Surface tension effects on the hydraulic conductivity of hydrophobic porous media

Abstract

Under certain conditions of natural or anthropogenic origin, the wettability properties of soils change from hydrophilic to hydrophobic. Such a transition results in substantial changes in water flow dynamics and soil hydraulic properties. Among them, the dependence of the hydraulic conductivity of hydrophobic porous media on the ponded water depth has been measured with the constant water head method and traditional hydraulic conductivity formula. A mechanism that can essentially explain the dependence is still missing. The current study aims to narrow this knowledge gap. Hydraulic conductivities of hydrophobic and hydrophilic glass beads were measured with two column outlet conditions: open to air and immersed in water. The measured hydraulic conductivity of hydrophobic glass beads depended on the ponded water depth with an open-to-air outlet. However, it was independent of the ponded water depth with an immersed outlet. The measured hydraulic conductivity of hydrophilic glass beads was independent of the ponded water depth for both outlet conditions. Surface tension effects at the column outlets decreased the volumetric flow rate of hydrophobic glass beads. Introducing the low volumetric flow rate into the traditional hydraulic conductivity formula without considering the surface tension effects leads to a dependence of hydraulic conductivity on the ponded water depth. This result calls for a new approach to calculate the hydraulic conductivity in hydrophobic porous media.

Keywords

• Hydraulic conductivity
• Constant water head method
• Hydrophobic porous media
• Column outlet condition
• Surface tension