Time-dependent contact angle and repellency persistence in water-repellent Japanese forest soils in relation to droplet geometry and line tension

Water repellency (WR) restricts the spontaneous wetting of soil and lowers the affinity between soil and water. Studies focused on temporal alteration of droplet contact angles on hydrophobic soil substrates and their relation to persistence of WR are highly limited. This study examined time-dependent contact angle and WR persistence considering geometry and line tension droplets using six hydrophobic forest soils. Soils were collected from Japanese cedar (Cryptomeria japonica, CED1, CED2), Japanese cypress (Chamaecyparis obtusa, CYP1, CYP2), and cedar/cypress mixed (MIX1, MIX2) forests. The time-dependence of apparent and geometric soil-water contact angles (θ_A and θ_G, respectively) and repellency persistence, using the water drop penetration time (WDPT), were determined. In all tested soils, both θ_A and θ_G showed a gradual continuous decline with increasing time to reach zero within a period of ~5–6 min, with strong negative exponential correlations (R² ≥ 0.90). The θ_A showed a strong positive linear correlation with θ_G (R² > 0.99; p < 0.05), indicating the preservation of a fair spherical segment shape of droplets during spreading irrespective of the possible roughness on granular soil surfaces. The line tensions at the periphery of the contact circle of the water droplet on the tested hydrophobic soil surfaces were in a range of 200–300 μJ m^–1, which was within the previously reported order-of-magnitude range (10⁻⁵–10² μJ m⁻¹). The timedependent nature of the contact angle in the present study can most likely be attributed to water molecule adsorption processes or amphiphilic molecular reorientation processes. The soil-water contact time until contact angle receded below 90° (CT-θ_90°) was not equivalent to WDPT, emphasizing that the time-dependent decline of contact angle would not correspond with water infiltration into the soil matrix. Still, the strong positive linear correlation between CT-θ_90° and WDPT (R² = 0.926; p < 0.05) indicated a close relation between these two parameters.