**1. Introduction**

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 defined in the system.

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 that there are two requirements for the equilibrium.

### **2. Static equilibrium**

The first requirement for keeping a balance of the three interfacial tensions in horizontal direction is described by Young's Eq. (1):

$$
\gamma\_{lv} \cos \theta = \gamma\_{sv} - \gamma\_{sl} \tag{1}
$$

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

#### **Figure 1.**

*Contact angle on various surfaces.*

interface intersects the solid surface as shown in **Figure 1**. For the surface of solid with high surface energy, *γ*sv > *γ*sl, *γ*lv directs to the side of *γ*sl and forms a contact angle smaller than 90°. This kind of surface is known to be hydrophilic as shown in **Figure 1a**. For a solid with low surface energy, *γ*sv < *γ*sl, *γ*lv directs to the side of *γ*sv and forms a contact angle larger than 90° which is known to be hydrophobic as shown in **Figure 1b**.
