The effect of contact angle on saturation overshoot

For certain porous media and initial conditions, constant flux infiltrations show a saturation profile that exhibits overshoot. This overshoot, which is the cause of gravity-driven fingering, cannot be described by standard models of unsaturated flow, and it is likely controlled by the exact nature of the pore filling at the initial front. These frontal dynamics have recently been studied as a function of infiltrating flux and infiltrating fluid and were present in our earlier paper. The overshoot behavior is found to disappear below a certain flux which has been named the overshoot flux. We found that, except for water, the overshoot flux q _over is a linear function of surface tension a divided by the viscosity μ, q _over ~ σ/μ. This is expected if the physics at the front that controls overshoot are the same physics that controls the capillary desaturation curve, but the overshoot flux for water did not fall on this curve. To resolve this discrepancy, we conducted additional experiments to test how wettability affects the saturation overshoot.