Water Potential and Aggregate Size Effects on Contact Angle and Surface Energy

Soil wettability affects hydrological processes like infiltration, percolation, preferential flow, and surface runoff. Wettability is related to the soil‐water contact angle, which in turn depends on the solid surface free energy. Little is known, however, about contact angles and their dependence on soil water potential. The main objective of this study was therefore to investigate the dynamics of contact angle due to variation of the water potential. Aggregate fractions of 2‐ to 4‐, 1‐ to 2‐, and <1‐mm diameter and corresponding homogenized material of a subcritical water repellent Orthic Luvisol were studied at water potentials of −1000, −154, −30, and −0.14 MPa. Wettability was assessed in terms of the advancing contact angle by the capillary rise method (CRM). Additionally, we calculated the surface free energy. Results showed, that the contact angle increased as water potential increased to a specific level. It was found for several soil samples, that above this water potential level, the contact angle decreased again. The change of contact angle due to variation of water potential reached nearly 90° for one sample. Contact angles of homogenized fractions were slightly larger than those measured for the aggregate surfaces. Surface free energy was consistently between 55 and 65 mJ m −2 with relative contributions of the dispersion and polar components to surface free energy of approximately 1/3 and 2/3, respectively. We conclude, that the assessment and physical description of the specific water potential for which a surface becomes wettable is a key factor for a better understanding of soil wetting.