  Recently, Roht et al. [J. Contam. Hydrol. 145, 10-16 (2013)] observed that
the presence of suspended non-Brownian macroscopic particles decreased the
dispersivity of a passive solute, for a pressure-driven flow in a narrow
parallel-plates channel at low Reynolds number. This result contradicts the
idea that the streamline distortion caused by the random diffusive motion of
the particles increases the dispersion and mixing of the solute. Therefore, to
estimate the influence of this motion on the dispersivity of the solute, and
investigate the origin of the reported decrease, we experimentally studied the
probability density functions (pdf) of the particle velocities, and
spatio-temporal correlations, in the same experimental configuration. We
observed that, as the mean suspension velocity exceeds a critical value, the
pdf of the streamwise velocities of the particles markedly changes from a
symmetric distribution to an asymmetric one strongly skewed to high velocities
and with a peak of most probable velocity close to the maximum velocity. The
latter observations and the analysis of suspension microstructure indicate that
the observed decrease in the dispersivity of the solute is due to particle
migration to the mid-plane of the channel, and consequent flattening of the
velocity profile. Moreover, we estimated the contribution of particle diffusive
motion to the solute dispersivity to be three orders of magnitude smaller than
the reported decrease, and thus negligible. Solute dispersion is then much more
affected by how particles modify the flow velocity profile across the channel,
than by their diffusive random motion.
