Redistribution of a passive tracer in a porous substrate under a surface washing flow at high Péclet number*

We study the cleaning of a liquid chemical contaminant from the surface of a flat, wet porous medium. We focus on quantifying the extent to which cleaning the surface of the porous medium can spread the contaminant over the porous material. This modifies the concentration distribution within the porous material - potentially increasing the area that has been contaminated - compared with the case if there were no cleaning flow. To gain an understanding of this redistribution phenomenon, we propose a simple model for a shear washing flow over a small, flat drop releasing a dilute passive tracer of constant concentration, initially situated on the surface of the porous medium. As the flow progresses over the porous surface, the tracer is transported from the drop into the flow through advective and diffusive processes, while molecular diffusion across the flow-porous interface encourages mass transfer into the porous medium downstream of the drop. We will discuss an asymptotic regime, based on an intermediate time scale for high-Péclet-number flow, valid at distances downstream of the drop, that can be used to study agent redistribution in the porous substrate. To substantiate this analysis, we performed finite element simulations using COMSOL. The results of these simulations will be compared with our experimental data to confirm the mechanism redistribution of contaminant during the decontamination of porous media.