Mechanism for Finger Persistence in Homogeneous, Unsaturated, Porous Media

Wetting-front instability, or gravity-driven fingering, can occur during vertical infiltration. Previous studies found that fingers persist over long periods of constant infiltration and in subsequent infiltration cycles. We present a physically based theory to explain finger persistence. The theory is tested using a new technique in which moisture contents are visualized by the transmission of light through unsaturated sand, recorded with a video camera, and then analyzed by video digitizing computer hardware. Theory and experiments show that hysteresis in the moisture characteristic relationship explains the persistence of fingers in time.