Understanding of sliding and contact angle results in tilted plate experiments

Tilted plate experimental methodology is studied in order to address misinterpretations and omissions that exist in the literature. The effect of methodology on subsequent sliding angle measurements is quantitatively assessed in two parts. The first part is a comparison of drop placement techniques in the conventional tilted plate method (drops placed on a level surface, then inclined). The second part is a comparison between the conventional tilted plate method and a modified method, in which the drop is placed on a surface that is already inclined. The first part of the study involved water drops placed on alkyl ketene dimmer (AKD) surfaces using a needle from above as well as below the surface (through a hole). For drops placed from below, three different drop locations (with respect to the hole in the surface) are included in the study. It was found that the drop placement technique had a statistically significant impact on sliding angle measurements, and the most consistent and unbiased measurements resulted when the drop was placed from below, with the hole leading (downhill from) the drop. The second part of the study involved water drops on both AKD (high contact angle hysteresis system) and fluorinated silicon (low contact angle hysteresis) surfaces. For either system, it was shown that sliding angles were lower when drops were placed on surfaces that were already inclined. Since the maximum and minimum contact angles were identical between the two experimental methods, the difference in adhesion was due to variation in contact line shape. The results of this study show that errors of 50–60% would result from using conventional tilted plate measurements to predict droplet mobility on fixed, inclined surfaces, such as those found in condensers. The important distinction between repellency (measured by the advancing contact angle) and drop mobility is also discussed. Further, it was observed that the advancing and receding contact angles do not always correspond to the maximum and minimum contact angles observed in a tilted plate experiment.