Resisting Wetting Transition by Using Concave Structure

Abstract

Water-repelling superhydrophobic surfaces are acquired from the combination of hydrophobic chemistry and rough topology. Specific properties of repelling water introduced acceleration on the industrial development and in-depth explorations for understanding the interfacial physicochemical behaviors. However, prior to the practical applications of superhydrophobic surfaces, the durability and stability of the surfaces under extreme conditions, such as impacting and underwater, should be figured out. Notably, superhydrophobic surfaces are ubiquitous in nature since several species live in various environments, thereby modifying their surface morphologies into proper features for their habitats. Owing to their peculiar surface features, they exhibit outstanding water-repellency, thereafter they have led the bio-inspired superhydrophobic surfaces and revealed crucial factors for the stable superhydrophobic surfaces. Afterward, the surface structures with hierarchy and reentrancy have been employed for designing stable superhydrophobic surfaces. In this thesis, investigations on superhydrophobic surfaces supported by a new structure, concavity, are discussed. The concave structures are inspired by the morphology of leaf beetles and some springtail species. Chapter 1 introduces the fundamental concept of droplet wetting on the surfaces and representative wetting models followed by crucial factors for wetting stability. Young’s equation, the Wenzel model, and the Cassie–Baxter model are introduced and the design concept of superhydrophobic surfaces based on them is discussed. In Chapter 2, the fabrication of concave pillar structures and normal pillar structures are introduced. From the specific etching characteristics of the silicon surface, the chemical etching process induced rough surface fabrication is illustrated. Chapter 3 presents the wetting stability of the superhydrophobic concave pillar structures under impact pressures comparing them to the typical superhydrophobic surfaces of normal pillar structures. In chapter 4, the wetting transition resistance of concave pillar structures underwater is introduced. The role of concave structure for stable superhydrophobicity underwater is unveiled. This thesis provides a comprehensive understanding of stable superhydrophobic surfaces and the crucial roles of newly introduced concave structures for wetting stability. These studies will offer a pioneering tactic for designing stable superhydrophobic surfaces advancing exploration and utilization of superhydrophobic surfaces in various industries.