Measurement of initial soil-water contact angle of water repellent soils

Water repellent soils rue common throughout the world. Water repellency significantly affects infiltration, evaporation, and other water-soil interactions. Various indices, such as the water-solid contact angle (theta), water drop penetration time (WDPT), and 90 degrees surface tension (gamma(ND)), have been proposed to characterize the degree of water repellency. The water repellency of many soils is not stable, but changes,vith time after contact with water. No method is available to measure the initial soil-water contact angle. The purpose of this study was to establish a technique to measure the initial soil-water contact angle. We combined previously published theoretical relationships to develop the equations cos theta = [(gamma(ND)/gamma(w))(1/2) -1] and h(p) = 2[(gamma(w)gamma(ND))(1/2) - gamma(w)]/r rho g, where gamma(w) is the water surface tension, h(p) is the breakthrough pressure head, ris the pore radius, rho is the water density, and g is the gravitational constant. The validity of these relationships was established by treating two sand materials with octadecylamine or solvent extracts from peat moss to create various levels of water repellency. An instrument was developed to measure h(p). A linear relationship was found between h(p), and gamma(ND)(1/2), as specified by the equation. The value of r tvas computed from the slope h(p) vs. gamma(ND)(1/2) curve, and this r value was combined with h(p) in the capillary rise equation to compute cos theta. Good agreement was found between measured and predicted relationships between cos theta and gamma(ND)(1/2). The major conclusion is that the value of theta can be determined by measuring gamma(ND), which is easily done in the field or laboratory.