Applicability of the Washburn theory for determining the wetting angle of soils

The Washburn theory is frequently applied to determine the wetting angle of soils. It is based on the porous medium being characterized as a set of cylindrical, straight capillaries. It is clear that this is an oversimplification as real soil pores vary at least as regards two important features: cross‐section and tortuosity. In this study, a mathematical model of meniscus movement in a tortuous, sinusoidal shaped capillary was developed and the obtained analytical expressions for the wetting angle and for the pore radius were compared with the appropriate formula concerning the Washburn theory. The results demonstrate that the wetting angle of water in soil determined on the basis of the measured wetting front kinetics and of Washburn equations is likely to have an overestimated value due to the wavy, tortuous nature of inter‐soil grain pores. The above conclusion was confirmed by capillary rise experiments. Wetting angles of methyl alcohol and of water measured on flat glass were 0 and 27·4° , respectively. Apparent wetting angles calculated from capillary rise in powdered glass kinetics and Washburn equation were 70 and 83° respectively. If the pore structure characteristics of porous media are not taken into account the applicability of the Washburn theory for soil wettability estimation seems to be very limited.