A traditional coating is usually applied either to protect the material beneath or to improve its visual appearance. More and more research interest is being put on coatings that in addition, can serve some other function. These types of coatings are generally called functional coatings. The list of different functional coatings is extensive but some of the most studied are anti-fouling, antimicrobial, conductive, self-cleaning, photo and thermochromic, self-healing and superhydrophobic coatings.
Extreme wettability is required for extreme functions
Often the purpose of the functional coating is to repel water or oil from the material surface. For example, different types of coatings on the textile are already utilized to improve their performance in outdoor clothing. Windows and solar cells are coated to make them easier to clean or even self-cleaning, and windshields are coated to prevent fogging. In all these example high contact angles between water and the surface are required.
Contact angle hysteresis and roll-off angle are important parameters in functional coating evaluation
Static contact angle measurements are often used to evaluate the quality of functional coatings. However, the static contact angle alone is often not enough to separate different surfaces from the other. When contact angles are high, even the advancing contact angles are relatively close to each other and the main difference comes from the receding angle or contact angle hysteresis.
To read how dynamic contact angle measurements have been utilized in the study of functional coatings, please download to the case study below.
The measurement methods can be divided into optical (needle and tilting methods) and force (Wilhelmy method) depending on the measurement principle used. All the methods are measuring advancing and receding contact angles which are used to calculate contact angle hysteresis.
Contact angle hysteresis (CAH) is an important physical phenomenon. When you look through the window on a rainy day, you have probably noticed that some of the water droplets stick to the glass even though gravity is pulling them down. The phenomenon can be explained by contact angle hysteresis.
Contact angles are routinely measured as they offer simple, yet extremely sensitive measurement method for surface characterization. Although, the static contact angle is an excellent tool, more in-depth understanding of the surface properties require dynamic contact angle measurements.