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Superhydrophobic fluorine-free hierarchical coatings produced by vacuum based method
Materials Letters 167 (2016) 30–33
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Superhydrophobic fluorine-free hierarchical coatings produced by vacuum based method M. Petr, J. Hanuš, O. Kylián n, J. Kratochvíl, P. Solař, D. Slavínská, H. Biederman Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics, Prague, Czech Republic
art ic l e i nf o
a b s t r a c t
Article history: Received 13 August 2015 Received in revised form 14 October 2015 Accepted 27 December 2015 Available online 29 December 2015
In this study it is presented fully vacuum-based, substrate independent method for fabrication of fluorine-free superhydrophobic surfaces based on combination of gas aggregation sources of nanoparticles and plasma polymerization. This method involves deposition of films of nanoparticles that are subsequently overcoated by plasma polymerized n-hexane. Two different kinds of nanoparticles that differ significantly in their sizes were used: small Cu nanoparticles (mean diameter 18 nm) and big plasma polymerized C:H nanoparticles (diameter 110 nm). It is demonstrated that superhydrophobic and slippery character may be achieved when surfaces with dual-scale roughness were prepared combining bigger C:H nanoparticles with smaller Cu nanoparticles. & 2015 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobicity Plasma polymerization Hierarchical coatings Nanoparticles Gas aggregation sources
1. Introduction Fabrication of surfaces with controllable roughness is the key step for production of biomimetic superhydrophobic surfaces inspired by the self-cleaning and anti-fouling properties observed in nature [1–3]. Based on numerous studies it is well established that the roughened hydrophobic surfaces have a larger area of contact between the surface and water droplet which enhances their hydrophobic character (homogeneous wetting regime [4]). In addition, certain level of roughness of a hydrophobic surface may result in trapping air beneath the water causing an increase of the contact angle (heterogeneous wetting regime [5]). The heterogeneous wetting is furthermore characterized by lower contact angle hysteresis that makes it more favorable for technological applications. However, heterogeneous wetting is in many cases not stable and may switch spontaneously or by external disturbance to homogeneous wetting regime that has often lower free energy and thus is thermodynamically more favorable [6]. One possible strategy for production of surfaces with stable heterogeneous wetting is based on fabrication of surfaces with hierarchical structures providing multi-scale surface roughness [7]. In this case the multi-scale surface protrusions form a high potential barrier between homogeneous and heterogeneous wetting regimes or even make homogeneous wetting physically n
Corresponding author. E-mail address: [email protected] (O. Kylián).
http://dx.doi.org/10.1016/j.matlet.2015.12.126 0167-577X/& 2015 Elsevier B.V. All rights reserved.
unrealizable [8]. As consequence various methods for production of surfaces with multi-scale roughness were developed. They are based for instance on chemical synthesis, imprinting, templating, e-beam or lithographic methods, layer-by-layer deposition, plasma etching etc. (e.g. recent reviews [9,10]). More recently also aerosolassisted atmospheric plasma deposition was employed for surfaces with multi-scale roughness [11]. Although it was demonstrated that surfaces produced in these ways are superhydrophobic and slippery, the use of above-mentioned techniques is often hampered by certain limitations (e.g. use of potentially harmful solvents, time consumption, high costs and complexity or limited applicability to wider range of substrate materials). Moreover, majority of studies were focused on the fabrication of fluorocarbon coatings, since they possess the lowest surface energy. However, the use of fluorocarbons is somehow problematic, i.e. because of their possible health effects [12]. In this study it is introduced an alternative, fully vacuum based strategy of production of fluorine-free coatings that have dualscale roughness and superhydrophobic character. Proposed strategy is based on the combination of two independent gas aggregation sources of nanoparticles and plasma enhanced chemical vapor deposition of hydrocarbon plasma polymer.
2. Experimental details Set-up employed for samples preparation consisted of deposition chamber pumped by diffusion and rotary pumps, two gas
M. Petr et al. / Materials Letters 167 (2016) 30–33
aggregation sources (GAS) of nanoparticles and a source of plasma polymer. Si polished wafers, polypropylene foil and Si wafers coated with gold layer were used as substrate materials in this study. Small copper nanoparticles (NPs) – 18 76 nm in diameter – were prepared by GAS based on a DC magnetron sputtering from a Cu target (51 mm in diameter) in Ar atmosphere. The current was set to 200 mA and the pressure of Ar in the aggregation chamber was 33 Pa. Deposition time of Cu NPs was 40 s. Similar GAS system was used also in case of plasma polymerized C:H NPs. The plasma polymerized particles were produced using a gas mixture of argon and hexane 7:1 at pressure inside the aggregation chamber 160 Pa. The radio frequency (RF) power applied to planar magnetron (51 mm in diameter) equipped with a carbon target was 40 W and the deposition time of C:H NPs was 300 s. The mean size of produced C:H nanoparticles was 110 nm. More details regarding the production of Cu and C:H NPs are given in [13,14]. Hydrocarbon plasma polymer film, which was used as a hydrophobic over-coating, was prepared by plasma polymerization in a mixture of Ar/n-hexane (7:1) at pressure 2 Pa. The RF power 80 W was delivered to the planar electrode 80 mm in diameter. Morphology of deposited coatings was studied by means of scanning electron microscopy (SEM) (Tescan Lyra) and atomic force microscopy (AFM) (Quesant Q-Scope 350). Chemical composition of the coatings was investigated by X-ray photoelectron spectrometer (XPS) equipped with Al Kα X-ray source (1486.6 eV, XR50, Specs). Wettability of produced coatings was measured by sessile water droplet method using a goniometer of custom construction.
3. Results Four sets of coatings were produced in this study: 40 nm thick smooth C:H films, Cu nanoparticles overcoated by 7 nm thick C:H films, C:H nanoparticles overcoated by 50 nm thick C:H films and, finally, hierarchical coatings that are composed of layer of C:H nanoparticles, 50 nm thick layer of C:H, layer of Cu nanoparticles and top 7 nm thick layer of C:H. The schematic representation of
31
all prepared coatings is together with their SEM images presented in Fig. 1. The thickness of C:H layer was close to the one half of the mean diameter of the underlaying nanoparticles in order to fix them on a surface. All deposited samples were characterized by XPS. It has been found that surface elemental composition is equal for all samples independently of the presence of base layer of NPs. Only detectable peaks in XPS spectra were carbon (atomic concentration 957 1%) and oxygen (atomic concentration 5 71%). The presence of oxygen may be most likely attributed to the surface oxidation that occurred during the transfer of the samples from the deposition chamber to XPS. The second studied parameter was surface roughness. As it is depicted in Fig. 2, the size of the nanoparticles that were used as the base layer has strong impact on the morphology of the samples and on their roughness. The coatings without any nanoparticles were smooth and characterized by root-mean-square (RMS) roughness below 1 nm. In contrast, coatings with Cu nanoparticle base layer had the RMS roughness 47 1 nm and films, for which big C:H nanoparticles were used, reached the value of RMS roughness 88 79 nm. The enhancement of surface roughness can be explained by the increase of the size of nanoparticles used as the base film [15,16]. In addition, AFM measurements revealed that coatings, for whose production were combined C:H and Cu nanoparticles, had surface roughness similar to the films prepared using solely C:H nanoparticles as the base layer (87 710 nm). Finally, the wettability of produced coatings was tested. Images of water droplets on all four kinds of samples deposited onto Si wafers are presented in Fig. 3. As can be seen the presence of Cu and C:H nanoparticles in the base layer led to substantial enhancement of the water contact angle by 24° and 58° as compared to smooth C:H film. This is consistent with increased surface roughness observed for Cu and C:H nanoparticles containing coatings. Nevertheless, the value of water contact angle did not overcome 150° even in the case of samples with the highest roughness and the samples retained rather high water contact angle hysteresis. This situation markedly changed when small Cu and big C:H nanoparticles were combined and the surface had dual-scale
Fig. 1. SEM images and schematic representation of prepared samples.
32
M. Petr et al. / Materials Letters 167 (2016) 30–33
Fig. 2. Sections of AFM images of A) Cu NPs coated by 7 nm of C:H film, B) C:H nanoparticles coated by 50 nm of C:H film and C) surface with dual scale roughness i.e. C:H NPsþ 50 nm C:H film þ Cu NPsþ 7 nm film of C:H plasma polymer. In order to visualize both small and big structures the AFM images presented in the upper row have smaller height range.
roughness; in this case the coatings exhibited superhydrophobic and slippery character (Fig. 3D). Taking into account the same surface chemistry and similar surface roughness of the coatings prepared using only C:H nanoparticles and samples that combined both C:H and Cu nanoparticles it can be concluded that observed
superhydrophobicity is connected with hierarchical character of the deposited films. In addition, the same result, i.e. superhydrophobic and slippery character of prepared coatings, was observed also when polypropylene foil and Si wafers coated with gold layer were used as substrate materials.
Fig. 3. Water droplets on A) C:H smooth film, coatings that have base layer of B) Cu NPs C) C:H nanoparticles and D) sequence of images of water droplet approaching and receding the surface with dual scale roughness.
M. Petr et al. / Materials Letters 167 (2016) 30–33
4. Conclusion Fully vacuum-based method that makes it possible to fabricate fluorine-free superhydrophobic coatings was developed. It is based on production of hydrocarbon surface with dual-scale roughness that is achieved by deposition of base layer of nanoparticles with different sizes. This approach brings certain advantages as compared to methods based on chemical synthesis or methods employing lithography. First of all, the process is relatively fast and since no potentially harmful chemicals are used also environmentally friendly. The second key advantage is possibility to coat virtually any substrate materials including polymers or metals, which enlarges the possible range of applications of this process.
Acknowledgments This work was supported by grant 13-09853S from the Grant Agency of the Czech Republic.
References [1] F. Xia, L. Jiang, Bio-inspired, smart, multiscale interfacial materials, Adv. Mater. 20 (2008) 2842–2858. [2] K. Liu, X. Yao, L. Jiang, Recent developments in bio-inspired special wettability, Chem. Soc. Rev. 39 (2010) 3240–3255. [3] E. Bormashenko, O. Gendelman, G. Whyman, Superhydrophobicity of Lotus Leaves versus Birds Wings: different physical mechanisms leading to similar
33
phenomena, Langmuir 28 (2012) 14992–14997. [4] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994. [5] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [6] C. Ishino, K. Okumura, D. Quéré, Wetting transitions on rough surfaces, Eur. Lett. 68 (2004) 419–425. [7] P. Roach, N.J. Shirtcliffe, M.I. Newton, Progress in superhydrophobic surface development, Soft Matter 4 (2008) 224–240. [8] N.A. Patankar, Mimicking the lotus effect: influence of double roughness structures and slender pillars, Langmuir 20 (2004) 8209–8213. [9] Y.Y. Yan, N. Gao, W. Barthlott, Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces, Adv. Colloid Interface Sci. 169 (2011) 80–105. [10] E. Gogolides, K. Ellinas, A. Tserepi, Hierarchical micro and nano structured, hydrophilic, superhydrophobic and superoleophobic surfaces incorporated in microfluidics, microarrays and lab on chip microsystems, Microelectron. Eng. 132 (2015) 135–155. [11] F. Fanelli, A.M. Mastrangelo, F. Fracassi, Aerosol-assisted atmospheric cold plasma deposition and characterization of superhydrophobic organic–inorganic nanocomposite thin films, Langmuir 30 (2014) 857–865. [12] K. Steenland, T. Fletcher, D.A. Savitz, Epidemiologic evidence on the health effects of perfluorooctanoic acid (PFOA), Environ. Health Perspect. 18 (2010) 1100–1108. [13] O. Kylián, J. Kratochvíl, J. Hanuš, O. Polonskyi, P. Solař, H. Biederman, Fabrication of Cu nanoclusters and their use for production of Cu/plasma polymer nanocomposite thin films, Thin Solid Films 550 (2014) 46–52. [14] P. Solař, O. Polonskyi, A. Choukourov, A. Artemenko, J. Hanuš, H. Biederman, D. Slavínská, Nanostructured thin films prepared from cluster beams, Surf. Coat. Technol. 205 (2011) S42–S47. [15] O. Kylián, O. Polonskyi, J. Kratochvíl, A. Artemenko, A. Choukourov, M. Drábik, P. Solař, D. Slavínská, H. Biederman, Control of wettability of plasma polymers by application of Ti nano-clusters, Plasma Process. Polym. 9 (2012) 180–187. [16] O. Kylián, M. Petr, A. Solař, P. Solar, O. Polonskyi, J. Hanuš, A. Choukourov, H. Biederman, Hydrophobic and super-hydrophobic coatings based on nanoparticles overcoated by fluorocarbon plasma polymer, Vacuum 100 (2014) 57–60.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Superhydrophobic fluorine-free hierarchical coatings produced by vacuum based method M. Petr, J. Hanuš, O. Kylián n, J. Kratochvíl, P. Solař, D. Slavínská, H. Biederman Charles University in Prague, Faculty of Mathematics and Physics, Department of Macromolecular Physics, Prague, Czech Republic
art ic l e i nf o
a b s t r a c t
Article history: Received 13 August 2015 Received in revised form 14 October 2015 Accepted 27 December 2015 Available online 29 December 2015
In this study it is presented fully vacuum-based, substrate independent method for fabrication of fluorine-free superhydrophobic surfaces based on combination of gas aggregation sources of nanoparticles and plasma polymerization. This method involves deposition of films of nanoparticles that are subsequently overcoated by plasma polymerized n-hexane. Two different kinds of nanoparticles that differ significantly in their sizes were used: small Cu nanoparticles (mean diameter 18 nm) and big plasma polymerized C:H nanoparticles (diameter 110 nm). It is demonstrated that superhydrophobic and slippery character may be achieved when surfaces with dual-scale roughness were prepared combining bigger C:H nanoparticles with smaller Cu nanoparticles. & 2015 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobicity Plasma polymerization Hierarchical coatings Nanoparticles Gas aggregation sources
1. Introduction Fabrication of surfaces with controllable roughness is the key step for production of biomimetic superhydrophobic surfaces inspired by the self-cleaning and anti-fouling properties observed in nature [1–3]. Based on numerous studies it is well established that the roughened hydrophobic surfaces have a larger area of contact between the surface and water droplet which enhances their hydrophobic character (homogeneous wetting regime [4]). In addition, certain level of roughness of a hydrophobic surface may result in trapping air beneath the water causing an increase of the contact angle (heterogeneous wetting regime [5]). The heterogeneous wetting is furthermore characterized by lower contact angle hysteresis that makes it more favorable for technological applications. However, heterogeneous wetting is in many cases not stable and may switch spontaneously or by external disturbance to homogeneous wetting regime that has often lower free energy and thus is thermodynamically more favorable [6]. One possible strategy for production of surfaces with stable heterogeneous wetting is based on fabrication of surfaces with hierarchical structures providing multi-scale surface roughness [7]. In this case the multi-scale surface protrusions form a high potential barrier between homogeneous and heterogeneous wetting regimes or even make homogeneous wetting physically n
Corresponding author. E-mail address: [email protected] (O. Kylián).
http://dx.doi.org/10.1016/j.matlet.2015.12.126 0167-577X/& 2015 Elsevier B.V. All rights reserved.
unrealizable [8]. As consequence various methods for production of surfaces with multi-scale roughness were developed. They are based for instance on chemical synthesis, imprinting, templating, e-beam or lithographic methods, layer-by-layer deposition, plasma etching etc. (e.g. recent reviews [9,10]). More recently also aerosolassisted atmospheric plasma deposition was employed for surfaces with multi-scale roughness [11]. Although it was demonstrated that surfaces produced in these ways are superhydrophobic and slippery, the use of above-mentioned techniques is often hampered by certain limitations (e.g. use of potentially harmful solvents, time consumption, high costs and complexity or limited applicability to wider range of substrate materials). Moreover, majority of studies were focused on the fabrication of fluorocarbon coatings, since they possess the lowest surface energy. However, the use of fluorocarbons is somehow problematic, i.e. because of their possible health effects [12]. In this study it is introduced an alternative, fully vacuum based strategy of production of fluorine-free coatings that have dualscale roughness and superhydrophobic character. Proposed strategy is based on the combination of two independent gas aggregation sources of nanoparticles and plasma enhanced chemical vapor deposition of hydrocarbon plasma polymer.
2. Experimental details Set-up employed for samples preparation consisted of deposition chamber pumped by diffusion and rotary pumps, two gas
M. Petr et al. / Materials Letters 167 (2016) 30–33
aggregation sources (GAS) of nanoparticles and a source of plasma polymer. Si polished wafers, polypropylene foil and Si wafers coated with gold layer were used as substrate materials in this study. Small copper nanoparticles (NPs) – 18 76 nm in diameter – were prepared by GAS based on a DC magnetron sputtering from a Cu target (51 mm in diameter) in Ar atmosphere. The current was set to 200 mA and the pressure of Ar in the aggregation chamber was 33 Pa. Deposition time of Cu NPs was 40 s. Similar GAS system was used also in case of plasma polymerized C:H NPs. The plasma polymerized particles were produced using a gas mixture of argon and hexane 7:1 at pressure inside the aggregation chamber 160 Pa. The radio frequency (RF) power applied to planar magnetron (51 mm in diameter) equipped with a carbon target was 40 W and the deposition time of C:H NPs was 300 s. The mean size of produced C:H nanoparticles was 110 nm. More details regarding the production of Cu and C:H NPs are given in [13,14]. Hydrocarbon plasma polymer film, which was used as a hydrophobic over-coating, was prepared by plasma polymerization in a mixture of Ar/n-hexane (7:1) at pressure 2 Pa. The RF power 80 W was delivered to the planar electrode 80 mm in diameter. Morphology of deposited coatings was studied by means of scanning electron microscopy (SEM) (Tescan Lyra) and atomic force microscopy (AFM) (Quesant Q-Scope 350). Chemical composition of the coatings was investigated by X-ray photoelectron spectrometer (XPS) equipped with Al Kα X-ray source (1486.6 eV, XR50, Specs). Wettability of produced coatings was measured by sessile water droplet method using a goniometer of custom construction.
3. Results Four sets of coatings were produced in this study: 40 nm thick smooth C:H films, Cu nanoparticles overcoated by 7 nm thick C:H films, C:H nanoparticles overcoated by 50 nm thick C:H films and, finally, hierarchical coatings that are composed of layer of C:H nanoparticles, 50 nm thick layer of C:H, layer of Cu nanoparticles and top 7 nm thick layer of C:H. The schematic representation of
31
all prepared coatings is together with their SEM images presented in Fig. 1. The thickness of C:H layer was close to the one half of the mean diameter of the underlaying nanoparticles in order to fix them on a surface. All deposited samples were characterized by XPS. It has been found that surface elemental composition is equal for all samples independently of the presence of base layer of NPs. Only detectable peaks in XPS spectra were carbon (atomic concentration 957 1%) and oxygen (atomic concentration 5 71%). The presence of oxygen may be most likely attributed to the surface oxidation that occurred during the transfer of the samples from the deposition chamber to XPS. The second studied parameter was surface roughness. As it is depicted in Fig. 2, the size of the nanoparticles that were used as the base layer has strong impact on the morphology of the samples and on their roughness. The coatings without any nanoparticles were smooth and characterized by root-mean-square (RMS) roughness below 1 nm. In contrast, coatings with Cu nanoparticle base layer had the RMS roughness 47 1 nm and films, for which big C:H nanoparticles were used, reached the value of RMS roughness 88 79 nm. The enhancement of surface roughness can be explained by the increase of the size of nanoparticles used as the base film [15,16]. In addition, AFM measurements revealed that coatings, for whose production were combined C:H and Cu nanoparticles, had surface roughness similar to the films prepared using solely C:H nanoparticles as the base layer (87 710 nm). Finally, the wettability of produced coatings was tested. Images of water droplets on all four kinds of samples deposited onto Si wafers are presented in Fig. 3. As can be seen the presence of Cu and C:H nanoparticles in the base layer led to substantial enhancement of the water contact angle by 24° and 58° as compared to smooth C:H film. This is consistent with increased surface roughness observed for Cu and C:H nanoparticles containing coatings. Nevertheless, the value of water contact angle did not overcome 150° even in the case of samples with the highest roughness and the samples retained rather high water contact angle hysteresis. This situation markedly changed when small Cu and big C:H nanoparticles were combined and the surface had dual-scale
Fig. 1. SEM images and schematic representation of prepared samples.
32
M. Petr et al. / Materials Letters 167 (2016) 30–33
Fig. 2. Sections of AFM images of A) Cu NPs coated by 7 nm of C:H film, B) C:H nanoparticles coated by 50 nm of C:H film and C) surface with dual scale roughness i.e. C:H NPsþ 50 nm C:H film þ Cu NPsþ 7 nm film of C:H plasma polymer. In order to visualize both small and big structures the AFM images presented in the upper row have smaller height range.
roughness; in this case the coatings exhibited superhydrophobic and slippery character (Fig. 3D). Taking into account the same surface chemistry and similar surface roughness of the coatings prepared using only C:H nanoparticles and samples that combined both C:H and Cu nanoparticles it can be concluded that observed
superhydrophobicity is connected with hierarchical character of the deposited films. In addition, the same result, i.e. superhydrophobic and slippery character of prepared coatings, was observed also when polypropylene foil and Si wafers coated with gold layer were used as substrate materials.
Fig. 3. Water droplets on A) C:H smooth film, coatings that have base layer of B) Cu NPs C) C:H nanoparticles and D) sequence of images of water droplet approaching and receding the surface with dual scale roughness.
M. Petr et al. / Materials Letters 167 (2016) 30–33
4. Conclusion Fully vacuum-based method that makes it possible to fabricate fluorine-free superhydrophobic coatings was developed. It is based on production of hydrocarbon surface with dual-scale roughness that is achieved by deposition of base layer of nanoparticles with different sizes. This approach brings certain advantages as compared to methods based on chemical synthesis or methods employing lithography. First of all, the process is relatively fast and since no potentially harmful chemicals are used also environmentally friendly. The second key advantage is possibility to coat virtually any substrate materials including polymers or metals, which enlarges the possible range of applications of this process.
Acknowledgments This work was supported by grant 13-09853S from the Grant Agency of the Czech Republic.
References [1] F. Xia, L. Jiang, Bio-inspired, smart, multiscale interfacial materials, Adv. Mater. 20 (2008) 2842–2858. [2] K. Liu, X. Yao, L. Jiang, Recent developments in bio-inspired special wettability, Chem. Soc. Rev. 39 (2010) 3240–3255. [3] E. Bormashenko, O. Gendelman, G. Whyman, Superhydrophobicity of Lotus Leaves versus Birds Wings: different physical mechanisms leading to similar
33
phenomena, Langmuir 28 (2012) 14992–14997. [4] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994. [5] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [6] C. Ishino, K. Okumura, D. Quéré, Wetting transitions on rough surfaces, Eur. Lett. 68 (2004) 419–425. [7] P. Roach, N.J. Shirtcliffe, M.I. Newton, Progress in superhydrophobic surface development, Soft Matter 4 (2008) 224–240. [8] N.A. Patankar, Mimicking the lotus effect: influence of double roughness structures and slender pillars, Langmuir 20 (2004) 8209–8213. [9] Y.Y. Yan, N. Gao, W. Barthlott, Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces, Adv. Colloid Interface Sci. 169 (2011) 80–105. [10] E. Gogolides, K. Ellinas, A. Tserepi, Hierarchical micro and nano structured, hydrophilic, superhydrophobic and superoleophobic surfaces incorporated in microfluidics, microarrays and lab on chip microsystems, Microelectron. Eng. 132 (2015) 135–155. [11] F. Fanelli, A.M. Mastrangelo, F. Fracassi, Aerosol-assisted atmospheric cold plasma deposition and characterization of superhydrophobic organic–inorganic nanocomposite thin films, Langmuir 30 (2014) 857–865. [12] K. Steenland, T. Fletcher, D.A. Savitz, Epidemiologic evidence on the health effects of perfluorooctanoic acid (PFOA), Environ. Health Perspect. 18 (2010) 1100–1108. [13] O. Kylián, J. Kratochvíl, J. Hanuš, O. Polonskyi, P. Solař, H. Biederman, Fabrication of Cu nanoclusters and their use for production of Cu/plasma polymer nanocomposite thin films, Thin Solid Films 550 (2014) 46–52. [14] P. Solař, O. Polonskyi, A. Choukourov, A. Artemenko, J. Hanuš, H. Biederman, D. Slavínská, Nanostructured thin films prepared from cluster beams, Surf. Coat. Technol. 205 (2011) S42–S47. [15] O. Kylián, O. Polonskyi, J. Kratochvíl, A. Artemenko, A. Choukourov, M. Drábik, P. Solař, D. Slavínská, H. Biederman, Control of wettability of plasma polymers by application of Ti nano-clusters, Plasma Process. Polym. 9 (2012) 180–187. [16] O. Kylián, M. Petr, A. Solař, P. Solar, O. Polonskyi, J. Hanuš, A. Choukourov, H. Biederman, Hydrophobic and super-hydrophobic coatings based on nanoparticles overcoated by fluorocarbon plasma polymer, Vacuum 100 (2014) 57–60.