Demonstration of Reversible Dispersion in a Darcy-Scale Push-Pull Laboratory Experiment

A solute transport experiment was conducted in a 50 cm × 50 cm × 4.4 cm apparatus filled with monodisperse glass beads and glycerin, using Rhodamine 6G dye as the solute. The dye movement was tracked through two cycles of a push-pull flow pattern, alternating between approximately radial outward flow and approximately radial inward flow. Depth-averaged concentration was measured during the experiment and integrated over the angular direction during post-processing to obtain a radial distribution of concentration. The experimental results were replicated using particle tracking, simulating advection and dispersion in the radial direction, including reversible dispersion during extraction. Results show that spreading of the measured concentrations follow a Fickian dispersion model during steps with radial outward flow. During steps with radial inward flow, spreading is reversed and can be modeled as reversible dispersion. The observed reversal in spreading is an indication of incomplete mixing within the pore space.