What is contact angle?

When it´s raining, compare a freshly waxed car to one that hasn’t been waxed for a long time. On a freshly waxed car, the drops prefer to stay as isolated droplets. On a car that hasn’t been waxed in a long time, the drops spread more and wet bigger areas of the car housing. This is because of different surface wettability and can be measured by contact angle .

Contact angle can be visually explained

Contact angle is easy to understand visually. It is the angle a liquid droplet forms in contact with a solid surface. If the angle is low, the interaction between the solid surface and the liquid is favorable, i.e. the wettability of the surface is good and the adhesion between the liquid and the solid surface high. On the contrary, a high contact angle indicates the interaction is unfavorable and the adhesion low. It is easy to understand visually, that the contact angle can be anything between 0° and 180°. The surrounding phase can be a gas, like air, or another liquid, immiscible to the droplet liquid.

Thermodynamic definition of the contact angle

What then dictates what kind of an angle is formed between the droplet and the solid surface? At the interface between each physical phase, typically solid, liquid and gas, exists a force caused by the imbalance of how strongly similar molecules adhere to each other (cohesion) and how strongly molecules of different phase adhere to each other (adhesion).

The balance between these forces is the thermodynamic definition of contact angle, known as the Young equation.

The γ_lv is the force per length for the liquid-gas interface, γ_sl is the force per length for the solid-liquid interface and γ_sv is the force per length for the solid-gas interface. The γ_lv is surface tension of the liquid, and γ_sv is surface free energy (SFE) of the solid. In practice, also other parameters such as surface roughness affect the measured contact angle.

To learn more about the contact angle, please download the contact angle white paper.