Experimental Visualisation of Solute Transport Processes in Rough‐Walled Rock Fractures Using Micro‐<scp>PIV</scp> Technique

ABSTRACT The difficulty in experimentally visualising solute transport leads to theoretical and numerical studies of solute transport in rock fractures. This study presents the comprehensive experimental method for investigating microscopic solute transport processes in rough‐walled fractures, including the setup of the micro‐PIV system combined with time‐lapse imaging, solute transport visualisation and a detailed calibration process to convert fluorescence intensity into solute concentration for solute transport analysis. The transition of the dispersion regime from macrodispersion to Taylor dispersion with increasing fluid velocity is experimentally observed for the first time. Increasing fluid velocity leads to channelised transport through high‐velocity flow channels and slower transport in low‐velocity regions, including eddies, near the walls, which creates higher concentration gradients across the fracture aperture, particularly for Re ≥ 1. These gradients account for Taylor dispersion and non‐Fickian transport behaviours. The observed spatiotemporal distribution of solute concentrations demonstrates, experimentally and visually, that mass transfer between the main flow channels and eddies is primarily advective, contrary to the common assumption of diffusive exchange between the two zones. The concentration measurements obtained using the micro‐PIV method combined with time‐lapse imaging show good agreement with concentrations calculated from numerical simulations conducted under idealised low‐velocity conditions, demonstrating the reliability of the experimental approach. The visualisation technique presented here provides the basis for experimentally elucidating solute transport processes in rough‐walled fractures.