Universality of a surface tension - Contact-angle relation for hydrophobic soils of different texture

Wettability of soil affects a wide variety of processes in soils like infiltration, percolation, preferential flow, and surface run-off. Even though efforts have been made to determine contact angles and surface tension or energy of smooth surfaces, the determination on granular materials like soil particles remains unsolved. One objective of this study was to test the consistency of contact angles (CA) measured with the newly modified and easy-to-apply Wilhelmy plate method by using solid particles and liquids with defined variations of surface tension. A second objective was to derive basic physical surface properties for the irregularly shaped and chemically heterogeneous soil particles. Advancing contact angles were determined by using model soils varying in texture from clay to coarse sand to check the impact of grain size on the CA measurements. Varying the solid-surface tension with dichlorodimethlylsilane treatments provided for pure-water wetting stages ranging from wettable to extremely hydrophobic. The surface tensions of the liquids were varied from 72 mN m^-1 to 25 mN m^-1 by using water or water-ethanol mixtures. The surface tensions of the model soils were determined with the zero degree-contact angle method following Zisman's critical-surface tension concept. Results show that the measured CA varied continuously with the variations of the surface tension of the liquid and the solid phase, respectively. A general interpretation of the results is possible by using the concept of the Equation of State Approach.