Column Flow: Connecting Pathways in Homogeneous Soils.

The occurrences of column or fingered flow in homogeneous sandy or water repellent soils has been well established during the last 30 years.  These columns wet up the soils partially and can transport water and solutes rapidly from the surface to the ground water.  The columns are formed by instability in the wetting front and then are preserved behind the wetting front by hysteresis in the soil characteristic curve. Although many experiments have been carried out on column flow, there is still no satisfactory explanation of the behavior at the wetting front itself.

In this presentation, we will show that the instability is a direct consequence of pore scale processes where the contact angle becomes less concave under transient conditions at the wetting front than under static conductions. This phenomenon is known under the general name of dynamic contact angle. Many contradicting theories have been proposed for explaining the dynamic contact angle.  In this presentation we will show that by solving the fluid flow equations at the pore scale with the incompressible Navier-Stokes’ equation, the dynamic contact angle is a function of the velocity of the wetting front (and thus the pressure applied) and to lesser degree by the static contact angle as long as it exceeds the pressure needed for water to enter the pore. Our results compare well with the experimental observations of Hoffman in the seventies in which the change in contact angle was measured as a function of flow velocity.