Many interfacial phenomena appear in situations where three phases meet. The most common situation is contact between solid, liquid and gas. The three phases meet at the so-called three-phase contact line. When a system is in steady state the three-phase contact line is motionless as there is an equilibrium of the tangential forces caused by the interfacial and surface tensions.
In the case of a liquid in contact with a solid and a gas the equilibrium contact angle ΘC forms at the three-phase contact line. The surface energy of the solid σS acts along the solid surface. The solid-liquid interfacial energy σSL acts in the opposite direction and the surface tension σL of the liquid acts tangential to the drop surface. The equality of vectorial forces (see figure 1) can be described by a simple scalar equation. A vector projection on the contact plane between liquid and solid yields the Young equation:
Figure 1: Contact angle at a solid-liquid-gas contact line
The surface energy of the solid and the interfacial tension between the solid and liquid are generally unknown. To determine these values models that take into consideration different types of interaction between liquid and solid are used.
The equilibrium contact angle has two extrema. At 0° the drop of liquid is completely spread and forms a thin (monomolecular) film. This is called complete wetting. At an angle of 180° the drop forms a sphere and touches the solid in only one single point. This is called complete dewetting. Depending on the application a contact angle as close as possible to either the minimal or maximal value is desired. An example for a material with a very high contact angle is ceramic with lotus effect which can be cleaned with little effort because water rolls off easily. An example for a small contact angle can be found in the dyes and varnishes industry which desires liquid formulations that distribute as evenly as possible.
The equilibrium contact angle can be measured optically with a contact angle goniometer and drop shape analysis system of the OCA series using the sessile drop method.
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