Light transmission measurement of solute dispersion in non-Brownian suspension flow

We characterize and employ a light transmission technique to measure the dispersivity of a solute in the flow of a neutrally-buoyant non-Brownian spherical particle suspension in a Hele-Shaw cell (parallel- plate axial flow). Particle radii (a )w ere 20 and 40μm, the particle bulk volume fraction φbulk was 0.2, and the cell aperture was 420 ± 10 μm. In each displacement experiment a suspension with a colouring solute displaces a transparent one at constant flow rates ranging from 0.721 to 0.928 mL/min (corresponding to solute Peclet numbers ( Pe s) between 350 and 450). A reference measurement, identical to the displacement ones but without particles in the flow (φbulk = 0), were performed in the same experimental assembly for comparison purposes. A light calibration related the transmitted intensity I to the solute concentration c for each combination of φbulk and a. The time variation of the solute concentration was found to be well-fitted by the solution of the advection-dispersion equation (ADE) in the range of Pe s studied, and consequently a dispersion coefficient D and a dispersivity ld of the solute were measured.