I. Introduction
Wetting property is one of the important parameters for considering the surface properties of a material. Surface wettability is useful for developing useful applications for artificial intelligence (AI) and Internet of Things (IoT) systems [1]. The wettability is defined in the contact angle, which is the angle between the liquid and the solid surface. The contact angle is defined by the following equation.\begin{equation*} cos\theta=(\gamma_{sv}-\gamma_{sl})/\gamma_{l} \tag{1} \end{equation*} is the contact angle. and is the solid-vapor interfacial tension, solid-liquid interfacial tension and liquid-surface tension respectively. When the contact angle is less than 90 degrees, it is hydrophilic, which reduce friction between solids [2], [3]. On the other hand, when the contact angle is 90 degrees or more, the wettability is hydrophobic, which can remove dirt by rolling water droplet [4]. Since the solid-air interfacial tension, solid-liquid interfacial tension, and liquid surface tension are determined by the solid surface and water droplet, the wettability of the solid surface depends on the material of the solid surface and the water droplet. Conventionally, the surface was coated to change the wettability, and the wettability was changed by changing the material of the surface. However, it is not possible to make a permanent change in wettability because the coating agent peels off the surface. Therefore, a new non-coating method must be used to achieve permanent hydrophobicity. So, at present, attention is being paid to changes in wettability due to the fine structure of the surface. The change in wettability due to the microstructure is shown in two types, the Wenzel model and the Cassie-Baxter model [5]. The Wenzel model is a model in which water droplet completely penetrate the microstructure. On the other hand, the Cassie-Baxter model is a model in which water droplet does not penetrate the microstructure at all. Here, the Wenzel model and the Cassie-Baxter model are shown in figure. 1, and the changes in wettability due to these models are shown by the following equations, respectively.\begin{align*} & \quad cos\theta_{W}=rcos\theta \tag{2}\\ & cos\theta_{CB}=f(cos\theta+1)-1 \tag{3} \end{align*} and are the apparent contact angles in the Wenzel and Cassie-Baxter models, r is the surface roughness , and f is the surface area ratio . Also, θ is the true contact angle on the solid surface. In the Wenzel model, the hydrophilic surface becomes more hydrophilic and the hydrophobic surface becomes more hydrophobic. On the other hand, the Cassie-Baxter model shows increased hydrophobicity \ on all surfaces, regardless of hydrophilicity or hydrophobicity. These models suggest that further hydrophobicity can be achieved by creating microstructures on the hydrophobic surface. However, on a hydrophilic surface, water droplet infiltrates due to capillarity. Therefore, it becomes a Wenzel model and becomes more hydrophilic. It turns out that the Cassie-Baxter model cannot be applied simply by creating a microstructure on a hydrophilic surface. In the study of molecular dynamics simulation, it has been shown that the capillary phenomenon can be suppressed and the hydrophobicity due to the surface tension at the molecular level also occurs on the hydrophilic surface of the nanostructure when the microstructure at the nanostructure is prepared [6].
Schematic diagram of (a) Wenzel model and (b) Cassie-Baxter model.