Fig. 4. Laboratory measurements of the electrokinetic coupling coefficient. (a) Sketch of the experimental setup (ZetaCad) showing pore fluid reservoirs R1 and R2 (1), sample tube (2), pressure sensors (3), voltage electrodes connected to an impedance meter (4), and measurements of the electrical conductivity of the electrolyte (5). The pressure level in the reservoirs is controlled by adjusting the pressure with N₂ gas. (b) Laboratory measurement of the electrokinetic coupling coefficient of the sand used in the sandbox experiment with tapwater.