Microscopic imaging technique for solute transport in rough-walled rock fractures using micro-PIV

Experimental observation and measurement are essential to fundamentally understanding the processes that govern fluid flow and mass transport in rough-walled fractures. The micro-PIV (micro-Particle Image Velocimetry) technique has been introduced for flow visualization inside microscale rough-walled fractures. However, the methodology for mass transport visualization has yet to be established, which is crucial for the analysis/quantification of mass transport and dispersion in rough-walled fractures. This study presented the improved micro-PIV technique to visualize mass transport and measure solute concentration in rough-walled rock fractures. Calibration processes for determining the solute concentration from the measured fluorescence intensity were presented, and measured concentrations were applied to the solute transport and dispersion analyses to validate the measurement technique. The microscopic imaging and analysis demonstrated the transition from macrodispersion to Taylor dispersion-dominant transport. As the fluid velocity increased, higher concentration gradients occurred across the fracture aperture, enabling the solute to break through rapidly along the main flow channel in the middle of the fracture aperture. We successfully visualized channelized solute transport associated with eddies that accounts for Taylor dispersion and non-Fickian transport. This technique enables phenomenon-based experimental research on fluid flow and solute transport in microscale rock fractures, which used to remain in the realm of numerical simulations. Our improved visualization technique will contribute to experimentally elucidating mass transport processes in rough-walled rock fractures.