C3. On Wettability and Unstable Flow in Porous Media
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The effect of contact angle (CA) between water and porous media on flow regime in water repellent and sub-critically repellent soils will be illustrated and discussed. Water flow pattern that was monitored in a thin transparent flow chamber that ensures a 2D flow for water repellent soils (initial contact angle θ > 900) and sub-critical repellent porous media (permanent contact angle 300 < θ < 750) was remarkably affected by the CA. Plumes that were initiated by a point source at the soil surface (local perturbation) had different shape, size and longitudinal and transversal water content distributions for different contact angles and water application rates. The contact angle and water application rate control in particular the development of sharp water content decrease along the peripheral wetting front and non-monotonous water content variation along vertical cross section directions that are associated with unstable flow. For the low water application rate, thin and long plumes with saturation overshot behind the wetting front were observed for higher contact angles and wide plumes with lower average water content flow for the lower contact angles. On the contrary, negligible differences among the plumes were observed in all media for the high water application rates. Differences among the plumes became apparent only during the following drainage stage. The plumes in the media of high contact angle barely changed during the drainage period while the water content gradient along vertical cross sections turned positive at the lower part and negative along the upper part of the plumes in the media of lower contact angle. The discussion will include simulations made by a model developed for a bundle of capillaries of constant and variable cross section area and different contact angles.
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