A fundamental appraisal of experimental methods to measure soil-fluid contact angle

While soil-liquid contact angles are important in unsaturated soil analyses, the reliability of existing experimental methods to measure contact angles is uncertain. Capillary rise method (CRM) and sessile drop method (SDM) tests were simulated using a computational fluid dynamics code. The simulation data were used to appraise the capacity of these tests to accurately measure contact angles. Data from simulations of capillary rise in capillary tubes appraised pore size- and reference liquid-dependent accuracy of the CRM test. While the particle configuration and angularity influence the wetting front kinetics due to variations in the vertical component of specific surface tension force, they have a minimal impact on contact angle measurements at a constant porosity in the CRM test. Air entrapment can compromise the accuracy of CRM measurements in densely packed materials. In the SDM, the measured contact angle decreased with increasing base-plate wettability and mostly differed from the particle-fluid contact angle specified in the simulation. The data indicate that the SDM cannot provide accurate soil-fluid contact angles if the droplet penetrates the particle layer and touches the base plate. Sparse particle arrangements and particle angularity promote droplet penetration and reduce accuracy.