Hysteresis in the Soil Water Retention of a Sand-Clay Mixture with Contact Angles Lower than Ninety Degrees

Soil water retention curves (SWRCs) are usually obtained on the assumption small water menisci spread in the surface of particles yielding contact angles near zero degrees. While this is true at high water contents, where pores are filled by water or a film of water covers the particles, it is less likely for drier states, where the adhesion of water menisci to the surface of the particles is controlled by the nature of the particle surface (chemistry and surface roughness) and the presence of organic matter. Here, we investigate hysteretic effects in the relation between water content and soil particle wettability versus suction for model samples following drying and wetting paths. A comparison is done for a model material (mixture of sand and clay) with and without water repellent substances, being the samples with water repellent substances in a subcritical water repellent state (contact angles < 90 degrees). Wettability was manipulated by treatment with organic acids that mimic the chemistry of natural water repellent substances. Suction was measured directly with a high suction tensiometer and wettability via contact angles by the sessile drop method. The results revealed lower water retention and greater contact angles for the subcritical water repellent samples, following both drying and wetting paths. Hysteresis was present in the relation between the contact angles and suction for the subcritical water repellent samples.