Calculation of the contact angle using the sessile drop method
As said, the specialized software can analyse the recorded camera images. By evaluating the contrast values of the image, the so-called baseline, i.e. the contact line between the drop and the solid, and the drop outline can be recognized (see Figure 3). Ultimately, various mathematical models can be used to determine the contact angle.
The simplest, automatic calculation basis for determining the contact angle is to apply tangents to the drop outline at the two intersections with the detected baseline. In further automatic evaluations, the detected drop outline is approximated using different geometric forms. These forms include, for example, a circular shape, an ellipse, or a higher order polynomial. This allows the drop outline to be described and the contact angle to be calculated.
It is also possible to use the Young-Laplace equation to determine the contact angle from the drop contour. In contrast to the geometrical forms, as presented above, the Young-Laplace equation considers the physical properties of the droplet, too. As a rule, the Young-Laplace equation provides the most reliable results for larger contact angles and large droplets. It should be noted, however, that this calculation assumes a symmetrical droplet.
Extended contact angle evaluations: surface energy determination
The optical determination of the contact angle using the sessile drop method also allows the determination of the surface energy of the solid with its disperse and polar components. To calculate it, the contact angles of at least two different test liquids, such as water and diiodomethane, are measured. The surface energy is important in many areas, namely whenever a solid surface is to be wetted with a liquid.