Impact of grain roughness on residual nonwetting phase cluster size distribution in packed columns of uniform spheres.

We imaged the pore-scale distribution of air and water within packed columns of glass spheres of different textures using x-ray microcomputed tomography after primary drainage and after secondary imbibition. Postimbibition residual air saturation increases with roughness size. Clusters larger than a critical size of about 15 to 40 pores are distributed according to a power law, with exponents ranging from τ=2.29±0.04 to 3.00±0.13 and displaying a weak negative correlation with roughness size. The largest cluster constitutes 7 to 20% of the total residual gas saturation, with no clear correlation with roughness size. These results imply that activities that enhance grain roughness by, e.g., creating acidic conditions in the subsurface, will promote capillary trapping of nonwetting phases under capillary-dominated conditions. Enhanced trapping, in turn, may be desirable in some engineering applications such as geological CO_{2} storage, but detrimental to others such as groundwater remediation and hydrocarbon recovery.

Fluorescence spectroscopy of Rhodamine 6G: concentration and solvent effects.

Rhodamine 6G (R6G), also known as Rhodamine 590, is one of the most frequently used dyes for application in dye lasers and as a fluorescence tracer, e.g., in the area of environmental hydraulics. Knowing the spectroscopic characteristics of the optical emission is key to obtaining high conversion efficiency and measurement accuracy, respectively. In this work, solvent and concentration effects are studied. A series of eight different organic solvents (methanol, ethanol, n-propanol, iso-propanol, n-butanol, n-pentanol, acetone, and dimethyl sulfoxide (DMSO)) are investigated at constant dye concentration. Relatively small changes of the fluorescence spectrum are observed for the different solvents; the highest fluorescence intensity is observed for methanol and lowest for DMSO. The shortest peak wavelength is found in methanol (568 nm) and the longest in DMSO (579 nm). Concentration effects in aqueous R6G solutions are studied over the full concentration range from the solubility limit to highly dilute states. Changing the dye concentration provides tunability between ∼550 nm in the dilute case and ∼620 nm at high concentration, at which point the fluorescence spectrum indicates the formation of R6G aggregates.