Abstract:The earth surface soil, especially cultivated soil, contact angles of soil may be changed by external factors. Hydrophilic soil will translate into hydrophobic soil because of the bigger contact angle. Repellent soil will bring a series of agricultural and environmental problems. To develop the study to the pore water-air shape distribution in different contact angle soils, some tests were carried out. The experimental results showed that the increase of contact angles between soil particles and pore water can make the thickness of the closed-water soil layer reduced. When the contact angle was increased to a certain value, the closed-water soil layer thickness would remain unchanged. With the increase of the contact angle, the thickness of closed-air layer was reduced. It was different from the continuous solid surface, the contact angel of soil granule was less than 90°, even reduced to 36°, it would also appear the water-repellent phenomenon obviously, but with the increase of the saturation degree in the sand, the water-repellent phenomenon would disappear. The corresponding saturation of water repellent and hydrophilic conversion was basically consistent with the corresponding saturation of closed-water system and bi-opened system transformation. Therefore, the critical water content of hydrophilicity to hydrophobicity was closely related to pore water-air configurations. The matric suction in different pore water-air configurations was analyzed with ideal models, the causes of this phenomenon was the disappearance of water-air surface centers of curvature in water side (contact angle was less than 90°), the matric suction caused by water-air surface was greater than zero. The results suggested that it was not enough to focus on chemical property of soil water repellency, the mechanics of interface was also an important aspect of repellent soil study.
