Molting Materials: Restoring Superhydrophobicity after Severe

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Molting materials: Restoring Superhydrophobicity after Severe Damage via Snakeskin-like Shedding Roland Hönes, Vitaliy Kondrashov, and Jürgen Rühe Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00814 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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Molting materials: Restoring Superhydrophobicity after Severe Damage via Snakeskin-like Shedding Roland Hönes, Vitaliy Kondrashov, and Jürgen Rühe∗ Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany. E-mail: [email protected]

Abstract The nanostructures which are required to generate superhydrophobic surfaces are always sensitive against shear and are easily damaged, especially by scratching with sharp objects. As a result of this destruction, the water repellency will be lost. We introduce a novel approach to restore the original surface properties after mechanical damage. In this approach the damaged layer is shed off like the skin of a snake. This is demonstrated with a threelayer stack as a proof-of-principle system: When the original, superhydrophobic surface layer is damaged, this leads to dissolution of a sacrificial layer below. Thus, the damaged layer is shedded, a new, unscathed surface is uncovered, and superhydrophobicity can easily be restored after a short washing.

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Introduction In nature, superhydrophobic surfaces (SHS) are quite common. 1,2 The wetting behavior of water striders, 3 the Stenocara beetle, 4 and the Lady’s mantle (Achimilla vulgaris) 5 are well studied, but the lotus leaf (Nelumbo nucifera) certainly is one of the most famous examples. 6 When water is dropped on such a surface, it rolls or bounces off and leaves the surface eventually. To create an SHS, the surface has to be composed of or coated with an intrinsically hydrophobic material and the surface topography must be highly rough, featuring appropriate micro- or nanostructures. 6–9 Motivated by a plethora of attractive potential applications, 1,8,10–12 tremendous efforts have been devoted to understand 2,10,13–16 and mimick natural superhydrophobicity. 2,8 In the process, SHS out of many different materials have been created. 17–21 Despite the strong scientific progress made in the past years, commercial examples of SHS are still rather rare. 11,18,22,23 This is because due to their small size, the nanostructures that are required for the generation of SHS are intrinsically mechanically weak, especially against shear. This high vulnerability is a fundamental problem, inherent to all such nanostructured surfaces: Even when a rather moderate force is applied, the pressures/stresses exerted onto the naturstructures are high due to the small cross-sectional area of the individual nanostructures. 8,10,19,24,25 In practically all known examples of nanostructured surfaces, even moderate shear forces such as those caused by rubbing with a finger result in severe surface damage. Nature shows that the generation of hierarchical structures, for example, the incorporation of larger, protective microstructures, is one way to improve the mechanical stability. 2,6,26 In such systems, the microstructures carry the mechanical load and the nanostructures generate the favorable wetting properties. Such structures have already been successfully implemented in artificial systems. 24,27,28 However, sometimes, the forces are so very strong that all protective measures are overcome and damage cannot be avoided. For example, this is the case when surfaces are scratched with sharp objects. To keep favorable wetting properties despite significant dam2


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age, the strategy pursued by most living organisms is to renew the surface. After serious damage, organisms can regrow the appropriate structures in order to restore superhydrophobicity. 2,6 In order to realize such restoration capacities in artificial systems, a number of approaches have been investigated. 2,25,29–33 For example, self-assembly of embedded colloidal particles on the surface has been used 34 where the removal of some of the structures yields a surface which is practically identical to the original one, as well as nanocomposite materials. 35–37 Another way has been based on highly porous materials, such as aerogels. 38 Here, shear removal of some fraction of the material will recreate a similar surface structure below the original surface. Alternatively, encapsulated hydrophobic low-molecular weight substances have been employed as curing agents. 39 In this case, damaging the surface also damages the capsules and releases the active compounds, thus restoring the system.

①

H2O

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③

Superhydrophobic Sacri�icial layer

⑥

⑤

+ H2O

H2O

④

Figure 1: Concept of skin shedding for the restoration of superhydrophobicity: After mechanical damage, the original surface is shedded off by dissolution of a sacrificial layer, and a new functional surface is uncovered.

In this article, we present an alternative, bioinspired approach to restore superhydrophobicity—shedding of the damaged, outermost part of the surface in a fashion similar to when snakes and lizards give up their outer skin (also known as ecdysis 35 or molting). Consider a three-layer stack. The topmost layer is a SHS. Below that, there is a water-soluble sacrificial 3


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layer, and below that, another SHS is hidden. When the topmost layer of such a system is mechanically damaged, the superhydrophobic properties will be lost for the moment. However, as soon as this happens, water can reach the sacrificial layer, leading to its dissolution. Thus, the original, damaged surface lifts off and a new SHS is uncovered. This concept is illustrated in Figure 1. Biomimetic and bioinspired principles have found application in almost all areas of science, technology, and even in art, 40,41 and using sacrificial layers is a well-established tool in MEMS technology (microelectromechanical systems) to create undercut and hollow structures and to release temporary support structures. Sacrificial layers with self-degrading behavior have been employed in surface regeneration, 42,43 for example, for the suppression of biofilm generation and for micromachining. Building up on our recent publication on polymer nanograss, here, we present a proof-of-principle system consisting of superhydrophobic silicon nanograss that was covered with water-soluble poly-N -vinyl pyrrolidone (PVP) as a sacrificial layer and a film of superhydrophobic poly-1H,1H,2H,2H -perfluorodecyl acrylate (PFA) nanograss on top. We describe the fabrication of such assemblies and the behavior of the system upon severe mechanical damage.

Experimental Silicon nanograss, PDMS negatives of Si nanograss were fabricated according to our recent publication. 44 Briefly, in a plasma of SF6 and O2 inside a cryogenic deep reactive ion etching (cDRIE) reactor, Si nanograss with additional, larger microcone structures was created in a single-step process after carefully adjusting the relevant process parameters (nanograss needle height 1–3 μm, microcone height 15–20 μm). Afterwards, the Si nanograss surfaces were coated with a layer of 1H,1H,2H,2H -perfluorooctyl trichlorosilane to reduce adhesion. A commercial polydimethyl siloxane (PDMS) kit (Dow Corning, Sylgard 184 silicone elastomer kit) was mixed according to the manufacturer’s recommendations, and poured over

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(1) Fabrication of PDMS Negative of Si Nanograss + PDMS

PDMS Negative

Silanised Si Nanograss

(2) Fabrication of PFA Nanograss Film Air Plasma Treatment

∆p, ∆T Lamination

+ PFA

upper part is upside down

PDMS Negative

+ H2O

PVP-covered Si

(3) Assembly of Three-Layer Stack + + PVP

PFA Nanograss Soft Transfer

Silanised Si Nanograss

Three-Layer Stack

Figure 2: Fabrication of the three-layer model system: Superhydrophobic coating of cDRIE Si nanograss and fabrication of a PDMS negative, fabrication of a free PFA nanograss film, and assembly of the three-layer stack.

a piece of silanized Si nanograss. After curing at room temperature, the Si nanograss and the PDMS negative were carefully separated by bending the PDMS away from the Si. Poly-1H,1H,2H,2H -perfluorodecyl acrylate (PFA) nanograss films were fabricated by double replication, as published recently. 44 Briefly, after PFA synthesis according to the literature procedure, 45–47 a piece of PDMS negative was covered with PFA solution and allowed to dry. Then, the back side of the continuous PFA film was hydrophilicized by 5


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plasma treatment. By stamping at elevated temperature, the PFA film (thickness ≈ 30 μm) with the replicated nanograss structures was laminated onto a water-soluble support (pieces of polished Si wafers covered with a thick layer of Poly-N -vinyl pyrrolidone (PVP)) and separated from the PDMS by bending. The resulting sample (flat Si/PVP/PFA nanograss) was submerged in water to dissolve the PVP, resulting in a free PFA nanograss film floating on the water surface. Three-layer stack assembly: A sample of silanized Si nanograss was preheated to 75–80 ◦ C and covered with PVP by drop deposition of solution (4 × with

1 2

h drying at 75–80 ◦ C in

between – layer thickness ≈ 0.3 mm). After additional drying for 1–2 h at 75–80 ◦ C, this sample was submerged in the water bath below the floating PFA film and lifted back out so that the PFA nanograss film was deposited on the PVP-covered Si nanograss. Each sample was thoroughly dried for 2 d at room temperature prior to the shedding experiment. Contact angles were measured using a OCA20 device (Dataphysics GmbH, Germany). Water drops of 10 μL were soft-landed on the sample. Based on video capture, , the advancing and receding contact angles (θadv and θrec ) were determined by adding/removing water from the drop at a rate of 0.5 μL·s−1 until the three-phase contact line started to move. The last video frames before the movement started were evaluated. All CAs were determined by manually fitting an ellipse to the drop profile using the instrument software (SCA 20, Version 4.1.12, Build 1019, Dataphysics GmbH, Germany). Because high static contact angles can be misleading, 9 samples were only labelled superhydrophobic when their wetting behavior fulfilled the rather strict criterion of θrec > 150◦ proposed by Korhonen et al. 48 and supported by Butt et al. in their recent study of microscopic contact line dynamics. 49 Scanning electron microscopy

(SEM) was performed using a Phenom Pro (Phenom-

World B. V., The Netherlands) at an acceleration voltage of 10 kV. Prior to SEM investigation, all samples were sputtered with Au (Sputter Coater 108auto, Cressington Scientific

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Instruments, United Kingdom; 0.1 mbar Ar plasma, 2 × 20 s operation time).

Results The entire process for the fabrication of such a three-layer system is visualized in Figure 2. First, silicon nanograss was produced by cryogenic deep reactive ion etching (cDRIE), as discussed previously. 44 In this mask-free single-step process, both “nanograss” needles (typical height of 1–3 μm, opening angle 19(3)◦ ) and additional “microcones” (height 10–15 μm, opening angle 37(3)◦ ) were created simultaneously (Figure 3c). This Si nanograss from cDRIE was originally superhydrophilic, but by chemical vapor deposition (CVD) of a perfluoroalkyl silane, the wetting behavior was switched to superhydrophobic (Figure 3a, 3b). Water drops deposited on such a surface assumed an almost spherical shape with advancing and receding contact angles of θadv = 165(2)◦ and θrec = 161(2)◦ , respectively, and negligible hysteresis. The drops could be dragged around on the surface with the dispensing needle, showing practically no deformation. The PFA nanograss films were created by double replication according to the method published recently: 44 Briefly, in the first step, 3D negatives of the original silicon nanograss were fabricated out of polydimethyl siloxane (PDMS). In the second step, these were coated with PFA from solution. The back side of the PFA layer was oxidized by air plasma to improve adhesion. Then, it was imprinted onto a silicon substrate covered with a thick sacrificial layer of PVP. This polymer is well-soluble in water. Thus, by submersion of such a stack in water, the PVP dissolved, and free-floating PFA nanograss films could be obtained, picked up, and transferred onto a new support of choice. The double replication process allows for an easy fabrication scale-up because both the original silicon nanograss and the PDMS negative stamps can be used multiple times. Normally, the size of the replica is limited to wafer size, but this can be overcome by a step-andrepeat process (“multistamping”). The typical opening angle at the cone tips increased from

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(c)

10 µm

(d)

(e)

(f)

10 µm

(g)

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50 µm

(i)

(j)

(k)

10 µm

Figure 3: Surface structures and wetting properties of (a)–(c) silanized Si nanograss, (d)–(f) PFA nanograss with additional “microcones” included, (g), (h) PFA nanograss after scratching, and (i)–(k) Si nanograss that was uncovered by skin shedding: (a), (d), (g), (i) Photographs of millimetric water drops; (b), (e), (j) Side views of drops of 10 μL used for contact angle determination; and (c), (f), (h), (k) Scanning electron micrographs of these surfaces, obtained at an angle of 45◦ . Videos of the dynamic contact angle measurement are shown in the supporting information (Figures S 1–S 3). Note that the images are from several repetitions of the experiment and do not all show the same sample.

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37(2)◦ to 59(4)◦ (Figure 3c, 3f). While this effect certainly reduced numerical roughness values, it did not lead to loss of superhydrophobicity: The dynamic water contact angles were very similar to those of the silanized Si original (θadv = 159(2)◦ , θrec = 157(2)◦ ), and the hysteresis was negligible again (Figure 3d, 3e). Additionally, the replicated PFA needle tips were found to be bent slightly to one side. This is explained by mechanical deformation during peel-off of the PDMS negative after lamination. To assemble a three-layer stack, the free-floating PFA nanograss films were transferred onto pieces of PVP-covered Si nanograss. This was achieved by briefly submerging the new support in the water below the PFA film and using the support itself to lift out the PFA film. As the water contact was rather brief, the PVP did not dissolve, it only became sticky, which was beneficial for the assembly process. Due to the plasma treatment of the back side of the PFA film during the fabrication process, firm contact between the PFA and the underlying PVP was established. To study how the system behaves when mechanically challenged, the PFA nanograss films were deliberately damaged by scratching them with a scalpel. The key idea behind this is to demonstrate what happens when a strongly excessive force is used to cut into the film, i. e., the force imparted is very much higher than what the sample can withstand. Water drops placed on the damaged area were strongly pinned and showed an extremely high contact angle hysteresis. While drops placed on the non-damaged areas rolled off upon very slight tilting of about 3◦ , drops on the damaged areas did not roll off, irrespective of how strongly the surface was tilted. Instead, water drops that were pinned at those defects established contact to the hydrophilic PVP layer below (Figures 3g and 3h). By simply submerging such a damaged sample in water, the entire PVP layer was dissolved. This typically took 1–2 hours for the given sample size (≈ 2 × 2 cm2 ) – the more the PFA was scratched, the faster the process. Upon dissolution of the PVP layer, the PFA film was lifted off and floated away. In consequence, the Si nanograss surface was uncovered (Figure 4; for a video of the process, see Figure S 4). 9


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111 min

①

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H2O PFA PVP

⑥

i

H2O

③

PrOH ⑤H2O + PVP(aq)

+ H2O ④

H2O

PFA

PVP

Figure 4: Shedding of a Si NG/PVP/PFA NG sample by submersion in water: (a) Video snapshots, documenting the progress of PFA liftoff. Video of the entire process in the supporting information (Figure S 4); (b) Scheme to explain the mechanism of shedding, combined with optical photographs for illustration: (1) Initial state: superhydrophobicity; (2) scratching the surface with a scalpel; (3) superhydrophobicity is lost, water reaches the water-soluble PVP layer; (4) the PVP layer swells in water and starts to dissolve, and thus, the PFA is pushed outwards; (5) completed PVP dissolution, the PFA film is lifted off; (6) uncovered and dried Si nanograss surface with restored superhydrophobicity.

When such a three-layer system was submerged in water without any scratches disrupting the PFA nanograss film, on the same time scale, no changes were observed. Only after several 10


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days of water exposure, the adhesion of the film to the substrate weakened, and the film eventually delaminated and floated off. The time it took for such a non-desired loss to happen depended quite strongly on the extent of drying of the system before immersion. The uncovered Si nanograss was taken out of the water and dried quickly in a stream of nitrogen. Alternatively, the sample can be left in the hood for a few hours for drying. After this, water drops were still strongly pinned on the surface, that is, water drops were in the Wenzel wetting state. However, by rinsing with soap water (standard household product; tenside concentration approximately 0.5 %), 2-propanol or any other solvent that completely wets the surface and is known to dissolve PVP (for example, other alcohols and chlorinated solvents), superhydrophobicity was reliably restored: Then, the dynamic contact angles were effectively identical to those of new Si nanograss (θadv = 163(2)◦ , θrec = 161(2)◦ ; Figures 3i, 3j). We believe that the initial water pinning after shedding can be explained by some remaining PVP residues on the surface. These residues might be explained by the sample geometry: During PVP dissolution, the tips of the nanograss needles will be uncovered right away, and the voids between the needles only at later stages. As more and more of the superhydrophobic Si nanograss is uncovered, water access to the remaining PVP in the voids is hindered more and more. The superhydrophobic needle tips and slopes of the needles make it unfavorable for water to descend down into the narrow voids to reach down to the PVP. This slows down the dissolution of the last bits of PVP significantly, especially when some air is trapped in the structures during the layer assembly. Thus, some PVP residues remain on the surface for a long time. For the wetting behavior on the other hand, even minimal contact of water and PVP at very few locations will already lead to significant pinning. This will break the superhydrophobicity, while the same amount of contact does not necessarily result in full PVP dissolution. In contrast to water, 2-propanol and many other solvents wet the superhydrophobic Si nanograss surface easily, and thus, PVP residues are dissolved quite easily and superhydrophobicity is reliably restored (Figure S 5 in the Supporting Information). Addition of a simple surfactant

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to water also succeeds in restoring superhydrophobicity – the surfactant reduces the surface tension of the water, and in consequence, it improves the wetting of the uncovered silicon nanograss by water. SEM investigations confirmed that the entire three-layer stack buildup and shedding process did not damage the Si nanostructures (Figure 3c versus 3k). Moreover, with any individual Si nanograss piece, it was possible to repeat the three-layer stack buildup and shedding process at least 2–3 times before eventual failure.

Conclusions When objects with acute edges or sharp protrusions scratch over a superhydrophobic surface, the surface will be damaged easily as nanostructures generally are mechanically rather weak, especially against shear. Shedding of the heavily damaged, uppermost layer is an approach to mitigate the adverse effects of intense mechanical damage and to restore superhydrophobicity. To demonstrate this, a multilayer assembly was generated consisting of two superhydrophobic surfaces with a sacrificial layer in between. After scratching of the original surface, water could access the water-soluble, sacrificial layer. Dissolution of this sacrificial layer lead to shedding of the damaged original surface: The topmost layer was lifted off completely, and a new nanostructured surface was uncovered. Superhydrophobicity was easily regained by a simple washing step. Scratch resistance of the sacrificial layer could be improved to avoid simultaneous damaging of more than one superhydrophobic layer. However, in any case, it would be beneficial to confine the shedding to a limited area of the substrate by releasing only small tiles from a parquet-like surface layer. We hope to report on such approaches in a follow-up publication.

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources We are grateful for financial support by the Bundesministerium für Bildung und Forschung (BMBF) and the VDI/VDE Innovation + Technik GmbH through project NanoTau (grant 16SV5348).

Acknowledgement We wish to thank Natalia Schatz for synthesizing the PFA polymer, and Johannes Auber, HSG IMIT, Villingen-Schwenningen, Germany, for his support using cDRIE.

Abbreviations CA – contact angle; cDRIE – cryogenic deep reactive ion etching; CVD – chemical vapor deposition; PDMS – Polydimethyl siloxane; PFA – Poly-1H,1H,2H,2H -perfluorodecyl acrylate; PVP – Poly-N -vinyl pyrrolidone; SEM – scanning electron microscopy; SHS – superhydrophobic surface; θadv – advancing contact angle; θrec – receding contact angle.

Keywords Superhydrophobicity; Mechanical stability; Bioinspired; Skin Shedding; Surface Restoration.

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Supporting Information Available Videos of dynamic contact angle measurements on new and used Si nanograss and on PFA nanograss; video of the shedding process and a video demonstrating how to restore superhydrophobicity. For detailed descriptions, see the supporting information cover file.

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Graphical TOC Entry ① H2O

④

②

③

H2O ⑤

⑥

Skin shedding is presented for restoring superhydrophobicity after it has been lost by mechanical damage of the original surface.

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Molting Materials: Restoring Superhydrophobicity after Severe

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