**1. Introduction**

Coatings are defined as mixtures of film-forming materials containing solvents and other additives that, when applied to a surface and after curing/drying process, yield a solid, protective, decorative, and/or functional adherent thin layer [1]. Surface coatings include paints, drying oils and varnishes, synthetic clear coatings and other products. Coating on substrates offer several advantages, such as the fact that coated materials have better durability against demanding contact conditions or aggressive environments than the raw materials [2]. Moreover, coatings allow the enhancement

of certain properties that the raw materials could not provide, such as antimicrobial, self-cleaning, superhydrophobic and antifouling effects, and so forth [3–5].

Over the past years, nanotechnology has emerged as a potential tool in the development of surface coatings [2], resulting in their use in fields such as electronics, medical, food, pharmacy and aerospace. These surface coatings can contain micro/nanoscale features that offer more optimal and processing properties than conventional coatings, such as higher opacity, better interaction between the coating and surface and higher durability of the coating [2]. Moreover, coatings containing topographical cues may impart hydrophobic and oleophobic properties, thus improving corrosion resistance and enhancing either insulative or conductive properties [4].

The generation of superhydrophobic coatings has been gaining a fair interest in the field of coating technology [6,7]. Superhydrophobic coatings are specifically referred to water repellent layers with a water contact angle >150◦ [8]. Such coatings can be prepared either by fabricating rough architectures on the surface or by chemically modifying rough surfaces containing low surface free energy materials [8,9]. These surfaces can often be accomplished by common techniques such as dip coating [10], lithography [11,12], chemical vapor deposition [13], among others [14]. However, the common processes to obtain such coatings are considered complex and their industrial applications are still limited. Therefore, new developments in coating technology are necessary in order to expand the accessibility of superhydrophobic coatings.

Electrohydrodynamic processing (EHDP), including electrospinning and electrospraying, is a very appealing technology that can be used to apply topographical structured (such as fibers or particles) layers on a wide variety of substrates [15]. These technologies utilize high electrostatic potentials to draw polymer solutions or polymer melts into fibers or particles [16,17]. Electrospinning/electrospraying processes present the advantage to work at atmospheric pressure and are, therefore, easily integrate into continuous production lines. Moreover, unlike vapor deposition, these novel technologies allow the application of a continuous layer with the required surface roughness necessary to exhibit superhydrophobic properties [4]. The structure and the physicochemical properties of the electrospun fibers and electrosprayed particles generated by EHDP can also be tailored according to the end application, for instance packaging [18,19]. Moreover, both techniques enable to process many synthetic and natural polymers either alone or blended with other polymers and/or additives (e.g., surfactants). Among them, polylactide (PLA) is one of the most attractive materials for coating fabrication by electrospinning/electrospraying because it is easily spinnable, biodegradable, and thermoplastic [4,20].

Likewise, functional coatings containing particles of titanium dioxide (TiO2), silicon dioxide (SiO2, also known as silica), carbon black (CB), iron oxide (Fe2O3), and zinc oxide (ZnO) have been developed [3]. Among them, SiO2-based coatings have been of particular interest as such coatings can offer self-cleaning properties as well as antimicrobial functionality. The use of SiO2 microparticles for coating deposition allows for a change on the surface roughness and surface energy, leading to a superhydrophobic behavior without the need to perform any chemical modification [7,9]. As these surfaces are also highly water-repellent, they can be used in mirrors, self-cleaning windows, frames, bricks, wall paint, tiles, flat glass, and so forth [21]. Recently, Lasprilla-Botero et al. [8] reported an interesting approach to the design of superhydrophobic films by means of electrospinning/electrospraying, depositing electrospun ultrathin poly(ε-caprolactone) (PCL) fibers followed by electrosprayed nanostructured silica (SiO2) microparticles onto low-density polyethylene (LDPE) films. The resultant coated films showed a high surface hydrophobicity with an apparent contact angle of 157◦. Moreover, they showed good adhesion between layers and improved thermal stability.

In the current work, a similar but a one-step continuous electrospinning/electrospraying process was applied to generate a superhydrophobic PLA/SiO2 bilayer on polyethylene terephthalate (PET) substrates with the additional aim of achieving maximum transparency. The process consisted of a deposition of PLA fibers and SiO2 microparticles sequentially onto a PET film by electrospinning and

electrospraying, respectively, and a subsequent annealing to improve the adhesion between the layers. To the best of our knowledge, this is the first time that PET films are coated with a PLA/SiO2 bilayer obtaining not only high superhydrophobicity but also a good adhesion between layers using a scalable one-step continuous process.

#### **2. Materials and Methods**

#### *2.1. Materials*

The substrate PET, with a thickness of 77.0 ± 0.6 μm, was kindly provided from Belectric OPV GmbH (OPVIUS-Organic Photovoltaic Solutions, Kitzingen, Germany). For the films deposition, PLA was an Ingeo™ Biopolymer 2003D with melting features determined by differential scanning calorimetry (DSC) from 145 to 160 ◦C as reported by the manufacturer, NatureWorks LLC (Minnetonka, MN, USA). The SiO2 microparticles (HDK® H18, Pyrogenic Silica) were obtained from Wacker Chemie AG (München, Germany). Polyvinylpyrrolidone (PVP), with a molecular weight (MW) of 40,000 g/mol, was provided by Sigma Aldrich S.A. (Madrid, Spain). Chloroform, acetone, and ethanol with purities >99.5% were supplied by Panreac Química S.A. (Barcelona, Spain).

### *2.2. Preparation of Multilayer Films*

#### 2.2.1. Solutions

The solution for electrospinning was prepared by dissolving 7.5 wt.% PLA in a chloroform/ acetone 7:3 (vol./vol.) solution and further stirring until complete dissolution. For electrospraying, a 1.5 wt.% of SiO2 microparticles were suspended in ethanol and ultrasonicated for 1–2 min prior to electrospraying. Concentrations were optimized in preliminary experiments based on a previous work [22].
