A Semi-Automated Technique for Repeatable and Reproducible Contact Angle Measurements in Granular Materials using the Sessile Drop Method

Contact angles (CAs) are used to measure the extent to which a material is wettable. Granular materials such as natural soils and crushed minerals, which are commonly assumed wettable, can exhibit non-wetting characteristics. The sessile drop method (SDM) is a direct method widely used to generate and measure CAs, however, the procedure involved in their determination is often overlooked leading to very large standard deviations in their measurements. In this study, a close examination of the steps involved in extracting the CAs on granular materials shows that two factors, the image exposure and the position of the baseline, can affect CAs measurements significantly. Seven methods of fitting CAs were compared. It was found that the discrepancy between the methods became more and more significant as CAs increase in magnitude. A semi-automated technique has therefore been proposed through this study to improve the standard deviations of CAs measurements. The new technique uses five steps and involves an adjustment of the image exposure and manual movement of the baseline. The proposed method was tested on flat surfaces as well as granular materials (chemically treated sand and a naturally occurring hydrophobic mineral). The results have shown that the method can be applied for both flat and granular materials with a wide range of CAs. In particular, the standard deviations of flat surfaces (e.g., hydrophobized microscope slides) with CA in the range of 90 to 135 degrees recorded improvements of 37%. For granular materials (e.g., fluorspar) with CA in the range of 105 to 120 degrees, improvements of 33% in standard deviations have been observed.