Clay effects on the contact angle and water drop penetration time of model soils

Clay is an important factor that affects soil water repellency. The purpose of the present study was to assess the effects of clay content on contact angle and water drop penetration time (WDPT), and to examine their relationship. Model soils were prepared with silica sand and kaolinite, and hydrophobized using stearic acid (SA). The effect of kaolinite on the contact angle changed with the hydrophobic organic matter content of the model soils. The contact angles of organic matter free samples increased with increasing kaolinite content. This was explained by a decrease in the surface-free energy of kaolinite with the adsorption of water at 57% relative humidity. In samples with SA content above 1.6 g kg-1, the contact angle decreased with increasing kaolinite content because the hydrophobizing effect of SA was lower at higher kaolinite content owing to the lower extent of hydrophobic organic coatings. Kaolinite increased the WDPT in non-repellent soils, but decreased the WDPT in slightly to severely repellent soils. Clay effects on the contact angle and WDPT were not comparable and, consequently, the relationship between contact angle and WDPT was found to be non-uniform at different clay contents.