斥水性砂土水-气形态及其对斥水-亲水转化的影响分析

处于地球表面的土壤,尤其是耕作层土壤,受外界因素影响其接触角可能会发生改变。通过试验对不同接触角的壤质砂土中的孔隙水-气形态分布状况进行研究,试验结果表明:砂土颗粒与孔隙水间的接触角增大会使表层土壤中水封闭土层厚度减小,但接触角增大到一定值后,水封闭层的厚度不再发生变化。随着接触角的增大,气封闭层的厚度不断减小。与连续固体表面不同,砂土颗粒的接触角小于90°甚至降低至36°也会出现明显的斥水现象,但随着砂土中饱和度的增大,斥水现象会消失,砂土斥水与亲水转化时对应的饱和度与水封闭向双开敞转化时对应的饱和度基本一致,因此,砂土亲水与斥水转化时对应的临界含水率与孔隙水气分布形态密切相关,通过理想模型对两种不同水-气形态下土壤的基质吸力变化分析可以发现,产生这种现象是因为当土壤由水封闭变为双开敞时,水-气交界面在液体侧的曲率中心消失(接触角小于90°),气-液界面引起的基质吸力恒为"正"。

Pore Water-Air Configurations in Water Repellent Sandy Soil and Its Effects on Transformation of Hydrophilicity to Hydrophobicity

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.