Liquids come in contact with solid surfaces in a variety of applications, including everyday situations like painting a wall or using medicine. In industry, wettability is important for example when manufacturing electronics or when planning how to extract oil from the oil reservoir. In all these applications, knowing the contact angle value gives a strong indication on the performance of the product or a successfulness of the process.
Contact angle is a measure of wettability
When a drop of water is placed on a solid, it will spread on the surface based on the intermolecular interactions between the solid and the liquid. Water contact angle will immediately give an indication of the wettability of the solid. If the measured contact angle is above 90 degrees, the solid is said to have poor wetting and is termed hydrophobic. If the contact angle is below 90 degrees, a term hydrophilic is used. As extreme cases, the water can either spread completely or form a sphere on top of the solid, in these cases terms complete wetting and superhydrophobic surfaces are used, respectively.
Contact angle can be linked to adhesion of the coating
Coatings are used for both decorative and protective purposes. Whenever coating is applied on the solid, the spreading of the coating will have a big impact on the adhesion. Good wetting is generally related to good adhesion. Wettability of the surface can be improved by various surface treatments such as plasma or corona treatment. Different types of adhesion layers are also often utilized. To determine the good-enough level for the surface treatment, contact angle measurements are used.
To read more how contact angles are used in industries, download the overview below.
Contact angle is easy to understand visually. It is the angle a droplet forms in contact with a solid surface. Thermodynamically, it is a balance between different forces acting on a three-phase contact point where solid, liquid and air meet.