Direct Imaging of Superwetting Behavior on Solid–Liquid–Vapor Triphase Interfaces

Abstract

has been established. The contact angle is the first parameter to directly and quantitatively characterize the wetting behavior of a surface. The apparent contact angle is mainly determined by the thermodynamic equilibrium among three coexisting phases: the solid phase, the liquid phase, and the vapor phase (Figure 1a), meanwhile influenced by the scale ratio between the measured liquid droplet and the hierarchical structure. In the ideal scenarios when the solid surface has a perfectly planar geometry and is chemically homogeneous (Figure 1a), the contact angle can be obtained via the well-known Young's equation. [1] However, in practical circumstances, the solid surface possesses different degrees of roughness, which deviates from the basic assumption of Young's equation. In 1936, Wenzel [2] proposed a typical wetting model for a solid with a rough surface. In Wenzel's model, the liquid completely infiltrates the microscale/nanoscale structure of the rough surface (Figure 1b), which successfully explained the contribution of surface roughness to surface wetting behavior. In addition, the Cassie model [3] is another typical wetting model, which was presented in 1944 by Cassie and Baxter. Their results indicated that the presence of air pockets between the liquids and the substrate can significantly affect the special wettability of surfaces (as shown in Figure 1c). Therefore, two typical models of wetting behaviors are applied to correlate the relationship between apparent contact angle and surface roughness. [4] The wettability of solid surfaces is dominated mainly by both chemical composition and geometric structures of the solid surface, as many experiments with multifunctional materials via bioinspired strategies have proven. As a result, studying multiphase wetting behavior is beneficial for developing wetting theory and providing guidance on the precise structure of pioneering materials. That is, the multiphase wetting behaviors of solid surfaces contain solid-liquid-vapor triphase interfaces, which are dominated by the three-phase contact line (TCL). Research on the TCL of solid-liquid-vapor interfaces includes not only fundamental investigations but also a broad domain of promising practical applications in the field of advanced materials, medical science, biotechnology, and electronic devices. [5] In the past few decades, the crystallization process dominated by the surface wettability has been confirmed, and the strategy for controlling surface wetting is a critical step to the fabrication of single-crystal arrays in the whole crystallization process. [6] Recently, Song and co-workers reported a promising method to A solid-liquid-vapor interface dominated by a three-phase contact line usually serves as an active area for interfacial reactions and provides a vital clue to surface behavior. Recently, direct imaging of the triphase interface of superwetting interfaces on the microscale/nanoscale has attracted broad scientific attention for both theoretical research and practical applications, and has ...

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