The Wettability and Permeability of Material Surfaces

**Chapter 7**

**Abstract**

**1. Introduction**

defined in the system.

**2. Static equilibrium**

**115**

*Yeeli Kelvii Kwok*

Wettability on Different Surfaces

Wettability has been explored for 100 years since it is described by Young's equation in 1805. It is all known that hydrophilicity means contact angle (*θ*), *θ <* 90°; hydrophobicity means contact angle (*θ*), *θ >* 90°. The utilization of both hydrophilic surfaces and hydrophobic surfaces has also been achieved in both academic and practical perspectives. In order to understand the wettability of a droplet distributed on the textured surfaces, the relevant models are reviewed along with understanding the formation of contact angle and how it is affected by the roughness of the textured surface aiming to obtain the required surface without considering whether the original material is hydrophilic or hydrophobic.

**Keywords:** wettability, droplet, hydrophilic, hydrophobic, surface tension, contact

It is well known that when a small droplet of liquid is deposited on the solid surface, it forms a shape with a contact angle to the solid. This phenomenon is firstly described by Young in 1805, and he proposed that surface energy is the interaction between the forces of adhesion and the forces of cohesion which determine whether the wetting occurs or not (i.e., the spreading of a liquid over a surface) [1]. If it does not occur the complete wetting, the liquid in a bead shape will be formed. In the same time, as a function of the surface energies, a contact angle is

When the liquid wets the solid, three different interfacial boundary surfaces, viz., solid-air (sv), solid-liquid (sl), and liquid-air (lv), are involved. The contact angle, which is included between the interfaces of sl and lv, has to reach a certain value to satisfy the equilibrium state of the three interfacial tensions. It is all known

The first requirement for keeping a balance of the three interfacial tensions in

where *γ* denotes the interfacial tension in the denomination of the force per unit length, or of the energy per unit area, which are equivalent in measuring the surface energy density, and *θ* is the contact angle at a location where the tangent along an lv

*γlv* cos *θ* ¼ *γsv* � *γsl* (1)

angle, textured surface, Wenzel model, Cassie-Baxter model

that there are two requirements for the equilibrium.

horizontal direction is described by Young's Eq. (1):
