Measurement of Initial Soil-Water Contact Angle of Water Repellent Soils

Abstract

Water repellent soils are common throughout the world. Water repellency significantly affects infiltration, evaporation, and other water-soil interactions. Various indices, such as the water-solid contact angle (θ), water drop penetration time (WDPT), and 90° surface tension (γND), have been proposed to characterize the degree of water repellency. The water repellency of many soils is not stable, but changes with 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 cosθ = [(γND/γw)1/2 − 1] and hp = 2[(γwγND)1/2 − γw]/rρg, where γw is the water surface tension, hp is the breakthrough pressure head, r is the pore radius, ρ 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 hp. A linear relationship was found between hp and γ1/2ND, as specified by the equation. The value of r was computed from the slope hp vs. γ1/2ND curve, and this r value was combined with hp in the capillary rise equation to compute cosθ. Good agreement was found between measured and predicted relationships between cosθ and γ1/2ND. The major conclusion is that the value of θ can be determined by measuring γND, which is easily done in the field or laboratory.