Electrokinetics dependence on water-content in sand

The electrokinetic potential results from the coupling between the water flow and the electrical current because of the presence of ions within water. This coupling is well described in fluid-saturated media, however its behavior under unsaturated flow conditions is still discussed. We propose here an experimental approach which can clearly describe streaming potential variations in unsaturated conditions. Several drainage experiments have been performed within a column filled with a clean sand. Streaming potential measurements are combined to capillary pressure and to water content measurements each 10 centimeter along the column. In order to model hydrodymanics during each experiment, we solve Richards equation in an inverse way which allows us to establish the relation between hydraulic conductivity and water content, and retention relation. The electrokinetic coefficient C shows a more complex behavior than it was previously reported and can not be fitted by the existing models. We show that the normalized electrokinetic coefficient increases first when water saturation decreases from 100% to about 80% - 95%, and then decreases as the water saturation decreases, whereas all previous works described a unifrom decrease of the normalized electrokinetic coefficient as water saturation decreases. We delimited two water saturation domains, and deduced two different empirical laws describing the evolution of the electrokinetic coefficient in unsaturated conditions. Finally, electrical potentials data from four different drainage experiments and hydrodynamics were jointly inversed, including electrical conductivity measurements in order to find a robust description of the electrokinetic coefficient behavior in unsaturated conditions.