The resemblance between flow patterns in submerged or open-to-air outlet hydrophobic capillary tubes and water infiltration in hydrophobic porous media

Zhongfu Wang, Rony Wallach*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Porous media (PM) flow is essential in many natural and industrial processes. The PM can be either hydrophilic or hydrophobic, substantially affecting its imbibition and flow. As opposed to a decreasing infiltration rate with time (concave shape) in hydrophilic PM, an increasing rate with time (convex shape) has been observed in hydrophobic PM. A commonly accepted mechanism to explain the convex infiltration pattern is still lacking. The current study elucidates the latter by focusing on flow in hydrophobic capillary tubes under different boundary conditions. A convex-shape meniscus propagation in a hydrophobic capillary tube was observed for low ponded water depth at the tube inlet, whereas a concave shape propagation was observed for high ponded water depths. To the best of our awareness, such a finding is innovative. Once the meniscus reached the tube outlet, the hydraulic conductivity of the hydrophobic tube depended on the driving head for an open-to-air outlet while having a constant value for a submerged one. The saturated hydraulic conductivity for an open-to-air capillary tube depends on the ratio between ponded water depth and tube water-entry head,R0, and reaches a constant value measured for a submerged tube for R0 > 20.8. While successfully predicting the wetting-front propagation for R0> 20.8, the modified Lucas-Washburn (LW) equation with slip boundary condition failed to predict the convex infiltration patterns. The observed infiltration patterns in hydrophobic porous media in general and soils in particular, seem inherent since capillary tubes are fundamental units in such media. Moreover, flow in PM has been frequently modeled using a bundle of nonuniform capillary tubes. The failure of the modified LW equation to predict the measured data for R0< 20.8 calls for a new approach to model water flow in hydrophobic tubes and PM.

Original languageAmerican English
Article number128008
JournalJournal of Hydrology
Volume611
DOIs
StatePublished - Aug 2022

Bibliographical note

Publisher Copyright:
© 2022 Elsevier B.V.

Keywords

  • Convex infiltration pattern
  • Hydrophobic capillary tube
  • Immersed and open-to-air tube outlet
  • Low driving head
  • The flow resistance of capillary tubes

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