Millifluidic investigations of solute transport and of its link to electrical conductivity

International audiencePore scale characterization of flow velocities and concentration spatial distributions is a key to understanding non-Fickian transport, mixing and reaction in porous media. We use millifluidic setups to investigate such processes in transparent saturated and unsaturated porous media, at the pore scale. The porous media are quasi-two-dimensional, consisting of a Hele-Shaw cell containing cylindrical grains. They are made by soft lithography with full control on a geometry containing thousands of grains. The setup allows for unsaturated flows, be it primary drainage/imbibition, or the injection of two fluids (f. e. water and air) at the same time. A camera positioned above the flow cell records the distributions of fluid phases, at regular times, providing also the position of solid tracers and spatially-resolved images of light emissions inside the flow cell. The pore scale velocity field is measured from the tracking of solid tracers (microspheres), while full pore scale concentration fields are measured accurately in passive transport experiments, using fluorescein and converting the emitted light intensities into solute concentrations. Using two chemo-luminescent liquids, the reaction of which produces photons in addition to the reaction product(s), we can also measure the local production rate of the reaction product as the reactive liquids flow through the system. We confront the experimental pore scale data to numerical simulations and models that upscale transport and mixing properties. We shall address two examples of studies based on this approach. Firstly, we shall discuss how medium desaturation affects solute mixing [1,2]. Secondly we shall discuss the link between the transport of charged chemical species and electrical signals that can be measured in the experimental cell [3], and the subsequent implications in terms of electrical imaging of the subsurface