Water repellency as conditioned by particle size and drying in hydrophobized sand

Abstract The effects of particle size and soil moisture on water repellency (WR) from hydrophobized sand are studied in this research. Quartz sand samples were separated into three sieve fractions: 0.5–2 mm (coarse sand, CS), 0.25–0.5 mm (medium sand, MS), and 0.05–0.25 mm (fine sand, FS). WR of sand was induced using different concentrations of stearic acid (SA; 0.5, 1, 5, 10, 20 and 30 g kg− 1). Moist samples have been exposed to two types of drying: air-drying at standard laboratory conditions and oven-drying at 50 °C. Change in moisture content, and water repellency has been monitored every 24 h for 10 days. After 1 day of drying, SA concentrations ≥ 10 g kg− 1 caused extreme WR in oven-dry samples, independently of sieve fraction. In air-dried samples, time of drying and decreasing soil moisture content increased WR, but an erratic behaviour was observed in MS and FS samples. All air- and oven-dried samples became extremely water repellent after 7 days of treatment. At all SA concentrations and drying temperatures, WR was extreme in the CS fraction after one day. Superhydrophobicity of CS samples is suggested as a possible explanation of this response. In MS and FS samples, water repellency showed an erratic behaviour at lower SA contents, which may be due to contact of water droplets with a high proportion of areas not covered by hydrophobic coatings. The higher severity of WR observed in CS is in agreement with the idea of hydrophobicity associated with coarser particles. Coarse-textured soils have a lower specific surface than fine-textured soils, and a limited amount of organic matter may cause higher WR than in finely textured soils.