Contact angles are routinely measured both in academia and industry as they offer simple yet extremely sensitive measurement method for surface characterization. Contact angle tells about wetting of the surface which can be related to adhesion and cleanability for example. Although, the static contact angle is an excellent tool in many cases, more in-depth understanding of the surface properties require other methods.
If you have ever measured contact angles you have probably noticed that the contact angle readings on different parts of the seemingly same substrate can vary a lot. This is because of the surface heterogeneity which is caused both by the chemical heterogeneity and roughness. Surface heterogeneity gives rise to a parameter called contact angle hysteresis. Contact angle hysteresis can be defined by measurement of advancing and receding contact angles also called dynamic contact angles. Dynamic contact angles are thus able to give you more information on your surface properties than the static contact angle.
Designing new material properties
Different types of hydrophobic and superhydrophobic coatings are gaining interest due to their unique anti-wetting and self-cleaning properties. As on these surfaces, the high contact angle alone is not enough to produce the desired property on the surface, contact angle hysteresis and thus dynamic contact angles are typically measured. Low contact angle hysteresis and roll-off angles i.e. the tilting angle at which the drop rolls off the surface show that the droplets don’t stick on the surface. This is beneficial as the water drops don’t accumulate on the surface, think of a window shield or lens for example. Also, the rolling water droplet will collect the dirt with it, making the surface self-cleaning.
If you are interested in reading more about how dynamic contact angle has been used to study quality of coating on a lens, please download 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.