# Study of the wettability texturing hydrophobic surfaces prepared with cross-linked hybrid solutions under UV

Maillard Mathieu, Izabel Caldeira, Bérangère Toury, Christian Brylinski, Valentin Gâte, Daniel Turover

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Maillard Mathieu, Izabel Caldeira, Bérangère Toury, Christian Brylinski, Valentin Gâte, et al. Study of the wettability texturing hydrophobic surfaces prepared with cross-linked hybrid solutions under UV. Surfaces, Interfaces and Coatings Technologies International conference, setcor, Apr 2023, Lisboa, France. hal-04876869

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# Study of Wettability: Textured Surfaces prepared with UV-Crosslinked Hybrid Solutions

Izabel Caldeira¹,², Bérangère Toury¹, Mathieu Maillard¹, Valentin Gâté², Daniel Turover², Christian Brylinski¹

¹ Laboratoire des Multimatériaux et Interfaces, UMR CNRS 5615
Univ Lyon, Université Claude Bernard Lyon 1, F-69622 Villeurbanne, France, izabel.moraes-caldeira@univ-lyon1.com ; berangere.toury-pierre@univ-lyon1.com ; christian.brylinski@univ-lyon1.fr ; mathieu.maillard@univ-lyon1.fr

² NAPA Technologies
382 rue Louis Rustin - Archamps Technopole, F-74160 Archamps, France, izabel.moraescaldeira@napa-technologies.com ; valentin.gate@napa-technologies.com ; daniel.turover@napa-technologies.com

## Abstract

The most recent developments around sol-gel processes to produce hybrid solutions (organic/inorganic) allowed us to find alternative routes to replace commonly used resins in nanoimprinting lithography processes. Another concern is high temperatures in the reproduction processes, so the solution addressed in this work is using UV light to polymerize the coatings. Here, we present a study on wetting a smooth and paternal surface using a hybrid solution by using a simple process to produce surfaces with superhydrophobic patterns via the Soft UV-NIL. The process involves the synthesis of a transparent hybrid silica-based solution that coats the substrate, is structured with micrometric patterns, and is cross-linked under UV light. Finally, static and dynamic wetting characterizations were performed to understand wetting theories.

**Keywords:** nanoimprint, wettability, contact angle, hydrophobic, micropatterning, Wenzel, Cassie-Baxter, hybrid materials.

## 1. Introduction

Many applications require surface texturization. As primary examples, living organisms have evolved many strategies to change their color and appearance. Some of these characteristics are related to the physical and topographical properties of the surface, enabling color modifications or adding a surface functionalization such as hydrophobicity[1].

A surface to be considered superhydrophobic needs to present a water contact angle greater than or equal to 150°, combined with low surface energy (10 mN/m), which is defined as the surface free energy that quantifies the rupture of intermolecular bonds that occurs when a surface is created [2].

For instance [3], a patterned surface can provide hydrophobic behavior by mimicking [Erreur ! Source du renvoi introuvable.], that is, trying to imitate the topographies of natural surfaces such as those mentioned above. Several techniques can be used to give a hydrophobic character to a surface, such as chemical vapor deposition (CVD) [4], plasma[5], direct laser interference (DLIP)[6], and, notably, nanoimprint lithography[7].

Chou proposed Nanoimprint lithography (NIL) [Figure 2] for the first time in 1995. NIL is a simple mechanical lithography technique in which a stamp or a template is pressed against a layer of deformable printed resist deposited on a substrate to create the template's outline. This low-cost technique is nowadays of the most used techniques for nanopatterning because it can easily and precisely reproduce micro and nanopatterning. [8]
<figure><img src="image_2.png" /><figcaption>Figure 1: Scanning electron microscope image of Velvety leaves of a Dahlia flower with convex, structured cells.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Principle of Nanoimprint technique.</figcaption></figure>

It can be added that NIL can be achieved by using several development variations, such as Hot Embossing Lithography, UV-based Nanoimprint Lithography, roll imprint process, laser-assisted direct imprint, among others [9]. In parallel, the use of soft lithography techniques, i.e., a replica impression using a silicone-based stamp, opened interesting alternative routes to replace the above-mentioned expensive and complex methods. However, two remaining major challenges with the NIL technique are extend the size of the patterned area (from cm² to m²) and to achieve the patterning of complex surfaces (2.5-3D geometries) leading to hydrophobic surfaces. Another challenge is to replace the use of thermally photopolymerizable resins with UV polymerizable ones, to broaden the variety of substrates which can be used, and thus develop new applications.

In this paper, we propose a soft process to produce surfaces with super-hydrophobic patterns using the UV-NIL technique on hybrid solutions. To characterize and understand the hydrophobicity of the surfaces, wettability characterizations have been developed and some modelling has been performed.

## 2. Wettability theories for material surface

The wettability of a material is primarily characterized in terms of contact angle (CA) of a liquid with respect to the solid surface (interfaces between the solid-liquid-gas phases). The further modelling and understanding of the adhesion and repellency characteristics on a surface have been mostly developed around the concepts describing the pinning of the droplet on the surface's patterns.

Four models for liquid spreading are commonly used: Young's, Wenzel, Cassie-Baxter and Marmur model, of which are schematized in the following Figure 3. [10]

<figure><img src="image_4.png" /><figcaption>Figure 3: Four classic theoretical models: (a) Young; (b) Wenzel; (c) Cassie-Baxter; (d) Marmur model.</figcaption></figure>

The Young model (a) describes the wettability of a flat or smooth surface in terms of the contact angle, Wenzel model (b) is used to describe a surface in which droplets penetrate the rough structures leading to high adhesive forces and the Cassie-Baxter Model (c) describes a rough surface that is non-penetrable by liquid droplets. The last model (d) is the intermediate state between Wenzel and Cassie, named Marmur Model [11]. These wettability models can be used to evaluate the hydrophilic/hydrophobic nature of a material's surface.

## 3. Experimental Techniques

The Soft UV-NIL is a well-known replication method capable to reproduce efficiently various shapes and sizes of patterns, with lateral dimensions from a few hundred micrometers to the nanometer range. This technique differs from other classical lithography techniques since it uses UV light to cure a monomer, thus allowing the use of thermally sensitive substrates. Another important factor is the feasibility of using flexible and reusable molds instead of glass molds, which are more commonly used.

The use of this technique demonstrates a high potential for application in solar cells, MEMS and NEMS, photovoltaics, microoptics, wettability, etc. [12], [13].

For the surface structuration on our samples, a Soft UV NIL technology was chosen. Patterns (circles and squares) from 10-25µm in diameter were produced by using a PDMS mold. Hybrid solutions with organic and
inorganic components, solvent and photoinitiators were used to coat a transparent layer on a glass substrate. The solutions were deposited by spin coating and cured with UV light (HAMAMATSU Lightningcure LC8) for 10 minutes. The thickness of the coating obtained at the end of the process is around 40 µm, measured by stylus profiler (DEKTAK 150). Static and dynamic wettability measurements (hysteresis angle) were performed with a goniometer equipped with a microscopic camera (KRUSS Easydrop FM40). The dynamic wettability measurements were performed with the “ALPIN”, a home-made device that demonstrates the angle a drop needs to slide over a surface (water repellency angle). Anti-gel and anti-fog measurements were obtained with the BCBG bench created by NAPA, which shows the appearance of fog and ice and the total ice time of a coating.

## 4. Results and discussions

A first visual evaluation of the replicated patterns was performed by using a SEM (ZEISS Merlin Compact VP). Images confirmed the well-reproduced patterns as shown in the Figure 4.

<figure><img src="image_5.png" /><figcaption>Figure 4: Examples of SEM images of textured surfaces with photocrosslinkable hybrid solution.</figcaption></figure>

For contact angle measurements, the Figure 5 shows a significant increase in the contact angle value passing from a flat with 80° to a textured surface with 150° (average obtained between circles and squares with no significant difference in values between the two types), proving that the reproduction method is valid for reaching hydrophobic surfaces with CA of 150°.

<figure><img src="image_6.png" /><figcaption>Figure 5: Contact angles for smooth and embossed surfaces based on a hybrid solution.</figcaption></figure>

The Owens, Wendt, Rabel, and Kaelble method is a standard method for calculating the surface free energy of solids from the contact angle with various liquids. These methods divide the interfacial interactions into two parts: polar and dispersive. Using the Owens-Wendt model, we get a value of 45 mN/m in a flat surface, slightly less than the surface energy of an untreated sheet of glass (49.7 mN/m). However, these values are considered high, that is, the mobility of the drop will be reduced, and it will have difficulty in sliding.

Using a goniometer with a microscopic camera, on a dynamic plateau, several tilts were reproduced up to an angle of 75° and measurements of the hysteresis angles were obtained as the difference between “forward” and “backward” sliding angles when the drop is subjected to variable inclination. Complementary measurements were also obtained using a home-made equipment by NAPA Technologies called “ALPIN”. It uses lasers and photodetectors to capture the inclination angle value for which a drop starts to slide over the surface. The results are shown in Table 1.

<table><caption>Table 1: Values to Dynamic wettability</caption><thead><tr><th rowspan="3">Coating</th><th colspan="4">Dynamic wettability</th></tr><tr><th colspan="2">Contact angle hysteresis</th><th colspan="2">Water repellency angle (ALPIN)</th></tr><tr><th>Flat</th><th>Embossed</th><th>Flat</th><th>Embossed</th></tr></thead><tbody><tr><td>Glass</td><td>35 ± 4</td><td>na</td><td>14±5</td><td>na</td></tr><tr><td>Hybrid solution</td><td>19 ± 8</td><td>57 ± 19</td><td>26±1</td><td>35±5</td></tr></tbody></table>
From the results shown in table 1, contact angle hysteresis on the hybrid coating is reduced compared to the bare substrate. We can also observe that both hysteresis and repellency angle are increased on the textured surfaces. Globally, the static wettability behavior improves with embossing, while the dynamic wettability worsens.

This can be explained by supposing a shallow water penetration within the patterning structures, making it more difficult for the drop to move. Based on these results, we may conclude that the wetting on our samples behaves likely along the Marmur Model (Figure 3d), which is an intermediate state between Cassie and Wenzel.

An application study for anti-frost purposes has started to be carried out. Preliminary results show that the appearance of ice was delayed from 55 seconds, for an untreated glass surface, up to 10 minutes for a surface covered by a textured layer of our hybrid solution.

## 4. Conclusion and perspectives

In conclusion, we have been able to produce surfaces with enhanced hydrophobic performance by using UV-curable hybrid solutions with a fast and simple process. The wettability of the samples demonstrated so far seem to behave along the Marmur model. For reaching super-hydrophobic behavior, the surface energy must be further reduced, which may be pursued using other precursors or changing the topography of motifs. As for the understanding of the gliding properties of the textured surfaces, complementary analyses will be needed.

## Acknowledgements

This work was carried out within the framework of the NAPASOL project, which received funding from the CIFRE (Industrial agreements on training through research) program of the National Association for Technological Research (ANRT). This work is carried out by the company NAPA Technologies in collaboration with the University of Lyon 1 within the Multimaterials and Interfaces Laboratory (LMI).

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