Background Water-repellent sand soil covering the surface of hydrophilic soil can effectively retain water and prevent evaporation. After water accumulation on the surface of water-repellent sand soil, water will penetrate the water-repellent sand soil to reach the hydrophilic layer. Water-repellent sand soil has a wide application prospect in arid areas due to its characteristics of arid water retention and rainfall permeability. It is also difficult for people to grasp the pattern of water repellent sand permeability and the recovery of water repellent after permeability. As a key parameter reflecting the interface characteristics of soil particles, the contact angle is of great significance for the study of soil-water characteristics of water-repellent soil.

Methods The changes of the contact angle of soil particles in different humidification starting points and the process of drying and wetting were studied by experiments, and the evolution pattern of the contact angle under the condition of the wet-dry cycle of soil particles was revealed. Sand soil samples with non-uniform water repellent and uniform water repellent were prepared by dry method and wet method respectively. The natural hydrophilic sand was used as a control. The octadecylamine was used to mix dry sand in dry method, and the dichlorodimethylsilane was used to soak sand in solution in wet method. The contact angle of sand soil surface at the same position with different humidification starting points was measured by laying drop method. The indirect antenna variation of sand particles during dehumidification and humidification was observed by direct observation method.

Results 1) The initial contact angle of the sand soil particles surface affects the size of the contact angle. 2) There is no significant difference in the variation of indirect antennae of soil particles during the dehumidification process of hydrophilic or repellent sand soil, but the variation of contact angle between the wetting of repellent particles and the wetting of hydrophilic particles is very different. The contact angles of the hydrophilic particles measured by the direct observation method have little difference between the humidification and dehumidification processes. The initial contact angle and average contact angle of repulsive samples are much larger than that of hydrophilic samples, and the contact angle of repulsive samples basically decreases with the increase of time. 3) The humidification (dehumidification) history has a reuse effect on the contact angle of the sand surface can be explained by Cassie equation. 4) The variation of contact angle of soil particles in the process of humidification or dehumidification can be summarized into five modes, namely, conventional mode, differential mode, abrupt mode, defect mode and soil and water repellent mode. Abrupt mode and sand soil and water repellent mode are not common, and the other modes cause the contact angle of soil particles to fluctuate in a small range. In the process of sand soil repulsion and humidification, there is no liquid bridge force connecting particles, so there is no sudden change mode in the process of contact angle change.

Conclusions Soil-water characteristic curve lag is not obvious due to contact angle lag, and the repulsive and hydrophilic humidification modes are completely different, which is the key to water seepage after repulsive water and sand soil. The research results will provide support for mastering soil-water characteristics of water-repellent sandy soil.