Contact angle hysteresis (CAH) is an important physical phenomenon. It is present in nature but also plays a role in many industrial processes. 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. CAH is also important in industrial processes such as immersion lithography, fiber coatings, and ink-jet printing. Measuring contact angle hysteresis can also provide additional information on your surface compared to the static contact angle alone.
Contact angle hysteresis is a difference between advancing and receding angles
Contact angle hysteresis arises from the chemical and topographical heterogeneity of the surface, or swelling, rearrangement or alteration of the surface by the solvent. Contact angle hysteresis cannot be measured directly but through the measurement of advancing and receding contact angles. Let’s think about the droplet of water on a tilted surface. If there would be no contact angle hysteresis, the drop would immediately, when hitting the surface, start to slide on it because gravity is pulling it downwards. However, the contact angle hysteresis causes some drops to stay even on a vertical surface, such as a window glass. When tilting of the surface is increased (or raindrop size on a window glass increases) at some point the gravity will win and the droplet starts to move. At this point, the advancing and receding angles can be measured. Advancing contact angle tells the maximum contact angle the surface can have and the receding contact angle the minimum.
To understand how contact angle hysteresis can be utilized in coating development, 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 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.