Methods for Laser-Induced Fluorescence Imaging of Solute Plumes at the Darcy Scale in Quasi-Two-Dimensional, Refractive Index-Matched Porous Media

Abstract

A spectrum of environmental, industrial, and biomedical processes depends on solute plume spreading in laminar flows within porous media. The literature includes a number of approaches to model solute movement in this context, but fewer techniques are reported to measure it experimentally. To address this gap, this study describes novel experimental methods that permit measurement of solute plume spreading at the Darcy scale using laser-induced fluorescence in a 50 cm × 50 cm × 4 cm quasi-two-dimensional apparatus filled with refractive index-matched porous media. Fluorescence is provided by Rhodamine dye; refractive index matching is provided by borosilicate glass in glycerin. The depth-averaged porosity distribution of the porous media is determined using a novel optical method, whose results are used to investigate solute transport in two spatiotemporally varying flow fields, (1) a push–pull pumping sequence and (2) a sequence to induce stretching and folding of the solute plume. Quantitative analysis of replicate experiments reveals an extraordinary degree of reproducibility based on spatial moments, reactor ratio, and correlation coefficient.