Epi-Fluorescence Imaging of Colloid Transport in Porous Media at Decimeter Scales

A noninvasive epi-fluorescence imaging technique was developed for real-time observation of colloid transport in porous media at decimeter scales. Fluorescent latex microspheres and translucent quartz sand were used as a model colloid-porous medium system. Various calibrations were performed for accurate conversion of fluorescence intensities to microsphere concentrations. Fluorescence intensities were found to linearly increase with microsphere concentrations (5 × 10⁵−5 × 10⁸ spheres/mL in saturated sand) and with camera exposure time. Fluorescence intensities also increased with sand thickness (saturated with microsphere solution), indicating that the fluorescence signals detected by the imaging system were integrated signals from the entire thickness (10 mm) of the sand. A set of microsphere transport experiments was conducted to demonstrate the versatility of the imaging system. Excellent mass recoveries (93−103%) were achieved in all transport experiments, demonstrating the robustness of the imaging system for quantitative study of colloid transport. The system allowed the change of flow velocity, ionic strength, and flow direction within one transport experiment and the real-time, quantitative monitoring of the movement of microspheres in packed sand, greatly reducing the time and effort needed for similar work with traditional column experiments.