Microfabrication of Highly Robust Superhydrophobic Glass Surfaces Via Combined Thermal Poling and HF Gas Etching
Authors: Yasuda Kohei , Mineyuki Nobuya , Sano Kohei , Hayashi Yasuo , Homma Takayuki 
Published: December 2024

Superhydrophobic glass affords a clear view by preventing the adhesion of raindrops and dirt. This type of glass can be used in automotive windows, sensor covers for automatic driving, solar panels, and building windows. A superhydrophobic surface has a water contact angle greater than 150° and is fabricated by combining micro- to nanoscale structures with low-surface energy coating materials. In the previous study [1], we demonstrated that the transparent superhydrophobic glass can be efficiently produced through HF gas etching. The robustness of its superhydrophobic surface is the most challenging issue in the practical application of the superhydrophobic glass. The recent breakthrough, namely, the development of superhydrophobic surfaces with “armor structures” [2], addresses this issue for conventional superhydrophobic surfaces containing nanostructures that would be easily destroyed by abrasion. The armor structures protect the fragile superhydrophobic nanostructures, resulting in highly robust abrasion resistance. In the previous study, the armor structures were fabricated using molding methods that impose strict limitations on the shapes that can be formed. A common microfabrication technique, which combines photolithography and reactive ion etching (RIE), can be applied to glass surfaces. This method allows for the formation of any desired shape on the glass surface. However, this method involves vacuum processes that require masks, resulting in extremely low productivity. Compared to electronic components, for which this technique is typically used, glass has a larger area and lower unit cost. The low production efficiency of the technique would therefore hinder its applicability.
In this study, we developed a unique microfabrication method that combines thermal poling and HF gas etching. In thermal poling, heated glass is sandwiched between electrodes, and a high voltage is applied to transfer alkali metal ions from the anode-side surface of the glass [3]. Microstructures formed on the surface of the anode used for thermal poling are transferred onto the glass surface as alkali metal-deficient regions. Subsequently, etching the glass facilitates the formation of these shapes on the glass surface owing to the difference in etching rates. At this stage, traditional solution-etching fabricated steps on the nanometer scale; larger steps on the microscale remain to be formed. We fabricated the steps on the micrometer scale via HF gas etching, which presents a large difference in the etching rate between the alkali-metal-deficient layer and the normal surface. Figure 1 presents the results of glass microfabrication using thermal poling and HF gas etching. Circular shapes with diameters of 2.5, 5, 10, 20, and 40 µm were created for the proof of concept. A glassy carbon microfabricated via RIE was used as the anode. For thermal poling, a voltage of 300 V was applied to the glass heated to 450 °C for 5 min. HF gas etching was performed at 500 °C with a small amount of HF gas for a few seconds. A highly robust superhydrophobic glass surface was thus fabricated. The surface produced using the proposed method successfully endured 1000 abrasion cycles, while that produced using the conventional method lost its superhydrophobicity after ten abrasion cycles.
[1] K. Yasuda, Y. Hayashi, and T. Homma, ACS Omega, 9, 12204–12210 (2024).
[2] D. Wang et al., Nature, 582, 55–59 (2020).
[3] S. Ikeda, K. Uraji, T. Suzuki, K. Yamamoto, and J. Nishii, J. Non-Cryst. Solids, 453, 103–107 (2016).

Figure 1