising the sample until contact with a hanging droplet was established. Based on video capture, advancing and receding contact angles were determined by adding/removing water from the drop at rate of 0.5 µL·s−1 until the 6 ACS Paragon Plus Environment

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three-phase contact line started to move. The last video frames before the movement started were evaluated. Rolloff angles were measured at the same device by soft-landing droplets of 10 µL on the horizontally aligned sample in the fashion given before and tilting the sample until the droplet rolled off. Contamination of the samples was performed by depositing the selected substance on the samples. Organic solvents (Sigma Aldrich or Carl Roth GmbH), motor oil, drops of aqueous solutions and particle dispersions were deposited by disposable pipettes, dry particles (Silica Gel 60, Carl Roth GmbH, and Nanosilica particles, Sigma Aldrich) were added by letting them rain down from a spatula. Where applicable, drying-in of liquids was done by leaving the sample at room temperature under cover until the liquid was gone. For the restoration experiments, first, the samples were washed (deionized water, detergent-containing water, organic solvents including isopropanol and acetone). Only if this did not restore superhydrophobicity, recoating was tested. Delibrate scratching was performed using a rheometer (Physica MCR 501, Anton Paar GmbH, Austria) equipped with a cylindrical test body for plate–plate geometry (PP08, Anton Paar GmbH, Austria) that could be moved up and down to any z position for contact with the sample at the desired normal force (see sketch of the setup in figure 7d). Subsequently, the test body was rotated at 10 min−1 for 30 s on the sample. Finally, the test body was raised back up so that contact was lost, and the sample was investigated.

Results Sample Fabrication As an example system, we have used fractal nickel “nanoflowers”, surfaces covered with aggregates of acute, pentagonal nanopyramids of nickel generated in an electroplating process. These structures were grown by anisotropic electroplating following an adapted literature procedure33–37 as described in detail in our recent publication.38 In this process, an electrolyte 7 ACS Paragon Plus Environment

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that contains a large amount of complexing agents is used so that the supersaturation of the solution during the deposition is low. Then, crystal growth will follow the screw dislocationdriven mechanism, and anisotropic growth can be achieved.39–41 The structures obtained depend on the chosen current density and deposition time. All samples in this work were fabricated at 30 mA·cm-2 for 20 min. For SEM images of the nanostructures, see figure 3a. After the fabrication, the obtained surfaces were superhydrophilic. To switch the wetting behavior into a superhydrophobic state, a stable, highly hydrophobic coating was applied. To obtain maximally stable attachment and durability of the coating itself, a surface-attached polymer network was chosen for the coating (cf.42–45). For an illustration of the coating process, see figure 3b. As an anchor layer, a monolayer of octadecane phosphonic acid (ODPA) was deposited first. On top of that, a thin layer of poly(1H,1H,2H,2H-perfluorodecyl acrylate-co-1-(4(methacryloyloxy)butyl)-3-methyl-2-diazomalonate), P(FAco-MAz), was deposited by simple dip coating. The diazomalonate comonomer can act as a crosslinker that can be activated thermally and photochemically.46 Thus, it is able to form links between adjacent organic molecules via the nearly universal C,H insertion crosslinking (CHic) process:47,48 The only requirement that the reaction partner has to fulfill is that there are aliphatic C,H groups available. For the mechanism of the CHic reaction, see figure 2.


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Figure 3: Ni “nanoflowers” surfaces and the coating designed to render them superhydrophobic: (a) SEM images of Ni “nanoflowers” produced at 30 mA·cm-2 for 20 min, magnifications 25000 × and 100000 ×. (b) Coating process: 1. Deposition of a self-assembled monolayer of octadecane phosphonic acid (ODPA) for stable bonding to the Ni, 2. Dip coating with P(FA-co-MAz), and 3. UV irradiation for permanent bonding to the alkyl chain of the ODPA and for chemical bonding between the PFA polymer chains. The result is a stable, surfaceattached polymer network. Figure (b) reproduced with permission from.38 When the “nanoflowers” coated with ODPA and P(FA-co-MAz) where irradiated with UV light, stable chemical bonds both between adjacent polymer chains and between the polymer and the ODPA below where formed, thus building up a surface-attached polymer network. Extraction experiments showed that the layers obtained like this were indeed perfectly stable. After the coating, these samples exhibited dynamic water contact angles of 161 ± 2° and 160 ± 2° (advancing and receding water contact angles), respectively.38 A rolloff angle of 4 ± 1° was found (droplet volume 10 µL).

Contamination and the Superhydrophobic State In any application scenario for SHS, the resistivity against contamination is a critical factor. To study this, samples are exposed to a number of different contaminations and the 9 ACS Paragon Plus Environment

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possibilities for restoring the original superhydrophobicity are elucidated. An originally superhydrophobic sample was brought into contact with a selected substance and the wetting behavior was tested again. If the substance had caused the superhydrophobicity to deteriorate, first, several simple washing steps were tried in order to restore the superhydrophobicity. When samples were rinsed with or immersed in pure organic liquids (e.g., ethanol, acetone, dichloromethane, α,α,α-trifluorotoluene, 1,1,1,3,3-pentafluorobutane; the last two are solvents for non-bound PFA polymer) and the solvent was dried away, this did not deteriorate the superhydrophobicity. The samples behaved just like before the solvent treatment. However, the situation was already different when technical solvents (purity ≈ 95%) were used and solvent drops were dried on the surface. In some cases, visible “coffee stains” formed, indicating a deposition of particles. At these spots, superhydrophobicity was lost, i. e., a Wenzel wetting state was established and drops were strongly pinned on the surface. However, immersion in common home use soap water followed by rinsing with fresh water was sufficient to remove the stains, and thus, superhydrophobicity was restored. Receding contact angles and rolloff angles of such samples were identical to those found on newly fabricated samples. A somewhat different situation was found when slowly-evaporating aqueous solutions like concentrated NaCl or sugar solutions (c = 1 g·mL−1) or common home use soap water were used. Here again, long-term immersion proved harmless – after rinsing with fresh water, superhydrophobicity was reliably restored. However, when droplets of any of these solutions were dried on the sample, even after washing, residues were observed. SEM investigations showed that these residues were not salt crystals (which should be aggregates of well-defined cubes in the case of NaCl), but contaminations of largely different appearance that were believed to be a mixed contamination of dust from the laboratory environment. The same contaminations were found after drying of soap water or sugar solution (see figure 4a). Washing the samples with large amounts of deionized water in order to remove the residues did not succeed in restoring superhydrophobicity, and prolonged immersion in water or soap 10 ACS Paragon Plus Environment

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water did not help either. In these cases, Wenzel-type wetting persisted. Water drops that were deposited on the contaminated area did not roll off even when the sample was tilted to fully vertical, both for small (10 µL) and large drops (≈100 µL). To study the influence of particulate contamination on the superhydrophobic properties of a surface in more depth, commercial silica gels with different particle size distributions (Silica Gel 60: 30–200 µm, and Nanosilica: ≈ 12nm according to the manufacturers) were used as a model contaminant. As silica particles are highly hydrophilic, any residues that might be left behind on the surface should be detrimental to the superhydrophobicity. It was found that when deposited as a dry powder, both types of particles could be largely removed in an N2 stream and the samples were superhydrophobic again. Apparently, such loosely attached particles are easily removed. On the other hand, when a thick suspension of the larger dust particles was dropped on the surface, over the course of 6–12 h, the dispersion sedimented and after rinsing with fresh water, a wettable spot (Wenzel wetting) remained. Here, drops did not roll off even when the sample was tilted to vertical. When suspensions of nanometric particles were employed, the maximum contact time after which a spotless removal was still possible became much shorter – partial loss of superhydrophobicity was observed already after only ≈ 30 s of contact.


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Figure 4: Contaminated surfaces that have lost their superhydrophobicity: SEM images of (a) contamination found on a sample on which soap water was dried, and (b) contamination found after exposure to a suspension of nanometric silica particles (Nanosilica, average particle size ≈ 12 nm according to manufacturer). In both (a) and (b), the images show only a small fraction of the area exposed to the contaminant (≈ 1 cm2). (c) illustration of the situation of particles having a varying number of contact points; (d) schematic energy diagram for the detachment detachment of particles according to the situations in c). SEM investigations have shown that in all these cases, the problem was caused by particles left behind on and in between the nanostructures (figure 4b). An analysis by energy dispersive X-ray spectroscopy (EDX) showed that when silica particle dispersion were dried, the contamination left behind was indeed composed of silicon dioxide. It proved effectively impossible to redisperse contaminating particles from superhydrophobic surfaces even when hydrophilic silica particles were used. This was the case even though here, the interactions between particles and dispersant are very strong and the interactions between the completely non-polar fluoropolymer surface and the particles should be rather weak. This showed clearly 12 ACS Paragon Plus Environment

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that while removing dry-deposited dust from an SHS is generally quite easy, this is completely different for dust that remains after a drop has dried up on the surface. During the drying process the capillary forces will bring the particles into close contact with the surfaces. The differences between the behavior of particles that were deposited as a dry powder and those left behind after drying a dispersion can be explained based on particle– surface interactions: The van der Waals forces between the two surfaces play an important role, but also other factors such as static electricity or capillary forces are important. Both an increase of the van der Waals forces and changes in the capillary forces cause a stronger interaction between surface and contaminant, which makes the pick-up of dust by the rolling water droplet more difficult as well.52 If a particle of irregular shape comes into contact with a macroscopically rough surface, it may form multiple points of contact to the surface, and then, the total energy of adhesion will be the sum of the contributions of all those contact points.53 Now, if we have a hydrophilic particle on an SHS that comes into contact with water, of course, it would be thermodynamically favorable for the particle to detach and be carried away by the water – this is the “Lotos Effect”. However, if the energy barrier Edes that has to be overcome for desorption is too high, the particle may be inert against removal. For a particle with multiple points of contact, the total energy barrier can be expressed as

des = ∙ adh + A

(1)

where n denotes the number of contact points, Eadh represents the adhesion energy (enthalpy) for an individual contact point, and EA denotes the excess activation energy. The situation and the symbols are visualized in figures 4c and 4d. The rate constant for particle desorption kdes may then be calculated following the simple Arrhenius approach as


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des ∝

des B

=

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∙adh A B

(2)

Accordingly, if the position of a particle on the surface is changed in such a way that the number of points of contact with the surface or the total area of contact is increased, the energy barrier of desorption of this particle will increase. In consequence, the detachment rate constant will be reduced exponentially and the particle will no longer be washed off in the given experimental time frame. Additionally, once the particles in the drying dispersion have reached positions where they strongly interact with each other and with the surface, the resulting forces will probably also lead to particle deformation and particle–particle fusion. This situation is essentially identical to coagulation of colloidal dispersions which is exploited technically, e. g., in wall paints. This effect will further increase the adhesion of the contamination to the surface. This simplistic model explains why particles that are deposited in the dry state can be removed much more easily than those deposited under wet conditions and why the contact time plays a big role for the ease of particle removal. Inside an aqueous dispersion that is slowly drying in on the surface, the particle is able to explore the energy landscape of the surface more thoroughly, and eventually, it will rest in places where it experiences a large number of contact points with the surface. This makes the adhering particles inert against removal by washing or by pick-up through rolling drops. As the self-cleaning is not effective any more in this case, the surface becomes permanently contaminated. This in turn leads to droplet pinning, and in consequence, the superhydrophobic properties are permanently lost. A second line of contamination stems from the adsorption of hydrophobic substances from the environment. For example, when a sample was contaminated with commercial motor oil, representing a typical daily-life contamination, washing was not enough to restore the original superhydrophobicity, and the sticky Wenzel-type wetting behavior remained (see figures 5a–d;


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in this case, even intense washing with and immersion in several organic solvents was not sufficient to remove the oil completely, and water drops still experienced strong pinning. While this may be surprising at first, similar effects have been observed and explained before.26 Small amounts of contaminant may remain in the voids between micro- and nanostructures for a long time because of their limited accessibility from the outside due to geometrical restraints. Moreover, it is possible that a monolayer of oil is strongly adsorbed to the underlying, original hydrophobic coating of the nanostructures. It may be that the enthalpy of desorption for this monolayer to detach from the surface is simply higher than the enthalpy of solution in organic solvents). Another possible source of contamination might originate from contact of the SHS with biological systems. Living organisms coming into contact with artificial surfaces might leave proteins behind. Proteins that adsorb to a surface can denature, and thus become completely insoluble. This situation was investigated by drying drops of a solution of bovine serum albumin (BSA; serum albumin is the most abundant blood plasma protein and it is well-known to adsorb strongly to artificial surfaces; this property is widely exploited during the “blocking” of bioanalytical surfaces). After drying, indeed, washing with pure water or water containing detergents was not sufficient to restore superhydrophobicity and the wetting behavior of the material was permanently changed (see figure 6a).

Restoring Superhydrophobicity In those cases where extensive washing did not suffice to restore superhydrophobicity, e. g., when particles were stuck on the surface, a new layer of P(FA-co-MAz) was deposited on the soiled sample by dip coating and subsequent crosslinking by UV light. This way, a surfaceattached fluorinated network was formed. The hydrophobic polymer PFA can be deposited on practically all surfaces, nearly independent of their chemical composition and surface structure. During the UV exposure or a short heating to ≥ 150 °C, the diazo groups in the polymer become


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activated and form carbene intermediates which can insert into practically any C,H bonds. This leads to crosslinking of the deposited thin fluorinated film (C,H insertion crosslinking, CHic), rendering it completely insoluble.46 If organic moieties are present on the surface below the polymer film such as those present on the uncontaminated parts of the surface, the newly forming layer will also become covalently bound to the surface below. In consequence, the polymer network is stable against dissolution and delamination, practically independent of the contamination present on the surface below the network. In some cases (organic contaminants), the network that covers the contaminant like a thin veil will bind to it, in other cases (inorganic contaminants), the contamination will be covered only physically by the “surface-glued” film. It was found that this process did indeed restore the desired superhydrophobicity in all tested scenarios: Dried soap foam, dried BSA (figure 6b) and sugar solution, motor oil residues (figure 5e), and nanometric silica particles. In all cases, after the coating process, the superhydrophobicity was restored and again, water drops were almost spherical, showed the high interfacial reflectivity typical for Cassie–Baxter-like wetting states, exhibited virtually no pinning, and rolled off the surfaces again very easily when the sample was tilted only very slightly (dynamic water contact angles of 161 ± 2° and 160 ± 2°, respectively, and rolloff angle of 4 ± 1°, as before). It is quite remarkable that the entire simple procedure took only a few minutes and required no specialized equipment other than that for the dip coating. Of course, this process will also have a limitation: If the whole SHS or extremely large areas on it are flooded with a liquid contaminant, the forming layer cannot be covalently anchored to the substrate and might become delaminated under appropriate conditions. On the other hand, the formed network also clings very closely to the nanostructures, which leads to a rather strong mechanical interlocking, rendering delamination not so easy. Accordingly, in all experiments we carried out so far where we varied quite strongly the type and the extent of deliberate soiling of the SHS, we never observed delamination. 16 ACS Paragon Plus Environment

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Moreover, it should be noted that such repair cycles can be performed multiple times, depending on the extent of contamination. It could be shown that for small-area contaminations (≈ 10 mm2), the recoating procedure was effective for at least 5 subsequent recoating steps. The sample shown in figure 5e was subdued to five of the described soil-andrepair cycles and still showed perfect superhydrophobicity. The dynamic contact angles and rolloff angles were effectively identical to what was observed directly after the fabrication. Of course, for a much larger number of recoating cycles, it can be expected that at some point, the coating will smoothen out the nanostructures and then, superhydrophobicity will degrade. However, as an individual layer of P(FA-co-MAz) is as thin as ≈ 7 nm, a significant number of repetitions is possible.

Figure 5: Restoring superhydrophobicity after contamination with motor oil: (a) Drops of motor oil on a sample of PFA-coated nickel “nanoflowers”; (b) the same sample after immersion in soap water in order to remove the oil; (c) water drops on the sample described in (b) after postrinsing with deionized water; (d) water drops on the same sample after additional rinsing with acetone; and (e) a drop of water on the very same sample after recoating with P(FA-co-MAz). The black, structured area of the sample is 3.0 × 3.0 cm.


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Figure 6: Restoring superhydrophobicity after contamination with a protein (bovine serum albumin, BSA): (a) water drops on a sample of PFA-coated nickel “nanoflowers”. At one spot in the left of the image, a drop of BSA solution was allowed to dry. The image was taken after extensive washing with soap water and rinsing with ethyl acetate; (b) water drops on the very same sample after recoating with P(FA-co-MAz). The black, structured area of the sample is 3.0 × 3.0cm. To some extent, this process can even be applied when mechanical impact has damaged the nanostructures that are required for superhydrophobicity: Upon mechanical damage of the nanostructured nickel surfaces used in this work, as the structures are deformed and/or partially removed, the hydrophobic PFA network is ruptured and the hydrophilic metal is exposed. In such a case, applying a new overcoat will restore the original hydrophobicity. In a simple experiment, a sample of PFA-coated nickel “nanoflowers” was scratched with a ring-like stylus such that a series of circular wear tracks appeared (for SEM images before and after scratching and a scheme of the scratching experiment, see figure 7a). At these areas, all nanostructures were completely abraded and the hydrophilic Ni layer was exposed, as discernible by the silvery, glossy appearance. This could also be seen by the naked eye as the surface became highly reflective here. Such damage is a most common case because if a hydrophobic bulk material like Teflon is used to fabricate nanostructures, they would be mechanically weak due to the low intermolecular forces in such a material (for which they are chosen in the first place). Therefore, to obtain an SHS, quite often, mechanically strong, polar substrates are used and coated with a hydrophobic layer to achieve hydrophobicity. When such


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a material is exposed to water after some damage, the influence of the now bare, hydrophilic areas on the wetting behavior becomes dominant. Accordingly, when water drops (≈ 50–100 µL) were deposited on these damaged areas (water drops from a Pasteur pipette were brought into contact with the sample and enlarged until they detached from the pipette tip), they became pinned instantly, and the sample shown in figure 7b could be tilted until the vertical was reached without the drop rolling off. Of course, the exact roll-off condition depended strongly on the size of the drop and the shape of the damaged area. Pinning also occurred when the sample was pre-tilted by 5° before a drop was deposited. However, after a recoating with P(FA-co-MAz), followed by UV irradiation, in the same experiment with pre-tilting by 5°, no more pinning was observed, and the drops rolled off immediately (figure 7c). Of course, when some nanostructures are removed, the surface cannot become perfectly superhydrophobic again. When the recoated sample was not tilted, limited pinning still occurred at the defects, but gently blowing some air over the sample was enough to remove the droplets and no water was left behind (figure 7d).


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Figure 7: Restoring superhydrophobicity after deliberate scratching: (a) Scratch test, leading to partial loss of superhydrophobicity – SEM images before and after the scratching (magnification 5000 ×), and a sketch of the setup used for the scratching. (b) Water drops on two samples of PFA-coated nanostructured nickel after scratching. In the case of the upper image, the drop was deposited in the horizontal, and then, the sample was tilted to the vertical. In the case of the lower image, the sample was tilted prior to drop deposition by 5° to show the pinning clearly. (c) Video snapshots of water drops on the very same sample after recoating with P(FA-co-MAz): The sample was tilted just as before, and now, the drops rolled off instantly upon detachment from the pipette. (d) Video snapshots of water drops on the very same sample without tilting. Some pinning is present, but by blowing some air over the sample, the water can be removed completely. In images (b)–(d), the diameter of the scratch rings is 8 mm.


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Naturally, this repair mechanism will not work for too large damaged areas or when a contamination is so intense that the roughness features are effectively smoothed out, because then, the overall roughness is just too low, and the recoating can no longer regenerate superhydrophobicity (advancing and receding contact angles > 150°). In these cases, the typical wetting properties of a more or less flat perfluorinated surface are obtained (advancing contact angle ≈ 120°). For restoring superhydrophobicity in such a scenario, alternative strategies should be pursued. These include surface regeneration by shedding the damaged layer similar to snake skin shedding,26 surfaces made from re-meltable wax with dispersed, highly rough and hydrophobic colloidal particles,54 or built out of many highly rough, hydrophobic metal particles compressed together,55–57 or surfaces with healing agents encapsulated inside the material to be released upon damage,58 to name just a few approaches. The positive effect of recoating even when the nanostructures were sheared off on a limited area is somewhat analogous to the situation of an SHS with circular, more hydrophilic spots.59 It was found that for a drop to overcome the wetting contrast and roll off a hydrophilic spot of given diameter d and a given volume V , the sample must be inclined more than a critical angle α given by sin =

∙ ∙∙

(cos "rec − cos "adv )

(3)

where γ is the surface tension of the test liquid (water, 72.8mN·m−1), θrec and θadv are the receding and advancing water contact angles, respectively, ρ is the density of the liquid, and g is the earth’s gravity constant. For the situation discussed here, d is equivalent to the diameter of the scratch rings (see figure 7: 8mm). For any given drop on a given damaged spot, the effect of the recoating can be expressed by the fraction '() *+ '() *,

=

-.' /rec, 2 -.' /adv -.' /rec, 1 -.' /adv


(4)

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where α1,α2 are the critical roll-off angles of the damaged and repaired surface. For the situation here, θadv = 161°◦ (undamaged area), θrec, 1 = 0° and θrec, 2 = 90° (damaged, flattened area, uncoated and recoated, respectively), and accordingly, sinα2/sinα1 = 0.486. If we assume a critical rolloff angle of α1 = 90° before recoating, we obtain a value of α2 = 29° afterwards, demonstrating a greatly improved wetting behavior achieved by the repair process. Of course, the described calculation is a somewhat simplified approach because of the assumptions about the nature of the damage that are implied. For example, it assumes a circular spot whereas the damaged areas had a somewhat irregular shape. Nevertheless, it can be seen in figure 7 that the predicted behavior change is indeed close to that observed in the experiment. Surfaces which have been scratched slightly leading to strong water drop pinning can indeed be rejuvenated.

Conclusions One of the very attractive features of superhydrophobic surfaces (SHS) is their self-cleaning property. As water drops come into contact with the surface and roll off, they take loosely adhering dust along. However, eventually, just like any other hydrophobic surface that becomes exposed to the ambient or other challenging conditions for prolonged periods of time, sticky, non-water-soluble substances will become adsorbed. This lowers the inherent hydrophobicity of the material, thus destroying the superhydrophobic effect. Additionally, liquids that do not roll off immediately and therefore are in contact with the SHS over longer times can pick up particulate contaminants from the environment and leave them behind after drying. Such particles can become stuck on the surface upon drying due to strong surface forces between particles and surfaces. As a result, in both cases, water drops become pinned on the surface. To overcome the effects of a contamination that cannot be washed away any more, a simple repair mechanism is to permanently encapsulate them by covering them with a thin 22 ACS Paragon Plus Environment

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layer of a strongly hydrophobic polymer (i. e., a fluorinated polymer) and crosslinking this layer by C,H insertion crosslinking (CHic). As the polymer network will also react with the underlying substrate resulting in multiple covalent attachment points, it is very strongly linked to the substrate and does not come off easily. Even when the fluorinated polymer cannot link to the contaminant itself, e. g., in case of silica particles, the forming polymer network will cling to the particle and cover it like a veil that is pinned down at all neighboring locations, wherever C,H groups are present on the surface. Thus, all contaminants will be trapped below the network and they cannot have detrimental effects on the surface properties any longer. Consequently, superhydrophobicity is restored. Such repair cycles can be performed multiple times with a single sample, and they can even be used when surfaces are slightly mechanically damaged by scratches and an underlying hydrophilic substrate is exposed. Those damaged areas become covered by the fluorinated network in the same way, and superhydrophobicity is again restored. Even when at some point, the damage becomes excessive and when all nanoscale features are getting removed, the surface will still become hydrophobic with flat-Teflon-like properties. This represents a good fallback solution. Having a flat, Teflon-like surface – though being not superhydrophobic – is still better than the situation directly after scratching the surface where water is very strongly adhering to the substrate.

Author Contributions Both authors take full responsibility for the content of the paper and have given approval to the final version of the manuscript.

Acknowledgement We wish to thank Kay Steffen for his kind help with electroplating, and Philip Kotrade and Andreas Walder for synthesizing the P(FA-co-MAz) copolymer.


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Abbreviations PFA – poly-1H,1H,2H,2H-perfluorodecyl acrylate; P(FA-co-MAz) – poly-(1H,1H,2H,2Hperfluorodecyl acrylate-co-1-(4-(methacryloyloxy)butyl)-3-methyl-2-diazomalonate); MAz – 1(4-(methacryloyloxy)butyl)-3-methyl-2-diazomalonate; SHS – superhydrophobic surface.

Keywords Superhydrophobicity; C,H insertion crosslinking; Lotos Effect; Contamination; Surface Restoration.

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Extending the Lotus Effect: Repairing Superhydrophobic Surfaces

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