Statistical approach for contact angle determination on inclining surfaces: “slow-moving” analyses of non-axisymmetric drops on a flat silanized silicon wafer

The determination and evaluation of contact angles on solid surfaces is a widely used procedure in industrial and scientific research. A number of good approaches exist, especially for axisymmetric drop shapes on horizontal surfaces. Generally, the reproducible analysis of contact angles on inclined surfaces is more difficult. Due to the non-axisymmetric drop shape, the contact angle determination using axisymmetric drop shape analysis (ADSA) is not possible. Commercial software fit one circle, ellipse or polynom on the somehow determined drop shape. The recently published routine high-precision drop shape analysis determines the drop shape in a comprehensible way. Afterwards, the contact angles are computed by a non-tangential method. In this study, we present a first detailed data evaluation of measurements on a silanized silicon wafer. In this regard, it will be shown that the movement of the triple line and therefore the determination of specific contact angles are more complex than often described in the literature. The reproducibility of contact angles measurements often suffers from the optical identification of the triple line movements. In this contribution, a special fitting and also a novel statistical analysis of the contact angles is introduced. The first results in an overall properties analysis whereas the second results inter alia in so-called global values E(p) for the contact angle, inclination angle, triple line velocity and covered distance of the first triple point. Both methods lead to contact angle data that are independent from “user-skills” and subjectivity of the operator.