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

Abstract

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°). 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.Abbreviations: SWRCs, soil water retention curves. Soil water retention curves or capillary pressure-saturation relations are a relation between soil water content and suction as soils dry or wet. The boundaries are the main drying and main wetting curves, which are enclosed by ascending or descending scanning curves (Tompsett et al., 2005). Soil water retention curves are usually obtained on the assumption that soils small water menisci spread in the surface of particles yielding contact angles near 0°. While this is true at high water contents, where water fills the pores 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. According to Young-Laplace equation (Fisher, 1926), a contact angle of 90° marks the theoretical threshold for attraction or repulsion between two particles or infiltration to occur or not in granular media. This is disputed and shown to be as low as 55° (Hamlett et al., 2011). This study focus on wettability conditions 0 to 90°, where water retention occurs but is likely to be influenced by the contact angles. Czachor et al. (2010) obtained the SWRCs for sands prepared at different wettability levels within the 0 to 90° range and up to suctions of 4 kPa by the hanging-column method. The results revealed differences between the drying and wetting paths. Liu et al. (2012) obtained the SWRC at low suctions with the hanging column method and the pressure plate up to t...

Water repellent soils: the case for unsaturated soil mechanics

“…Notably, hydrophilic and water-repellent retention curves were similar in the low (<AEV) and high (greater than residual) suction ranges. Similar results were found by [16] and [18]; lower gravimetric water contents were found for hydrophobised granular soils than for the same soils in a hydrophilic condition for a given suction. Possible desaturation mechanisms for a hydrophilic and water repellent granular soil are shown schematically in Figure 3.…”

“…Notably, tested soils in [16] to [18] displayed positive suctions. Soils in [17] were not tested in wetting, which is more likely to produce higher contact angles.…”

“…Soils in [17] were not tested in wetting, which is more likely to produce higher contact angles. Samples in [16] and [18] were tested in both drying and wetting. However, hydrophobising reagents were selected to mimic natural water repellent substances, whose contact angles were either <90 degrees or reduced from ° in the presence of water [4].…”

“…Slope models were prepared by mixing hydrophilic and water repellent sand ( =0.13 mm) in known ratios from 0 to 100%, in 10% increments, creating soils of heterogeneous wettability. Water repellent sand was prepared via silanisation, as per [16,18]. Results are shown schematically in Figure 4.…”

Effect of particle size on the measurement of the apparent contact angle in sand of varying wettability under air-dried conditions

“…Lourenço et al [5] demonstrated that for a mixture of sand and clay, its ability to retain water decreased as the ACA approached 90. Shear strength of non-wettable soils as evaluated by direct shear tests performed by Bardet et al [6] using soils coated by wax demonstrated that there was no change in the angle of shear resistance compared to wettable samples.…”