Imbibition of saline solutions into dry and prewetted porous media

Infiltration of saline solutions and pure water into homogeneous packs of prewetted and air-dry silica sands was investigated using a light transmission system. Four sand grades and five solutions were considered. Narrow fingers with a sharp, almost saturated, wetting front were observed in the air-dry sands. The water content left behind the fingertip of saline solutions was higher than for pure water, resulting in a greater lateral expansion of the saline fingers over time. The rate of lateral expansion scaled with the square root of time, likely due to classic liquid sorption with the possible addition of water vapor diffusion. At early time, the salty fingers moved faster, but were ultimately overtaken by the pure water fingers. In prewetted sand, the wetting fronts were diffuse and never exceeded 26% saturation, less than third that seen in the initially air-dry media. The plumes in the prewetted sand were also much wider and their shape varied. In the prewetted sand the elevated surface tension of the saline solutions was the major cause for the observed differences in finger width and velocity, yet appeared to be insignificant in air-dry sand. Here, in addition to the density effect, absorption of the saline solution to the silica sand influenced the depth of wetting, finger velocity, and subsequent lateral expansion.