An electrolyte is to move through a capillary or porous plug by a pressure gradient. The excess charges around the particles or wall are carried along by the liquid. Their accumulation down stream causes the build-up of an electric field which drives an electric current back (by ionic conduction through the liquid) against the direction of the liquid flow. A steady state is quickly established and the measured potential across the capillary is called the streaming potential.
The streaming potential E is measured at different gas pressures P. The Helmoltz-Smoluchovski formula is used to correlate this relation to the Zeta potential :
where ε is the medium permitivity, λ the medium conductivity, λs the surface conductivity, η the medium viscosity, ζ the Zeta potential, and r the capillary radius.
ζ is extracted from the simplified formula :
(ζ is expressed in mV, E in mV, P in mbar, and, λ in S.cm-1), where C is a constante including the dielectric constante of the medium and the solution viscosity, both depending on the température T. C is phenomenologically given by the following relation :
C = 16,32 – 0,35197 T + 0.00351 T2
(T is expressed in °C).