Self-cleaning and wear-resistant polymer nanocomposite surfaces

Accepted Manuscript Self-cleaning and wear-resistant polymer nanocomposite surfaces

P. Cully, F. Karasu, L. Müller, T. Jauzein, Y. Leterrier PII: DOI: Reference:

S0257-8972(18)30515-2 doi:10.1016/j.surfcoat.2018.05.040 SCT 23415

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

13 February 2018 1 May 2018 21 May 2018

Please cite this article as: P. Cully, F. Karasu, L. Müller, T. Jauzein, Y. Leterrier , Selfcleaning and wear-resistant polymer nanocomposite surfaces. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.05.040

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ACCEPTED MANUSCRIPT Self-cleaning and wear-resistant polymer nanocomposite surfaces

P. Cully1,2, F. Karasu1, L. Müller1, T. Jauzein2, Y. Leterrier1*

(1) Laboratory for Processing of Advanced Composites (LPAC), Ecole Polytechnique

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Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

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(2) SICPA SA, Av. de Florissant 41, CH-1008 Prilly, Switzerland

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(*) corresponding author

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ACCEPTED MANUSCRIPT Abstract

Superhydrophobic self-cleaning and wear-resistant nanocomposite surfaces were produced by mimicking the hierarchical structure of the lotus leaf using a combination of rapid selfassembly and a UV nano-imprint lithography (UVNIL) process with a silicone master. Two

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different acrylate formulations containing acrylated silica nanoparticles and an acrylated

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silicone surfactant were used. The presence of the silicone master did not suppress the

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spontaneous migration of the surfactant to the polymer surface, which increased its hydrophobic character. Adding acrylated silica particles considerably increased the viscosity

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of the acrylate suspensions and led to a shear-thinning behavior. However the particles did not prevent the fast migration process of the surfactant and further increased the hydrophobicity

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of the material, due to increased nanoscale roughness of the nanocomposite surface. The largest increase of hydrophobicity was achieved for the UVNIL printed lotus surfaces using

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the acrylate formulation with lowest viscosity. These surfaces became superhydrophobic for

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the highest investigated concentration of silica. These nanocomposite lotus surfaces were, in addition, very hard with a microhardness above 400 MPa and particularly wear-resistant, and

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Keywords

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were self-cleaning with respect to hydrophobic contamination.

Bioinspired; self-cleaning; wear resistant; polymer composites; photopolymerization; silica

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ACCEPTED MANUSCRIPT 1. Introduction

Natural surfaces possess unique properties with specific evolutionary benefits. In particular, the well-known lotus leaf is self-cleaning for improved photosynthesis owing to dust removal by rain droplets [1]. Detailed knowledge gained over the last decades on the structure-

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property relations of natural surfaces has motivated considerable efforts to synthesize artificial

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surfaces with similar, or even improved properties [2–6]. Today, bioinspired materials and

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surfaces represent one of the fast-growing research fields, with considerable potential for cost and resources savings. For instance reduction of fouling of ship surfaces by sea organisms

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using superhydrophobic textures may lead to a more than a twofold decrease of running costs of sea transportation [7]. Further examples include soft robot skins and various biomedical

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devices and implants, food and pharmaceutical packaging, support and tools to reduce bacterial contaminations, improve durability, minimize waste, and improve reuse or recycling

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through reduction of cleaning operations.

Self-cleaning with water is associated with superhydrophobicity and results from the

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combination of a low surface energy and a hierarchical, sub-micron scale roughness [8]. Superhydrophobic surfaces display contact angles with water (WCA) greater than 150° and a

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sliding angle (WSA) below 10° as critically reviewed by Bhushan [1]. The first condition to achieve a self-cleaning surface is to create a hierarchical morphology [9]. Surfaces with hierarchical textures can be produced using nanofabrication methods [10], primarily through ‘top-down’ approaches evolving from microfabrication [10–12] such as thermal imprint [13,14] and nanoimprint lithography [15,16]. The second condition is that the surface energy is as low as possible, which is ideally realized with fluorine (surface energy as low as 6 mJ/m2 for organized -CF3 surfaces [17,18]). Notice that this second condition maybe relaxed for

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ACCEPTED MANUSCRIPT special self-affine surfaces [19]. Most works indeed rely on conventional surface treatment of previously texturized materials using fluorinated moieties [20–22] however with concerns over their biopersistence and cost-effectiveness [23]. An alternative approach demonstrated in the 1990s to reduce the surface energy of polymers is based on the spontaneous, enthalpydriven migration of surfactants and resulting segregation at the polymer-air interface [24–29].

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We recently reported the development of superhydrophobic surfaces based on the

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combination of this segregation phenomena with a cost-effective, low-pressure UV replication

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process of various plant surfaces [30]. An acrylated hyperbranched polymer was used to benefit from its low shrinkage behavior and led to a very high replication fidelity of the

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natural surfaces and a glass transition temperature well above room temperature. However,

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the mechanical endurance of these surfaces was not particularly investigated.

A drawback of synthetic polymer surfaces is their lack of mechanical robustness. The addition

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of hard inorganic particles to soft polymers is classically used to improve the hardness [31]

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and wear resistance [32] of the polymer. In fact, reports of self-cleaning surfaces that are also hard and wear resistant are rather scarce. Present developments are based on metals [33,34]

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and ceramics [21], however with a lack of adequate durability and moreover based on complex, energy intensive processes as reviewed in [35], and on polymer nanocomposites.

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Examples of the latter type of materials include composites with surfactant-functionalized silica particles [36], colloidal silica-based raspberry-like particles synthesized with a multiplestep method [37], or coatings produced using facile spray processes [38]. These are nevertheless limited to particle fractions below few vol%, hence with limited improvement in wear resistance [38,39]. Interestingly, the addition of nanoparticles usually increases the roughness of the polymer surface [36,40]. This is in turn expected to increase the hydrophobicity of the surface, providing that it is intrinsically hydrophobic [1].

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ACCEPTED MANUSCRIPT The objective of the present work was to develop polymer/silica nanocomposite replica of superhydrophobic plant surfaces, which would be far more mechanically robust than the original plants, and polymer surfaces in general, using fluorine-free precursors and a resource efficient UV printing process. The main challenges were twofold. First, the addition of nanosized particles into low viscosity oligomers usually leads to a problematic liquid-to-solid

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transition at very low particle loadings [41], which may have a catastrophic effect on the low-

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pressure, nanoscale replication fidelity. Secondly, the dense network of nanoparticles may act

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as a filter for the migration of the surfactant molecules, and therefore suppress the

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superhydrophobic effect.

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2. Experimental section

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2.1. Materials and processing

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Two different polyester acrylates were used. The first (Acryl-1) was a hyperbranched acrylate with a theoretical functionality, i.e. number of reactive end-groups of 16 and a real one of 13

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(CN2303, Sartomer, density 1.13 g/cm3), and which features reduced shrinkage upon curing [42,43]. Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO, Esacure) was used as a

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photoinitiator at a concentration of 6 wt%. The second (Acryl-2) was a formulation based on an amine modified polyester acrylate (Photocryl DP143, 48, Miwon Specialty Chemical, density 1.10 g/cm3, at a concentration of 48 wt%), with two reactive diluents (1,6-hexanediol diacrylate, HDDA, Allnex at a concentration of 15 wt% and 4-acryloylmorpholine, ACMO, Sigma-Aldrich at a concentration of 12 wt%), a mixture of pentaerythritol tri- and tetraacrylate (PETIA, Allnex, at a concentration of 10 wt%) and ditrimethylolpropane tetraacrylate (Ebecryl 140, Allnex, at a concentration of 10 wt%) and a difunctional alpha hydroxy ketone

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ACCEPTED MANUSCRIPT photoinitiator (KIP 160, IGM Resins, at a concentration of 7 wt%). An acrylated siloxane surfactant (Tego Rad 2010, Evonik) was added to both Acryl-1 and Acryl-2 formulations at a concentration of 0.8 wt%. The chemical structures of the compounds are given in Figure 1 (at the exception of the amine modified polyester acrylate Acryl-2, which was not provided by the supplier). In the following, Acryl-1S and Acryl-2S will refer to the acrylate-surfactant

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mixtures. Acrylated fumed silica particles (Aerosil R7200, Evonik) were added to both Acryl-

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1S and Acryl-2S at a concentration of 25 vol%. Aerosil R7200 is a powder of aggregated

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particles with a specific surface area of about 150 m2/g and a primary particle size of 12 nm. Homogenisation was carried out by manual stirring and the composite suspension was

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degassed by centrifugation during 5 min at 5000 rpm. Suspensions with silica fractions in the range 5 to 20 vol% were subsequently obtained by dilution of the initial suspension in the

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oligomer formulation. The volume concentrations were calculated from the weight fractions using the above-mentioned densities of the acrylates and a density of 1.9 g/cm3 for Aerosil.

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We did not investigate particle loadings beyond 25 vol% due to the emergence of a liquid-to-

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solid transition and associated yield stress in the nanocomposite suspension. Notice that the acrylation of silica enabled to postpone the occurrence of yield stress, which in the case of

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untreated silica was found to occur at a concentration as low as 5 vol% [41]. This phenomenon is rather common for submicron particles due to the very large specific

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interfacial area between the particles and the suspension medium and practically results in severe mixing problems.

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Figure 1. Representations of hyperbranched polyester acrylate (Acryl-1, a) and acrylated siloxane surfactant (Tego Rad 2010, d); chemical structures of pentaerythritol tri- and tetraacrylate (PETIA, b), ditrimethylolpropane tetraacrylate (Ebecryl 140, c), 1,6-hexanediol diacrylate (HDDA, e), 4-acryloylmorpholine (ACMO, f), Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO, g) and difunctional alpha hydroxy ketone photoinitiator (KIP 160, h).

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The surface of fresh lotus leaves (Nelumbo Lutea) was replicated in the composite formulations using an intermediate negative polydimethylsiloxane master (PDMS, Sylgard

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184, Dow Corning) and a UV-nanoimprint lithography process (UVNIL), as depicted in

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Scheme 1 and detailed in [44]. This soft, vacuum-free and ambient molding technique preserved the delicate biological surfaces from damage, and enabled to accurately reproduce

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their nanometer scale features [30]. In short, leaves were cleaned with deionized water and gently air-blown, using the self-cleaning property of the leaf. Square samples (2 cm x 2 cm)

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were then cut from the lotus leaf and were fixed onto Petri dishes. PDMS mixed with hardener (10:1 ratio) was poured onto the samples and air bubbles were removed under a reduced pressure for 2 h. The PDMS was subsequently cured at room temperature for 4 days. The resulting PDMS master (which is the negative of the lotus leaf surface) was demolded just before the UVNIL process in order to avoid contamination from air. A UVNIL tool equipped with independent control of UV exposure and pressure was used to print the composite surface with the PDMS master. The liquid composite formulation was first poured

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ACCEPTED MANUSCRIPT on a glass slide and the PDMS master was rapidly placed on the top, so that the time available for the expected migration of the surfactant to the polymer-air interface was limited to few s. Curing was then performed with illumination through the UV-transparent master using a 200 W high-pressure mercury lamp (OmniCure 2000, EXFO, Canada) during 30 s under a light intensity of 75 mW/cm2 and a pressure of 3 bars. The light intensity between 230 and

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410 nm was measured using a calibrated radiometer (Silver Line, CON-TROL-CURE,

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Germany). The printed composite surfaces were finally carefully demolded. A similar

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procedure was applied to create flat, control composite surfaces. In that case a flat PDMS master was prepared by pouring and curing the PDMS resin in direct contact with the flat

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surface of a Petri dish. Notice that for the Acryl-1S surfaces polymerized under air, the

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surfaces were non-sticky and hard.

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concentration of photoinitiator was high enough to overcome oxygen inhibition and the cured

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Scheme 1. Schematic representation of the PDMS replication and UVNIL processes.

2.2. Characterization methods

The viscosity of the composite suspensions was determined using oscillatory shear rheometry (AR2000, TA instruments). Suspensions were tested using a plate-plate geometry of diameter 2 cm and a gap of 560 μm. A strain amplitude sweep was performed first for all silica

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ACCEPTED MANUSCRIPT concentrations to determine the limit for the linear viscoelastic range. A frequency sweep was then carried out between 1 and 69 rad/s.

The migration of the silicone surfactant at the surface of Acryl-1S samples polymerized in contact with the PDMS template or in contact with air, was analyzed using X-Ray

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photoelectron spectroscopy measurements (XPS, PHI VersaProbe II scanning XPS

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microprobe, Physical Instruments AG, Germany). Acryl-1 samples (i.e., without surfactant)

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were used as controls. The analyses were performed using a monochromatic Al Kα X-ray source of 24.8 W power with a beam size of 100 µm. The spherical capacitor analyzer was set

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at 45° take-off angle with respect to the sample surface. The pass energy was 46.95 eV yielding a full width at half maximum of 0.91 eV for the Ag 3d 5/2 peak. Curve fitting was

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performed using the PHI Multipak software. The amount of Si was calculated from the Si2p peak of oxidized Si at 103.3 eV since its intensity is higher than that of the Si2s peak, using a

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relative scale factor equal to 0.368.

The WCA of the flat and printed composite surfaces was measured using a contact angle

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meter (EasyDrop, Krüss GmbH) at room temperature, with deionized water and a droplet volume of 10 μl. Four WCA measurements were made on each sample and the values were

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averaged. The WSA of printed composite surfaces was measured using 5 µl deionized water droplets, as the angle of the surface when the droplet started to roll with respect to the horizontal reference.

Atomic force microscopy analyses of the flat composite surfaces (i.e., non-texturized with the lotus surface) were performed in tapping mode (Multimode AFM, Nanoscope IIIa) with a silicon cantilever (HQ:NSC16/No Al, Micromash). The root mean square (RMS) roughness,

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ACCEPTED MANUSCRIPT defined as the RMS average of the height deviations from the mean value was measured on 1 x 1 µm2 areas. The printed surfaces were too rough and could not be characterized with the AFM. Instead, they were analyzed using scanning electron microscopy (SEM, Zeiss Merlin). Samples were attached to a conductive carbon sticker and a 8 nm thick carbon coating was

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deposited on the surface to avoid charging effects.

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The cross section of printed composites was imaged by SEM, after preparation using a focus

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ion beam method (FIB, Zeiss NVision 40). Samples were attached to a conductive carbon sticker. A 30 nm thick gold layer was deposited first on the printed surface to emphasize

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contrast. A 1.2 µm thick carbon layer was subsequently deposited by ion-beam to protect the sample during further FIB processing. After aligning e-beam and ion beam, gallium ions were

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accomplished with a current of 3 nA.

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accelerated at an energy of 30 keV to etch material, and polishing of the cross section was

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The Young's modulus and hardness of the composite surfaces polymerized in contact with a flat PDMS mold were measured by nanoindentation using a Berkovich indenter (Anton Paar

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NHT3 nanoindenter). Flat, 200-300 µm thick coating samples on a glass slide support were tested. Indentation was carried out with a load of 10 mN, a loading time of 6 s, a creep time of

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15 s and a final unloading time of 6 s. The indentation depth was in all cases below 3 µm so that there was no influence of the glass substrate. At least 20 measurements were carried out on each sample. The Oliver and Pharr method was used to determine the elastic modulus and hardness of the samples [45], assuming a Poisson’s ratio of the polymer and composite samples equal to 0.3. Attention was moreover paid to the possible influences of creep and of indentation depth on the calculated Young's modulus and hardness values. The ratio between the strain rate at the beginning of unloading and the strain rate at the end of the 15 s creep

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ACCEPTED MANUSCRIPT period was found to range from 13 for unfilled Acryl-2S to 20 for Acryl-2S with 25 vol% of silica. These values are well above the threshold value of 10 so that elastic unloading could be assumed [46]. This was further checked by analyzing the influence of creep on the apparent contact stiffness, following the correction procedure devised by Feng and Ngan [47]. It turned out that the Young's modulus values extracted with the Oliver and Pharr method slightly

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overestimated the corrected values by 6-7% for unfilled resins, and less than 5% for the

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highly filled composites, whereas the reported hardness values underestimated the corrected

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values by 5% for unfilled resins and by 10% for highly filled composites. The influence of penetration depth was examined for the Acryl-1S resin (without silica). Four different loads

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were used, namely 10, 15, 20 and 30 mN, corresponding to indentation depths of 2.3, 2.9, 3.3 and 3.9 µm. A slight increase of Young's modulus (15 kPa/mN) and hardness (520 kPa/mN)

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was obtained, which resulted from increasing creep effects, possibly combined with hardening. The Young's modulus and hardness reported in the following were obtained for the

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smallest load of 10 mN.

The wear behavior of the printed composites was determined using SEM observations and

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WCA measurements of samples subjected to an oscillating sand abrasion test, following ASTM standard F735 − 17. This method is relevant to characterize the wear endurance of

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polymer surfaces. Samples were attached to the bottom of a 15 cm x 16 cm box, which was filled with 600 g of sand following the requirement of the ASTM standard. Sand oscillated circularly at 700 rpm during 10 min, corresponding to 7000 abrasion cycles.

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ACCEPTED MANUSCRIPT 3. Results and discussion

3.1. Rheology of suspensions

Figure 2 shows the complex viscosity of Acryl-1S and Acryl-2S suspensions, for silica

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volume fractions ranging from 0 to 25%. The two acrylates were Newtonian fluids with

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viscosity in the range 0.1 to 1 Pa.s, Acryl-2S being three times less viscous owing to the

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presence of reactive diluents. For both resins a considerable increase of viscosity upon loading with silica due to colloidal interactions is evident, but it was much less pronounced in

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case of the Acryl-2S. At the maximum investigated silica fraction of 25 vol%, the increase was around three orders of magnitude and 300 times for Acryl-1S and Acryl-2S, respectively,

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and the viscosity of the Acryl-2S suspension was five times lower than that of the Acryl-1S suspension. It is also evident that the composite suspensions were shear-thinning. The inset in

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the two graphs shows the influence of silica on the shear-thinning exponent, n. For the Acryl-

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1S suspensions the exponent n progressively decreased from the Newtonian regime (n = 1) to rather low values below 0.3. In contrast, the Acryl-2S suspensions kept a Newtonian behavior

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up to a silica concentration of 10 vol%, beyond which they also became shear-thinning, but with exponents above 0.5. Such large viscosity increases and the emergence of a non-

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Newtonian behavior at rather low concentrations of particles is characteristic of nanocomposite suspensions, and result from the considerable interfacial area in these systems [41]. In the present case, the acrylation of the silica surface was assumed to suppress H-bonds between OH-terminated silica and acrylate functions of the resins. This was found to postpone the occurrence of gelation to silica concentrations above 25 vol% [41].

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Figure 2. Complex viscosity * vs. angular frequency  for Acryl-1S (a) and Acryl-2S (b) suspensions, for silica volume fractions ranging from 0 to 25% as indicated. The inset shows the shear-thinning index n vs. silica fraction .

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3.2. Migration of surfactant

The XPS spectra of Acryl-1 and Acryl-1S polymerized in contact with air or with PDMS are

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shown in Figure 3. The corresponding atomic concentrations are reported in Table 1. A

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similar amount of Si, close to 1 at%, was detected when no surfactant was added to the acrylate, irrespective of the presence of PDMS or air at the surface during curing, and in fact

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resulted from various contaminations. This shows that PDMS did not contaminate the polymerized surface. The superficial concentration of Si in the resin cured in contact with air

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or with PDMS was well above the initial concentration of Si in the Acryl-1S mixture, which was close to 0.1 wt%, using a concentration of Si in the surfactant equal to 18 wt%. This is the signature of the spontaneous migration of the surfactant at the polymer-air interface, driven by the gradient in surface energy [24], which occurred within the few seconds between the application of the liquid formulation on the glass carrier and the onset of photopolymerization. Notice that the concentration of Si at the surface was reduced in the case of

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ACCEPTED MANUSCRIPT contact with PDMS, because the silicone elastomer (surface energy 19.8 mJ/m2 [48]) reduced

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the surface energy contrast responsible for the migration.

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Figure 3. High-resolution XPS spectra of Acryl-1 and Acryl-1S polymerized in contact with air or with PDMS. The spectra have been shifted vertically for legibility. The inset highlights the Si2p peak.

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Table 1. Integrated peaks of high resolution XPS (Atomic concentration, %) of Acryl-1 and Acryl-1S polymerized in contact with air or with PDMS. Contact

C1s

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Si2p

No (Acryl-1)

Air

68.54

30.06

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Yes (Acryl-1S)

Air

59.63

28.93

11.44

PDMS

68.12

30.17

1.71

PDMS

63.83

30.36

5.81

No (Acryl-1)

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Yes (Acryl-1S)

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Surfactant

3.3. Wetting analysis

Figure 4 shows the WCA of flat and printed composite surfaces vs. acrylated silica particle volume fraction for both types of acrylate resins, polymerized in contact with a flat or a negative lotus PDMS master, respectively. The WSA of highly filled, printed composite surfaces is also reported in the Figure. The WCA of the two acrylates with surfactant, but

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ACCEPTED MANUSCRIPT without silica, was very close to 90°, at the limit of hydrophobicity. This value is approximately 20° higher than that of Acryl-1 (i.e., without surfactant) equal to 72° [30], which confirmed the presence of the silicone surfactant at the surface of the polymerized material as already shown with XPS. The addition of silica enabled to further increase the WCA for both resins, to values of 103° and 97° for the Acryl-1S and Acryl-2S with 25 vol%

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of silica, respectively. This result is interesting and implies first, that the surface roughness

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was increased, as shown in the following with AFM data, and second, that the surfactant was

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allowed to migrate within the dense silica network. Indeed, the WCA of Acryl-1 with silica, but without surfactant was found to be in the range 73°-77° depending on silica concentration,

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slightly higher than the value of the plain acrylate, due to the influence of sub-micron scale

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roughness.

The creation of a lotus texture at the surface of the composites led to a much higher increase

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of the WCA. Similar values were obtained for both types of composites, ranging from 135°

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for the printed resins without silica up to a value of 152.3° for the Acryl-2S composite with 25 vol% of silica. We should point out that these contact angle values are underestimates of

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the true values because the surfaces were not perfectly flat and water droplets would actually roll away from the place they were deposited to a local minimum of the surface height (see

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Figure SI1 in Supplementary Information). Nevertheless, the large WCA increase obtained after printing implies again that silica did not prevent the surfactant to migrate. The addition of silica itself led to an overall increase of WCA around 10°, similarly to the flat case, although here the increase was more pronounced for Acryl-2S composites compared with Acryl-1S. The WSA was found to be well below 10° for the two composite surfaces when the silica fraction was above 23 vol% as shown in Figure 3, and in fact could hardly be measured for Acryl-2S composites, for which water droplets started to roll almost immediately upon

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ACCEPTED MANUSCRIPT tilting the sample. In other words, the Acryl-1S composite surfaces with WCA ~ 144° were close to being superhydrophobic, and the Acryl-2S composite surfaces were remarkably superhydrophobic. Control tests were also carried out with printed Acryl-1 composite surfaces with 25 vol% of silica but with no surfactant. Water droplets had an initial WCA close to 150°, but were not stable. After few seconds, they switched from Cassie-Baxter state to

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Wenzel state due to wetting of the printed structure and the contact angle dropped to 119±19°.

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Figure 4. WCA of flat and printed composite surfaces vs. acrylated silica particle volume fraction. The WSA of several printed composite surfaces are also reported in the graph.

The wetting behavior of the printed surfaces shown in Figure 4 was analyzed using the classic

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Cassie-Baxter model [49], which considers that there exist air pockets trapped in the rough morphology. This results in a composite solid-liquid-air interface, responsible for the observed increased in hydrophobicity:

cos θ = 1 + f (1 + r cos θ0)

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[1]

ACCEPTED MANUSCRIPT where  and 0 are the WCA of the printed and flat surfaces, respectively, f represents the wet area fraction of the solid-liquid interface (1f being the fraction of the surface in contact with air under the water droplet) and r is the roughness ratio between the real area and the projected area, assumed in the present analysis to be equal to 3, i.e., the value for the lotus leaf [50]. Figure 5 shows the wetted area fraction f vs. volume fraction of silica, , for both types

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of printed composite surfaces. In spite of comparable WCA values (Figure 4), an opposite

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behavior is evident, where f increases with increasing fraction  for Acryl-1S composites, and

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vice-versa for Acryl-2S composites. Increasing the value of r, to account for the additional silica-induced roughness detailed in the following, would greatly complicate the analysis

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because the relation between roughness and area is non-trivial [51], but would not change the

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overall trend.

Figure 5. Wet area fraction vs. silica fraction for Acryl-1S and Acryl-2S composites, calculated using Eq. (1) based on measured WCA of flat and printed surfaces, and assuming a constant roughness factor equal to 3.

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ACCEPTED MANUSCRIPT 3.4. Morphological analyses

The opposite wetting behavior of the two composites shown in Figure 5 was puzzling since the roughness of the flat Acryl-1S composites was twice as high compared to that of the Acryl-2S composites as found from AFM measurements reported in Table 2. The higher

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roughness at sub-micron scale of the former composites should indeed favor the emergence of

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superhydrophobicity, which was not the case.

Table 2. Root mean square roughness, RRMS, of acrylate composite surfaces vs silica volume fraction  Acryl-1S RRMS

Acryl-2S RRMS

[vol%]

[nm]

[nm]

0

1.1

0.3

8.4

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5.2

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

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This paradoxical result was in fact resolved with the morphological analyses of flat and

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printed surfaces using AFM and SEM. Figure 6 shows AFM images of the Acryl-1S and Acryl-2S resins and composites with 25 vol% of silica polymerized in contact with a flat

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PDMS master. The surface of the plain acrylates was smooth and homogeneous with a RMS roughness of 1 nm or less and no significant features in the phase image. On the contrary, the surface of the composites was much rougher, with domains of sizes of few tens of nanometers, characterized by a large phase contrast with their surroundings. The phase image of the Acryl-1S composite reveals local heterogeneities of sizes around 10 nm, which matches the size of the acrylated silica particles, and which are homogeneously distributed at the surface. The phase image of the Acryl-2S composite also shows heterogeneities, of larger

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ACCEPTED MANUSCRIPT sizes of few tens of nm. The combination of the height and phase images reveals that these domains were silica outcrops. Adding 25 vol% of silica led to an order of magnitude increase

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of the roughness of Acryl-1S, and a 17-fold increase for Acryl-2S.

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Figure 6. AFM images of the surface of the Acryl-1S resin (a: height; b: phase) and composite with 25 vol% of silica (c: height; d: phase) and Acryl-2S resin (e: height; f: phase) and composite with 25 vol% of silica (g: height; h: phase) on scan areas of 1 µm x 1 µm. The colored z-scales of all the height and phase images is 0-100 nm and 0-20°, respectively. All surfaces were polymerized in contact with a flat PDMS master.

Figure 7 shows the FIB cross section of the printed Acryl-1S composite with 25 vol% of

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silica. The replica of three lotus papillae, the thin conducting Au layer on the top of the

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composite surface and the 4 µm thick protective carbon coating are visible. It is evident that the replication fidelity was very good at the micrometer scale, in particular in terms of the aspect ratio of the lotus epidermal cells. However, the sub-micron features characteristic of the natural plant were not clearly reproduced. It is moreover evident that silica was present in the replicated papillae, in the form of aggregates with a broad size distribution as previously reported [41]. A closer look at the very surface of one papillae (Figure 7b) reveals the presence of a high density of silica, in agreement with the AFM observation of silica outcrops on the flat composite surface.

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Figure 7. FIB cross-section of the lotus replica of the Acryl-1S composite with 25 vol% of acrylated silica particles at two different magnifications. The 4 µm thick protective carbon coating is visible in (a) and the thin conducting Au layer is visible in (b).

SEM top views of the printed polymer and composite surfaces are shown in Figure 8. In all

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cases the hierarchical roughness of the lotus surface, characterized by papillae of diameter close to 10 µm and sub-micron features was evident. In the case of the Acryl-1S resin and

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composite, and Acryl-2S resin the latter features were not as sharp as the epicuticular wax crystalloids on the lotus leaf surface (see Figure SI2 in Supplementary Information). In contrast, the surface of the Acryl-2S composite was characterized by the presence of a rough topography in the 100-500 nm range, that was comparable with the size of the wax crystals of the lotus surface. To elucidate the difference in surface morphology at the sub-micron scale between the different materials, the negative lotus PDMS template was examined by means of SEM. This analysis revealed that the lotus papillae structure was accurately reproduced, but

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ACCEPTED MANUSCRIPT the wax crystalloids were not well-defined and wax tubules were absent (see Figure SI3 in Supplementary Information). This lack of nanoscale replication fidelity with PDMS may be the consequence of the unfavorable processing conditions, i.e., ambient temperature hence high viscosity and absence of pressure. Nevertheless it appears that the addition of nanosized acrylated silica particles favored the emergence of sub-micron roughness of Acryl-1S and

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even more of Acryl-2S composites.

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Figure 8. Scanning electron micrographs of the printed lotus surface of Acryl-1S resin (a) and composite with 25 vol% of silica (top view, b; 88° tilted view at two different magnifications, c,d), and Acryl-2S resin (e) and composite with 25 vol% of silica (top view, f; 88° tilted view at two different magnifications, g,h).

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3.5. Mechanical properties and wear resistance

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The Young's modulus and hardness of the composites determined by nanoindentation are shown in Figure 9. Silica stiffened and hardened both resins with a similar twofold increase of these two properties between 0 and 25 vol%. Acryl-2S composites were twice as stiff and hard compared to Acryl-1S composites, with Young's modulus and hardness as high as 6 GPa and 400 MPa for a silica fraction of 25 vol%. As mentioned in the Experimental Section, the nanoindentation test protocol enabled to minimize the influence of creep. Taking this influence into account would reduce the modulus values by 5-6% and would increase the

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ACCEPTED MANUSCRIPT hardness values by 5-10%. Such an increase in mechanical properties is outstanding, and the Acryl-2S composite outperformed most of the polymer nanocomposites reported since more

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than 15 years [31].

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Figure 9. Young's modulus (a) and hardness (b) of Acryl-1S and Acryl-2S composites. The dotted lines are guides for the eye.

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The wear resistance of the superhydrophobic Acryl-2S surface with a printed lotus texture was investigated based on WCA measurements and microscopy observations, and found to be

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greatly improved with the addition of acrylated silica nanoparticles. The texturized Acryl-2S resin without silica had a contact angle of 132.7° ± 1.7° before abrasion. After abrasion the

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surface was still hydrophobic but with a much lower contact angle of 105.3° ± 2.5°, similarly to silica-based spray coated surfaces [52]. In contrast, the superhydrophobic behavior of the printed Acryl-2S composite with 25 vol% of silica (WCA of 152.3 ± 1.5°) remained unchanged after abrasion with a WCA above 150° and a WSA close to 3°. Figure 10 shows the morphology of the surfaces after abrasion. The plain Acryl-2S lotus texture appeared to be damaged with brittle breakage of the papillae and micron size debris, whereas the composite texture looked intact, with no damage even on the replicated sub-micron cuticular features.

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Figure 10. Texturized plain Acryl-2S resin (a) and composite with 25 vol% of silica (b,c) after sand abrasion.

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3.6. Self-cleaning and optical performance

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The self-cleaning character and optical transparency of the texturized Acryl-2S composite with 25 vol% of silica are shown in Figure 11. The surface contaminated with pepper grains

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(Figure 11a), which are hydrophobic hence more challenging to remove with water, was

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totally clean after letting 2 ml of water roll on the surface, and all pepper grains were captured by water (Figure 11b). The ~1 mm thick sample was slightly hazy with a pale yellow tint, due

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to the presence of photoinitiator. A thinner sample (approx. 200 µm) was placed atop a color

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printed paper (Figure 11c), showing a good level of transparency.

Figure 11. Demonstration of the self-cleaning ability of a synthetic lotus replica made out of Acryl-2S with 25 vol% of silica (a, inclined composite sample with surface covered with black pepper grains, b, clean composite surface after dropping 2 mL of water and trapping of pepper), and visual appearance of an Acryl-2S/silica composite sample placed above a color printed paper (c, a dotted line has been added on the periphery of the sample to facilitate visualization), with a water droplet on the sample (indicated by the arrow).

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ACCEPTED MANUSCRIPT 4. Conclusions

Superhydrophobic self-cleaning nanocomposite replica of a lotus leaf were produced using a combination of self-assembly of a silicone surfactant at the surface of acrylate/silica suspensions and a UVNIL process with a PDMS master. The migration of the surfactant to

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the surface occurred within few seconds after application of the liquid formulations. It was

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marginally affected by the contact with PDMS and led to a 20° increase of the WCA to values

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close to 90° for the Acryl-1S and Acryl-2S resins. The addition of acrylated silica nanoparticles increased the viscosity of both resins by several orders of magnitude, however

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to a considerably different degree, the Acryl-2S suspensions being five time less viscous than the Acryl-1S suspensions. Suspensions were also shear-thinning, which nevertheless did not

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compromise the replication fidelity. The presence silica at concentrations up to 25 vol% did not inhibit the rapid and spontaneous migration of the surfactant, and led to a further 10-15°

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increase of the WCA, which was attributed to increased roughness due to the formation of

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superficial submicron silica outcrops. The creation of a lotus texture led to an even greater increase of the WCA, and Acryl-2S composites with 25 vol% of acrylated silica particles

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were superhydrophobic, with a WCA equal to 152° and a SA well below 10°. It turned out that the nanosized wax crystalloids at the surface of the lotus papillae were not replicated in

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the negative PDMS master due to its high viscosity and insufficient pressure. Nevertheless submicron silica outcrops were again observed at the surface, particularly in the case of the Acryl-2S composite. These synthetic nanocomposite lotus leaves were, in addition, among the hardest within polymer composite surfaces ever reported, with a microhardness above 400 MPa, and were particularly wear-resistant, with no change in superhydrophobic behavior after 7000 sand abrasion cycles. These remarkable surfaces were moreover self-cleaning with respect to hydrophobic contamination.

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ACCEPTED MANUSCRIPT Acknowledgments

The authors acknowledge EPFL's Integrated Food and Nutrition Center for funding part of the work. Lausanne botanical garden is also acknowledged for the supply of lotus leaves and

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Pierre Mettraux for XPS analyses.

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Bioinspired self-cleaning and wear resistant polymer nanocomposite surfaces Combination of ambient, energy efficient self-assembly and UV-replication processes The network of silica nanoparticles does not hinder the migration of the surfactant to the composite-air interface Silica nanoparticles outcrops contribute to the superhydrophobicity of replicated synthetic lotus surfaces

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