Capillary rise method for the measurement of the contact angle of soils

The contact angle quantitatively describes the contact on the liquid-solid interface and is thus critical to many physical processes involving interactions between soils and water. However, the role of the contact angle in soils is far from being adequately recognized. This paper reports a comprehensive study on the application of the capillary rise method (CRM) to measure the contact angles of soils. The deviations of analytical solutions to various forms of the Lucas-Washburn equation were presented to offer a detailed study on the theoretical basis for applying CRM to soils, which is absent in existing studies. The disadvantages of the conventional CRM investigations were demonstrated with experiments. Based on a comparative study, a modified CRM was proposed based on the analytical solution to one form of the Lucas-Washburn equation. This modified CRM exhibited a reliable performance on numerous unsieved and sieved (different average particle sizes) specimens made of a subgrade soil and a silicon dioxide sand. Procedures for the specimen preparation were designed and strictly followed, and innovative apparatuses for the preparation, transport, and accommodation of soil specimens were fabricated to ensure repeatability. For the modified CRM, experimental results for unsieved specimens exhibited good repeatability, while for sieved soils, clear trends were observed in the variations of the contact angle with particle size. Contact angles much greater than zero were observed for all of the tested soil specimens. The results indicate that the assumption of perfect wettability, which is adopted in many existing geotechnical studies involving the contact angle, is unrealistic.