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

Water repellent soils rue common throughout the world. Water repellency sig nificantly affects infiltration, evaporation, and other water-soil interact ions. 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 wate r repellency of many soils is not stable, but changes,vith time after conta ct with water. No method is available to measure the initial soil-water con tact angle. The purpose of this study was to establish a technique to measu re the initial soil-water contact angle. We combined previously published t heoretical relationships to develop the equations cos theta = [(gamma(ND)/g amma(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 pre ssure head, ris the pore radius, rho is the water density, and g is the gra vitational constant. The validity of these relationships was established by treating two sand materials with octadecylamine or solvent extracts from p eat moss to create various levels of water repellency. An instrument was de veloped 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 agreem ent was found between measured and predicted relationships between cos thet a and gamma(ND)(1/2). The major conclusion is that the value of theta can b e determined by measuring gamma(ND), which is easily done in the field or l aboratory.