Wettability refers to the ability of a surface to be wetted by a liquid. If the liquid spreads evenly, the surface is considered wettable. If the liquid partially wets or does not wet at all and even beads up, the surface is non-wettable.
The key factors are the physical-chemical properties of the surface and the liquid. Adhesion and cohesion forces are at play here. When the adhesion forces between the liquid and the surface are greater than the cohesion forces within the liquid, the liquid will wet the surface. Conversely, poor wettability is observed when the cohesion forces within the liquid dominate.
To control wettability, Contact angle measurement in various configurations has been established.
The contact angle is a measure of the wettability of a surface by a liquid and indicates the angle at which the liquid meets the surface. The contact angle is typically measured in degrees. A contact angle of 0° indicates complete Wetting, while at an angle of 180°, the liquid beads up spherically.
In more modern measurement setups, the droplet on the surface is illuminated. High-resolution cameras capture images of the droplet on the surface. These images are then analyzed to calculate the contact angle. Software can automatically extract the angle by tracking the shape of the droplet.
The mobile contact angle measuring device SITA SurfaSpector automatically places a water droplet on the surface via a dispensing unit. The droplet is side-lit, casting a shadow on an inclined measurement surface. The image taken by a high-resolution camera is perspectively corrected, and then the contact angle is calculated from the droplet contour. Due to the indirect image capture, the method is independent of the reflective properties of the surface. The baseline for contact angle calculation is predefined and does not need to be manually controlled afterward.
