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) were 20 and 40 mu m, the particle bulk volume fraction phi bulk was 0.2, and the cell aperture was 420 +/- 10 mu m. 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 (P is an element of(s)) between 350 and 450). A reference measurement, identical to the displacement ones but without particles in the flow (phi bulk = 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 phi bulk and alpha. 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 P is an element of(s) studied, and consequently a dispersion coefficient D and a dispersivity l(d) of the solute were measured.